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INTERNATIONAL LABOUR OFFICE

STUDIES A N D REPORTS
Series F (Industrial Hygiene) N o . 17

SILICOSIS
Proceedings of the I n t e r n a t i o n a l Conference
held in Geneva
from 29 August to 9 September 1938

A

Published in the United Kingdom for the International Labour Office,
by

P. S. K I N G and

SON,

Ltd.,

Orchard House, 14 Great Smith Street, Westminster, London, S. W. 1

The thanks of the International Labour Office are due to the
Milbank Memorial Foundation, New York, whose generous assistance has made possible the printing and general distribution of
this volume.

P R I N T E D IN GENEVA BY IMPRIMERIES

POPULAIRES

CONTENTS
Page
INTRODUCTION

1

L I S T OP MEMBERS OF THE CONFERENCE

3

AGENDA OF THE CONFERENCE

5

PROCEEDINGS OF THE CONFERENCE
FntST SITTING. — Item 1 : Recent Advances in the Knowledge of the
Pathology of Silicosis, including the Effect, if any, of Non-Siliceous
Dusts admixed with Silica or Silicates in the Production of
Pulmonary Fibrosis or Tuberculosis
SECOND SITTING. — Item 1 (continued)

8
13

THIRD SITTING. — Item 1 (continued)

15

FOURTH SITTING. — Item 1 (continued)

24

F I F T H SITTING. — Item 2 : Pneumoconiosis of Workers in Coal
Mines and of other Workers exposed to Coal Dust

28

SIXTH SITTING. — Item 2 (continued)

33

SEVENTH SITTING. — Item 3 : Pulmonary Disease (or Disability) due
to Inhalation of Dust in Industries other than those covered by 1
and 2 (with Quali- and Quantitative Estimation of the Composition
of the Dust in Question)

36

EIGHTH SITTING. — Item 4 : Methods and Standards of Early Diagnosis of Pneumoconiosis, with or without Infection, including
Determination of t h e Silica Content of the Blood, Urine andFaeces.
Value, if any, of Early Diagnosis of Simple Silicosis in. regard to
the Question of Removal from Dusty Occupations with a View to
the Arrest of the Disease

43

N I N T H SITTING. — Item 4 (continued)

52

Item 5 : (a) New Methods of Dust Investigation as regards Sampling, Concentration, Composition and Size Frequency Distribution. Possibility of Standardising these Methods. . . .

52

Item 5 (continued) : (6) Intensity of Exposure to Dust a n d Means
of Estimating I t

57

TENTH SITTING. — Item 5 (b) (continued)

Item 6 : What Contributions from Other Scientific Fields are
Essential to the Solution of the Pneumoconiosis Problem ? . .
SPECIAL SITTING

60

62
64

2

INTRODUCTION

on Industrial Hygiene a sub-committee on pneumoconiosis, composed of a few of the most competent experts on the subject.
This sub-committee m e t in September 1936 and drew up a list of
the essential questions t o be placed on the agenda of the proposed
international meeting.
The Governing Body decided a t its 77th Session (November
1936) to convene this international conference and a t its 82nd
Session (February 1938) it approved the agenda submitted by t h e
Office.
As a preliminary measure the Office circulated to the various
experts a questionnaire dealing with the items placed on t h e
agenda.
The Second International Conference on Silicosis was held
in Geneva a t the International Labour Office from 29 August
to 9 September 1938. Reports had been sent in by a number
of experts. The proceedings took the form of a round table
conference, following the usual procedure adopted at meetings
of the Correspondence Committee on Industrial Hygiene.
This compilation summarises the discussion and gives the text
of t h e reports submitted.

LIST OF MEMBERS OF THE CONFERENCE

Members of the Committee
Australia
Dr. Charles, Medical Officer of Industrial Hygiene, Depart»
ment of Public Health, Sydney, N. S. W.

BADHAM,

Belgium
Dr. A., Inspecteur Général, Chef du Service médical
pour la protection du travail, Ministère du Travail, Brussels.

LANGELEZ,

Canada
Dr. A. R., Clinical Specialist, Industrial Hygiene Branch,
Department of Health, Toronto (Substitute for Dr. J. Grant

RIDDELL,

CUNNINGHAM).

Denmark
Dr. Skuli, Chief of the Health Service of the Factory
Inspection Department, Copenhagen.

GUDJONSSON,

France
POLICAED,

Dr. A., Professeur d'Histologie, Faculté de Médecine,

Lyons.
Great Britain
KING, Dr. E. J., British Post-Graduate Medical School, University
of London, Hammersmith Hospital, London.
MIDDLETON, Dr. E. L., Medical Inspector of Factories, London.
Japan
Dr. Jisaku, Mining Inspector, Bureau of Labour, Department of Welfare, Tokyo (Substitute for Dr. Seiji ONISHI).

TSUKATA,

Union of South Africa
IEVINE, Dr. L. G., Chairman, Miners' Phthisis Medical Bureau,
Johannesburg.
OBENSTEIN, Dr. A. J., Chief Medical Officer, Rand Mines Ltd.,
Johannesburg.
United States
Dr. Leroy IL, Director, The Saranac Laboratory for the
Study of Tuberculosis (Trudeau Foundation), Saranac Lake, N. Y.
SAYERS, Dr. R. R., Senior Surgeon, Chief of the Division of Industrial
Hygiene, National Institute of Health, Public Health Service,
Washington.
GABDNEB,

4

LIST OP MEMBERS OF THE CONFERENCE

*

*

*

MACKENZIE, Dr. M., Health Section, League of Nations (substitute
for Dr. L. RAJCHMAN).

Representatives

of the Governing

Body

Mr. KITAOKA, Juitsu (Government Group).
Mr. TZATJT, Ch. (Employers' Group).
Mr. ANDERSSON, G. (Workers' Group).
Observers
Australia
Dr. W. E., Medical Officer in Charge, Bureau of Medical
Inspection, Broken Hill.

GEORGE,

Belgium
Dr., Vice-président de la Commission des Médecins Spécialistes de la Fédération des Associations Charbonnières, Brussels.
VAN BEÏTEDEN, Dr. J., Professeur d'Hygiène à l'Université, Liège.
QUINET,

France
Dr. G-, Directeur de l'Institut d'étude et de T)révention
des maladies professionnelles, Paris.

HAUSSER.,

Netherlands
Dr. A. H., Chief Medical Officer, United Collieries,
Limbourg, Heerlen.

VOSSENAAR,

United States
Dr. L., Executive Director, Division of Industrial
Hygiene, New York State Department of Labor, New York.

GREENBTJRG,

*

The organisation of the Conference was carried out by D R . L. CAROZZI,
chief of the Industrial Health Service and technical adviser, I. L. O.,
assisted by D R S . V. DHERS, A. STOCKER and Miss MACRAE, who effected
the secretarial work.

AGENDA OF THE CONFERENCE

1. Recent advances in t h e knowledge of the pathology of silicosis,
including the effect, if any, of non-siliceous dusts admixed with silica
or silicates in the production of pulmonary fibrosis or tuberculosis.
2. Pneumoconiosis of workers in coal mines a n d of other workers
exposed to coal dust. •
3. Pulmonary disease (or disability) due t o inhalation of dust in
industries other t h a n those covered b y 1 and 2 (with quali- and quantitative estimation of t h e composition of the dust in question).
4. Methods and standards of early diagnosis of pneumoconiosis,
with or without infection, including determination of t h e silica content
of the blood, urine and faeces. Value, if any, of early diagnosis of
simple silicosis in regard t o the question of removal from dusty occupations with a view to the arrest of the disease.
5. (a) New methods of dust investigation as regards : sampling,
concentration, composition and size frequency distribution.
Possibility of standardising these methods.
(b) Intensity of exposure to d u s t and means of estimating it.
(c) Possibility of establishing experimental criteria for determining the degree of pathogenicity of dust.
6. W h a t contributions from other scientific fields are essential to
the solution of the pneumoconiosis problem?
7. Determination of disability a n d assessment of t h e degree of
UUV1I

W1UUIFJAIAV«

(1J.

•JJU.V-' U J . J L 1 V ^ V J U 1 1 1 V I J V O

•

8.

Prevention : (a) Initial examination of workers in dusty industries and methods a n d standards for it.
(b) Campaign against dust in general a n d siliceous dust in
particular.
(c) Personal protection (by means of masks, etc.).
9. The specific t h e r a p y of silicosis and results of any investigations
into methods for stimulating elimination of silica.
10. The most practical means b y which the Sub-Committee can
assist in the anti-dust campaign (through co-operation with other
organisations and individuals).

1
Beyond defining standards of disability, the question of compensation
is not being dealt with by this meeting, since the numerous factors which
would have to be taken into consideration would necessitate the presence
of additional experts who cannot on this occasion be convened.

PROCEEDINGS OF THE CONFERENCE

F I R S T SITTING
(Monday,
Chairman :

MR.

Harold

29 August 1938, 10 a.m.)
BUTLEB,

followed by

DR.

A. J .

ORENSTEIN

The Second International Conference on Silicosis, held a t the
International Labour Office, was opened b y the Director of the
Office, Mr. Harold Butler, who, after welcoming the members present and referring to the Silicosis Conference held in Johannesburg
in 1930, summarised the subsequent progress achieved both in
the medical sphere and in compensation legislation in various
countries.
The International Labour Conference, which in 1934 revised
the Workmen's Compensation (Occupational Diseases) Convention,
1925 (No. 18), added silicosis to the schedule by means of wording
which was sufficiently wide to enable States to adapt their legislation to the Convention.
Ten States had so far ratified the Workmen's Compensation
(Occupational Diseases) Convention (Revised), 1934 (No. 42)
(Brazil, Cuba, Great Britain, Hungary, Ireland, Japan, Mexico,
New Zealand, Norway and Sweden) and other countries would
certainly have done so had it not been t h a t the schedule includes
a certain number of other diseases upon which the competent
authorities reserve the right to collect information concerning
the occupational risk involved before taking a decision.
On the proposal of Dr. MIDDLETON, Dr. Orenstein was elected
Chairman of the Conference.
after thanking his colleagues for the honour
conferred on him, paid a tribute to the memory of Professor Kettle
of London, Member of the Committee.
He stated t h a t the Johannesburg Confsrence had established
the fact t h a t silicosis was unquestionably a definite clinical entity
T H E CHAIRMAN,

8

SILICOSIS

affecting workers in certain occupations, causing disability for
work and calling for the adoption of preventive measures to
reduce its incidence. The present Conference was not therefore
asked to discuss the existence or non-existence of the disease in
general or in a n y given industry or processs, but to study the progress in knowledge of the disease made since the Johannesburg
Conference, within the framework of the agenda placed before
the Conference. The Conference would at the end of its proceedings be called upon to submit a report and recommendations
to the Governing Body of the International Labour Office.
On the proposal of D R . MIDDLETON, Drs. Say ers and Langdez
wire elected Yice-Chairmen, and on the proposal of P R O F . P O L I CARD, Dr. Middleton was elected Reporter.

Item 1
Recent Advances in the Knowledge of the Pathology of Silicosis,
including the Effect, if any, of Non-Siliceous Dusts admixed with
Silica or Silicates in the Production of Pulmonary Fibrosis or
Tuberculosis.
D R . L. U. G A R D N E R said t h a t the histological findings affecting human beings, as well as the results of animal experiment,
showed t h a t t h e admixture of certain mineral elements with silica
tended to inhibit the action of free silica and to modify the resulting
anatomic injuries.
He had no experience in regard to the action of the so-calied
accelerators. Histologically, study of the lungs of workers exposed
to the dust of abrasive- soap powders had revealed the existence
of injuries resembling "acute silicosis" among workers engaged on
sand blasting. These injuries were, he considered, to be attributed
not so much to t h e presence of free alkalis as to very high concentrations of particles of extremely fine silica, in a very pure state.
The minerals known as retarders exerted a two-fold effect—in
the atmosphere and in the human organism. I n the atmosphere
these non-siliceous elements tended to combine with the quartzitic
elements forming particles, so large t h a t they were precipitated
in the atmosphere or were caught up in the nostrils and upper
respiratory t r a c t .
Workers with a working experience exceeding 35 years in
handling "sanded plaster" and exposed to a mixture of siliceous

PROCEEDINGS OP THE CONFERENCE : FIRST SITTING

9

sand and gypsum dust showed no evidence of silicosis or any
significant pulmonary reaction. In animals exposed to the
inhalation of artificial mixtures of fine quartz and calcined gypsum
dust a pulmonary reaction was noted, quite different from t h a t
caused by pure silica. Animal experiment was carried out by
injecting measured amounts of pure quartz and quartz associated
with iron (ferruginous chert composed of 50 to 60 per cent, of
crypto-crystalline silica and iron oxide : haematite) or granite
(one-third quartz and two-thirds mostly mica and feldspar).
The results showed t h a t a given quantity of silica in pure form
produced much more reaction in a given period of time t h a n the
same quantity when associated with other minerals. The presence of the "inhibiter" minerals modified the form of the anatomic
reaction ultimately found in the lungs or other organs. Instead
of sharply circumscribed masses of laminated hyaline tissue, as
in the case of pure silica, the form of reaction to the mixed dust
had an irregular ill-defined border. The peripheral zone was
composed of a more cellular connective tissue, in which there were
great numbers of dust cells filled with non-siliceous dust particles.
Bands of such pigmented cellular connective tissue surrounded
the central zone of hyaline fibrosis radiating into the surrounding
tissues.
PROFESSOR POLICARD said t h a t the experiments carried out
with mixtures of silica and alkalis had not been productive of
result in the sense of accelerating the action of the silica. The
principal element in these experiments was the number of silica
particles rather than the action of the alkalis present. The alkalis
did not appear to accelerate the action of the silica in bringing
about forms of monocytic alveolitis. Nevertheless the action
of these alkaline dusts on the pulmonary alveoli was far from
harmless, though not so far understood.
The harmful effect of granite appeared to vary with its place
of origin : it was very intense in the United States and not particularly dangerous in other countries. This fact might well be
connected with the composition of "granites", which consisted
of such complex mineral elements. I t was a problem which for
the present was still without solution as regards its details.

said t h a t the work a t the Banting Institute
and observations at the Mclntyre Mine (Canada) showed t h a t thé
admixture of aluminium dust to the silica appeared to modify
and retard its action.
DR. RIDDELL

10

SILICOSIS

D R . TSTXKATA said t h a t Mr. Jones's sericite theory called for
further research with a view to its substantiation ; such research was
doubly essential because some authorities had called in question
the efficacy of t h e method adopted for analysing the dust particles.
D R . VOSSENAAR

then read his report entitled "The Biochemistry

1

of Silicosis " .
D R . K I N G stated t h a t it was first necessary to decide whether
such a thing as "acute silicosis" actually existed or whether the
term was not a misnomer, and also whether the acute injuries
consequent on mass exposure to dust had much in common with
the slow insidious changes noted in ordinary silicosis with chronic
evolution.
The part played by the alkalis might find its explanation
in the action peculiar to those substances (independent of any
accelerating effect on the silica) but it must be admitted that
very little was known regarding the action on human beings of
acute exposure to mass doses of strong alkalis, such as soda and
potassium.
The majority of authorities on the subject considered at present t h a t silica alone exerted a pathogenic effect and t h a t the
addition of silicates merely repressed t h a t action. There was the
question of mechanical, chemical or possibly electrical or radioactive action, or the further possibility of modification of the
infective processes in the lung implying once more action of analogous nature (mechanical, chemical or electrical). Discussion
of these possibilities led more or less exclusively to elimination
of any but chemical action, implying almost inevitably solubility (reaction in a solution or between a solid and a solution).
In considering solubility it was not so much a question of the
amount of silica in solution but rather of a constant "leaching"
of particles. Gelatinous silica for instance dissolved very quickly,
producing rapid changes, b u t its action was soon terminated,
because once in solution it soon disappeared, with the result
t h a t no more of it was left to produce the chronic changes due
to constant leaching.
Free silica dusts were much more soluble than silicates in the
body fluids, particularly at 7.5 p H , as might be expected on chemical
grounds. The free silicas were in effect simply the anhydrides
1

See p . 106.

PROCEEDINGS OF THE CONFERENCE : FIRST SITTING

11

of silicic or metasilicic acids, and, as such, more soluble in mild
alkaline fluids than their natural crystalline salts (silicates of
potassium, aluminium) which in their turn were more rapidly
attacked by acids than by mildly alkaline fluids.
The majority of silicates were then not only less soluble in
alkaline liquids but actually depressed the solubility of free silica.
1 g. silica (very finely powdered quartz pebble or Brazilian rock
crystal : 100 per cent. SiOa) exposed for about two weeks t o blood
serum finally reached the very considerable solubility of 9 mg.
per 100 c.c. The calcium concentration in the blood was only
9 to 10 mg. per 100 c.c. and the phosphorus only 2 to 3 mg.
No such considerable concentrations of silica in the serum could
be found in the blood of the intact animal, since the silica was
excreted in the urine as fast as it entered the blood. I n certain
experiments it had been found possible to raise the concentration
in the urine to 170 times its original figure without any appreciable
increase in the silica content of the blood (King). The author
and co-workers had noted this phenomenon by self experiment
(taking large doses of magnesium trisilicate). Consequently he
considered determinations of the silica content of the blood to
be useless.
On the other hand it was not possible to obtain experimentally
similarly high concentrations in the blood serum with sericite,
most clays or any natural crystalline silicas. 2 g. of natural
stone dust or shale dust, such as t h a t used for dusting in coal
mines, with a 50 per cent, free silica content, gave a solubility
in blood serum of only about 1 to 2 mg. per 100 c . c , pointing to
the presence in these minerals of an element which depressed the
solubility of free silica; this applied to mixed rock dust as well
as to sericite.
Coal dust too appeared to depress the solubility of free silica
almost in direct proportion to the amount of silicate, or indeed
even of aluminium salt it contained. The work of the Canadian
investigators appeared to indicate t h a t aluminium was the most
effective depressor of the solubility of silica, being much more
effective than natural silicates, silica oxides or iron in any form.
Yet the preliminary results so far published, though dramatic,
were scanty and must be accepted with reserve.
Some authorities (King and Whitehouse) had worked with
the dusts used for stone dusting in coal mines in Great Britain.
From the outset Haldane had insisted, contrary to prevailing
opinion, that these dusts actually diminished the silicosis risk

12

SILICOSIS

and considered t h a t many natural rocks contained an antidote
for silicosis, advocating t h a t silicosis could be reduced by the
inclusion of shale dust in the harmful dust breathed by the workers.
I n conclusion, Dr. King said t h a t he did not believe t h a t solubility per se could suffice to provide the explanation of silicosis.
Though many experiments tended to indicate t h a t the action of
silica could be eliminated simply by depressing its solubility.
evidence was still too scanty and further investigation was essential, while at the same time it was necessary to be very cautious
in applying to man experimental results and conclusions affecting
laboratory animals.
D E . I R V I N E considered t h a t the term "acute silicosis" was a
misnomer. He had experienced among sewer miners in South
África the incidence of a very rapidij^ developing silicosis occurring
in four stages, ranging from the most acute type of dust pneumonia
with profuse cellular reaction, through a condition showing nodulation, which h a d not however become hyaline by the time of
death, to a third condition with numerous hyaline nodules accompanied by diffuse fibrosis; while other cases exposed to less dust
showed a fourth form analogous to gold-miners' silicosis.
I t therefore appeared essential to consider the existence of a
clinical picture comprising various clinical forms, which appeared
to be connected with a degree of concentration of the fine particles
inhaled and also to be perhaps subject to possible influence from
nitrous fumes liberated in blasting. The greatest advance made
on the Rand in t h e reduction of the silicosis rate had been through
the control of exposure to blasting dust and nitrous fumes.
I n regard to the effects of stone dusting in mines on the
aetiolog3r of silicosis, it had to be remembered that fine dust tended
to settle on the ground, gradually coagulating the coarser particles,
so t h a t it was not afterwards easily stirred up and disseminated
in the atmosphere; it was essential to consider this fact together
with the gradation of the composition of the dust used.
Dr. Irvine placed a t the disposal of the members of the Conference two albums of photographs showing macro- and microscopic
injuries in cases of acute silicosis seen on the Rand (Dr. Simpson).

The Conference adjourned at 12.30 p.m.

PROCEEDINGS OP THE CONFERENCE : SECOND SITTING

13

SECOND SITTING
(Monday, 29 August 1938, 2.40 p.m.)
Chairman : D B . A. J.

ORENSTEIN

Item 1 (continued )
The meeting opened with the reading of a report by D R .
entitled "Protective Action of Various Minerals against
Free Silica", accompanied by lantern slides 1.
GARDNER

congratulated Dr. Gardner on his excellent research work, which constituted a valuable contribution
to the progress of experiment in regard to pneumoconiosis.
H e desired to make certain statements on what could or could
not be effected by experimental work. The research conducted
by exposing animals to dust was indisputably extremely difficult,
since the production of sclerosis demanded a very long lapse of
time.
Dr. Gardner had dusted animals for two or three years, which
was an extremely long time, in the course of which the animals
might contract infection, as frequently occurred, involving a
very high mortality rate. Animals dusted at once offered resistance to the atmospheric changes by almost ceasing to breathe,
with the result t h a t it was necessary, in order to cause a n animal
to absorb rapidly large 'quantities of dust to render it dyspnoeic
by adding 5 to 10 per cent, of carbon dioxide to the atmosphere.
I t was also highly important to remember t h a t animals were
very largely nose-breathers, thus enabling an enormous part of
the dust to be arrested in the nasal passages. Therefore, in order
to dust the lungs adequately, it was essential to make the animal
breathe formol in order to cause nasal irritation and induce mouth
breathing.
Another difficulty was connected with the interpretation of
certain injuries observed, since the lungs of an animal were of a
very special nature. I n fact, the lungs of a rabbit differed from
those of a guinea-pig as regarded the state of the lymphatic system,
which was of primary importance; the rabbit had hardly any
PROFESSOR POLICARD

1

See p. 108.

14

SILICOSIS

lymphatic glands, but possessed a series of small lymphoid nodules
situated along the bronchi. These constituted an intra-pulmonary
glandular system not possessed by the guinea-pig which, on the
other hand, had lymphatic glands, as seen in the slides shown
by Dr. Gardner, and this considerable difference explained the
varying pathological results obtained.
I n human beings conditions were about midway between the
guinea-pig and t h e rabbit, since the interior of the lungs contained
small lymphoid nodules which were, however, relatively scarce.
On the other hand, recent anthropological knowledge pointed to
the existence of variations in the lungs in the different races.
Certain Mongols, for instance, possessed a greater number of small
lymphoid nodules t h a n other races.
Dusting for a long period with dust rich in silica undoubtedly
nrodnced fibrosis, but after a very long lapse of time. The difficulty of the experimental and clinical problem in silicosis lay in
the fact t h a t it was a chronic disease, and t h a t little was known
of such diseases, particularly of their mode of evolution; silicosis
was only a particular instance of this general statement.
Mineral dust more or less rich in silica set up in practice in the
laboratory animal two types of injuries : monocytic alveolitis,
and dust-cell alveolitis, with changes in the lymphatic glands,
where any existed, in the lung. I t appeared t h a t a pathological
structure had never been obtained similar to the silicotic nodules
found in human beings (consisting of a hyaline centre very poor in
cells, surrounded b y a layer of fibrous tissue rich in mineral particules and in cells). The pulmonary fibrosis obtained in the guineapig, the rat or t h e rabbit, was due to another mechanism, for
it was the usual fibrosis of all broncho-pneumonic lesions, with
alveoli filled with dust cells and undergoing gradual change into
a fibrous mass through production of fibrous tissue in accordance
with a commonly occurring phenomenon. Fibrotic transformation of the lymphatic nodules in the lung might be noted, simñar
to those affecting the lymphatic glands in man and in all animal
species. The silica arrived in the glands, setting up there fibrous
lesions, but enormous quantities of dust were required to effect
this. Experimental conditions were therefore quite different
from those encountered in industry.
I n accordance with Dr. Gardner's practice, it was essential
t o use dust with a very high silica content, t h a t is to say practically pure silica, chalcedony or quartz". The dusting of the animals,
during a period of over two years, for eight hours per day and

PROCEEDINGS OF THE CONFERENCE : SECOND SITTING

15

six days per week, with dust from a mine where silicosis occurred,
was without result.
There was therefore ground for assuming t h a t some element
other than dust played a part in the evolution of silicosis in human
beings, since dust alone did not suffice to produce the disease in
animals. Where dust set up fibrosis, t h e result was obtained under
conditions other than those encountered in practice, where, nevertheless, very definite silicosis or fibrosis had occurred. I n agreement with Dr. Gardner's experience, Professor Policard h a d never
met with experimental reproduction of silicosis due to rock dust
which was not pure silica. With pure silica sclerosis might perhaps be observed, but no sclerosis with silicotic nodules.
The Conference adjourned at 5.40 p.m.

T H I R D SITTING
(Tuesday,

30 August 1938, 9.40 a.m.)

Chairman :

DR.

A. J .

ORENSTEIN

Item 1 (continued)
D R . LANGELEZ asked Dr. Gardner if, in the experimental
work done by him, all question of superimposed infection could
be excluded, if, in carrying out the experiments, he had added
infective agents to the dust, and, further, if, in the absence of this
procedure, he had met with silicotic nodules with caseous transformations (tendency to the appearance of tubercular nodules).
D R . GARDNER replied t h a t since in animal experiments the
risk of accidental infection was very high, a very large number
of animals were used. In 100 guinea-pigs there was, in fact,
a reduction of 20 to 25 per cent, a t the end of two or three years.
I n a large number of the experiments carried out, the animals
were purposely infected by means of tubercular bacilli of an
attenuated virulence. In animals so infected and exposed to
siliceous dust, infection invariably became progressive, while

16

SILICOSIS

in those exposed to non-siliceous dust the infection might progress
for a month or so, then cease to spread and become healed with
formation of excessive scar tissue.
With dust rich in calcium, calcification of the tubercles commonly occurred. Infection, whether tuberculous or not, accelerated the development of the fibrotic reaction to the silica dust,
the form of t h e resultant reaction differing only slightly from
that seen in non-infected animals, with no greater tendency to
lamination of t h e hyaline fibres.
PROFESSOR POLICARD showed some slides which he had himself
prepared. B y using silica, it was possible to obtain in animals
fibrotic lesions which never were (in his personal experience, as
in t h a t of Dr. Gardner) of the same type as the characteristic
l e s i o n f m i n d i n h n r n a . n Twinors

said t h a t Professor Policard, like himself, had
encountered great difficulty in attempting experimental reproduction in animals of fibrotic lesions by exposure to mixed dusts
known to be harmful in industry. On account of the short-lived
animals experimented on (five years for the rabbit, three for the
guinea-pig, and two for the rat) excessive concentrations had to
be used. Professor Policard had stressed the fact t h a t they were
even much higher than any met with under industrial conditions.
Concentrations such as the maximum used in the experiments :
800,000,000 particles per cubic foot and 100,000,000 in the control
experiments with quartz dust, were however very often exceeded
in American industry.
While recognising the difficulty, mentioned by Professor
Pohcard, of making the animals in question (rodents) inhale
dust, he had never found it necessary to apply forced acceleration
of the breathing rate. Exposures had lasted over a period of
eight hours per day for one, two or three years. At night the
animals remained in the same rooms, and although the dustgenerating machinery was turned off, considerable quantities
of dust still remained in the atmosphere.
The anatomical and physiological differences referred to had
induced him to extend his experiments to various animal species,
mammals (guinea-pigs, rabbits, white rats, cats, dogs, etc.) as
well as birds, amphibians and fish, and the specific response to
silica was found to be a general property of animal life, with
variations in accordance with the species, and even within the
DR. GARDNER

PROCEEDINGS OF THE CONFERENCE : THIRD SITTING

17

same species, due, possibly, t o anatomical peculiarities or special
characteristics of the tissues.
Professor Policard had claimed—and rightly so—that the
lesions reproduced were not hyaline in character, but it could
hardly be expected that changes induced in animals in a period
of a year or two should approximate to those requiring twenty
to thirty years, or longer, to develop in human beings. These
lesions, too, showed unique fibrous modification not commonly
encountered with any other irritant known to him. The nodules
in the rabbit's liver resembled more closely the silicotic nodules of
the human being than the pulmonary lesions observed.
Professor Policard had also raised the very important point
t h a t the reaction might be largely due to accidental infection;
but he claimed to be able to recognise the presence of infection
by histological evidence of its presence or b y checking by repeated
culture of the lungs to detect contaminating infections. Chronic
pneumonia presented a nodular type of reaction only when due
to tubercular bacilli, and a diffuse type when due to other agents
(pneumococcus, Friedlander's bacillus). I t might be concluded,
therefore, t h a t infection was not essential. At least, the effect
had been reproduced in the liver and other organs without
infection.
Intravenous injection of the various dusts set up perfectly
typical silicotic nodules in the lungs and liver and also in the
spleen, bone marrow and lymphatic glands, justifying the conclusion t h a t the reaction to silica was certainly a specific one, not
produced by any other mineral, which might occur in the absence
of all infection, whether tubercular or not. He deprecated unsupported experimental observation, which required confirmation by the study of human beings roentgenologically, clinically,
or by means of autopsy, since, while experiment might indicate
the guiding principles involved, it could never furnish a decisive
answer, which must depend ultimately on examination of the
human material.
D R . BADHAM said t h a t experiments in dusting animals (rabbits,
guinea-pigs), undertaken five years ago with large concentrations of sandstone and, later, quartz dust free from sericite, had
produced, in a period of time depending on the dust concentration (six months at most), extensive massive and macroscopic
fibrosis in a large number of animals. I n most of the cases there
was no evidence of infection, its absence being confirmed by the

•SILICOSIS

18

culture of sections; but where the dusting had been done over a
long period at high concentrations, there was marked evidence
of heart failure. On examination the lungs proved to be almost
solid, showing on microscopic examination a concentration of dust
cells. I n the case of white guinea-pigs with pink eyes, the progress of the fibrosis could be determined by observing the cyanosis
present in the blood vessels of the eye.
The results of dusting with coal dust (containing y2 to 1 per
cent, free silica), lasting in some cases for four years and over,
showed the lungs loaded with dust-marked perivascular deposition
and a small amount of fibrosis, but not the massive fibrosis so
easily produced with quartz. The results of these experiments
would be published shortly. He had also used aluminium in the
dust concentration described by the Canadian authors, without
obtaining, after six months, any fibrosis, which moreover could
hardly be expected to develop in the guinea-pig in t h a t time.
D B . HAUSSEE. enquired whether experiments had been carried
out with monkeys or with animals kept at the working post, and
whether heart troubles had been noted in the animals.
H e referred to four fatal cases of acute silicosis with death
from heart failure, which had occurred in a workshop in which
celluloid toys were cleaned by sandblast.

stated t h a t he had used monkeys, b u t had
failed to produce in them nodular fibrosis, as in the case of the
other animals, and t h a t in another series of experiments he had
observed guinea-pigs exposed in a coal mine in West Virginia,
but the procedure had yielded unsatisfactory results.
Right-side dilatation of the heart was found in animals with
massive lung fibrosis, especially where co-existing infection complicated the reaction, and in consequence large areas of the lung
were replaced b y scar tissue. The change was not found in presence of merely discrete isolated nodules scattered throughout the
lungs.
DB.

GARDNER

D B . GKEENBTTEG invited the Conference to define "acute
silicosis" and t o decide for or against the use of this term. I t
was, in his opinion, hardly an acute disease in the ordinary sense,
and he would prefer the expression "rapidly developing silicosis"
or some similar term.

PROCEEDINGS OF THE CONFERENCE : THIRD SITTING

19

D R . LANGELEZ asked Professor Policard whether it was possible
t o reproduce experimentally the silicotic nodule or, in other words,
the fibrous silicotic nodule, as opposed to the nodule with a hyaline
centre.
PROFESSOR POLICARD replied that confusion between the initial and fundamental action of silica must be avoided. Particles
of pure silica absorbed by the alveolar cells destroyed these cells,
silica constituting, beyond all doubt, a cell poison. The cells
affected by silica and having in consequence the changed aspect
described by Mavrogordato were lodged in the alveoli.
The reactions of the pulmonary tissue, properly so-called,
varied : they might be considered as a specific response t o silica
or of common type, since all tissues in which cells died showed
reaction of the surrounding elements, with formation of fibrosis
in particular. Where any point of the tissue was undergoing
necrosis, and more especially gradual necrosis, encystment by the
surrounding connective tissue developed. This had been demonstrated, for instance, by Dr. Gardner, in the liver, occurring around
masses containing quartz particles and surrounded with fibrosis.
The same phenomenon, though of less intense degree, might be
noted around foci of hepatic necrosis, for instance, surrounding
a dead parasite. Silica'gave rise to a form of fibrosis, which
was indubitably intense, for it was possible to obtain by means of
silica cirrhosis of the liver.
Among the varieties of fibrosis there was first of all a commonly occurring type : interstitial fibrosis with thickening of
the walls and formation of more or less numerous alveolar masses.
I n this manner nodules might.be set up, t h a t is to say, fibrous
masses of homogeneous structure without a hyaline centre or
peripheral fibrosis. The pathological agent capable of constantly
setting up nodular formation with a hyaline centre was essentially tuberculosis. The round masses seen in Dr. Gardner's
preparations were nodules in the etymological sense of the term,
b u t not silicotic nodules. Other less known effects, due t o reaction of the tissues to poisoning of the cells.by silica, and consisting of interstitial fibrosis, might be found. I n certain cases an
increase in the pulmonary fibrous network might be noted, with
pathological change of the lymphoid formations, showing at the
outset reticulation (well known to pathologists), followed later
b y fibrosis. I t must therefore be remembered that, along with
pure silica, another toxic agent, a bacillus, or other harmful

20

SILICOSIS

cause, there was always the same.sequence of histological phenomena, t h a t is to say, reticulosis, followed by fibrosis. The former
appeared as a commonly occurring phenomenon which might be
set up by silica.
Fibrosis of t h e glands might occur under the influence of coal,
silicates or other minerals; it was merely a question of density.
Yet it was of primary importance to distinguish the relatively
rapid and acute cell injury, as contrasted with the more or less
gradual pulmonary reactions varying with the factors involved :
disturbance of t h e lymphatic circulation, individual constitution
(fibrotic temperament, Rössle).
D E . SAYEES recalled how, in 1923, he came to study types of
stone dust siiitablp. for IIBB in or>al rm'neg as a protection against
explosion. Not much was known at that time about schist dust,
except in Great Britain (work of Haldane).

After investigation in various countries as to the best dust to
be employed, he h a d used the intra-peritoneal method of studying
experimentally t h e effects of various dusts : each animal was
given a single injection of 2 dg. of a sterile physiological
salt solution suspension of the dust to be studied, the animals
being sacrificed at periods ranging from one week to one -year
after injection. I t was decided empirically to discard any dust
setting up a reaction similar to t h a t produced, by silica and to
retain those in which the reaction resembled that of limestone
dust, while a dust showing a reaction similar to coal dust might
be used, b u t was not preferred. Silica dust first set up a peripheral reaction with migration or infiltration of phagocytes, which
engulfed the dust and in addition produced proliferative reaction
visible macroscopically after a few days and microscopically
after two weeks or one month, followed by necrosis, older fibrous
tissue, fatty degeneration and some calcium deposition, the reaction appearing t o be almost complete in a year's time.
With limestone and similar dusts (gypsum, dolomite) the
particles disappeared from the tissues after a few weeks without
setting up any special reaction. Dusts such as haematite, iron
oxide, carborundum and coal did not disappear, but produced
no proliferative reaction, becoming incorporated in the peritoneal tissue a n d remaining stationary, the so-called "inert
reaction".

PROCEEDINGS OF THE CONFERENCE : THIRD SITTING

21

This peritoneal reaction was found to be a good index to the
possible reaction of any dust on the lungs, and on the whole it
correlated very well with clinical experience.
D R . IRVINE stressed the difficulty of comparing experimentally induced silicotic lesions in animals with those which occurred
in the human subject. Dr. Kettle had obtained in animals, by
introduction of large quantities of siliceous dust in watery suspension, groups of alveoli filled with siliceous dust and phagocytes,
but little fibrosis. Dr. Irvine had never met with this type of
injury in human beings nor with the alveolitis referred to by
Professor Policard, except in the case of acute silicosis contracted
by sewer miners. I n these cases the alveoli were not primarily
affected ; particles of siliceous dust set up lesions at points where
lymphoid tissue was aggregated. There was, in all cases of silicosis, definite bronchitis and the disease should always be thought
of in terms of associated bronchitis.
No one but Dr. Gardner had so far succeeded in reproducing
experimentally, by intravenous injection of quartz a n d flint
dust, nodular lesions practically identical with those found in
human subjects.
The Conference ought to come to a decision on the use of the
term "acute silicosis", which he considered to be a misnomer.
I t would be preferable, he considered, to differentiate between
the different types of silicosis—pure silicosis (caused by varying
degrees of concentration of silica dust), and modifications produced
in silicosis by infection, particularly tubercular. He believed t h a t
aggregations of siliceous dust in the lungs precipitated t h e development of tuberculosis up to a point at which the joint condition
produced an excessive fibrosis, resulting in definite retardation
and quite frequently in arrest of the infectious process. The modifications caused by association of coal dust and silica dust were
known. Pure coal dust (containing less than 1 per cent, pure
silica) loaded up the lung with dust (coal-trimmers), giving gross
radiographic pictures, b u t it had not yet been proved b y postmortem examination how far fibrosis and clinical disability accompanied these changes.
D R . MIDDLETON said t h a t the Medical Research Council's
Committee on Industrial Pulmonary Diseases in Great Britain
had carried out an investigation to determine the effect of pure
coal, prior to a study on mixed dusts in coal mines. The enquiry

22

SILICOSIS

covered coal-trimmers employed in the port of Swansea, handling
anthracite containing very little ash but rich in carbon. Clinical
and radiological examination of <40 men with 5, 10, 15, 20, and
in one case, 40, years' working experience, indicated t h a t coal
per se did not produce fibrosis of the lungs.
PROFESSOR POLICARD insisted on the importance of the form
in which silica was. present. Kieselguhr (Tripoli), for instance,
caused extraordinary cell reactions differing from those set up
by quartz. Kettle's method (intra-tracheal injection of a suspension of silica or mineral matter) produced in the alveoli an
exudative reaction, still but slightly known, and different from that
caused by introduction of th'e dust by inhalation, which would
indicate t h a t such experimental results are not comparable with
those occurring in indus trial practice. Dr. Sayers' method (intraperitoneal) appeared preferable.
Alveolitis occurred rarely in the human lung, though it was
of almost constant occurrence round fibrous lesions, nodules or
tubercular lesions in cases of silicotic tuberculosis; in presence
of alveolitis the circulation and lymphatic drainage of the lung
was less effective. On the other hand, in the experiments carried
out, alveolitis was both of constant occurrence and massive in
form.
He agreed with Dr. Irvine t h a t there was a tendency to exaggerate the importance of the silicotic nodule. I t s mechanism and
formation was a matter of interest to pathologists, but it was
dangerous to base diagnosis of this disease, which was known to
exist, and designated silicosis, on the appearance of the nodule.
H e could not subscribe to the Johannesburg definition according
to which silicosis was characterised b y the presence of the silicotic
nodule, and he was ready to admit the fact t h a t there might exist
in fibrosis forms connected with silicosis, and in particular in the
final phases of infection, enormous fibrous masses (pseudo-tumoral
form), only found under the influence of silica.

said t h a t in Denmark research carried out
with pure silica (silica, sericite or a mixture of them) enabled
production in animals of massive nodular fibrosis to be effected
after a period of 18 to 26 months, the time factor remaining the
same whatever dust was used.
I t might be assumed t h a t in experimental silicosis other factors
than dust constituted an important element in bringing about
D R . GUDJONSSON

PROCEEDINGS OF THE CONFERENCE : THIRD SITTING

23

the injuries, and merited further study : such factors as infection,
temperature, moisture of the atmosphere, state of nutrition, and
age of the animals.
As regarded the factor age, he thought it opportune to mention
that, in one investigation effected among pottery workers, there
seemed to be no doubt that older individuals more readily developed silicosis of a type other than t h a t affecting younger workers,
a phenomenon which might explain the rare occurrence of the
nodular form in animals, since the guinea-pig dusted for 3 to 4
years was already very old.
D R . GARDNER said t h a t he had not been able to demonstrate
any difference of reaction capacity to dust between adult and young
animals, either guinea-pigs or white rats. He did not personally
believe that the age factor, as far as animals were concerned,
was significant, apart from the fact t h a t it might influence the
appearance of injuries of the lymphatic system due t o former
exposure to other dusts. H e had undertaken research into the
possible influence of exposure to various dusts—coal, gypsum,
marble—before exposure to quartz dust, on the capacity of the
lungs to retain dust and on the more or less rapid development
of the silicotic lesions, but had n o t been able to draw a n y definite conclusion.
He had noted t h a t in any group of animals (100 to 125 guineapigs, for instance) exposed under approximately the same conditions for a long period of time, the last survivors showed less disease
than some which had died or were killed previously. They seemed
to have less dust in their lungs, having apparently possessed better
respiratory tracts. He had noted t h a t different breeds of the same
animal (rats, rabbits) did not possess the same capacity for inhaling dust. The blue Chinchilla rabbit, an excellent animal for
intravenous injections, developed silicosis by inhalation less
readily than the short-eared, black, white and tan Australian
rabbit. Infection might prevent the development of silicosis,
as in the case of certain animals (white rats) with chronic pulmonary infections (purulent deposits in the broncheoli) which did
not develop silicosis because the silica dust inhaled was caught
in a heavy pus in the broncheoli and thus did not reach the terminal air spaces.
He was not of opinion t h a t in animals the age factor was of
any significance. Individual differences noted were t o be explained
by constitutional capacities to react, variations in the effective-

24

SILICOSIS

ness of the respiratory tract, and certainly by t h e presence of
pulmonary infections.
D E . K I N G said t h a t his experience in experimental work was
that pure sericite did not cause—contrary to the results obtained
by Dr. Gudjonsson—reactions comparable to those caused by
quartz alone, while a mixture of both dusts caused a reaction
differing from t h a t of quartz even where the animals were exposed
a t the same time to the action of intense nitrous fumes (King,
Irwin and Robson). Experiments with inoculation gave the
same results. Intra-cutaneous—not
subcutaneous—inoculation
of a suspension of quartz dust into the breast skin of a chicken
caused in two months a lump as big as the thumb and sometimes
even the size of a pigeon's egg, while in the case of mixed sericite
and quartz dust (25 per cent, oí quartz) there was no reaction.
Animal experiment, therefore, showed t h a t free silica did
produce a reaction while the silicates did not, and t h a t the admixture of silicates with free silica did modify and perhaps even
retard the action of the free silica. These experimental results
seemed parallel with industrial experience, since authenticated
cases of occupational silicosis were those in which the action of
free silica was responsible, wThiie cases in dispute were usually
those following exposure to mixed dust.

The Conference adjourned at 12.40 p.m.

F O U R T H SITTING
(Tuesday,

30 August 1938, 3.5 p.m.)

Chairman :

DR.

A. J .

ORENSTEIN

Item 1 (continued)
D R . BADHAM exhibited to members of the Conference a series
of lungs of guinea-pigs exposed to coal dust from four to five years
(1,630 to 1,767 days with dusting during 5,595 and 5,988 effective
hours). The lungs showed characteristic perivascular massive
fibrosis without nodules. The average concentrations used were

PROCEEDINGS OF THE CONFERENCE : FOURTH SITTING

as follows : coal (containing less t h a n 1 per
2.463 mg. per m. cu.; coal and schist : total
m. cu. (after incineration 750 mg. per m. cu.);
1.349 mg. per m. cu. of fine dust (subsequent

25

cent, of free silica)
dust : 1.812 mg. per
quartz from Orange :
to sedimentation).

D R . MIDDLETON read the p a r t of his report dealing
"fuller's e a r t h " 1 . He stated t h a t this industry had for
centuries been considered harmless in Great Britain until
were first reported during recent years. He also showed
radiographic slides.

with
many
cases
some

PROFESSOR POLICARD said t h a t fuller's earth belonged to a
group of smectic clays, the essential physical property of which
was their absorption capacity for fats and probably other substances. Dr. Middleton's statements were highly interesting,
but phenomena of absorption, though extremely important,
were still but slightly understood in the development of pneumoconioses. The dust studied was deposited along the peri-bronchial
and peri-vascular layers of fibrous tissue in the lungs. I n the
radiographs shown, these layers of fibrous tissue presented an
accentuated picture giving the impression of nodulation, as in
the case of haematite miners, though in their case radiography
showed the existence of dense anatomic structures due to metallic
deposits with a high atomic weight (capable of arresting X-rays).
Fuller's earth, however, contained also about one-third of metals
with a high atomic weight (aluminium, iron oxide, magnesium,
calcium, barium).
D R . MIDDLETON replied that he did not intend to insist on
the question of the absorbent properties of fuller's earth—which,
in his view, provided the explanation both of its utilisation in
industry and the injuries to which it gave rise, as well as t h e types
of radiographs obtained—for he believed t h a t Dr. King had a
communication to make on this subject.
H e was inclined to support Professor Policard's explanation
of the radiographic findings (presence in the dust of heavy metallic
compounds arresting the X-rays and producing shadows interpreted as fibrosis). A second radiograph made with a harder
tube had shown in fact definite penetration of what had appeared
to be nodujes on the first plate, while the true silicotic nodule
1

See p . 134.

26

SILICOSIS

was, of course, densest in the centre. Further pathological examination of a fatal case showed small deposits of dust in certain
parts of the lung.
Professor Policard had drawn attention to radiographs of the
lungs of haematite miners. These workers had been found to
suffer from true fibrosis in Great Britain, with the result t h a t this
disease had now been made compensable.
Another condition which produced shadows due to heavy
metals was found in arc welders (oxide of iron liberated during
burning off of t h e steel), but so far as experience in Great Britain
went these signs had not been accompanied by clinical changes
or reduced working capacity.
D R . I R V I N E insisted on the fact t h a t one of the radiographic
pictures presented showed the lung to be sown with pseudonodules represented by small ring-like shadows suggesting that
they were peri-bronchial and peri-vascular lesions. On the other
hand, it was a well-known fact t h a t iron workers in Cumberland
did contract a form of silicosis after exposure to a dust which,
as far as he knew, contained 30 per cent, of free silica. He wished
to know whether Dr. Middleton in speaking of haematite referred
to this oust or to pure uaeniatite.
'
D R . M I D D L E T O N replied t h a t his sole object in showing these
radiographs was t o demonstrate the appearance produced by this
dust, which might be considered innocuous on grounds of its
chemical analysis alone. The annular shadows indicating bronchial deposits were of special interest in Great Britain at present,
as an investigation among coal miners had revealed the existence
of a radiographic picture, arbitrarily termed "reticulation", associated with certain periods of exposure but unaccompanied by
any disability or clinical signs up till a certain age.
By haematite he meant a particular iron ore, b u t . not the
carbonate iron ore occurring in Nottingham and the east of England,
where as yet no evidence of pulmonary disease had been found.
This ore did contain free silica in the form of quartz crystals,
perhaps infiltrated by solution through the limestone after the
haematite was laid down.
D R . K I N G drew attention to the absorbent and hygroscopic
properties of fuller's earth, which from this point of view resembled
bentonite. The relation between these properties of absorption

PROCEEDINGS OF THE CONFERENCE : FOURTH SITTING

27

and the biological processes was not so far very clear, b u t might
be connected with base exchange produced in the pulmonary
tissues. Further, though there was no quartz, these dusts contained at least 8.5 per cent, of free sihca in a form represented by
some such formula as Si0 2 - N ( H 2 0 ) which might be readily extractable and in a highly reactive condition. Examination of
dust isolated from the lung by the hydrogen peroxide decomposition method (stewing of the dried lung with strong Irydrogen
peroxide : a neutral reaction) showed the existence of these base
exchanges by revealing a difference between the composition
of the dust accumulated in the lung and the original dust. The
total amount of silica and aluminium in the dust from the lung
had diminished with a smaller net loss for the aluminium dust,
there being a marked disturbance of the silica-alumina ratio
(5 : 7 in the original, whereas in t h e dust isolated from t h e lung
it was 3 : 7), so t h a t there was apparently definite evidence t h a t
the silica dust had undergone partial dissolution in the tissues.
PROFESSOR POLICARD said t h a t experiments a t present in
progress with fuller's earth had led him to conclude t h a t i t was a
highly important element in the study of the mechanism of pneumoconiosis. This earth contained minerals belonging to the
zeolite family, which could be roughly defined as bodies constituted by molecules having a silicon skeleton within which base
displacement might occur. I n a medium containing calcium the
zeolite element would fix this body, liberating the potassium or
sodium which it contained. Zeolites were also the source of
interesting chemical discoveries; for instance, permutite used
as a water softener, since it absorbed calcium from t h e water,
yielding a certain quantity of other ions which passed into solution.
H e had effected research which indicated t h a t permutite dust
caused in animals very considerable pulmonary injuries connected
with base exchanges in the pulmonary tissue, similar t o those
set up by the minerals contained in fuller's earth.

D R . GARDNER stressed the necessity of distinguishing between
shadows given by dust accumulations in the lung and those due
to biological reactions to dust.
He had for long been of the opinion t h a t an a t t e m p t should
be made to find out whether dust accumulations in the lungs were
capable of casting shadows. I t was well-known t h a t the density
of the shadow cast b y a given volume of dust was directly propor-

28

SILICOSIS

tionate to the atomic weight of the mineral in question; hence
he considered t h a t in the case of fuller's earth it would be more
profitable to consider the atomic weight of the various substances
rather t h a n their chemical composition.
read in succession the second part "hearthstone"
and the third part "alkali" of his report 1 .
D E . MIDDLETON

The Conference adjourned at 5.40 p.m.
V I E W OP THE F I L M " S T O P SILICOSIS"

The members of the Conference then visited the League of Nations
cinema to view t h e film "Stop Silicosis", prepared by t h e Federal
Department of Labor of the United States, to disseminate information on the means of prevention to be applied in the antisilicosis campaign.

F I F T H SITTING
(Wednesday,

31 August 1938. 9.35 a.m.)

Chairman : D B . A. J .

OBENSTBIN

Item 2
Pneumoconiosis

of Workers in Coal Mines and of other Workers
exposed to Coal Dust

D B . BADHAM said t h a t in 1930 477 coal miners working in the
same pit had been examined and 25 per cent, of them were found
to show some evidence of fibrosis; 33 of these cases had since
been compensated. In New South Wales, out of a total of about
10,000 coal miners employed underground, 116 had received
compensation in t h e course of recent years. He had lately published
an article on the chemical analysis and pathology of a number of
1

See p p . 136 a n d 137.

PROCEEDINGS OF THE CONFERENCE : FIFTH SITTING

29

coal miners' lungs 1 . After showing two radiographs, Dr. Badham
stressed the necessity for examination of t h e lungs b y a dissecting
microscope of a power from 10 to 30, and summarised his conclusions as follows :
1. Of the 76 lungs referred to in his publication, 69 came from
workers in various dusty occupations; 7 were immune and 62
showed varying degrees of pneumoconiosis.
2. Five lungs came from fatal cases of tuberculosis, having had
no known dust exposure, and two others from individuals definitely not exposed to dust.
3. Of coal miners' lungs there were 32; metalliferous miners,
11 ; sandstone workers and miners, 8; and of miners who h a d done
both coal and metalliferous or sandstone work, 13; together
with a small group of workers belonging to various dusty occupations.
4. These lungs were described and classified on t h e basis of
pathological and chemical findings, including separation of free
and combined silica, and were grouped according t o carefully
recorded industrial history.
5. "Coal dust", as distinct from a mineral content of free a n d
combined silica which it might contain, was capable of producing
the fibrotic changes seen in some coal miners' lungs.
6. Contrary to generally accepted opinion, it was suggested
t h a t the entrance of coal dust and probably other dusts into a
lung already affected b y pulmonary fibrosis was small a n d possibly
negligible as a cause of increase of already existing fibrosis.
7. The chief disabling factor in t h e pulmonary fibrosis of coal
miners in New South Wales was shown to be emphysema, and it
was believed t h a t this was more often s e c o n d a ^ to fibrosis than
to bronchitis, which appeared t o be accepted as t h e incapacitating factor in European and American coal miners.
8. Comparison with, the results of chemical analyses of lungs
by various authors was made, and the ambiguity resulting from
the use of the term "total silica" in different senses was stressed,
together with the need for some standard method of analysis.
PROFESSOR P O I J C A R D expressed himself in agreement with
Dr. Badham on the necessity of using a dissecting microscope
1

BADHAM, C. and TAYLOR, H . B. "The Lungs of Coal, Metalliferous,

and Sandstone Miners, and other Workers in New South Wales. Chemical
Analysis and Pathology." Annual Report of the Department of Public Health,
1936. Sydney, 1938.

30

SILICOSIS

with 8—10-fold enlarging power during autopsies, since it provided
much more enlightening information than the usual method of
examination. He was also opposed to the method followed in
certain countries of sending a small fragment of lung to the histologist and favoured on the contrary Dr. Badham's method of
studying the whole lung under water with the aid of binoculars
or a dissecting microscope.
Numerous examinations of fibrous masses found both in
coal miners and quarrymen with or without tuberculosis had not
led him to distinguish any pathological differences, since they
all showed absolutely the same anatomical structure. He agreed
with Dr. Badham that there was a connection between emphysema and fibrosis, emphysema even being considered as a form
of pulmonary fibrosis.
D E . MIDDLETON asked Dr. Badham if the radiological appearances designated in Great Britain at the moment, "reticulation"
—i.e. not nodulation, but a network of shadows—were associated
with disability in the subjects, and if there were any special a,ge
periods at which such disability, if any, appeared.
D E . BADHAM emphasised the presence of emphysema in coal
miners' lungs, his view being strongly supported by the work
of Professor Lyle Cummings. The question was, however, whether the emphysema had already existed or was a consequence
of the fibrosis. Were emphysematic lungs those which in coal
miners developed fibrosis and caused disablement ? Reticulation, he believed, might or might not be associated with disability, which depended on the presence of emphysema.
The problem of providing equitable compensation for lung
diseases in coal miners was in his view dependent on whether
there was emphysema or not. He had little faith in the opinion
of medical men and radiologists in regard to this point, and the
only solution known to him, apart from post-mortem examination,
was the method followed by Hurtadel and his colleagues in America, which might be termed a rectified vital capacity test, allied to
the so-called oxygen test.
D E . IEVINE regarded as essential the point raised by Dr. Badham,
namely the question of deciding whether there was such a thing
as pure anthracosis. Coal dust had hitherto been regarded as
an inert accumulation in association with fibrosis, produced by

PROCEEDINGS OF THE CONFERENCE : FIFTH SITTING

31

silica, b u t Dr. Badham's contribution seemed in strong contradiction to this conception.
submitted and commented on his report on
"Belgian Miners' Silicosis" '.
D R . LANGELEZ

said t h a t the annual occurrence of a pulmonary trouble consisting of chronic bronchitis and emphysema
affecting a small number of miners had for long been noted in t h e
Liege district. The disease was diagnosed as anthracosis and
those contracting it were entitled to an indemnity. A statistical
enquiry carried out in 1933 by one of his assistants, and covering
various categories of patients belonging to the anti-tuberculosis
dispensaries in the district, showed the following returns concerning average age at death, classified according to occupation :
43 for housewives; 36 for factory workers; 35 for students and
employees, 4 5 for miners, and 51 for the so-called anthracotic
miners.
After emphasising the extreme care with which the enquiry
in the Belgian mines was effected, he described the results obtained : presence of positive radiological pictures, both in workers
engaged on the coal seam and in those employed on stonework,
perhaps even with more frequent occurrence among the former—
a most unexpected discovery. The workers examined enjoyed
full working capacity and in certain cases no correlation could
be discovered between cardio-vascular phenomena, radiological
pictures, and the age of the workers or length of their working
experience. The medical men were faced with a truly devastating
problem, either from the practical point of view of clinical
diagnosis, or t h a t of the pathogenesis, which was once more
completely open to doubt. All the earlier problems of t h e older
anthracosis and the more recent silicosis were tending to draw
together.
The radiological picture was merely an anatomical document
giving no indication of the gravity of the pathological syndrome,
which could only be deduced in accordance with the physiopathological phenomena and clinical findings.
D R . VAN B E N E D E N

D R . H A U S S E R said t h a t a study effected by Prof. Leclercq
on the Anzin and Lens mines revealed the existence among the
workers of extremely discrete lesions only.
i See p . 140.

32

SILICOSIS

said that the health situation in the Netherlands
differed considerably from t h a t in Belgium, which might possibly
explain the different conditions found in the two countries. While
the mortality rate from tuberculosis was, according t o Dr. Langelez,
very high among Belgian miners, it was 3 per cent, in the Netherlands, and even as low as 2.2 per cent, in a small town of 35,000
workers, where 5,000 workers were employed in century-old mines.
D R . VOSSENAAR

D E . TSUKATA said t h a t in the Japanese mines at Hokaïdo a
close connection had been established between the work and
the incidence of respiratory diseases, which increased with the
length of working experience. As regards tubercular infection,
coal dust was less dangerous t h a n ordinary mineral dust and
especially siliceous dust. Nevertheless in the case of a worker
suffering from tuberculosis, the disease was aggravated by inhalation of coal dust. The connection between the two was, however,
extremely difficult to prove statistically, since working conditions
played an important part, both in the outbreak and the evolution
of the disease. I n regard to the figures quoted by Dr. Langelez
showing t h a t for those other t h a n coal miners the age of death
from tuberculosis was about 34, while for coal miners it was said
to be from 35 to 65, he would like to ask whether t h e conclusion
to be drawn was t h a t the outbreak of tuberculosis had been retarded b y the d u s t or whether it was rather believed t h a t tuberculosis became superimposed at a later age in the case of individuals affected by anthracosis.
D R . LANGELEZ replied that his assertion must be restricted
to noting in the case of coal miners the considerably retarded
outbreak of tuberculosis, the cause of which was difficult to explain :
action of coal dust or complication of silicosis in miners who contracted tuberculosis at a more advanced age. Moreover, coal
workers working in surroundings very rich in coal dust but outside
the mine, did not develop silicosis and showed no changes of the
lungs on the radiographic picture, while among those who worked
underground, as had already been demonstrated by Dr. Badham,
very numerous changes as revealed by radiography were noted.

Du. H A U S S E R drew attention to cases of silicosis which had
been reported in t h e mines at Ales in the Gard basin in France.
The Conference adjourned at 12.35 p.m.

PROCEEDINGS OF THE CONFERENCE : SIXTH SITTING

33

S I X T H SITTING
(Wednesday, 31 August 1938, 3.30 p.m.)
Chairman :

D R . A. J . ORENSTEIN

Item 2 (continued)
D R . SAYERS referred to the methods used in the enquiry carried
out in 1933 b y members of his service, which had necessitated
the collaboration of medical men, technical experts (engineers,
chemists and physicists), statisticians, etc. The enquiry covered
2,711 miners and had revealed in 23 per cent, of them the presence
of anthraco-silicosis 1 . Urine analyses showed increased excretion
of silica, probably signifying t h e increased silica exposure.
Dr. Sayers presented and commented on a number of slides
taken from his report on the subject.
The enquiry had led to t h e conclusion t h a t t h e disease and
resulting disability found in t h e anthracite miners were due t o
inhalation of a mixed dust composed of silica a n d coal ;. t h a t
the pathological changes were due primarily to silica being subsequently modified by the presence of the coal dust; a n d t h a t the
condition was one which could be controlled by methods now
available in t h e mining industry.
T H E CHAIRMAN, summarising t h e discussion, remarked t h a t
one school of thought believed that anthraco-silicosis constituted
a disabling disease, while another considered t h a t tuberculosis
played the predominant role in the infection.

D R . K I N G raised t h e point of dissimilarity in t h e expression
of t h e results of chemical analyses of t h e lungs as found in t h e
works of different authors, rendering comparison impossible, and
referred to the great advantage which would accrue from uniform
expression of these results in milligrammes per cent, of dry weight
of t h e lungs, together with a standard method of drying, accompanied, wherever possible, by the figures for the total silica content
1

OF

U . S. T R E A S U R Y D E P A R T M E N T , P U B L I C H E A L T H S E R V I C E .
INDUSTRIAL H Y G I E N E AND SANITATION. "Anthraco-silicosis

OFFICE
among

H a r d - C o a l Miners". Public Health Bulletin
No. 221, D e c e m b e r 1936.
G o v e r n m e n t P r i n t i n g Office, W a s h i n g t o n , 1936, 114 p p .
2

34

SILICOSIS

of the lungs. I n his view the free silica should not be determined
by chemical analysis alone, since chemical methods could not
always be relied on to give trustworthy results, but should be
completed by petrological analysis.
The amounts of silica in the miners' lungs, mentioned by Dr.
Badham, were of the order of those found in silicosis and, without
calling in question the role attributed by Dr. Badham to coal
dust, he would like to point out t h a t it was possible to have enormous amounts of silica in the tissues without any fibrosis, but
that in presence of fibrosis, which had to be attributed either to
carbon or to silica, it seemed to him in the present state of knowledge t h a t silica was the more likely of the two to be responsible.
D E . GUDJONSSON, while recognising the importance of Dr.
Langelez's and Dr. Sayer's communications, thought it necessary
to "be very careful in drawing any general conclusions on the relation between coal dust or other dust and tuberculosis, since tuberculosis was influenced in its development by circumstances of the
most varied kinds ; nutrition epidemiological conditions social
and economic circumstances, housing and so on.

stressed the fact t h a t the chief value of his research lay in the pure industrial history of the cases in question,
the workers having been employed solely in coal mines, with the
result t h a t the chemical analysis of the lungs was of particular
value. I n eight men who had worked all their lives at the coal
face, in measures containing 0.5 per cent, of free silica or less, the
dust in the lungs was of about the same order of composition.
D B . BADHAM

PROFESSOR POLICARD said t h a t Drs. Sayers, Gudjonsson and
Badham had raised the highly complicated problem of chemical
analysis and its value in the study of silicosis and pneumoconiosis..
A lack of uniformity, primarily in the methods used in all the
documents available on the subject, must be admitted. Usually
what was intended b y silica was the residue from treatment
of an alkaline silicate with hydrochloric acid after incineration.
This older method, still adhered to, rendered present-day results
comparable with those obtained formerly. A further point
was t h a t chemical analysis of the whole lung was of no very great
value from the pathological point of view. I t was preferable to
effect chemical analysis of parts which were anatomically and
pathologically similar inside the lung. Such "regional" analysis
rat.

PROCEEDINGS OF THE CONFERENCE- : SIXTH SITTING

35

gave quite different results which ranged from one to five and
even to 10 for the quantity of silica. Chemical analysis of a fibrous
mass gave amounts of silica very much greater t h a n those found
in the case of silicotic nodules.
A curious and almost unknown fact was t h a t a sort of selection
of the various dusts occurred inside the lung. These were in the
majority of cases absorbed, localised in the lung and transformed,
for part of the dust was soluble and what remained represented
the insoluble elements. In agreement with Dr. King, he would
like the Conference to recommend the study of chemical analysis
of the lungs.
D R . SAYERS said t h a t in the American enquiry "regional"
analyses of the lungs were effected and revealed different amounts
of silica according to the location. In one case the percentages
were as follows: solid portion: ash 11.7 with 2.5 free silica;
while a sample of the remaining useful portion of the lung gave :
5.2 ash with 1 per cent, free silica, the average for the entire lung
being : 8.5 ash and 1.8 free silica.
D R . GREENBURG said that it should be remembered in estimating the p a r t played by coal dust in the cases reported on by
Dr. Badham and Dr. Sayers that the workers were exposed to
under 5 per cent, of free silica. Neither in medical literature nor
in his own personal experience had he encountered, except very
rarely, disability with such exposures ; this implied t h a t the additive
factor of a tremendous exposure to coal dust had played a n important part in the disability affecting the miners in question.
D R . MIDDLETON said that the enquiry effected in anthracite
mines in South Wales had shown in the case of 560 miners t h a t
there was a considerable amount of pulmonary disease, with two
fatal cases, in the absence of stone dusting and of work in hard
headings or rock containing free silica, in the sense of the compensation schemes.
In reply to Dr. Gudjonsson's remarks upon tuberculosis,
he would add that in the village in which this mine was situated
the general population was remarkably free from tuberculosis
and their standard of living was higher than t h a t in other mining
areas in the country.
The distribution of cases of death from silicosis for the period
1 June 1931 to 31 December 1935 in England had been 22 fatal

36

SILICOSIS

cases among half a million underground workers, while the deaths
in South Wales amounted to 147 per 140,000 miners employed.
In South Wales t h e incidence of the disease was moreover higher
in t h e east t h a n in the west. This distribution remained unchanged
during the years after 1935 and no adequate explanation for the
differences in morbidity and mortality on the grounds of geological formation of the coalfields had been discovered, with the
result t h a t t h e y must be based rather on a higher proportion
of work in hard headings. The object of the enquiry at present
in progress was to discover whether a peculiar geographical distribution of silicosis existed and likewise of other pulmonary
diseases leading to disablement and connected with exposure to
dust.
The Conference adjourned at 5.20 p.m.

SEVENTH SITTING
(Thursday,

1 September 1938, 9.40

Chairman :

DB.

A. J .

a.m.)

ORENSTEIN

Item 3
Pulmonary Disease (or Disability) due to Inhalation of Dust in
Industries other than those covered by 1 and 2 (with Quali- and
Quantitative Estimation of the Composition of the Dust in Question)
T H E CHAIRMAN reminded the Conference that only dusts containing free silica were involved in the discussion of this item.
D R . GUDJONSSON referred to the work he had done in connection with pulmonary fluorosis due to inhalation of cryolite containing free silica.
Cryolite was obtained in Greenland only and was worked in
two factories, one in Canada and the other in Copenhagen (80
per cent, of the total production), where it was ground to fine
powder and freed from impurities. I t was used for medical

PROCEEDINGS OF THE CONFERENCE : SEVENTH SITTING

37

purposes, as an insecticide and as a constituent of enamel and opal
glass, etc.
Examination of workers with a working experience of five
years in the Copenhagen mill, where enormous quantities of dust
were liberated, revealed massive fluorosis of the bones and ligaments, and also, in 28 out of 100 workers examined, pulmonary
changes shown by radiography which he was inclined to diagnose as
silicosis, not of the nodular, but rather of the well-marked fibrotic
type, affecting all parts of the lungs.
The raw mineral contained about 90 per cent, aluminiumsodium fluoride and only about 2 per cent, free silica, but it might
be that the dust inhaled really contained more free silica t h a n the
mineral itself. Subsequently certain preventive measures were
introduced, with the result t h a t concentration of the dust in the
work-rooms fell to a minimum.
Pathological findings a t the autopsy of one of the cryolite
workers two years later, who died of a disease not connected with
the state of his lungs, showed only a very slight interstitial fibrosis
and some pigmentation and anthracosis not exceeding, however,
what one might expect to find in a man between 50 and 60. W h a t
had appeared as fibrosis on the radiograph two years previously
had entirely disappeared.
Three years after the first examination, namely, some months
ago, a re-examination of the workers failed to reveal in any of
them changes that could be called fibrosis typical of the action
of silica.
He wondered what the changes shown on the first examination
had really been. He had always doubted whether they were due
to inhalation of silica dust, and since they had subsequently
cleared up he was certain t h a t they must have been attributable
to some other cause.
I t should be remembered, however, t h a t the workers in question
really suffered from respiratory derangement and t h a t the mortality and morbidity rate for respiratory troubles was about three
times as high in the workers exposed to cryolite as in the other
workers in the same factory. On the other hand, the clinical
and experimental research carried out by Roholm pointed t o the
fact that the inhalation of fluorine dust containing free silica
might cause abnormal conditions of very chronic inflammatory
type in the lungs and mucous membranes which, on first examination, looked like commencing fibrosis, but disappeared on
cessation of exposure to dust, without leaving permanent injury.

38

SILICOSIS

These were t h e explanations he had so far given in connection
with t h e two examinations effected, b u t after having heard Dr.
Middleton's communication regarding workers exposed to fuller's
earth, he wondered if the lung pictures at the first examination
were not perhaps shadows due to the dust in the lungs rather than
to changes in the tissues.
I n fact, r a w cryolite really did contain a certain amount of
heavy minerals (lead, iron), so t h a t his cases might serve t o supplement Dr. Middleton's observations and he therefore suggested
to him t h a t h e should remove his workers from contact with
fuller's earth a n d examine them after a certain interval to see if
the changes shown on the X-ray picture actually disappeared,
as in t h e case of his own cryolite workers.
He did not find any lung changes or fluorosis of t h e bones in
about 50 other workers examined, who h a d been working with
artificial fluorine not containing any free silica, with enamels
containing both artificial fluorine a n d cryolite, or with fluoric
acid in the glass and bottle industry; the workers in question
V\ n »-I t i r» /-i r\ TfTriti I n vi nr Q i r r i i i * i i A r m A r\4- -HIT-I +• *-v A C\ Y t r i r i -nn
i±ai\* J.1CUVJ ou vv \jíi\.ííLi'
0 - A . Í J V Í X I Ü I I V ^ Ü \JX. W-M u u m\j
y Odilo.

So far the bone changes and t h e mysterious changes in the lungs,
if any, had only been observed in workers exposed to large amounts
of cryolite dust containing about 2 per cent, of free silica.
D R . LANGELEZ agreed with Dr. Gudjonsson.
H e found that
workers exposed t o v e ^ large quantities of dust free from silica
did not show lesions typical of silicosis.
D E . GABTOÏEK said t h a t he had had occasion to examine between
300 and 400 workers engaged in extraction of sodium fluoride
containing 6 per cent, of free silica.
Surface men engaged in
grinding t h e purified ore containing relatively little or no silica
showed on examination no pulmonary changes of any significance, though in a few cases there was linear exaggeration not
considered t o be of specific origin. On the other hand the miners
showed a certain percentage of silicosis with nodulation of an unusually fine type, though this percentage was not as high as that
generally found among hard-rock miners. The high incidence of
tuberculosis noted among both groups was, after investigation,
attributed, n o t to working conditions, b u t t o social conditions.
There was no later evidence of the disappearance of the lesions
and a few of t h e men who h a d been away from work on account

PROCEEDINGS OF THE CONFERENCE : SEVENTH SITTING

39

of disability for a period not exceeding five years still showed
nodular shadows.
Animal experiment with crude fluorspar containing 6 per cent.
of silica gave rise, after inhalation during 18 months, to a few
silicotic nodules in the tracheo-bronchial lymph nodes, but there
was no nodulatiön in the lungs. Intravenous injection (rabbit)
caused tubercle-like nodules of proliferation in the liver which
did not, however, develop to the stage of fibrosis.
He was not able to confirm the hypothesis t h a t fluorspar
increased the solubility of silica and produced a more rapid reaction
than silica alone.
T H E CHAIRMAN asked Dr. Gudjonsson whether the respiratory
affections mentioned in cryolite workers had disappeared or were
greatly reduced after the introduction of preventive measures,
and whether the workers who had complained of respiratory
troubles ceased their complaints after a reduction of the dust
contained in the atmosphere.
D R . GUDJONSSON replied that he had as yet no answer to offer,
since it would be essential to examine the sick rate after a lapse
of two years before forming conclusions.
D R . HAUSSER reported on several cases of pulmonary silicosis :
one a worker of 56 years of age engaged in making emery-paper
(slow development complicated by tuberculosis), t h e second a
worker aged 19 employed on an emery-grinding wheel, and the
third a sandblaster cleaning celluloid toys. H e also mentioned
four cases of pneumoconiosis affecting electric arc welders, which
had been made the subject of a recent communication, the cases
being regarded as due to siliceous dust liberated b y the electrodes.

D R . R I D D E L L spoke of pulmonary injuries due to talc powder.
Examination of 37 workers engaged in grinding ore (enormous
liberation of dust) for 5 to 18 years (average 12 years) revealed,
.in the case of 13 workers, fibrosis resembling silicosis and, in 3
workers, tuberculosis (1 fatal case). In one case, under observation for 8 to 10 years, radiography showed t h a t the injury had
not receded, but on the other hand t h a t it had not progressed,
as usually occurred in the case of exposure to pure silica. Unfortunately, no autopsy was effected.

40

SILICOSIS

Dr. Riddell demonstrated and commented on a number of
slides.
D R . G A R D N E R mentioned the enquiry carried out in the talc
industry by Dr. Sayers 1 . Examination of 300 films of talc
workers in the same region had enabled him (Dr.°Gardner) to note
in about 50 some evidence of pulmonary reaction which, in 17
cases, was suggestive of a nodular type. The free silica content
of the ore worked varied according to the mine from 2 to 20 per
cent. I n a young man exposed for 8 years to a talc with a rock
content of 20 per cent., far-advanced discrete nodulation was
noted.
D R . G R E E N B U R G referred to the study made in the State of
.New York on silicosis in the foundry industry, the results of which
had just been published 2 . Out of 311 undertakings, 80 were
selected for study. X-ray examination of 5,000 workers by a
standardised technique yielded a rate of 2.7 per cent, for silicosis,
"while the tuberculosis rate was 0.7 per cent.
D R . I R V I N E stated t h a t in workers from a very dusty ironstone
mine (haematite) in the Transvaal, including candidates for the
initial certificate for entry into the gold mines, very marked radiographic changes had been noted, developing after one, two or
three years' work in the iron mine. Subsequent investigation
showed additional cases, though no fatal cases had occurred,
and hence the exact pathology was still unknown, though there
was obviously a residue of dust impervious to X-rays after a very
short period in this occupation.
Dr. Irvine showed several slides demonstrating the early
changes affecting haematite miners and consisting of smaller and
more numerous lesions t h a n those which occurred in ordinary
silicosis.
D R . BADHAM said t h a t a small investigation affecting iron
foundry workers led him to believe t h a t it was only as a result
of certain processes t h a t fibrosis developed, contrary to Dr. Green-.
burg's opinion t h a t its distribution was general. He considered
1
DBEESEN : "Effects of Certain Silicate Dusts on the Lungs." Journal
of Industrial Hygiene, March 1933, pp. 66-78.
2
Silicosis in the Foundry Industry.
N. Y. State Department of Labor,
Division of Industrial Hygiene, Special Bulletin No. 197, N. Y. 1938, 44 pp.

PROCEEDINGS OF THE CONFERENCE : SEVENTH SITTING

41

t h a t it was largely a question of ventilation. I n New South Wales
the work could be performed under more or less open-air condit i o n s / For these reasons he was opposed to the institution in
his own country of a special compensation scheme for silicosis
affecting foundry workers. The results he had met with proved
how very difficult it was to effect direct comparison between
studies of the same subject carried out in different countries.
D R . VOSSENAAR made some remarks upon the mechanism of
the radiographic picture in the so-called "silicosis of pseudotumoral form". Radiographic examination of 5,639 miners, of
whom 1,567 were rock cutters, effected in 1937, revealed t h e fact
that dispersed nodules were at times liable to show on the radiograph as more massive localised forms. These radiographs, in
fact, presented superposition on a plain surface of shadows which
in reality were distributed throughout three dimensions. A
stereoscopic plate would eliminate all doubt, while autopsy would
eventually throw light on the existing lesions. On the other hand,
massive shadows might also be noted in cases of tuberculosis
sclerogenous from the outset.
Dr. Vossenaar showed a certain number of radiographic slides.
PROFESSOR POLICARD said t h a t Dr. Vossenaar's communications emphasised a point which he considered the Conference would
do well to insist on—that was to say, t h a t it was not yet possible to
deduce the existence of anatomical pulmonary lesions from the
examination of a radiographic film, however excellent it might
be : just as in the case of tuberculosis and many other pulmonary
diseases the connection between the radiograph and the anatomical pathology of silicosis was still incomplete.

reminded the Conference t h a t many medical
men in all countries were awaiting the results of the Conference
in the hope of finding in its conclusions an enumeration of the
obvious signs on which to base the diagnosis of silicosis. H e considered t h a t it would be of great service in the study of silicosis to
assert that it was a scientific fallacy to base diagnosis on the
simple reading of a radiograph. Very vast experience might
enable Dr. Irvine to determine the nature of a lesion from examination of a radiograph, but t h a t was not the case of the ordinary
medical man, who might be tempted to venture diagnosis based
on radiography.
D R . LANGELEZ

42

SILICOSIS

DR. IRVINE agreed that without correlation in a large number
of individual cases between the radiographic findings and pathological changes found at death, it was impossible to give more
than a provisional opinion on the basis of radiographic appearances.
In his opinion, it was for the radiologist to describe what appearances he saw, leaving it to the physician, or the pathologist in
case of death, to state definitely any underlying structural lesions.
At the beginning of the history of silicosis, perhaps in all
countries radiologists had been apt to construct a hypothetical
dust pathology on the basis of radiographic appearances. In
South Africa, opportunity was given of correlating the radiological appearances with the pathological condition after death
in 1,500 cases in which clinical and radiographic examination
had been effected within six months of death. It had been possible
to confirm the accuracy of the radiological diagnosis at autopsy
in 97 per cent, of the cases of simple silicosis without tuberculosis
and 86 per cent, of the cases of infective silicosis. It was true that
it was not always possible to say with certainty whether the consolidation shadow was due to an active tubercle or to a chronic
tuberculous silicosis arrested or latent, but even then a pretty
near estimate could be arrived at. Where, in a large number
of individual cases in a given industry, careful correlation had been
made between the radiographic appearances and the pathological
condition, he was convinced that the radiograph was undoubtedly
the most important single item of evidence in the diagnosis of
silicosis.
In the radiographs of ironstone miners already discussed,
the pictures might be due to the accumulation of inert haematite
dust or might represent fibrosis due to an accumulation of haematite and silica dust, but in the absence of a post mortem no decision was possible. Once the post-mortem results were available
and could be correlated with the radiograph it could then be
stated with fair accuracy what the condition was.
THE CHAIRMAN said that the Committee on Occupational
Diseases of the International Labour Conference in 1934 had
stressed this very point, namely that it was not possible to depend
on any single method of diagnosis for the definite establishment
of silicosis, but that it was essential to make use of every available
means (industrial history, nature of the dust, its concentration,
length of exposure to the dust, clinical findings, physical examination, disability and radiography), though it might be said that

PROCEEDINGS OF THE CONFEBENCE : EIGHTH SITTING

43

radiography played a very important role, perhaps the most
important.
A medical expert with a narrow experience of silicosis in one
industry was only properly competent to deal with the disease
occurring in t h a t industry. I n South Africa the position of medical
men in this connection was most fortunate. They were dealing
with a single industry (gold mines) where conditions were more
or less uniform and had a centralised medical bureau which had
been studying the disease for over 22 years. He was, however,
certain that in countries with many small industries involving
exposure to silicosis, but with varying conditions, the problem of
diagnosis presented much greater difficulty.
The Conference adjourned at 12.30 p.m.

E I G H T H SITTING
(Thursday,

1 September 1938, 3.5 p.m.)

Chairman :

DB.

A. J .

OBENSTEIN

Item 4
Methods and Standards of Early Diagnosis of
Pneumoconiosis,
with or without Infection, including Determination of the Silica
Content of the Blood, Urine and Faeces. Value, if any, of
Early Diagnosis of Simple Silicosis in regard to the Question
of Removal from Dusty Occupations with a View to the Arrest
of the Disease
D E . GEOBGE described the three mainstays in early diagnosis
as radiography of the lungs, especially when a radiograph of the
worker on entering the industry was available for comparison;
history of exposure to silica-containing dust; and the results of
clinical examination.
A standard diagnosis was easy enough in cases uncomplicated
by infection—presence of uniform discrete marking, sometimes
coarse and sometimes very fine.

44

SILICOSIS

Difficulty arose when infection was present from the beginning,
as was now usually the case at Broken Hill, and an area of consolidation developed in some portion of the lung while the worker
was under observation. The question was whether the condition
was pure tuberculosis or, where there was an element of silicosis
present in the radiograph, the condition described as silico-tuberculosis. When the worker had been subjected to long exposure
to silica dust he should be given the benefit of the doubt.
Certain cases of uncomplicated tuberculosis were very easy
to recognise, others very difficult; cases originally designated
as uncomplicated tuberculosis frequently showed in a few
years' time a typical radiograph of silicosis complicated with
tuberculosis.
Great caution was required in diagnosing pneumoconiosis
or tubérculo-silicosis when there was extensive consolidation
in one lung only with no changes at all in the other. In tuberculosis the sputum was always positive, if the consolidated area
•was large, and there was always some area of consolidation in the
other lung. If this were not so, the condition was probably a
new growth. Some cases were very atypical from the outset, and
resembled new growths until a positive sputum for tubercle bacilli
and detection of the condition enabled the diagnosis of silicotuberculosis to be made.
At Broken Hill uncomplicated pneumoconiosis with discrete
uniform mottling was now not nearly so common as the case
showing evidence of infection from the outset. Even when the
worker died before consolidation broke down and a positive sputum
was obtained, symptoms in presence of these consolidations almost
always suggested tuberculous toxaemia (progressive loss of weight,
loss of energy, feeling of exhaustion, etc.).
Clinical observation of the disease led with increasing insistence to the conviction that most cases of silicosis had a tuberculous
element in them from the outset. Even where uncomplicated
pneumoconiosis showed no signs of tuberculosis on withdrawal
from work, sooner or later signs and symptoms of the tuberculosis
complication set in.
. The frequency with which tuberculosis developed later and the
number of cases which showed evidence of tuberculosis at the
time of diagnosis constituted in his opinion an incontestable
argument for the withdrawal of men affected from their dusty
occupations on detection of the earliest signs of the disease. Withdrawal from the work would not prevent the development of

PROCEEDINGS OF THE CONFERENCE : EIGHTH SITTING

45

^tuberculosis, but it might be assumed t h a t open-air work would
delay its onset.
The only attempt to draft men into other occupations in Australia was a failure. Of 34 men suffering from silicosis in the
early stage placed on the land (growing fruit and grapes), 15
developed tuberculosis and after 16 years only 10 remained on
their farms. Eight were alive and living elsewhere, 5 of them
being tubercular. All the others had died. They found the
work too hard for muscles trained to hold other tools t h a n agricultural ones, and while they had no disability or very slight
disability for mining, they displayed marked disability behind
the plough.
The symptoms alleged by the worker were of very little value
and depended almost entirely (in the absence of other disease)
on whether he was seeking compensation or not. Men suffering
from silicosis, when seeking employment, denied the slightest
symptoms, while those applying for examination with a view
to compensation always alleged symptoms. The question of the
amount of compensation to be obtained, in comparison with the
wages earned on continuing, work very often determined the
presence or absence of symptoms. When these two amounts
approximated applications for examination for compensation
increased in number, and when they differed widely (in times of
big bonuses) the claims fell off.
Shift bosses and other staff men who had not to work so strenuously remained at work much longer t h a n the miner, who
rapidly developed incapacity for his task on account of the
disease.
Cases suffering from other medical conditions (chronic nephritis, senile changes, myocardial insufficiency, etc.), gave rise
to great difficulty as regards withdrawal from the occupation
and compensation.
Initial examination with periodical examination a t regular
intervals solved the difficulty of early diagnosis in the case of new
workers in any given industry, and enabled t h e effect of t h e occupation on the lungs to be studied and an estimate to be made of
the efficacy of dust-prevention measures adopted.
Unfortunately, at Broken Hill there was no periodical examination of employees a t regular intervals. However, underground
workers might apply for examination at six-monthly intervals
and many men had been examined after periods of underground
work since the Bureau of Medical Inspection was opened in 1922.

46

SILICOSIS

In no instance had a case of pneumoconiosis with or without/
tuberculosis been discovered in an applicant for mining worla
when passed "fit" by the Bureau of Medical Inspection, the number
of those examined totalling upwards of 10,000.
Í
D R . H A U S S E R asked whether the Conference could not draft
a type of general questionnaire on the detection of silicosis and
determine a standard technique for radiography and chemical
research.
D R . BADHAM said t h a t within certain limits it was not the
amount of free silica in the lung t h a t determined the onset of
tuberculosis, which was just as common with a very small amount
as with large amounts. I n his view, little was to be gained by
removal from dusty occupations following early diagnosis of simple
silicosis. The result to be aimed at was reduction of the dust
exposure. He further believed t h a t there was a limit to the
amount of free silica which could be taken into the lungs and
referred to metalliferous miners, whose lungs were already fibrous
and who did not, when p u t into coal mines, take much coal dust
into their lungs.
D R . I R V I N E said t h a t in approaching the question of the diagnosis
of silicosis it was necessary to have a clear idea what specific
changes might be encountered and how far available diagnostic
facilities were of assistance in determining the presence or absence
of these conditions. I n the report presented by him \ he had
ventured to give a succinct account in three propositions of the
general conception of the pathology of silicosis, and to indicate
the variations of the different factors involved as explanatory
of the various clinical types caused.
The diagnosis of silicosis post mortem depended on the presence
of palpable nodular or massive lesions, distinctive of simple or
infective silicosis a n d of no other disease.
During life, t h e most important single element of the different
diagnostic elements (occupational history showing sufficient
exposure to siliceous dust, adequate radiography, results of clinical
examination), was a good radiograph when it could be interpreted
by means of sufficiently long experience of its correlation with
post-mortem evidence. A combination of an extended clinical
experience and a n extended radiological experience rendered
1

See p. 151.

PROCEEDINGS OF THE CONFERENCE : EIGHTH SITTING

47

l t h e diagnosis as accurate as was practically attainable in any
¡ordinary disease, and enabled differentiation between t h e three
ttypes of silicosis at present recognised : the simple type, unaccomianied by any obvious infection ; the infective type without active
uberculosis; and the infective type with active tuberculosis 1 .
The cardinal symptom of silicosis was exertion dyspnoea,
passing on in advanced cases to resting dyspnoea, and the clinical type of disability in silicosis was t h a t of a chronic emphysema
and a dry bronchitis, with—in more advanced cases—myocardial
changes and slow heart failure. In early diagnosis these symptoms were not decidedly present, and reliance had to be placed
primarily on the radiograph.
Three characteristics of the radiographic findings indicated
the presence of silicosis :

Ï

(1) The appearance of an abnormal increase in t h e linear
striation visible in the lung fields, accompanied nearly always
by enlargement of the hilar shadows.
(2) Appearance of definite mottling, t h a t was to say, of
small discrete shadows scattered throughout the lung fields, which
in simple cases were usually sharply defined, circumscribed and
more or less symmetrically distributed and in the infective type
of case were more irregular in size and distribution. These could
be graded into small, medium and large mottling.
(3) Appearance of larger and diffuse opacities indicative
of a definite pulmonary consolidation significant of a massive
fibrosis of simple silicotic or infective silicotic nature, or due to
tuberculosis, infiltration and consolidation.
I t was of interest to discuss more particularly the gradation
of successive radiographic pictures from the appearance of abnormal increase in striation to a genuine silicotic nodule.
1
The incidence of simple tuberculosis among European miners on the
Rand is at present a little over one per thousand. Tubérculo-silicosis was
always classified as silicosis and there were no separate figures for these cases.
It may be said that in European miners the infective type and the simple
type represent about half and half.
Among European miners the factor of tuberculosis does not appear to
play a significant role unless and until the lung had become effectively
occupied by siliceous dust to an extent capable of producing silicotic lesions.
When that is the case, the factor of tuberculosis expresses itself generally,
not in the production of an unmodified simple tuberculosis, b u t in t h a t
of the characteristic lesions of tubérculo-silicosis (mixed lesions of silicotic
and infective origin). I t may be, however, that the threshold of facilitation
of silicosis is lower in the Native than it is in the European and t h a t in such
cases unmodified or practically unmodified tuberculosis may occur much
more commonly than in the European.

48

SILICOSIS

The earliest unequivocal radiographic picture indicative oil
silicosis was the appearance, against the background of well-/
marked increase in striation, of the significant amount of pin-|
head mottling, usually in the upper lobes of one or both lungs!
Increased striation nearly always preceded definite symmetrical
mottling. Post-mortem examination showed t h a t abnormal
increase in striation was coupled with an increasing amount of
significant palpable macroscopic silicotic nodulation 1 , t h a t was
to say t h a t in 50 per cent, of the cases in which there was wellmarked increase in linear striation, post-mortem examination
showed a significant amount of incipient silicosis. The appearance of increased striation in the lungs of a miner in which such
appearances were not present in earliest radiographs, together
with disability of the emphysematic and bronchitic type, were
accepted as typical of incipient silicosis. Such conditions were
to be found in cases of chronic heart disease or chronic bronchitis,
but not to the extreme degree found in those working underground.
Early removal from work of the silicotic miner should be
considered in relation to certain factors connected with the evolution of the disease.
Even in the ease of simple silicosis the condition would progress
up to a limit which, it was believed, was determined by the amount
of dust originally retained in the lung, and when t h a t limit was
reached, the disease underwent a substantial degree of arrest.
Early removal from work did not prevent t h a t progression but
merely had the effect of slowing down progress, with probably
a slight reduction in the ultimate amount. The second factor
in producing progression was infection.
I n his opinion, t h e only way to arrest a case was to make sure
that the amount of dust to which the worker was exposed was
limited to such an extent that it would no longer be productive
of silicosis.
„
H e had noted with great interest in Dr. Sayers' report 2 t h a t he
had raised the issue whether it was desirable to remove workers
1
He had had occasion in the earlier part of 1938 to conduct an investigation among Rhodesian miners, in the course of which over a thousand
men from various mines were radiographed. He found progressive increase in
the percentage of cases showing abnormal development of striation in proportion to the duration of their service (up to 30 per cent, for a working experience of 15 to 20 years). No such increase appeared in miners in a coal
mine in which pure coal was being exploited in a seam over 20 feet wide
and in which no work was carried on in the surrounding rocks.
2
See pp. 202-203.

PROCEEDINGS OF THE CONFERENCE : EIGHTH SITTING

49

suffering from simple silicosis from underground occupation and
had indicated the necessity of taking into consideration both
the medical and the economic and social aspects of the question.
He was inclined to agree with Dr. Sayers on the advisability of
refraining from removing silicotic miners from work as long as
they were free from tuberculosis.
T H E CHAIRMAN reminded the Conference t h a t the opinion
just expressed coincided with the decision arrived at by the Johannesburg Conference in 1930 on removal from work of cases of
simple silicosis.

asked Dr. Irvine what was his opinion
regarding "medium mottling", and the terms sometimes used to
designate similar appearances. Where this appearance of mottling was no longer progressive, might not the increased striation
be due to crossing of the striae, giving an effect of intersections
on the radiograph ? I n silicotic lungs it was possible to distinguish palpable hard nodules which could be enucleated with a
knife, and more irregular nodular formations, often elongated,
resulting from peribronchial transformations, due to sclerosis
and blocking of the bronchioli and sometimes, but more rarely,
of the smaller blood vessels.
He was anxious to know which of these types of nodules were
found in 50 per cent, or upwards of the cases.
PROFESSOR POLICARD

D R . LANGELEZ said the concentration and the quantity of
dust were most important, both from the point of view of diagnosis
and of prevention. Medical literature revealed very great divergencies as regards the amounts cited as corresponding to safety
limits (1 - 20, even 30 mg. per m. cu., and over), so much so, in
fact, t h a t it was essential to have more definite guidance on the
subject. I t was true of course t h a t it greatly depended on the
size of the elements and their silica content. Nevertheless he
thought it was essential to be able to determine the minimum
amount of pure silica which could be tolerated.

thought that the approximate weight of dust
present in the atmosphere which could be regarded as safe had
not so far been determined. On the Rand they had had counts
giving 20, 30 or 40 mg. and even more per m. eu., but during the
last ten years only 1 mg., and still there was a certain amount
of silicosis produced. Estimation by weight was entirely depenDR.

IRVINE

50

SILICOSIS

dent on the size distribution of the particles. Even 150 particles
per c . c , if the particles were sufficiently large, would give 1 mg.
per m. eu., b u t would not be in any way dangerous, for the danger
lay in the abundance of fine particles of which estimation by
weight gave no account. At present attempts were being made
on the R a n d t o achieve estimates of dangerous sizes, from 2 ¡i,
downwards.
I n reply to Professor Policard he recalled the fact t h a t the
earliest appearances of increased striation showed annular forms
of shadows, due to thickened bronchi and in some cases possibly
to thickened blood vessels. I n the most recent cases the silicotic
nodule was found to be small, hard and palpable, could be cut
out with a knife and retained this character throughout the whole
evolution of t h e disease, except in areas of massive fibrosis. I n
OlUIIlV

VAC- V *Dl\jijlX±g>

OliUjUOlO

KJllU ll**J\A.\A.l*3B

U 1 U ll\J

V XKJL III

YV l Vil

CU

ULKJlllillLÍO

or a blood vessel a t their centre, though they might rarely enclose
terminal bronchioles or more frequently arterioles. The clean-cut
hard silicotic nodule could be clearly distinguished from the
irregular, more difficult to eviscerate, usually infected nodule—•
mostly of tuberculo-silicotic nature.
B R . G A R D N E R asked whether in the case of aiscreie nodulation without conglomeration cardiac changes were found, and even
fatal cases of cardiac origin.
D R . I R V I N E thought t h a t cardiac changes in silicosis were in
general in proportion to the amount of emphysema and cardiac
degeneration resulting from it. I n cases of advanced silicosis mild
cardiac degeneration might be partly toxic in origin, due to chronic
bronchitis. He had seen cases of very marked massive fibrosis
without any enlargement of the heart a t all and although the
presence of large areas of massive fibrosis of infective type was a
marked cause of emphysema, he did not think it exerted any
effect on the heart apart from that. Formerly when there was
much massive fibrosis not apparently related t o active tuberculosis
and much peripheral emphysema—often bulbous—there were
cases of fairly rapid heart failure followed by fatal congestion,
but with the massive fibrosis now occurring it was rather a case of
slow heart failure of the myocardiac type.
D R . H A U S S E R asked Dr. Irvine whether the cardiac troubles
were not noted more particularly in cases of massive siliceous

PROCEEDINGS OF THE CONFERENCE : EIGHTH SITTING

51

poisoning. He had observed in a worker engaged in cleaning
celluloid toys by sandblast and suffering from silicosis intense
cardiac dilatation and total hyposystolia.
D R . IRVINE repeated that heart failure now found among
silicotics was rather more toxic than mechanical in origin. Cases
with massive fibrosis did not seem to show cardiac changes more
frequently than others; such changes depended on the amount
of emphysema present.
D R . SAYERS stated t h a t in the earlier diagnosis of silicosis,
where there was the appearance of a well-marked increase in
linear markings, it was still impossible to determine whether the
worker was having sufficient dust exposure to infect his lungs
and unless the man was under observation it was not possible to
state definitely whether he was developing pathological changes
due to dust exposure. He had not seen many cases of cardiac
injuries unless in the presence of infection—not necessarily tuberculous infection. In the majority of cases of simple silicosis
occurring at the present time heart affections rarely depended on
the condition of the lungs.

said t h a t the most important element for early
diagnosis was radiography. In Japan it constituted the essential
basis for fixing the extent of development of the disease, the
proper moment for removal from work, and the right to compensation.
The Japanese Industrial Hygiene Association had set up a
Committee for studying the standardisation of radiographic
technique. On the other hand, active research was in progress
into early diagnosis of silicosis and the importance of removing
workers from dusty surroundings with a view to arresting the
disease. Certain tests pointed to the conclusion t h a t in a considerable number of cases the disease had ceased to progress, but
the early stage in its development at which the disease could
most readily be arrested had still to be discovered, since according
to certain experts, once the stage of respiratory oppression had
been reached, the disease was bound to continue its course.
D R . TSUKATA

The Conference adjourned at 5.35 p.m.

52

SILICOSIS

N I N T H SITTING
(Friday,
Chairman :

DB.

2 September 1938, 9.35 a.m.)

A. J .

ORENSTEIN

and later

D E . LANGELEZ

Item 4 (continued)
read his report entitled "Silica in Body Fluids in
Relation to Silicosis "1. He felt it only right to add t h a t analysis
of body fluids could only be considered as accessory, although
perhaps interesting, evidence in the study of the disease, and to
emphasise the fact t h a t these results should not be made use
oí in an improper manner. I t had occurred t h a t certain persons
had been approached by litigants in cases before the courts and
asked to say t h a t the presence of abnormal amounts of silica in
the urine, for instance, constituted evidence of silicosis. Decision
in such circumstances ought to rest on the considered opinion of
experts and not merely on laboratory findings.
DR. KING

D R . H A U S S E R stated t h a t research into the silica content of
the normal lung had been effected by examining victims of street
accidents.

Item 5
(a) New Methods of Dust Investigation
as regards
Sampling,
Concentration, Composition and Size Frequency
Distribution.
Possibility of Standardising these Methods.
D R . MLDDLETOST drew the attention of the Conference to
recent papers published in Great Britain on instrumental methods
of collecting dust for subsequent examination 2A satisfactory instrument for dust sampling should enable
samples of atmospheric dust, as nearly as possible hundred per cent.
complete, to be collected. About six or seven years ago the Medical
Research Council's Committee, after a survey of existing instru1
2

See p. 167.
Papers on Dust Sampling Examinations.
The Institution of Mining
and Metallurgy, Excerpt from the Transactions. Volume XLVI, 46th
Session, 1936-1937, London, 1938.

PROCEEDINGS OF THE CONFERENCE : NINTH SITTING

53

ments, showed t h a t none of these provided a complete picture
of the atmospheric dust as it existed at the breathing level of the
workers exposed.
Research had shown t h a t an instrument depending upon
thermal precipitation could be adapted to this purpose (H. L.
Green's Thermal Precipitator). This instrument gives t h e whole
of the dust content of the atmosphere for purposes of microscopic counting, t h a t is, enumeration and particle size distribution,
up to particles of any size which are of pathological significance.
Two further types of instrument were evolved by Professor Briscoe
(at the Imperial College of Science, London), one of which depended
on the passing of comparatively large quantities of air through
a filter, the medium of which was solid and volatile at comparatively low temperature or soluble in alcohol (media found suitable
were anthracene, naphthaline or salicylic acid). This instrument
gave a complete cross-section of all atmospheric dusts and enabled
them to be submitted to micro-chemical and, to some extent,
pétrographie examination.
The other instrument devised by the same expert, to give
larger quantities of atmospheric dust for ordinary chemical analysis and to include particles large enough for pétrographie examination—that is, having over 3 ¡A and if possible over 101* in diameter
—consisted essentially of a cylinder interrupted by flanges or
fins to deflect the dust. I t possessed the advantage of enabling
very large volumes of air to pass through in a comparatively
short time, b u t had the drawback of not removing all t h e dust
from the atmosphere.
By using these three instruments, the nearest possible approach
could be made to the whole of the dust content of t h e atmosphere,
providing thus for enumeration, size particle distribution, chemical
analysis and pétrographie analysis.
D R . BAD HAM stated that there were two main spheres of action
in dust sampling : intensely specialised complete investigation
of the dust, and routine work.
He therefore asked Dr. Middleton what instruments he would
like at present to see in the hands of a mining or factory inspector
on the one hand, and of an expert on the other.
I t seemed to him that it was essential to p u t forward some
useful routine method of dust sampling productive of results
which could be considered as having a certain ratio to t h e more
intensive specialised work.

54

SILICOSIS

D E . M I D D L E T O N agreed t h a t instruments of great precision
necessary for research might not be the best for routine work.
He had found Owens konimeter very useful for giving some idea
of the relative quantities of atmospheric dust, but an instrument
the efficiency of which varied in different types of atmosphere
wa$ somewhat faulty, because it provided an unequal sample
in different types of dust cloud. The American methods had
been very thoroughly tried and had been found suitable for routine
use. In Great Britain Owens konimeter was used in factories
and the Kotze or Zeiss konimeter in mines, where they had proved
useful, though n o t sufficiently accurate in the research sense of
the word. At present the Factory Department had appointed
an inspector to use the thermal precipitator instrument for special
investigation in t h e pottery industry, to determine the dust hazard
in new industrial processes. Ke did not consider it possible to
answer off-hand Dr. Badham's question, but it should be stated
that if dust sampling was to be scientifically accurate the use
of an accurate instrument in the hands of a skilled observer was
essential and the more accurate the instrument used for evaluating
the hazard, the better would be the idea obtained of the real
risks to be combated.

(Dr. Langelez here replaced Dr. Orenstein in the GJiair.)
D R . O K E N S T E I N requested the Conference to discount South
African experience in dust sampling for the reason t h a t it was
entirely confined to one industry, the chemical and geological
composition of t h e rock of which was sufficiently well known not
to require any further detailed study.
B y application of water and ventilation the total dust content
of the air had been very materially reduced, b u t nevertheless the
incidence of silicosis had not fallen in the same ratio. Assuming
that it was the smallest particle which was the most dangerous,
it became essential to have an instrument which would give, with
the greatest possible precision, the size frequency distribution
of the dust in t h e various operations. The former predominance
of hand drilling (involving application of relatively small amounts
of energy to the end of the drill) had now given way almost entirely
to the use of efficient mechanical drills. I t was a physical law
t h a t the fracture of any given material was a function of the energy
applied to the point of fracture, so t h a t with a great deal of energy
the production of a great number of very small particles was

PROCEEDINGS OF THE CONFERENCE : NINTH SITTING

55

inevitable. Drills used in South Africa a t present drilled 90 lbs.
to the square inch at the rate of 9 ins. or 23 cm. a minute; a t 50 lbs.
pressure they could be made to drill at about 3 % ins. or 9 cm.
per minute. By reducing the speed of drilling from 9 ins. to 3 %
a minute the production of dust particles was reduced from
1,600 t o 700 and size frequency distribution tended towards t h e
production of smaller and smaller particles. These technical
data rendered advisable the use of t h e thermal precipitator,
which collected the finest dust particles (See Mr. Patterson's
report, p. 169).
Dr. Orenstein then showed and commented on some microphotographs illustrating dust structure and distribution of particles
of various sizes.
SIZE FREQUENCY DISTRIBUTION OF PARTICLES
< 3 ¡J.
Per cent.

Wet drilling machine
Hammer " )
pompom

machine

< 0.25 ¡j.
Per cent.

78

46

20

82

56

27

96

88

74

92

82

66

73

35

10

("Jack

Lung dust (digested tissue) . . . .
Dry

< 1 i*
Per cent.

(dry

In blasting there was a predominance of large particles, probably due to aggregation. Despite the tendency towards the
presence of large particles, it was nevertheless very dangerous
dust, since there was much more of it present t h a n in drilling.
I t was possible that nitrous fumes developed during blasting
might also account for this.
With the konimeter it would not have been possible to establish the size frequency distribution, as indicated above, since
it was unreliable as far as t h e finest particles were concerned in
dust clouds in which the size frequency distribution varied. I t
tended to collect a larger proportion of large particles and a smaller
proportion of the smaller particles.
Aggregation had been ascribed to the electric charge (Patterson),
which was definitely a probability, because the reciprocal motion
of a dry drill was likely to develop a great deal of electric potential,

56

SILICOSIS

which was probably discharged by the water in the case of wet
drilling.
This aggregation occurred in certain mines in the Transvaal
and in Rhodesia in which dry drilling was used and in which the
incidence of silicosis was claimed to be non-existent. I t was
considered t h a t in certain conditions dry drilling might produce
these aggregations so t h a t the dust became too large to be heavily
toxic. I t was well known t h a t very small particles could pass
through any quantity of water, and it was thus possible t h a t the
wet method in use in South Africa did not stop the very particles
which were extraordinarily toxic.
D R . VOSSENAAR, referring to Mr. Patterson's report, stated
that in the Netherlands research had yielded figures slightly
different from those given by Mr. Patterson for the size particles
present after drilling and blasting, which might perhaps be
accounted for by various factors such as composition of the rock,
peculiarities of t h e drills or of the explosives used, the methods
of blasting followed, etc,
The dust sampling instrument used in the Netherlands enabled
the dust cloud to be estimated by counting and permitted elimination of the largest particles by sedimentation.
Incineration of the dust samples permitted- fragmentation
of the largest particles into numerous small particles, more especially
after previous absorption of water. Where the sample contained
numerous coal particles incineration might exert an effect on the
determination of t h e number of small particles.
Optical analysis suggested by Mr. Patterson seemed open
to criticism. An ideal result could not be obtained except with
dusts of homogeneous composition and containing particles of
identical dimensions.
He referred in conclusion to the existence of "external waterfed drilling machines" which could not be operated until the water
had been turned on.
D K . BADHAM said t h a t enumeration by similar methods effected
twelve years ago, with the less efficient instruments then available,
revealed a risk of exposure to between 100 and 200 particles for
men using a drill in which Mr. Patterson counted 700 particles,
a figure which he himself would expect to find if he were to make
. these counts with t h e thermal precipitator, proving that the greater
accuracy of the counting method adopted would carry considerable

PROCEEDINGS OF THE CONFERENCE : NINTH SITTING

57

weight in formulating future ideas of the amount of dust likely
to produce silicosis. He would like to enquire whether the thermal
precipitator had been used in coal mines where it was a case of
grossly dusty conditions. For comparison of such conditions in
various countries, for instance, he himself would prefer t o use the
Greenburg-Smith impinger, in conjunction with sedimentation.
D R . IRVINE stated t h a t neither the konimetric methods nor
the impinger instruments yielded the exact results furnished
by the thermal precipitator, more especially as regards differentiating dust of different types of size frequency distribution.
But for the same dust (dust liberated in drilling for instance)
and under the same conditions, it was possible to establish some
kind of routine ratio between the konimeter and the thermal
precipitator using the impinger as a routine instrument a n d checking the results with the thermal precipitator. I t seemed t h a t
this would probably become the ordinary practice. Thus at one
of the mines on the Rand, Owens konimeter was used for routine
work and its results were checked once a month by use of the
thermal precipitator.

Item 5 (continued)
(b) Intensity of Exposure to Dust and Means of Estimating

It

D R . TSITKATA stated that, in Japan, the impinger konimeter
and the cotton wool filter were generally used for dust counting.
The weight of the dust was calculated in mg. per c.c.
The Japanese Industrial Hygiene Association was attempting
to introduce "standardisation" of : the weight expressed as mg.
per m. cu. (examination of dust content of 300-600 1. of air);
the number of particles per c.c. (analysis of 50 c.c. of air); the
dimension of particles calculated for at least 200 particles (arithmetical mean and standard deviation); the constituents of the
dust.
PROFESSOR POLICARD remarked t h a t the proportion of particles
revealed by methods of disassociation of lung tissue did n o t present
a quite accurate idea of the true distribution of the particles in
the lung. Microscopic examination (3,000 diameters) of thin
sections had revealed t h a t 40-46 per cent, of the particles included

58

SILICOSIS

in the cells and in the tissues were under four-tenths of a y., t h a t
24-27 per cent, were of the order of four-tenths to eight-tenths•[*
and t h a t 26-28 per cent, were over eight-tenths. On the other
hand, chemical methods (counting of the particles in the precipitate) for the study of distribution of the particles in the same
lungs revealed t h e fact t h a t 13 per cent, were under five-tenths ¡A
—a much lower figure than the above.
Another highly important point, from the biological aspect,
was the p a r t which ultra-microscopic particles of silica might
play in setting up the injuries, t h a t was to say, a kind of colloidal
silica in the sense of an aerosol.
D R . O R E N S T E I N reminded the Conference t h a t the question
raised b y Professor Policard was t o . some extent dealt with in
Mr. Patterson's r e p o r t l ; he had used as a method of extraction
digestion b y papain and not b y acids. Figures given b y him,
in regard to distribution of dust particles, were approximately
the same as those obtained by Professor Policard from his microscopic sections.
Mr. Patterson h a d studied the separation of the dust during
the process of inhalation and its deposition in the various parts
of the respiratory tract and arrived a t the conclusion t h a t the
factors in question (rate of fall, Brownian motion) acted in such
a manner t h a t t h e proportion between the size frequency distribution in the air and t h a t in the lung remained more or less constant. He could n o t say how true this conclusion might be. Mr.
Patterson considered t h a t the ultra-microscopic particles exerted
a very small influence, from the pathogenic point of view, but he
begged leave to doubt this assertion until it should be more conclusively proved.
D R . GARDNER said t h a t the capacity to provoke reaction was
inversely proportionate to the size of the particles as proved by
injection of particles. Retention of dust in the lungs was subject
t o a threshold dimension of the particles involved. Dr. Sayers,
on the basis of experiments with very fine particles, believed
that there was a certain size below which particles were exhaled
in a form more or less resembling a gaseous state. Experiments
had been made with silica of extremely fine subdivision (maximum
dimension estimated a t 20 Angström units) on animals (8 to 10
1

See pp. 179-183.

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59

guinea-pigs) with an exposure of six to seven months. The lungs
of the animals showed no gross changes macroscopically or
gross evidence of fibrosis. Microscopically, tremendous numbers
of very large giant cells were scattered through the air spaces,
indicating that a very considerable amount of the very fine
silica had been retained in the lung and had provoked a cellular
reaction there.
D R . IRVINE said that Dr. Simson had told him t h a t he had
obtained the same results as Dr. Gardner, after intravenous injection of finely divided silica. No significant lesions were found,
because there was a lower limit of practical fibrosis-producing
property in the size of the silica particles.
D R . GREENBTJRG stated t h a t there was no evidence to indicate
t h a t the ultra-microscopic particles did not constitute a very
great hazard. He agreed with Dr. Gardner that, in m a n y cases
a t least, these particles might act like a gas in penetrating the
lung, but he thought that by far the largest percentage of them
were exhaled, which could be demonstrated b y the Tyndall beam
effect. On the other hand, when dust was produced in a n y large
amount or in any fair amount, it always contained a very large
number of ultra-microscopic particles which were, therefore,
very abundant in all normal atmospheres, so t h a t if such particles
did do damage, there would be much more silicosis among the
general population than actually existed.
PROFESSOR POLICARD said t h a t if the small particles of silica
really penetrated neither into the tissues, nor into the cells, i t would
constitute a unique case, since all substances liable to occur in
an ultra-microscopic state acted on the protoplasm.
D R . GREENBTJRG stated that he did not mean t h a t silica dust
in fine suspension was different from any other colloidal material
adsorbed when in contact with cellular surfaces, but he presumed
t h a t 70-90 per cent, ultra-microscopic, colloidal or semi-colloidal
particles inhaled were subsequently exhaled.

The Conference adjourned at 12.25 p.m.

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SILICOSIS

T E N T H SITTING
(Friday,

2 September 1938, 3.5 p.m.)

Chairman : D R . A.

LANGELEZ

Item 5 (b) (continued)
D B . VOSSENAAR read his report on "Devices for Determination of the Dust Content of the Atmosphere in Coal Mines in the
Netherlands" a n d showed some slides relating to results which
he had obtained with the Leitz Tyndallometer and with the

said t h a t one of the conditions laid
down under Belgian legislation for the compensation of pneumoconiosis was t h a t the claimant must "present in the sputum phagocytes containing industrial dust". He invited the members
of the Conference to express their views on the value of this criPROFESSOR VAN B E N E D E N

D R . G A R D N E R said t h a t detection of mineral particles in the
sputum was of decidedly secondary importance, in comparison
with radiography, physical examination and the history of exposure of the patient, which constituted the threefold basis on which
diagnosis should rest.
PROFESSOR POLICARD said t h a t the finding of phagocytes
containing mineral particles in the sputum was merely indicative
of the presence of these particles and of the phagocytes in the
sputum itself, b u t threw no light on phenomena occurring inside
the lung.
T H E CHAIRMAN said t h a t silicosis had been defined in Johannesburg in 1930 as a disease due to inhalation of SiO¡¡. At the Inter-:
national Labour Conference in 1934, in consequence of more recently
developed theories, it had been accepted t h a t silicosis might
also be due to silicates and, in particular, to sericite. The present
Conference ought to express its considered opinion on this problem.
I n some countries silicosis had not yet been placed on the schedules

PROCEEDINGS OF THE CONFERENCE : TENTH SITTING

61

of occupational diseases to be compensated, because there was
opposition from those who believed that silicosis was due t o sericite and not to silica. For such countries, the opinion of the present
Conference would constitue an authoritative ruling likely t o lead
to extension of compensation for silicosis.
said t h a t experimental research with sericite
and 30 other silicates had invariably .failed t o produce fibrosis,
except in the case of asbestos (owing to its fibrous structure).
Nor did he believe, from his own experience in injecting sericite
in combination with quartz or other forms of free silica, t h a t this
substance had any accelerating action on free silica.
D R . GARDNER

PROFESSOR POLICARD said t h a t intense dusting of animals
with mica (a silicate very closely resembling sericite) bad enabled
him to demonstrate changes in the alveolar cells and the formation
of patches very similar to those caused by silica, b u t never traces
of pulmonary fibrosis.
D R . K I N G said t h a t both experimental work and industrial
experience pointed to the fact t h a t sericite probably never produced
a n y very marked hazard. There was an idea recently current
among geologists t h a t all the sericite found in the lung had not
necessarily got there primarily as such, but had developed by a
process of disintegration of other minerals, representing a stable
"end product".
D R . BADHAM said t h a t by means of chemical analysis it was
possible, even 15 to 30 years after exposure, to determine accurately where the subject had worked. He did not believe that,
once the dust arrived in the lungs, there was much removal,
whether it was a question of silica, silicates or coal.
PROFESSOR POLICARD asked Dr. Gardner whether it was
possible to obtain particles of similar size with the different silicates and whether the form varied in accordance with these
silicates.
D R . GARDNER replied t h a t particles of the same size were produced by grinding the minerals in a steel ball mill until all particles
were reduced to less than. 3 i* in diameter. Those below 1 ¡A
were separated out from the solution by sedimentation a n d discarded. The form of the particles was not controllable.

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SILICOSIS

Item 6
What Contributions from Other Scientific Fields are Essential to
the Solution of the Pneumoconiosis Problem ?
D E . O E E N S T E I N reminded the Conference t h a t the Johannesburg
Conference had emphasised the importance of team-work by the
various scientists in the solution of the problem. H e believed
that Dr. Jones' sericite theory had served a very useful purpose
in stimulating co-operation between medical men and other scientists, because medical men had previously rather pre-empted the
field of silicosis and made it their own preserve. H e would like
to see further extension of co-operation with the modern physicists who had recently studied "micro-physics".
D E . SAYEES agreed t h a t silicosis research constituted a team
problem, demanding correlation of various scientific studies and
the co-operation of physiologists, pathologists, chemists, biochemists, engineers in various spheres, geologists and petrographers, yet silicosis was in the last resort a medical problem,
demanding guidance and correlation of data, which should be left
to the medical profession.
PEOFESSOR POLICABD referred to the advantage to be gained
in various spheres from meetings of biologists, physicists and chemists, for the study of special aspects of the problem.
D E . MIDDLETOÍT stated t h a t in Great Britain the Committee to
investigate industrial pulmonary diseases, set up b y the Medical
Research Council, whose members represented different branches
of medicine, had immediately found it necessary to co-opt representatives from other scientific spheres. In an enquiry recently
undertaken into pulmonary diseases in coal mines, the work
of physicists, chemists, and geologists had been co-ordinated and
he considered the assistance of practical persons, such as mining
engineers and statisticians, to be also essential.
D E . BADHAM expressed the hope t h a t the Conference would
fix statistical standards for the study of silicosis. What was meant,
for instance, by "incidence of tuberculosis" ? Was it clinical.
or radiographic ?

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63

T H E CHAIRMAN, summarising the debate, stated t h a t the
Conference was of the opinion t h a t : in the study of a question as
complex as silicosis, it was desirable, not to say indispensable,
that the co-operation of scientists in different fields should be
assured. A resolution in this sense would be included in the
report of the Conference.
PROFESSOR POLICARD referred to the present tendency to
consider the influence of a vitamin factor (ascorbutic) in the
formation of fibrous tissue. Might it not be possible to suggest
the intervention of a similar factor in pathological fibrosis ?
He would like to know whether members of the Conference had
had an opportunity of studying or a t least considering the part
played by a nutritional factor in the development of pulmonary
fibrosis.
Another important factor was t h a t of individual constitution,
and there was also the racial factor. Apart from pulmonary
fibrosis, there was the well-known tendency of certain individual
temperaments towards fibrosis. H a d similar phenomena been
noted in regard to pulmonary fibrosis ?
As regards the racial influence, very interesting facts had been
revealed, for instance, in regard to cancer and thyroid diseases.
Had there come to the knowledge of members of the Conference
any difference as regards susceptibility to silica connected with
race or constitution ?
D R . ORENSTEIN said t h a t in South Africa, where people of
several anthropological groups were employed, an answer to
the last question might be expected, but, unfortunately, other
factors confused the issue. The Natives, for instance, did not
work continuously in the gold mines as the Europeans did and
few of them remained long enough to contract silicosis. The
total number of cases of silicosis among Natives with a working
experience of 15-16 years was too low to provide reliable statistical
data. For these reasons, he was of opinion t h a t it could not be
stated t h a t the Native in South Africa was racially less susceptible
to silicosis.
D R . IRVINE

expressed himself in agreement with this opinion.

The Conference adjourned at 5.30 p.m.

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SILICOSIS

SPECIAL SITTING
(Lucerne, 3 September 1938, 2.30 p.m.)

The members of the Conference visited the headquarters of
the Swiss National Accident Insurance F u n d a t Lucerne and
learnt the methods adopted in compensation for silicosis. They
were received b y Mr. Bohren, Director, and Dr. Lang, Acting
Medical Director of the F u n d 1 .
I t was stated that those cases of silicosis which occurred before
the coming into force of compulsory compensation (1 May 1938)
were not covered, but would nevertheless be granted benefits
weviouslv accordeon on a voluntary basis in lieu of compensation.

E L E V E N T H SITTING
(Monday,

5 September 1938, 9.40 a.m.)

Chairman : D R . A. J. OEENSTEIIST

Item 6 (continued)
D B . G R E E N B U E G asked permission to make certain observations t h a t , h e was not able to make at an earlier sitting on the
subject of dust sampling. I n order t h a t any instrument should
be of value, it had to fulfil the following conditions : it should be
able to take a sample directly from t h e atmosphere; it should
permit enumeration of the particles in an unchanged condition
and allow of dust sampling in the case of low as well as high concentrations; it should possess high efficiency with reference to
large particles as well as small particles; and, finally, it should
not modify the nature of the dust or particle size distribution
during manipulation of sampling and counting. There were
two or three hundred dust-sampling instruments, but none of
them quite fulfilled all the above requirements. •
1

See p. 184.

PROCEEDINGS OF THE CONFERENCE : ELEVENTH SITTING

65

Í

He had used the South African sugar tube method, the Kotze
konimeter, the Owens jet dust counter and the impinger method
(Greenburg's), b u t not the thermal precipitator. He was greatly
impressed by the photographs shown by Drs. Irvine and Orenstein,
demonstrating the results obtained with a thermal precipitator,
sampling over a large area in a very uniform manner, so t h a t
microscopic examination was rendered exceedingly simple and
practical.
• Since the standards used in the various countries were connected
with the instruments currently employed there, he thought it
would be wise to continue the use of the instruments to which they
were accustomed until enough experience had been gained with the
thermal precipitator. At some later date (perhaps in five years'
time) the results obtained by using the same type of apparatus
in various countries could be compared.
Item 7
Determination of Disability and Assessment of the Degree of such
Disability in Pneumoconioses
The CHAIRMAN reminded the Conference t h a t this question
was to be considered from the medical point of view and not
from the legal aspect of compensation.
D R . GEORGE said t h a t a few general considerations should be
taken into account in the estimation of disability in silicosis :
(1) Disability was not proportionate to the degree of change
present in the lung radiograph.
(2) Very marked radiographic changes might be present
with surprisingly little disability.
(3) When silicosis with infection was present, the disability
was usually greater than when there was no evidence of infection.
(4) Disability could only be estimated after a full clinical
examination, not only of the respiratory system, but of the other
systems of the body.
(5) If the applicant suffering from disability was working,
it was essential to enquire into the nature of the work he performed
and its effects on him.
(6) In estimating subsequent disability, the value of a good
clinical record of the worker at the time of initial or subsequent
examinations was obvious and a strong argument for such examin3

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SILICOSIS

ations. I n t h e absence of periodical examination, the worker
could be advised t o reapply for examination in three, six or nine
months.
At Broken Hill, disability was usually expressed as a percentage
of the capacity of the worker for employment in a non-mining
industry, and compensation was awarded by the compensating
body appointed under the Act on the basis of a certificate furnished b y the medical authority \
expressed himself in entire agreement with the
previous speaker, especially on the lack of correspondence between
the radiographic picture and the amount of clinical disability
present. H e t h e n made certain comments concerning the part
of his report dealing with this question 2.
He referred to an interesting observation made by Dr. Badham
on the subject of cyanosis, which had replaced the pink character
of the conjunctivae in guinea-pigs and which he considered as
indication of the amount of respiratory disability. Progressive
cyanosis was a useful sign in silicosis, beginning with a slight
blueness of t h e lips and cheeks and going on to obvious cyanosis
and breathlessness.
The various symptoms of functional disability could be estimated clinically, but he thought the Conference might usefully
attempt to discover what was the best foolproof and rapid method
of estimating respiratory disability. Physiologists unfortunately
were only able to suggest elaborate tests (staircase and bicycle
methods of exertion testing). I n South Africa the Miners' Phthisis
Bureau resorted t o a simple measurement of acceleration of the
respiration a n d the pulse rate, and finally of the time taken to
return t o normal after a short spell of exertion (lifting a 20 l b .
weight from t h e floor a certain number of times, or chair-stepping).
DR. IRVINE

1

The certificate covers the following points :
(1) Existence of pneumoconiosis and/or tuberculosis to such an extent
that the applicant should not be re-engaged or should be. withdrawn from
employment in t h e mines.
(2) Capacity or incapacity of earning the amount stated by the statutory
authority as the living or basic wage for the locality in which he resides.
(3) Determination of fitness for specified work (selected work, moderate
work, etd.).
(4) The percentage of impairment of the general physical capacity of
the applicant for employment, due to pneumoconiosis or tuberculosis.
I n the case of other disabling conditions, increasing to 100 per cent.
the worker's physical disablement, the supplementary disablement due to
disease other t h a n pneumoconiosis or tuberculosis is made the subject of
a supplementary statement.
2
See pp. 162-164.
•

PROCEEDINGS OF THE CONFERENCE : ELEVENTH SITTING

67

The tests in question were not very satisfactory, b u t no simple
alternative had yet been devised. Whatever method was followed
for measuring disability, it should be expressed as a percentage
reduction of working capacity in ordinary manual occupations.
I n South Africa the three stages of the disease recognised were
the ante-primary stage, in which disability should not exceed
25 per cent, and might even be absent altogether; the intermediate
stage, not exceeding 50 per cent.; and the final stage of grave
incapacity, ranging from 60 to 100 per cent.
D R . GREENBURG said that in the United States the majority
of Acts accorded compensation only for total disability. Therefore the whole problem resolved into estimation of this disability,
which was exceedingly difficult.
D R . IRVINE agreed with Dr. Greenburg t h a t determination
of disability was an extremely difficult problem. Since all cases
of silicosis, no matter how slight, were u p to a point progressive,
the Conference had to consider whether it was right to compensate
in cases showing positive disability only, or whether the very
fact of a person having a definite amount of such an intrinsically
progressive disease should not render all cases showing clear
evidence of structural change compensable, without there being
necessarily any mensurable disability. I n South Africa, compensation was awarded to those showing any significant degree of
silicosis, whether accompanied b y disability or not.
D R . SAYERS asked Dr. Irvine whether he considered the chairstepping test simply as a measure of physical fitness, or, when
repeated frequently enough, as a test of disability. I n the United
States this test had been used as a test of physical fitness (for
aviators and rescue workers in mines). I t was then applied for
the same purpose in connection with silicosis, and, if repeated
frequently, was believed to constitute a useful means of indicating
physical disability.
D R . IRVINE agreed t h a t the chair-stepping and weight-lifting
tests did not obviously indicate the measure of muscular or respiratory disability, and had to be used, so to speak, "cum grano
salis". Nevertheless, in a class of workers engaged in the same
type of work, and discounting other obvious factors of disability,
either muscular or general and the cardiac condition, it might

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SILICOSIS

be of some value, though it was difficult to distinguish cardiac
and respiratory factors from other factors causing physical unfitness in such cases.
D R . MIDDLETOST said t h a t in Great Britain Dr. MeMichael
(Edinburgh) had carried out research, the results of which would
be published shortly, concerning respiratory incapacity affecting
coal miners detected by the Knipping spirometer or modifications
of it. This method appeared satisfactory for estimating disability
in those affected clinically and radiologically by pulmonary disease.
I t was certain that selection of practical or routine methods
of assessing incapacity for purposes of compensation constituted
a most difficult problem. I n Great Britain compensation was
awarded on the basis of the capacity for work of the individual
at the time of examination. The certificate could be reviewed
either on the representation of the workman or of the employer.
I n cases of tuberculosis with silicosis, total incapacity for the
usual employment was assumed.
D K . R I D D B L L said t h a t there were two conceptions of disability which did not always coincide : working disability for the
man's accustomed work, and disability in the open labour market.
An affected worker might be capable of carrying on his usual
occupation and yet be hopelessly handicapped if forced to compete
for work to which he was unaccustomed. This had repeatedly
occurred in the case of miners removed from underground work
who had rapidly broken down when p u t on surface jobs, even
when those jobs seemed less arduous.
The whole clinical picture, together with information afforded
by X-ray examination, physical examination of the patient and
exercise tolerance tests, was essential t o arrive at an estimate
of disability, a n d in addition to these a great deal of information
could be obtained from watching the patient closely during the
examination, a method likely to render great assistance to those
sufficiently experienced in the art, which had to be acquired by
practice.
I n the Province of Ontario it was considered impossible to estimate the disability more closely t h a n as expressed by the following
scale : 20 - 30 per cent., 30 - 40 per cent., t h a t is to say within a 10
per cent, margin. All persons with pneumoconiosis and active
tuberculosis were considered to be disabled at least temporarily
and received compensation. Tuberculous suspects were placed

PROCEEDINGS OF THE CONFERENCE : ELEVENTH SITTING

69

under observation and might even be sent to hospital, in which
case they received the allowances provided for. Where incapacity
exceeded 75 per cent, it was considered to be total, apart from
certain special cases.
I t was felt t h a t the determination of disability, particularly
in regard tó the payment of compensation, carried too heavy
a responsibility to rest on the shoulders of one single individual,
and t h a t the applicant should therefore be examined b y a t least
two independent experts, their findings correlated and the final
estimate of disability arrived a t preferably by a medical board.
D R . LANGELEZ said t h a t in Belgium the problem of deciding
whether disability due to pulmonary infection was caused by
silicosis or by tuberculosis had been solved by considering in
principle tuberculosis to be consequent on silicosis only where
the tubercular picture was shown to coexist with pseudo-tumoral
lesions. On the other hand, in the case of discrete silicosis lesions,
(disseminated nodules), tuberculous lesions (confirmed b y radiography or sputum analysis) were not considered as due to silicosis.
He hoped that light would be thrown on the problem of determining the exact circumstances in which tuberculosis could be
considered as a consequence of silicosis in order t h a t disability,
not really due to silicosis, should not become a charge on the
compensation funds for t h a t disease. H e could not subscribe to
the principle under which the mere presence of tuberculosis was
to be defined as constituting permanent disability.
D R . R I D D E L L said t h a t in Canada compensation was not
provided for simple tuberculosis, but where tuberculosis was associated with specific signs of silicosis it was deemed to be a part
of the disease and no further effort was made to decide whether
it had been present previously or had occurred after the appearance
of silicosis.

drew attention to so-called primotubercular infection and wondered whether, in the case of tuberculosis apparently due to the inhalation of silica, it might n o t in
reality be a question of re-infection by tuberculosis.
PROFESSOR

POLICARD

D R . MIDDLETON said t h a t in Great Britain the situation was
similar to that in Canada. Simple tuberculosis was not accorded.
compensation. Silicosis accompanied by tuberculosis was deemed

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SILICOSIS

to be one disease for the purposes of compensation and compensated as such.
T H E CHAIRMAN remarked t h a t no definite reply had been
given to the point raised by Dr. Langelez, namely, at what stage,
in a country in which tuberculosis per se did not entitle a man to
compensation, should tuberculosis be considered as arising out
of silicosis and compensated as such ?
The basis of the 1934 Convention was t h a t silicosis, with or
without pulmonary tuberculosis, should be compensated, provided
t h a t silicosis was an essential factor in causing the resultant incapacity or death. I t was therefore a matter for each country to
interpret in t h e light of its own particular experience and he did
not consider t h a t the Conference could lay down the way in which
it should be interpreted.
D R . I R V I N E said t h a t if he might speak on behalf of Sotith
Africa, the view there was that tuberculosis might complicate
any degree of silicosis, however slight, as confirmed by the usual
radiographical and clinical signs. I t had also been found t h a t
cases which, appeared clinically and radiologically to be pure
tuberculosis proved on post-mortem examination, in a t least
50 per cent, of t h e cases, tó be tubérculo-silicosis. For this reason
the Miners' Phthisis Bureau was very diffident in excluding cases
of apparently pure tuberculosis, in a miner with more t h a n five
years' service, from the category of silicosis.
PROFESSOR VAN B E N E D E N emphasised the necessity of stimulating future research in the physiological field to complete present
knowledge of, determination of disability affecting silicotic subjects.
T H E CHAIRMAN said t h a t Dr. Irvine had expressed the wish
t h a t the Conference would give its opinion on a question already
raised by him \ Considering t h a t silicosis was a progressive
disease, should cases of silicosis be compensated in such a way
t h a t disability as such played a more or less secondary role, the
pathological changes in the lung being regarded, as -more important •?
I t had to be remembered t h a t in some countries the basic, consideration in compensation was inability of the worker to continue his
own employment, while in other countries it was disability for any

iSeëp. 67.

PROCEEDINGS OF THE CONFERENCE : ELEVENTH SITTING

71

sort of employment, i.e. general incapacity in the employment
market.
D R . R I D D E L L said t h a t in Ontario the worker who had made
a claim and who had had his claim established a t a given moment
on the grounds of the presence of silicosis, even in the absence of
disability, was later on entitled to compensation should disability
develop. No compensation was paid for silicosis without disability.
D R . IRVINE said t h a t the situation of a miner suffering from
slight silicosis, in receipt of compensation and continuing a t work
under certain conditions, raised problems of practical difficulty
should his disability increase later on.
T H E CHAIRMAN said that the point raised by Dr. Irvine was
t h a t of the circumstances in which a man with simple silicosis
should be compulsorily withdrawn from work. The opinion
expressed at the 1930 Conference was t h a t it depended very largely
on the economic status of the country concerned, the possibility
of finding other employment, etc., and that it would be desirable
not to enforce retirement in the case of simple silicosis. As far
as South African experience was concerned, they believed t h a t
compulsory retirement in the earlier stages of silicosis was certainly
not justified under the conditions in that country.
D R . SAYERS referred to the opinion of the Division of Industrial
Hygiene of the United States Public Health Service as expressed
in his r e p o r t 1 , namely : in the case of silicosis, no withdrawal
of the worker, but elimination of the dust from the working environment ; in the case of active tuberculosis only, withdrawal of
the worker.
Education of employers in this respect had been carried to
such a stage in the United States that they would not dismiss
workers even if they were found to have silicosis. H e referred
to one large firm which even engaged men shown to be suffering
from silicosis on initial examination, placing them in working
conditions where they would no longer be exposed to the risk.
D R . K I N G asked what had been the experience of members
of the Conference in the method of estimating nasal filtration and

^ e e pp. 202-203.

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SILICOSIS

any possible parallelism between it and the individual's susceptibility to the action of dust in the lung.
Dit. VOSSENAAR stated t h a t in the Netherlands they had not so far
had an opportunity of establishing any connection between serious
silicosis and the state of the nose. Examination of a great number
of individuals h a d shown very considerable variations in the state
of the nose from one day to another, in accordance with variations
in the degree of humidity of the mucous membrane.
D R . R I D D E L L said t h a t Dr. Russell had undertaken experiments
with apparatus obtained from Germany, b u t had been unable to
show any relation between the state of t h e nose and susceptibility
to silicosis.
H e asked whether a n y of the members present had seen disability in a case showing only widespread exaggeration of the trunk
and linear shadows presumed to be caused by dust, and if so,
what in their opinion caused the disability.
D R . SAYERS replied t h a t in cases of this kind observed, disability Ayas attributable to other causes t h a n the increased linear
shadows.

D R . I R V I N E stated t h a t in certain cases of minor degrees of
pulmonary fibrosis accompanied by chronic bronchitis and characterised b y increased linear striatum, b u t without obvious
indication of nodulation, disability did arise from bronchitis and
emphysema. Bronchitis appeared to him to be an essential element in cases of silicosis which was too often overlooked, while
there was a tendency to place too much exclusive dependence
on the question of nodulation. I n South Africa there was a
tendency towards a marginal inclusion of such cases under silicosis
and to compensate them.
English mortality statistics for silicotic industries tended to
show, first, a high mortality in middle life from pulmonary tuberculosis and, secondly, a high mortality from conditions generally
included under the term "bronchitis" (dyspnoea and chronic
cough)- in the later decades of life. The same phenomenon had
been noted by Dr. Badham as affecting his coal miners. I t was
true t h a t miners of long service and fairly advanced years did
suffer from disability owing to bronchitis and emphysema, and

PROCEEDINGS OF THE CONFERENCE : ELEVENTH SITTING

73

among, them there were cases which showed on radiography
no definite indications of nodulation, but well-marked increased
linear striation.
D R . MIDDLETON asked the Conference on the subject of departure from nodulation as a standard requirement in diagnosis of
silicosis, whether it would not be an opportune moment for
reviewing the existing standards, adopted a t the Johannesburg
Conference.

said t h a t he wondered where it would lead to
if nodulation as a symptom of silicosis was called in question
and emphysema and chronic bronchitis were recognised as symptoms.
I n his view, if nodulation was no longer to be considered as a
characteristic element, they would be still further from a n exact
idea of the disease than they were at the beginning of the Conference or even at the beginning of the 1930 Conference.
D R . LANGELEZ

T H E CHAIRMAN said t h a t Dr. Irvine simply referred to a condition which occurred among miners, and might or might not be due to
the inhalation of silica dust, or might be connected with numerous
other little-known factors (humidity, exposure to sudden changes
of temperature, action of other dust). That did not fundamentally
affect the definition of silicosis as a disease due to the inhalation
of silica.
Discussion had revealed the fact t h a t it was extremely difficult to
establish definite and positive standards for the assessment of
disability. These standards depended on the methods applied,
some of them very far-reaching and demanding time (methods
of scientific research), while others, which were practical and
intended for application during current examination of large
numbers of workers, were more or less empirical. I t was obvious
that in South Africa, where all the miners were examined before
entering the occupation and at six-monthly intervals afterwards,
it was comparatively easy to compare records and to arrive at
relatively satisfactory conclusions, but the same could not be said
in the case of countries where the worker was only examined once
he claimed to have contracted the disease.
I t would therefore be unwise to lay down rigid standards of
examination at the present time. I t would be preferable to
await the results of research being carried on in the various countries, which would perhaps enable progress to be made before the

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SILICOSIS

next Conference. He thought that the feeling of the Conference
appeared to be against removal from work.
The Conference adjourned at 12.30 p.m.

TWELFTH SITTING
(Monday, 5 September 1938, 3.10 p.m.)
Chairman :

DR.

A. J.

OBENSTEIN

and later

D E . LANGELEZ

Item 8
í reveniion .• \&/ initial Examination o¡ rrovtcers in Dusty Inu/ustvtes
and Methods and Standards for it
expressed thé hope that the Conference would
consider existing, data for. standards in initial examination. No
medical man was competent to say that a given individual would
or would not develop pneumoconiosis. Attention had been called
in this connection to the part played by the filtering capacity
of the nose, by blocking of the lymphatic glands and by bronchitis,
but he felt there was no justification for discrimination based on
such criteria and was of the opinion that in the majority of cases
initial medical examinations were introduced less to protect the
health of the workers than to eliminate those who might ultimately
become a burden on the compensation funds. On this account
certain cases of tuberculosis had been excluded, though in his
opinion the presence of a mere scar or of "more fibrosis than usual"
should not justify exclusion of a worker from a particular industry.
It might well be asked whether the standards for initial examination so commonly laid down had any scientific justification.
In his view, such examinations were largely guesswork.
DR. BADHAM

D B . GEORGE said that the initial medical examination effected
at Broken Hill was based on the South African one. They had
a list of conditions which the men had to satisfy and the standard

PROCEEDINGS OF THE CONFERENCE : TWELFTH SITTING

75

fixed was approximately what could be described as t h a t of a
first-class life insurance company,, plus the possession of a normal
chest radiograph.
The average proportion of rejections was about 15 to 17 per
cent, of those examined. Temporary rejection was sometimes
resorted to in the case of certain surgical conditions such as hernia
(pending cure b y operation). The initial examination was sometimes
modified for certain occupations by issuing a certificate endorsed
"for surface work only".
He believed t h a t initial medical examinations and periodical
examinations afterwards were of great value as a means of prevention as well as from the point of view of compensation. These
examinations ought to be applied wherever compensation was
granted for silicosis, since in their absence old miners might enter
such industries and after a certain time demand medical examination. I t would then be impossible for any doctor to say whether
silicosis diagnosed at t h a t time was or was not due to the industry
in question. Provision was made for initial examination a t the
expense of the employer and was effected by appointed specialists
in various capital cities and the results of such examinations
were usually accepted by the medical bureaux.
Dr. George submitted to the Conference forms and clinical
charts used for examinations.
D R . R I D D E L L stated that it was believed in Canada that,
even considered from the standpoint of the rejection of tuberculosis
cases alone, an initial examination was worth while. Initial examination constituted in general a protection both for the employer
against unjustified claims and also for the worker in substantiating
his claim.
D R . TSUKATA said t h a t thorough medical examination on
engagement of miners and workers in dusty trades in J a p a n
comprised the following : pulmonary capacity, chest measurement,
capacity to effect a given piece of work, and radiographic findings.
The workers' record was completed by the results of later periodical
examinations held a t least once a year. Efforts were being made
to introduce standard methods of diagnosis and to further their
general adoption. Research effected in J a p a n pointed to the
fact t h a t susceptibility to the action of dust varied very much
in individuals, though it was extremely difficult to distinguish
constitutional types presenting the marked susceptibility alleged

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SILICOSIS

in the case of leptosomes and others. The practical value of such
research had, however, still to be determined.
In Japan it was considered that risk connected with dust was
increased by its action in favouring the outbreak of pulmonary
tuberculosis, and an important preventive measure consisted in
the rejection of all subjects suffering from respiratory diseases,
particularly tuberculosis as well as chronic nasal affections.
DR. VOSSENAAE presented his report on "Measures of
Prevention of Silicosis in Dutch Coal Mines" 1 , with special
emphasis on guiding principles for initial examination.
He stated in conclusion that examination of some hundreds of
workers had not revealed any distinction between those with
good and those with bad nasal filtering capacity in the sense of
Lehmann's research.
DR. IRVINE agreed with Dr. George in his view that initial
examination protected the employer against the importation
among his employees of tubercular subjects or those actually
affected by fibrosis or other pulmonary lesions (bronchitis), all
of which might become aggravated in the course of their employment. He could not subscribe to Dr. Badham's contention that
medical examination on engagement was mainly guesswork.
The examination in South Africa of some ten thousand men per
annum had revealed the fact that the three physical types distinguished by anthropologists (muscular, asthenic, pycnic) were of
great value in estimating varying susceptibility to silicosis, the
asthenic type being particularly liable to tuberculosis, the pycnic type showing a definite tendency to early emphysema and
bronchitis 2 . Dr. Badham had stated that initial examination
was meant to safeguard employers rather than employed, but he
did not see how the protection of the former could be effected
without at the same time covering the latter. On the other hand,
in selecting men of good phj^sical type for a dangerous trade,
it was obligatory on employers and Governments controlling
them to introduce the fullest possible measure of prevention
against disease.

The system pursued in South Africa involved, it was true,
a considerable waste of time. Of the total number of candi1
2

See p . 204.
See p. 165.

PROCEEDINGS OF THE CONFERENCE : TWELFTH SITTING

77

dates examined by the medical bureaux, 30 or 40 per cent.
passed, and of t h a t proportion only about 50 per cent, found
employment. I t would be preferable to introduce the method
followed in Australia, namely, to restrict medical examination
to those having obtained at least provisional employment. Further,
he considered t h a t periodical examination every six months, as
was the custom in South Africa, was unduly frequent a n d should
not be adopted by a country introducing medical examination
for the first time. Annual examination would be quite sufficient.
D R . HAUSSER asked Dr. Vossenaar on what grounds removal
of workers from work on rock was advocated after twelve to
fifteen years' employment and what was the ultimate history
of such workers. Since it was true t h a t silicosis continued to
develop after removal from work, wherein did the efficacy of such
a measure lie ?

replied t h a t the rock dust in the Dutch mines
presented a relatively slight risk, b u t continuous employment
for decades was nevertheless in most cases apt to leave traces of
its effect in older workers and on t h a t account it was strongly
recommended t h a t work on rock should be limited to a period
of about fifteen years. Where this was done the workers still
retained sufficient physical strength to engage in useful work
of another kind, and by remaining in the mine the worker was
able to maintain the rights due to him under his contract.
Examinations carried out between 1934 and 1938 revealed t h a t a
certain number of workers still showed a normal pulmonarypicture after twenty to thirty years of employment on rock work.
Precautions were nevertheless essential, since there was a tendency
with advancing age to the development of dyspnoea without,
in most cases, however, any other apparent cause. I t was therefore considered advisable to remove the worker before the appearance of subjective symptoms.
D R . VOSSENAAR

D R . LANGELEZ said t h a t if criteria of medical examination
as strict as those applied in South Africa were to be adopted in
Belgium, conditions would become quite impossible, as it would
immediately lead to a shortage of workers. H e would welcome
some indication on the part of the Conference of methods and
standards of preventive examination of a much simpler character,
insistence, for instance, on exclusion from the work in question

78

SILICOSIS

of tubercular subjects—-a point which should in his view be emphasised in the report of the Conference.
(Dr. Langelez here replaced Dr. Orenstein in the Chair.)
Item 8 (continued)
(b) Campaign against Dust in Generaland Siliceous Dust in Particular
T H E CHAIRMAN said t h a t before recommending the application
in detail of preventive devices which were often extremely costly,
it was very necessary to make certain of their efficacy.
D R . M I D D L E T O N referred to the possibility of abolishing a
dangerous process altogether, for instance, cleaning metal
castings by a wet method instead of by sand-blasting. A modification of this method was the substitution of a safe substance
for a dangerous one, such as the use of metallic instead of siliceous
abrasives in sand-blasting, or again of alumina instead of flint
in china-bedding in the pottery industry, as the result of an enquiry
carried out in Great Britain by the Medical Research Council,
and covering 50 workers "exposed" to alumina dust (in the manufacture of aluminium from alumina), which revealed no evidence
of fibrosis of the lungs in consequence of exposure to t h a t dust.
The campaign against dust was largely a problem for technical
engineering experts. Localised exhaust ventilation to remove dust
as near as possible to its point of origin represented an extremely
complicated problem. Dust counting enabled the efficacy of
the methods employed to be adequately controlled.

said t h a t Mr. Patterson's report 1 dealt partly
with dust prevention in mines. Without mentioning in detail
the preventive measures discussed, he wished to refer to two
methods of a certain interest adopted in South Africa since 1930
as a protection against dust coming from rock bins. By one of
these—the so-called flannel ventilation method—it was possible to
achieve an efficiency of nearly 90 per cent, of the dust calculated
as particles. I t consisted in surrounding the opening of the bin
or large box containing the rock by frames on which was mounted
a textile of the flannel type. The other method, which had proved
DR. ORENSTEIN

1

See pp. 178-179.

PROCEEDINGS OF THE CONFERENCE : TWELFTH SITTING

79

equally efficient, consisted in closing the bin and causing the
dust to pass through a large duct containing a Cottrell precipitator.
The really difficult matter was to control production of dust
in rock drilling, usually done with the so-called water-feed jack
hammer 1 . Though no air was passed down the drill deliberately,
a certain amount inevitably escaped into the water, with the consequent formation of small bubbles which carried the fine dust
to the surface. Where the passage of air into the water could
be prevented, the volume of the dust emitted was reduced by
95" to 97 per cent 2 .
Another possible solution was to apply some machine which
would catch the sludge formed b y the mixture of water and dust
coming from the hole in the boring tool and then to pass t h e
sludge through a filter. All kinds of filters, including centrifugal
filtration, had been tried in South Africa, but proved so inefficient
or inconvenient in practice t h a t they had been abandoned. One
method only was found technically possible and effective, b u t
involved changing of the filters every 20 or 30 minutes, besides
necessitating a 50 per cent, increase in the compressed air required
for the jack hammer.
I t was well known t h a t the use of water was not essential to
drilling. Dry-drilling, however, was a most dangerous process
unless the dust could be removed at the mouth of the drilling
hole, and it was impossible in South Africa for two reasons. First
of all, the use of water in drilling was a legal obligation ; secondly,
propaganda had been engaged in for years to prove t h a t water
was essential as a means of protection, and no one would take
the responsibility of changing the method, more particularly
since it would take from 20 to 30 years to discover whether the
new technique was really effective or not.
Blasting dust, though it contained a larger proportion of large
particles, was very much more intense. On this account workers
were not permitted to work immediately after blasting, which
was done at the end of the shift, and a second method applied was
the use of atomised water curtains at the end of the drive.
The ultimate solution of the problem of mine dust was above
1
See p . 178.
' 2 In the "external feed" machines the water did not pass through the
machine itself, but entered the drill below the machine by a transverse
channel leading to the central longitudinal channel. So far, however,
this system had led to almost immediate fracture of the drill, b u t it was
stated that quite recently an attempt to evolve a drill which Would stand
up to the strain had been successful.

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SILICOSIS

all efficient ventilation, which unfortunately
difficult to apply in some mines.

was

extremely

D R . GREENBTJRG asked Dr. Orenstein what was the air-water
ratio in the South African drills. The actual concentration of
dust in the South African mines was, comparatively speaking,
low, and he wondered whether it would not therefore be possible
to reduce the dust level, either mechanically—by filtration—
or electrically.

replied t h a t no measurements had been made
of the air passing through the drill; the machine was designed to
prevent the passage of air, and the amount which passed into
the water did so inadvertently and varied considerably with the
wear of t h e machine and the speed a t which it was operated.
I n reply to t h e second query, in South Africa they considered,
rightly or wrongly, t h a t the risk was connected with the dust
immediately produced by the drill, for practically no one was
exposed to dust present in the drives, which was most heavily
diluted by ventilation, rendering the risk negligible, as was proved
by the fact t h a t officials not working on the drills did not develop
miner's phthisis t o any extent. The technical difficulties connected
with any precipitation processes in the drives and galleries were
extremely great and they could scarcely be justified in view of
the small concentration of dust involved.
D E . ORENSTEIN

The Conference adjourned at 5.40 p.m.

THIRTEENTH
(Tuesday,

SITTING

6 September 1938, 9.40 a.m.)

Chairman : D R . LANGELEZ

Item 8 (b) (continued)
said t h a t anti-dust activities in Ontario were
directed by the Department of Mines when connected with mining,
and by the Industrial Hygiene Section of the Health Department
D R . RIDDELII

PROCEEDINGS OF THE CONFERENCE : THIRTEENTH SITTING

81

when connected with industries other than mining. Activities
in these other industries were mainly educational and practical
(efforts to interest employers by personal visits, conferences and
through accident prevention associations). The practical side
comprised specially planned surveys by the Division's engineers,
followed by specific recommendations regarding ventilation,
and dust removal equipment and inspection and testing by
inspectors.
I n the gold mines men were not allowed to return to the blasted area until four to eight hours after a blast had been fired,
during which time water sprays and compressed air were employed
to disperse the dust and gas. In one mine a special mask had been
designed for certain operations. I n all mines the principal consideration was general ventilation. I n the various gold mines
individual activities were correlated through the efforts of a
committee set up in 1934 and composed of qualified employees
in different mines.
Dr. Rid dell placed at the disposal of the Conference several
reports dealing with anti-dust activities prepared by members of
the gold-mining group.
D R . TSUKATA said t h a t determination of the quantity and nature
of the dust in mines was effected in J a p a n each season in accordance with the conditions in the mines. I n this way they hoped
t o be able to discover a safety limit on the basis of which i t would
be possible to regulate and control the dust content of t h e mine
atmosphere.
He recalled methods for dust removal adopted in certain
dusty trades and in mines; among others, t h e careful removal
of dust by washing with water after blasting. I n conclusion,
he emphasised the importance of limiting working hours so as
to prevent fatigue.
D R . VOSSENAAR then presented the conclusions (Nos. 5 - 1 1 )
of his report, entitled "Measures of Prevention of Silicosis in
Dutch Coal Mines" \

agreed t h a t the problem of dust in mines was
largely a question of ventilation. I n open air work, ventilation was
D R . BADHAM

i See pp. 206-207.

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SILICOSIS

usually excellent and there were also certain methods of work
which protected the worker.
I n regard to the campaign against dust in general, in the inte- rests of factory workers, it should be remembered that, in the
present state of knowledge, it was often difficult to say whether
a certain dust was injurious or not. To go back to Sir Thomas
Legge's well-known dictum : "Dust should be eliminated at its
point of origin'^ he thought the Conference should enunciate
a rule to the effect t h a t "the worker was entitled to breathe an
atmosphere free from dust ".
D R . M I D D L E T O N remarked t h a t to meet the lack of knowledge
about certain dusts, the British Factories Act, 1937 (in force since
1 July 1938) required removal of any substantial quantity of dust
of any kind from the breathing level of the workplace.

D R . H A U S S E R recalled the French legislative provisions on the
subject (Book I I of the Labour Code).

D R . I R V I N E deprecated the inefficacy of general formulae and
stated his belief in t h e necessity for a detailed specific study
of each process and the prescription of specific regulations of
prevention to be applied to it, as had been done in South Africa
(blasting, drilling, handling and transport of broken rock). As
a result of the measures enforced, the liability of miners to contract
silicosis in 1936-1937 had been reduced by about 64-65 per cent.,
as compared with the period 1920-1923, while the reduction
reached about 80 per cent, as compared with the period 1918-1920.
Figures for "new R a n d miners" (who began work on dates after
1 August 1916 a n d all of whom had passed the initial examination
of the Medical Bureau) showed t h a t their liability to contract
silicosis, at all durations u p to the twentieth year of work, had been
reduced to the extent of 90 per cent, as compared with liability
of miners in general for equal durations of service in the period
1920-1923. However, it had been found that, though the machine
miner had shared proportionately to a very large extent in t h a t
reduction, the liability of the rock drill miner was, nevertheless,
about three times as great as t h a t of the non-machine miner,
indicating the necessity for concentrating upon further control
of dust produced in drilling, which remained the most important
problem in South Africa.

PROCEEDINGS OF THE CONFERENCE : THIRTEENTH SITTING

83

D R . GREENBTTRG said t h a t in the United States the problem
was more a general dust problem. I n the State of New York,
in 1936, as a result of the disclosures in connection with the Gauley
Bridge Tunnel cases, additional legislation was enacted providing
certain funds for the study of the silicosis dust-hazard and requiring
certain regulations to be enforced for the control of t h e dust
hazard in certain industries. I t was on the basis of this legislation t h a t the foundry survey already referred to \ was established,
as well as other enquiries covering thorough examination of workers
in various industries with the aid of a specially built automobile
X-ray truck.

Under the same legislative provisions, it had been found possible
t o require control of dust at the point of origin, as far as practicable, and submission of plans of dust-removal apparatus for
approval to the Department of Labor. At present, approximately
1,200 to 1,500 sets of plans of ventilating devices were examined
annually by the service concerned, coming from all types of establishments (buffing dust, casting dust in linotype machines, foundry
dust, grinding dust, etc.).
Under the Silicosis Act and with the aid of special committees,
a code had been promulgated for the control of silica dust in rock
drilling. The essence of this code consisted in its classification
of rock formations into two classes, those containing less than
10 per cent, by weight of free silica, and all other rock containing
more than 10 per cent, by weight of free silica. For t h e first
category the code fixed a permissible limit for dust concentration
at 100 million particles per cubic foot, while for the second category
the permissible maximum limit was 10 million particles per cubic
foot. The code also stipulated t h a t prevention of dust should
be effected by suction or exhaust methods, by wet working or
any other methods approved by the Industrial Board.
I n practice, the administration of the code was very simple.
The field inspector took a sample of t h e rock and had it analysed
for classification under one of the above categories, for the purpose
of declaring whether or not approved methods must be adopted
to keep the dust production within- the limits prescribed. Other
similar codes were in preparation, dealing with the foundry industry
and the stone-crushing industry.
1

See p . 40.

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SILICOSIS

Item 8 (continued)
(c) Personal Protection (by Means of Masks,

etc.)

D E . R I D D E L L observed t h a t the filter types of mask might be
of service under certain circumstances, as a supplementary means
of protection, though they ^should not be substituted for other
general methods of protection (ventilation).
H e showed a mask designed a t one of the Canadian gold mines,
for certain work, which reduced the concentration of particles
in the neighbourhood of 3,000 to about 300 per c.c.

said t h a t there were still many working
processes in which the only method available for protecting, the
workers was the use of a mask or respirator.
Filtering masks became very quickly filled with dust, if the
filter was tight enough to retain the smallest and most dangerous
particles, with t h e result t h a t it rendered breathing difficult
and most likely favoured the development of silicosis. On the
other hand, when the wearer stated t h a t his respiration was not
impeded, it very often meant t h a t filtration was incomplete and
the mask ineffective. The conclusion indicated was t h a t most
respirators did not constitute sufficient protection against
silicosis.
I n theory, masks with an .air supply should be effective when
used as long as there was any dangerous dust in the atmosphere,
but experience with these masks had not been satisfactory.
Examination of eight workers engaged in sand-blasting big
iron and steel pieces for bridge building, who were provided with
masks with an air supply, revealed in one of them signs of early
fibrosis after three years of work and well-marked silicosis, with
partial disablement, after four years. The other workers would
probably contract the disease in the near future. Once the special
process was finished, t h e workers were in t h e habit of wearing
their masks only so long as dust was visible to the naked eye,
forgetting t h a t invisible dust, which alone was dangerous, still
remained in the atmosphere.
DB.

GUDJONSSON

The protection afforded by masks or respiratory apparatus
was therefore limited and their use might delay the adoption of
other and more effective measures; this was the most serious
disadvantage which they presented.

PROCEEDINGS OF THE CONFERENCE : THIRTEENTH SITTING

85

They could only be used successfully for very short periods
of work, and then only provided t h a t the workers themselves
were interested in using them and understood the necessity of
them, on account of the presence of dangerous amounts of dust
in the air even when they could not see it.
I n protecting workers against silicosis, the question was not
one of theoretically effective masks, which were relatively easy
t o construct, b u t rather of real protection during daily work.
I n conclusion, it seemed to him t h a t the only basis on which
the effectiveness of masks in preventing silicosis could be discussed was not the experimental one, b u t rather their value as a
protection against the presence of dust to any considerable degree
in actual practice.
D R . ORENSTEIN said t h a t with masks, there was a difference
in point of view between the purely experimental worker and the
man called upon to apply the device under practical working
conditions. H e had never yet seen a mask capable of retaining
the dangerous dust particles of one !->•.
An effective mask should be capable of retaining ordinary
cigarette or tobacco smoke. I t should have a resistance of well
under one inch of water and should be such t h a t it could be worn
with reasonable comfort in a hot moist atmosphere. Apparatus
supplied with air from an external source could only be used in
industries where the worker was reasonably stationary and where
air could be obtained from a really clean source with a mask
so constructed t h a t the effort of respiration did not draw dust into
the mask.
The personal factor of the worker had also to be considered.
Speaking generally, it was very risky to depend upon a large
number of workers carrying out a protective measure which they
were free to abolish in the case of discomfort. For this reason,
except under special circumstances, masks could not be considered
of any value for use in mining.
D R . VOSSENAAR did noi; agree Avith Dr. Orenstein. He considered it useless and impossible to aim at retention of 100 per cent.
of the dust produced even in the case of the best type of mask,
in view of the circumstances under which t h e miners at t h e rock
face worked, including t h e duration of their exposure. H e believed
t h a t masks might render highly useful service, after blasting,
for instance, and during loading of rock fragments.

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SILICOSIS

I h t h e Netherlands the miner engaged on rock drilling did
not in general show a n y symptoms after a period of exposure
lasting ten years, and very little change after even twenty years.
I t was therefore permissible to assume t h a t the achievement of
a marked reduction of the dust inhaled would enable the physical
capacity of the worker to be maintained for a still longer period,
assuring him a life expectancy equivalent to t h a t of other workers.
He described the conditions essential to effective testing of masks
and described t h e apparatus constructed in the Netherlands 1.
said t h a t personal protection against inhalation of dust remained the last resort when substitution, suppression
or removal of dust failed. Two main types of personal protection
existed : the breathing apparatus of the hose type with air supplied
from a distance or by a pump, or provided with an oxygen supply;
and the respirator dependent on filtration.
During revision of a Code of Refractory Materials Regulations, the Home Office requested the Department of Scientific
and Industrial Research to undertake the construction of a respirator ensuring adequate protection against the finest industrial
dust. After several years of research and submission of models
to field tests, under t h e supervision of inspectors of the Factory
Department and t h e Mines Department, such a mask had been
perfected by Professor Jones. I t had now been manufactured
by three large firms and placed on the market. H e described
the technical details of this respirator, designated Mark IV.
D R . MIDDLETON

D R . H A U S S E R referred to the utility in the case of certain
dusty processes, where other methods of prevention proved futile,
of organising work in shifts and restricting working hours. He
urged the Conference to emphasise its belief t h a t mechanical means
of dust prevention constituted the only rational method of dealing
with the problem.
D R . SAVERS said t h a t owing to the very large number of types
of respirators on the market, the United States Bureau of Mines
had found it expedient t o work out a schedule specifying the
requirements to be fulfilled b y respirators with a view to their
approval. Two types were covered, one which would stop smoke,
and another which would stop dust of larger-sized particles such as
1

See r e p o r t , p . 2 1 8 .

x

PROCEEDINGS OF THE CONFERENCE : FOURTEENTH SITTING

87

was ordinarily encountered in industry. So far the United States
Bureau of Mines had not given its approval to any type of mask
that would stop smoke. It had, however, been claimed that a
quite recent model would do so, but it had not so far passed the
test.
The Conference adjourned at 12.35 p.m.

FOURTEENTH SITTING
(Wednesday, 6 September 1938, 3.5 p.m.)
Chairman :

DR.

A. J.

ORENSTEIN

Item 9
The Specific Therapy of Silicosis and Results of any Investigations
into Methods for Stimulating Elimination of Silica
PROFESSOR POLICARD referred to the remarkable auto-cleansing
capacity of the normal lung which, however, broke down in the
case of very intense dust. But there was reason to believe
that the interspersion of rest periods during exposure to heavy
dust concentration would lend effective assistance to the normal
auto-cleansing mechanism, a supposition based on theoretical
physiological considerations.
He asked the views of the members of the Conference on this
point and desired to know whether in their experience the effect
of adequate rest periods had shown practical results as a means
of prevention and therapy in the case of pneumoconiosis.
DR. IRVINE agreed that intermittency of occupation prolonged
the duration of the effective exposure to dust required to cause
silicosis, but not however in proportion to the amount of intermittency. In the case of a man who worked alternate years,
the effective exposure was, in fact, more than half that for a man

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SILICOSIS

who had worked continuously over the same total period. Account
had to be taken of the amount of dust already accumulated in the
lungs. I t was possible t h a t intermittency of exposure enabled
the lungs t o get rid of dust which h a d lodged in the alveoli or
bronchi but h a d not so far penetrated the lungs, though once
dust entered the lung the disease followed its usual course.
D E . K I N G addressed the Conference on the subject of "The
depression of silica solubility in body fluids by mineral dusts" 1,
and illustrated his address by a number of slides.
I n conclusion, he referred to the interesting parallel between
experimental experience and experience in the practical field
presented by the results described. The stone dusts used in the
investigations were those recommended formerly by Dr. Haldane
for stone dusting in mines, and considered b y him to possess the
effect of a sort of antidote to silicosis. The results also seemed
to some extent t o resemble those experimental results presented
b y Dr. Gardner, showing that, when sufficient silicate dust was
mixed with free quartz, the reaction to the silica in animals seemed
to be modified or perhaps retarded.
D K . R I D D E L L stated that a preliminary "Report on the prevention of silicosis due to metallic aluminium dust" had been
published in 1937, dealing with a series of experiments carried
out b y Denny, Robson and Irwin at the Banting Institute. The
' addition of metallic aluminium to silica (proposed b y Mr. Denny,
Metallurgical Engineer at the Maclntyre Gold Mine) seemed
to have t h e effect of inhibiting the usual silica reaction in animals'
lungs. Later experiments had been carried out a t the Porcupine
Quartz Mine a n d would be described in a report to be published
shortly. The main conclusions were to be found in the report
entitled "Prevention of Silicosis b y Metallic Aluminium" 2.
Dr. Riddell t h e n showed Dr. Irwin's slides, one series of which
contained microscopic sections of lungs of guinea-pigs exposed
t o dust for 16 hours per day for a period of six months with quartz
dust alone or quartz dust plus one per cent, aluminium. No
lesions were shown to occur in the latter case. Another series
showed sections of skin of the guinea-pigs demonstrating reaction
to subcutaneous injections of silica and silica plus aluminium.

»2 S e e p . 211.
See p . 208.

PROCEEDINGS OP THE CONFERENCE : FOURTEENTH SITTING

89

The addition of aluminium to the quartz reduced solubility of
the silica in water, as shown b y the following table :
Quantity of aluminium added
0.0
0.09
0.17
0.33
0.85
1.10
3.40

Solubility in water of SiO¿
[pts. per million]
107
20
8
7
1
1
1

g.
..
..
»
..
»
»

D R . GARDNER said t h a t he believed t h a t Dr. King's chemical
observations had a practical bearing on t h e subject of silicosis in
industry. I t was known t h a t the rate of development of silicosis
was not the same in all industries, which tended to indicate t h a t
various substances present in the dust did inhibit the action of silica
and this fact was probably responsible for the variation in thè
rapidity with which the cellular changes occurred. His experience
with different kinds of mineral inhibitors seemed to show that, in
certain cases where there was sufficiently long contact between
the mixture of silica and other substances with the body, a modified
type of fibrosis ultimately occurred. Only continued observation
could determine whether aluminium was the perfect inhibitor
and could prevent any reaction,, whatever the length of time.
With other inhibitors, such as iron, it was found t h a t while there
was no specific reaction at the end of one year, reaction did begin
to occur after three years. Not enough was known about the principles involved to justify more than conjecture as to the ultimate
results, yet the observations made by Dr. King, as well as the
Canadian experiments, constituted various parts of one picture,
probably of practical significance for more thorough understanding
of the disease.

D R . GREENBTJRG handed to the officers of the Conference a
complete draft of the report on the "Second National Silicosis
Conference" held in the United States 1 .

1

U". S.

DEPARTMENT

or

LABOR,

DIVISION

OP

LABOUR

STANDARDS.

B u l l e t i n N o . 2 1 , P a r t 1 : " F i n a l R e p o r t of t h e C o m m i t t e e o a t h e P r e v e n t i o n
of Silicosis t h r o u g h Medical C o n t r o l " . P a r t 2 : " F i n a l R e p o r t of t h e Comm i t t e e on t h e P r e v e n t i o n of Silicosis t h r o u g h E n g i n e e r i n g C o n t r o l " . P a r t 3 :
" E c o n o m i c , Legal a n d I n s u r a n c e , P h a s e s of t h e Silicosis P r o b l e m " . P a r t 4 :
" R e g u l a t o r y a n d A d m i n i s t r a t i v e P h a s e s of t h e Silicosis P r o b l e m " . W a s h i n g t o n , 1938.

90

SILICOSIS

D E . BADHAM reported that he had been dusting guinea-pigs
for over 6 to 7 hours a day for a period of six months with quartz
alone, or mixed with aluminium in accordance with the Canadian
formula. He had been unable to detect any difference between
the lungs of the guinea-pigs dusted with aluminium quartz and
those dusted with quartz alone. Both alike showed accumulations
of dust cells. He had not, however, expected to produce any
fibrosis with the concentrations used—4,000 to 8,000 particles
per c.c, in six months' time 1 . The experiments were being
continued.
THE CHAIRMAN reminded the Conference that admixture of
other materials with silica had already been discussed.
At the present time he did not suppose anybody knew of any
such thing as specific therapy for silicosis nor of any way of eliminating by therapeutic measures silica which had accumulated in
the body.

Item 10
The Most Practical Means by which the Sub-Committee can assist
in the Anti-Dust Campaign (through Co-operation with other
Organisations and Individuals)
THE CHAIRMAN reminded the Conference that the Sub-Committee on Pneumoconiosis had a very useful task to fulfil in stimulating propaganda in the various countries and providing
Governments and those concerned with information enabling them
to introduce adequate measures of compensation.
In spite of the extensive literature available it was always a
matter of surprise at a meeting, such as the present, to discover
how relatively little of the knowledge acquired in one place had
penetrated in detail to another. The International Labour Office
could do highly useful work by promoting interchange of detailed
knowledge among research workers, that was to say, by collecting
and redistributing documents—not only bibliographical information but even progress reports of work in course of completion—
a task beyond the scope of professional journals or societies. He
1
The dust concentrations used were as follows : Orange quartz arid
aluminium : total dust subsequent to sedimentation, quartz only : 210 mg.
per m.cu. quartz plus aluminium : 107 mg. per m.cu.; percentage of aluminium : 6 per cent, (for the pure quartz see above (p. 88)).

PROCEEDINGS OF THE CONFERENCE : FOURTEENTH SITTING

91

also felt that a Conference on silicosis, such as the present, attended
by experts on the subject, should be held at regular intervals,
say once every three years.
D R . CAROZZI (Technical Adviser to the International Labour
Office) stated that, though the Health Service of the Office was
in the habit of making extracts from about 250 periodicals, a
great number of original publications nevertheless escaped their
notice, because they did not appear in any of the periodicals devoted
to the special subject in question. Other important articles came
to their attention too late or in an incomplete or inaccurate form
and experience had shown t h a t when t h a t was the case it was
very often too late to obtain reprints. He expressed his desire
for co-operation on the part of members of the Conference in
facilitating the preparation of a list of persons dealing with the
question in the various countries. They could then be approached
and requested to furnish the Office with a summary of their original
articles as soon as they were published, with an indication of the
publisher or of the publication containing them. Communications received in this way could be roneoed by the Office and
distributed to national members of the Committee and thus
important information could be made available to all those
concerned. He thought t h a t a plan of this kind could be carried
out with the present staff and t h a t the expenses involved in roneoing the documents would not be very high.

D R . HAUSSER intimated t h a t the "Archives des Maladies professionnelles" of which he was secretary, would be glad to provide
the Office with the number of reprints necessary of all articles
published by t h a t organisation on the subject.

M R . TZAUT said t h a t he was impressed by the reference to the
dispersion of publications dealing with silicosis and would like to
know whether it would not be possible to introduce the procedure
followed by the Committee for the Prevention of Accidents,
which consisted in having monographs drafted on various subjects.
He expressed himself in full agreement with the remarks made
by Dr. Carozzi. The Chairman's proposal to convene the Conference at least every three years was, of course, feasible and they
might be assured t h a t the Governing Body would certainly give
it favourable consideration.

92

SILICOSIS

PROFESSOR POLICARD supported the proposal to create a
centre for exchange of information on forms of pneumoconiosis;
it would render great service to all concerned. Scientific progress
was subject to rapid development, and it was essential t h a t information on any new work on the subject and more especially on its
contents, should be transmitted with less delay than at present.
He begged his colleagues to undertake to submit as rapidly as
possible any work they published on the subject. He considered
that such a service for the collection and redistribution of information could be usefully completed by maintaining their collection
of documents on the subject in a manner similar to t h a t already
followed for certain sciences. I t might be possible, for instance,
to develop it in such a way t h a t it could eventually furnish photographs or films of articles, memoranda, etc., often unprocurable
elsewhere.

remarked t h a t quite a large amount of very
important work was not published for a very considerable time
and some of it was never published, more especially where the
results were negative, though such results were nevertheless useful,
if only to prevent the repetition of the same fruitless research.
He thought it might be possible to set up a system of correspondence between t h e various countries by which one or two persons
in each country might be responsible for keeping the Office in
touch with what was being done in the way of work and research
there.
T H E CHAIRMAN

D R . SAYERS supported the Chairman's proposal and emphasised
the necessity of holding more frequent meetings on the subject
of silicosis, as he believed t h a t those attending such a meeting,
as that just held, could learn a great deal by it.

The Conference adjourned at 6.10 p.m.
The officers of the Conference met, to discuss and revise the
draft report drawn up b y its Reporter, Dr. Middleton, on Wednesday, 7 September (morning and afternoon) and on Thursday
morning, 8 September.

PROCEEDINGS OF THE CONFERENCE : SIXTEENTH SITTING

93

F I F T E E N T H SITTING
(Thursday,

8 September 1938, 2.40 p.m.)

Chairman :

DR.

A. J.

ORENSTEIN

T H E CHAIRMAN called upon the Conference to discuss the draft
report, as revised by the officers of the Conference, embodying
the conclusions to be transmitted to the Governing Body of the
International Labour Office.
He reminded those present t h a t only the members of the
Conference, and not observers, could take p a r t in the discussion.
The Conference adopted Sections 1, 2 and 3 of the report after
adopting various amendments on points of detail.

The Conference adjourned at 5.30 p.m.

S I X T E E N T H SITTING
(Friday,

9 September 1938, 9.40

Chairman :

DR.

A. J .

a.m.)

ORENSTEIN

The Conference continued its discussion of the draft report
and adopted Sections 8 and 9.
reminded the Conference, as regards radiological diagnosis, t h a t the Johannesburg Conference had laid down
certain radiological standards, but that a t the present time, in
view of the vast and rapid progress effected in this sphere, the
present Conference had not been of opinion t h a t it was advisable
to fix new standards, which would very rapidly become out of
date.
T H E CHAIRMAN

The Conference adjourned at 12.20 p.m.

94

SILICOSIS

S E V E N T E E N T H SITTING
(Friday,

9 September 1938, 3.5 p.m.)

Chairman :

DE.

A. J .

ORENSTEIN

The Conference concluded its discussion of the draft report
and adopted t h e 10th and last Section. The report as a whole
was then adopted unanimously.
M E . K I T A O K A , on behalf of the Governing Body, thanked
the members of t h e Conference for the work they had accomplished
and referred t o w h a t had. been done in J a p a n in the field of industrial hygiene. H e submitted t o the Conference a book recently
published in J a p a n on Theory and Practice of Dust Hygiene.
M E S TZAUT, representing the Employers' group on the Governing
Body, expressed himself in agreement with the remarks made by
Mr. Kitaoka and emphasised the great interest with which he had
followed the work of t h e Conference.
M E . A N D E R S S O N , representing the Workers' group on the Governing Body, expressed his agreement with the remarks made by the
former speakers.

M R . HAROLD BUTLER (Director of the International Labour

Office) said t h a t h e h a d read with great interest the report adopted
by the Conference. The Office owed a considerable debt of gratitude to all t h e members of the Conference for the work they had
accomplished.
•.
H e expressed t h e hope t h a t t h e XJìoverning Body would favourably consider t h e suggestion to hold a special Conference of t h e
kind once every three years, a n d t h a t t h e necessary funds would
be available t ö enable the Office to print the proceedings of the
Conference a t a n early date.
T H E CHAIRMAN drew, t h e attention of t h e Director and t h e
members of t h e Governing Body to the importance attached by
those concerned t o rapid publication of the proceedings of t h e
Conference, even in an abbreviated and summarised form.

BBPOKT O í THE CONFERENCE

95

He then expressed his thanks to the members of the Conference,
to its officers, and to the members of the staff, and declared the
Conference closed.
The Conference adjourned sine die at 3.55 p.m.

R E P O R T O F T H E CONFERENCE
I t is evident t h a t in the time at its disposal the Conference
could not prepare a sufficiently elaborate report of its deliberations to be of service to research workers and t o those
called upon to deal with the many detailed and complex
problems of silicosis. This requirement could only be met by
the publication of the whole of the proceedings of the Conference,
including the papers submitted to it, which the Conference strongly
recommends should be done as early as possible.
The present report is intended solely to give the members
of the Governing Body, in as non-technical and concise a manner
as possible, a general idea of what the Conference has dealt with,
together with some of its conclusions and recommendations on
the more important questions.
Item 1. — Recent advances in the knowledge of the 'pathology of
silicosis, including the effect, if any, of non-siliceous dusts admixed
with silica or silicates in the production of pulmonary fibrosis or
tuberculosis.
For the purposes of this report the terms "free silica", "pure
silica", and "silica" are used to designate free—i.e., uncombined—
silicon dioxide.
The Conference reaffirms the decision arrived a t by t h e 1930
Conference in Johannesburg t h a t "silicosis is a pathological condition
of the lungs due to inhalation of silicon dioxide. I t can be produced experimentally in animals". Experimental work undertaken since 1930 confirms this statement, inasmuch as pure or.
nearly pure silica provokes similar reactions in animals and in man.
Most experimental work has shown t h a t in animals silicates
and dusts other t h a n silica do not appear to be capable of producing the lesions usually found in silicosis. Such dusts may

96

SILICOSIS

produce lesions of other types and, on the other hand, they may,
when present with free silica, modify the action of the latter.
It is generally accepted that the action of silica on tissues is related
to its solubility. This fact has a bearing on the preceding observation that under laboratory conditions certain dusts admixed
with silica depress the solubility of the latter and modify its action
on the tissues.
Clinical experience, supported by experimental evidence, shows
that infection, whether by the tubercle bacillus or by other organisms, has an important bearing on the pathological and clinical
development of silicosis. Tuberculous infection plays an important
role in this development.
I t is realised that under certain conditions the development
of silicosis may be very rapid. The term "acute silicosis" has been
used to designate this condition.
the use of this term should be avoided and that the words "rapidly
developing silicosis" would be preferable.
Item 2. — Pneumoconiosis of workers at coal mines and of other
workers exposed to coal dust.
(a) Silicosis occurs among workers in coal mines when
the dust to which they are exposed contains free silica.
The minimum proportion of silica necessary to produce
the disease is not, in the present state of knowledge, determinable.
(6) Coal dust alone does not, either in animals or in man,
produce lesions similar to those of silicosis.
Item 3. — Pulmonary disease (or disability) due to inhalation of
dust in industries other than those covered by 1 and 2 (with qualiand quantitative estimation of the composition of the dust in
question).
There are many industries and occupations in which silicosis is produced. These are always associated with exposure to
the inhalation of silica dust with or without admixture of other
dusts. Examples of these industries and occupations are : mining,
sand-blasting of metal and other articles, certain processes carried
out in metal foundries, and the manufacture of certain articles
of pottery and of abrasive powders containing silica. It must
be emphasised that these are only a few examples of a very large
number of industries and occupations in which silicosis is produced.

BEPORT OF THE CONFERENCE

97

Item 4. — Methods and standards of early diagnosis of pneumoconiosis, with or without infection, including determination of the
silica content of the blood, urine and faeces. Value, if any, of
early diagnosis of simple silicosis in regard to the question of removal
from dusty occupations with a view to the arrest of the disease.
The early diagnosis of silicosis is based upon :
(a) the employment history, including conditions involving
exposure to silica dust;
(6) a complete clinical examination;
(c) a technically satisfactory X-ray examination.
The first functional symptom to appear is dyspnoea on exertion.
Radiologically, there are small discrete circumscribed shadows
tending to uniformity in size and density, of bilateral distribution
and accompanied by increased striation.
I n a case of early silicosis complicated by tuberculosis or other
infection, there may be present, in addition, the clinical and radiological manifestations found in such infections.
The determinations of the silica content of blood, urine, faeces
and sputum, including the finding of dust in phagocytic cells in
the sputum, cannot be considered at present to have, of themselves,
a n y value in the diagnosis of the disease.
Early removal from the dusty occupation of a case of simple
silicosis may slow down the process and, within a limit determined
generally by the amount of dust in the lung, m a y lead to a substantial degree of arrest. Nevertheless, in view of the consequences
which may follow the compulsory removal of cases of simple
silicosis from their work, such removal is not advocated.
The Conference is of opinion t h a t further research should
be instituted into the determination of respiratory capacity.
The data obtained from such an investigation would be of great
assistance in the early diagnosis of silicosis.
For the satisfactory diagnosis of early silicosis, it is essential
t h a t the examinations be carried out by specially trained
and competent medical personnel, equipped with satisfactory
X-ray apparatus, and preferably these examinations should be
centralised.
Item 5. — (a) Methods of dust investigation as regards sampling,
concentration, composition and size frequency distribution. Possibility of standardising these methods.
The Conference notes with much satisfaction t h a t during
the last eight years marked progress has been made in developing
4

98

SILICOSIS

instrumental methods for the sampling of dust in the atmosphere
of workplaces. By these means it is now possible to determine
with greater precision the concentration, size frequency and
composition of the dusts, and to enable chemical and mineralogical
analyses to be carried out with much greater accuracy than was
possible in the past.
In view of the fact that such apparatus is now available,
the Conference recommends that, when enquiries relative to the
incidence of silicosis are carried out, there should at the same
time be made investigations into the characters of the dust present
in the workplaces.
The Conference recommends that the different countries be
requested, when publishing the results of their investigations
into dust counts, to express these results in terms of particles per
cubic centimetre.
The Conference recommends that, for routine purposes, each
investigator should use whatever instruments he prefers, but
that at the same time research with various available types of
instruments be carried out, so that at a future time comparable
results may become available.
(b) Intensity of exposure to dust and means of estimatine it.
The Conference considers that it is important to measure
the intensity of exposure to dust as bearing on the production
of silicosis, both in regard to the dust concentration, its composition and size frequency distribution. It notes that the large particles (approximately above 5 \>-) probably play a part of secondary
importance in the production of the disease and that on the other
hand the influence of ultra-microscopic particles has not yet been
determined. I t considers, therefore, that in estimating intensities of dust it is sufficient to have regard to the particles Which
are visible by the present available means.
Intensity of exposure is a function of both dust concentration
and time. At present effects of exposure to pure silica are best
known. The effects of exposure to mixed dusts still require
further investigation in each industry and occupation.
In studying concentrations of dust it must be borne in mind
that the quantity of silica found in the dust in the air does not
always correspond to the proportion of silica found in the rock
or in the dust at the point of origin.
The Conference recommends that work be encouraged on
methods of- estimating "free silica", particularly to enable more

REPORT OF THE CONFERENCE

exact comparison of direct chemical procedures with
analyses and mineralogical determinations.

99
rational

(c) Possibility of establishing experimental criteria for determining
the degree of "pathogenicity of dust.
The Conference notes t h a t various methods are in use b y
investigators in experimenting on the pathogenicity of dusts.
The Conference recognises the great value of this work, but, while
appreciating that standardisation would be of assistance, does
not consider t h a t sufficient information is at present available
to make definite recommendations.
Item 6. — What contributions from other scientific fields are essential
to the solution of the pneumoconiosis problem ?
The attention.of the Conference was directed t o the various fields
from which useful contributions have been received. Among
these are valuable contributions made by physicists, geologists,
chemists, engineers, biologists and other professional men.
The Conference, acknowledging these contributions, trusts t h a t
such co-operation will be instituted in every country and in every
possible direction.
However, as the problem of silicosis is essentially a medical
one, the Conference is of opinion t h a t the co-ordination and
correlation of all these contributions should be in the hands of
competent medical authorities.
Item 7. — Determination of disability and assessment of the degree of
such disability in pneumoconiosis.
The assessment of incapacity can only be determined by a
complete examination of the applicant, which must include an
X-ray examination, bearing in mind, however, t h a t the radiographical findings do not necessarily bear any definite relation
to the degree of incapacity. I n evaluating incapacity due to
the occupation it is also desirable to have regard t o t h e dust
exposure concentrations, to the duration of these and t o the
influence t h a t this dust may have had on the particular worker.
The presence of other diseases and their influence on the applicant's
general condition must also be assessed. The degree of incapacity
is incapable of being expressed in exact percentages. The existence of a record of preliminary and periodic examinations of the
applicant is of great assistance in determining the origin of the

100

SILICOSIS

incapacity. I t is not always possible to determine incapacity
merely by one examination, and repeated examinations, with
perhaps observation in hospital, may be necessary in a certain
number of cases. When chronic infection, particularly tuberculous, is present, the degree of incapacity is increased beyond that
which would be present in a case of simple silicosis in a similar
stage of development.
Item 8. — Prevention :

«

(a) Initial examination of workers in dusty industries and methods
and standards for it.
Initial examination of workers entering industries in which
they are liable to be exposed to silica dust is justified as a routine
IJJ,WVVJ,UXV

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the man from an added risk and his fellows from possible infection.
The minimum standard for such examination should therefore
be based upon preventing the entry into such industries of individuals suffering from active pulmonary tuberculosis.
(6) Campaign

against dust in general and siliceous dust in particular.

The Conference has given a great deal of attention to the
procedures applicable to the suppression of dust, which are,
however, too numerous and diverse to be discussed in detail
in a report of this nature. In general, the Conference is of opinion
that much has been achieved in recent times in the recognition
of the danger of dust, as evidenced by various laws and regulations
that have been introduced to control dust. The Conference
considers that the principle underlying the necessary measures
in this direction is to ensure by such means, as are applicable to
the particular process, that-the atmosphere to which the workers
are exposed is free from harmful concentrations of silica dust.
(c) Personal protection by means of masks, etc.
The Conference has had its attention directed to the improve-.
ments recently achieved in dust-protection masks. I t notes
that such masks should be of low resistance to breathing, and
provide a high degree of protection against particles of dust found
in the industry. They should have a good face fit and be reasonably comfortable to wear.
There are also available appliances which supply the workers
with air from a dust-free source.

REPORT OP THE CONFERENCE

101

The Conference considers t h a t these means of protection
should be made use of only when other methods, i.e. the suppression of dust at its source, are not applicable.
Furthermore, the Conference notes t h a t in certain conditions the effective wearing of a mask for a prolonged period is
difficult, e.g. where great muscular effort is called for, or in hot
and humid atmospheres.
Accordingly, the Conference urges t h a t the radical suppression
of dust be carried out wherever possible.
Item 9. — The specific therapy of silicosis and results of any investigations into methods for stimulating elimination of silica.
The Conference is not cognisant of any specific therapeu.tic measures for the treatment of silicosis, nor, of any method
of stimulating the elimination of silica from the lungs.
Item 10. — The most practical means by which the Sub-Committee1
can assist in the anti-dust campaign through co-operation with
other organisations and individuals.
(a) The Conference considers t h a t the progress of investigations into pneumoconiosis, and particularly into silicosis, would be facilitated if arrangements could be made
whereby the members of the Correspondence Committee on
Industrial Hygiene of the International Labour Office and
other interested persons would forward to the International
Labour Office reprints or abstracts of articles which are
published or are about to be published from time to time on
this subject. Further, it would be of great advantage to
workers in this field if the results of experiments carried
out by others, even though the results were negative, could
be made known. Much valuable material is frequently left
unpublished. The International Labour Office should therefore be charged with collecting such information, whether
published or otherwise, and distributing it as soon as
possible to all interested parties.
(6) The Conference recommends t h a t the records of its proceedings be published as soon as possible for the information
of all interested parties, including Governments and workers
in this field.
1
The Sub-Committee on Silicosis of the Correspondence Committee on
Industrial Hygiene.

102

SILICOSIS

(c) The Conference considers that similar Conferences should
be convened periodically, say at intervals of three years.
The principal subjects with which the next Conference could
advantageously deal are :
Methods of measuring the concentration and particle size
distribution of dust;
The determination of the pathogenic properties of various
dusts;
The application of scientific methods to the diagnosis of pneumoconioses and the determination of incapacity.
The experience of the present Conference has shown that it
would be of great advantage if the date of the next Conference
were made known as early as possible to the Correspondence
Committee and others who might be invited to the Conference.'
It has been a definite handicap in the work of the present
Conference that notice of it was received a relatively short time
before the date of the meeting, especially in the case of overseas
members.

APPENDICES

REPORTS AND NOTES
SUBMITTED TO THE CONFERENCE

The Office desires to express its regret that it has not been possible
to include in the Appendices the following communications and reports
submitted to the Conference. It should be stated, however, that, as far as
possible, prominence has been given to the views expressed on these, in the
Beport of the Conference, and that some of them have already been published
in the scientific press of the countries to which their authors belong.
IRWIN, D. A. and GIBSON, C. S. : An Experimental Study of the Toxicity
of Various Types of Quartz.
ISHIKAWA, T., AKATSTJKA, K. and MATOTUZI, H . : Investigation on Pneu-

moconiosis in Iron Founders.
KUBODA T. : Beiträge zum Blutbild des Staubarbeiters.
îtîitt. 1 i Das Hsemoírramm und die Senl£UHorsnfesc!iwiiidiOPkeit des roten
Blutkörperchens bei Staubarbeiter.
Mitt. 2 : Basophilia of Workers in Dusty Trades.
Hygienische und klinisch-roentgenologische Untersuchungen der
Lungen-Silicosis in einer japanischen Hütte.
POLICABJD, A. : E t u d e analytique, comparative des lésions élémentaires
observées dans les pneumoconioses minérales, humaines et animales.
SAYBES, R. R. : Pathology of Silicosis.
SiMSON, F . W. a n d STBACHAN, A. S. : Contribution t o the Pathology of
Silicosis (Summary).
SNLPTEN, R. C. a n d IBWIN, B. A. : The Modification of Quartz Fibrosis by
Silicates.
TSUKATA, G. : Silicosis in Japan.
VOSSENAAH, A. H . : La lutte contre la poussière dans les houillères néerlandaises et la valeur attribuée aux masques.
Existe-t-il une anthracoseî
Silicose, situation sanitaire générale de la population et nature de
la roche avoisinante.
•

Contribution au mécanisme de la soi-disant silicose dans sa forme
pseudo-tumorale.
Appareils pour la détermination de la teneur en poussières de l'air
dans les houillères néerlandaises.

WILLIAMS, H . E . a n d IRWIN, B. A. : Solubility of Various Types of Quartz.

APPENDIX I
BIOCHEMISTRY OF SILICOSIS
By

Dr. A. H.

VOSSENAAK.

In the Netherlands Dr. Nieuwenhuizen has made an interesting
attempt to study, by experimental research, the different ways in which
silica acts in the lung, as compared with other dusts.
He tried to solve the question whether the varying phagocytability of different dust particles could be partially explained by the
varying charge of those particles.
He examined three kinds of dust of importance from the technical
point of view, silica, carborundum and emery, which were known to
have very varied phagocytability. In order to establish the charge
of these particles he determined the speed of cataphoresis, a measurement
of the potential difference between the particles in suspension and the
liquid in which the particles are in suspension 1. These determinations
were made with the Reinders and Bendien apparatus. The speed of
cataphoresis was determined in pure water, in dialysed ox serum (that
is to say ox serum free from electrolytes) and in dialysed ox serum to
which calcium chloride had been added in sufficient quantity to give
a calcium chloride content corresponding to that of blood.
He mentions the speed of cataphoresis found, rectified in accordance with the viscosity of the water and a potential difference of 1 volt
per cm. While with silica cataphoresis in the dialysed serum containing calcium chloride, in a given interval of hydrogen ion concentration
(pH), the cataphoresis pursued its course in the same direction as in
pure water, carborundum became entirely discharged under the same
conditions. That is to say, the combined influence of calcium «chloride
and of the proteins of the serum diminished the charge more effectively
in the case of carborundum than in that of silica. The particles of
silica and carborundum were always negatively charged in a pH interval
of 6.6 to 8.1.
In the emery suspension in dialysed serum the influence of the
positive ion of iron and aluminium is predominant.
Above
a pH of 7.8 this influence of the positive ions is no longer manifest.
The charge of emery particles is thus modified. Prom becoming positive
it becomes negative. In the presence of calcium chloride the emery
charge is even inverted into a weaker hydrogen ion concentration.
In consequence Dr. Nieuwenhuizen found as a result of these experiments a manifest difference between silica and carborundum on the one
hand, which retain their charges during the experiments, and emery,
1

Cataphoresis or electrophoresis is the phenomenon by which particles
move in relation to a liquid as a result of the different tension produced.
The speed of these particles is in proportion to their respective charge.

106

SILICOSIS

on the other, which receives an opposite charge to that which it first
possessed.
Nevertheless no conclusion can be drawn concerning the charge of
these types of dust in a medium which apart from serum proteins possesses the physiological content in electrolytes. I t was not possible
to carry out experiments in this connection with a Reinders and Bendien
apparatus because of the vibration set up as a result of the high content
of the liquid in electrolytes. The experiments were made with the
Bungenberg de Jong microcatophoretic tray, in which vibrations are
not produced.
Research was effected to discover with this apparatus how much
time was taken by a dust particle to cover a certain distance. The
speed of cataphoresis was then equal to the reciprocal value of the time
measured.
The results found immediately showed that the differences of speeds
of cataphoresis for silica, carborundum and emery, very clearly manifested in the dialysed serum to which a given quantity of calcium
chloride was later added, disappeared entirely on account of the high
physiological content in sodium chloride.
In the physiological medium the powders examined are seen to have
practically identical charges, any differences found being due to experimental errors in observation.
The hypothesis of Höber and Kanai that the degree of phagocytability
is attributable to the charge of the particles is therefore completely
refuted as far as regards silica, carborundum and emery particles examined by Nieuwenhuizen.
Further, after having found that silica as well as carborundum and
emery show similar electrical properties in the physiological medium,
it only remained to attribute the action of the silica to dissolution of
traces of colloidal silicic acid. The only objection seemed to be the low
chemical sensitivity of silica.
Contrary to what was first believed, it was found that pulverised silica
dust possessed at body temperature a remarkable solubility in Ringer
solutions to which arresting mixtures of bicarbonate of soda of 6.8 to
7.4 pH had been added and that it even became dissolved in a physiological solution of salt and distilled .water.
Finely pulverised emery and purified carborundum seemed to be
insoluble in the same solutions. The demonstration of the presence
of silicic acid in these solutions by means of sensitive reactions did not
meet with success.
The experiments of Mills showed that spikes of fresh water spongy
silicic acid (spongilla fragilis)' are visibly attacked after a relatively
short lapse of time in the pulmonary tissue of the dog. The microscopic
image is said to show disseminated areas of fibrosis and the spikes in
the form of needles had lost their pointed extremities and were cut
obliquely. The cut edges were noted to be red.
The harmful action of siliceous dust would therefore be attributable
to its colloidal chemical properties.
Nieuwenhuizeri studied the biochemical properties of colloidal
silicic acid and showed that silicic acid is a normal element in the body.
On the one hand it is an element necessary to the formation of fibrous
tissue and on the other it is possessed of intensely toxic properties.
Colloidal silicic acid in solution appeared to exert a strongly flocculating action on all the positive proteins biologically concerned.
Dissolved colloidal silicic acid coming from the particles of phagocyted silica also exerts, as the same author has demonstrated in colla-

APPENDIX I : BIOCHEMISTRY OF SILICOSIS

107

boration with Professor Bungenberg de Jong, an intensely flocculating
action on the positively charged proteic compounds of the protoplasm
in the cell, even where they occur in the protoplasm as a complex of
colloidal systems. By complex formation between silicic acid and
positively charged proteins, the former acquires hydrophobe properties,
with the result that a part of the original water of hydration is separated
in the cell, forming a vacuole (experiments conducted with Miss de
Bruyn and Dr. Ouboter).
It is obvious that wastage of the cell occurs concomitantly with
the destruction of its fine structure. The products given off as a result
of this wastage set up proliferation of the surrounding fibrous tissue
elements of the body and are to be considered as a cause of the pulmonary
fibrosis.
Even if it be admitted that necrosis of the phagocytes under the
influence of toxicity of the dissolved silicic acid may occur, the difficulty
still remains of providing an explanation for the fact that the waste
products of the phagocyting cells exert a stimulating influence on the
increase of the cells of the surrounding fibrous tissues, or in other words
are liable to give rise to fibrosis.
Necrotic tissue is, nevertheless, resorbed as a general rule by the
tissue plasma without giving rise to lasting local reactions.
The difficulty is therefore not solved by Dr. Nieuwenhuizen's
research ; it is merely transposed.

APPENDIX II
PROTECTIVE ACTION OF VARIOUS MINERALS
AGAINST FREE SILICA
By Dr. Leroy U.

GARDNER

and Thomas M. DUBKAN, M. D.

The idea that other minerals may protect the lungs against injury
by silica dust is not new. As long ago as 1898 Haldane observed that
the coal miner's exposure to siliceous shale dust caused no serious
pulmonary reaction. Mavrogordato's animal experiments seemed to
demonstrate evidence of protection when the coal dust was inhaled
at the same time as the silica ¿ The Haldane concept of inhibitory action
gained general acceptance, but in the course of time it was gradually
supplanted by the growing conviction that only the silica in atmospheric dusts was dangerous and that any other components acted
merely as inert diluents. By 1929 Mavrogordato himself had apparently abandoned his former beliefs, for he expressed the opinion that
coal dust has no protective action against silica, but that it is merely
responsible for pigmentation of the silicotic fibrous tissue.
More recent studies, however, have brought to light certain evidence which would seem to substantiate the Haldane concept of protective or at least modifying action. Men working in atmospheres
polluted by mixtures of silica with other mineral dusts have frequently
exhibited less evidence of silicosis than would be anticipated from the
analysis of the dust that they breathed. The cases of silicosis which
are detected often present pathologic lesions that are atypical in form.
Results of animal experimentation have been confusing. By the
inhalation of pure silica, it is possible to produce a specific type of nodular
fibrosis, but when the experimental pathologist works with silica in
association with other minerals his results are often disappointing.
In most instances the lungs of animals exposed to such dusts have
revealed only foci of pneumonitis with little fibrosis and no suggestion
of the modulation that the same kind of dust apparently provokes in
man.
The unfavourable results have generally been attributed to the
inadequacy of the experimental method. The writers, however,
have sought to explain the reasons for their own apparent failures
and have reached the conclusion that the method is just as satisfactory
for the "mixed dusts" as it is for pure silica. The "failures" are not
failures, but merely demonstrations of principles of pathogenesis that
have been overlooked in previous studies of pneumoconiosis. This
communication brings together a considerable amount of experimental
data and points out the possible application of these principles to
industrial exposures. The evidence indicates that particles of other
minerals mixed with silica in the form of dust tend to inhibit the inhalation of such silica and to some extent neutralise its injurious effects
upon the body.

APPENDIX I I : PBOTECTIVE ACTION OF VARIOUS MINERALS

109

An extended investigation of the behaviour of air-borne suspensions
of silica and various other mineral particles and a study of their effects
upon experimental animals seems to justify the following conclusions :
1. The composition of a dust suspended in the atmosphere usually
differs from that of the rock from which it is produced when the
rock contains appreciable quantities of different minerals.
2. The composition of a mixed dust does not necessarily remain
uniform, as it is dispersed from the point of generation. Dust
settled upon the floor or rafters often has a different composition
from that which remains suspended in the atmosphere.
3. The composition of a mixed dust that has been inhaled into
the lungs of experimental animals is not necessarily the same
as that in the atmosphere.
4. Exposure to mixtures of silica and certain other kinds of mineral
particles, for a time at least, results in the inhalation of smaller
quantities of dust than exposure to equivalent concentrations
of pure silica.
5. The inhalation of silica is retarded by the presence of particles
of other minerals in the same atmosphere.
6. When exposures to mixtures of süica and other minerals are
sufficiently prolonged effective quantities of silica may finally
accumulate in the lungs.
7. The tissue reactions which then ensue may result in a typical
silicotic fibrosis. The other minerals modify the form of the
resultant lesion in direct proportion to their quality and quantity.
8. Physical or chemical reactions between silica and inhibitor
particles already inhaled into the lungs may prevent or retard
the development of fibrosis.
Confirmation of these observations would modify the prevailing
belief that the hazard in any industry is solely and directly defined
by the amount of free silica in the rock being worked. But before
such knowledge can be applied in the formulation of equitable codes,
many more detailed studies of mixed industrial dusts and their effects
upon exposed workmen will have to be made. At the present time,
generalisations are impossible arid existing codes can only be modified
as warranted by empirical experience. In' the writers' opinion there
is still no justification for attempts to prevent silicosis by the addition
of "protector dusts" to the industrial atmospheres. The most effective
method for the control of silicosis remains the suppression of siliceous
dust at the point of generation. The objective of the industrial hygienist should be the reduction of all dust concentration regardless of composition.
ExPEKIMENTAL. DATA

The basis of the above-mentioned conclusions is a long series of animal
experiments performed over a period of twenty years, clinical and
roentgenographic surveys of groups of workmen exposed to carefully
studied mixtures of silica and other minerals and a considerable number
of post-mortem examinations. The data are not always complete,
as some of the experiments were finished years ago before all of the
desiderata were appreciated, but much of the material is pertinent.
Guinea-pigs, rabbits, white rats and other animals rapidly develop
nodular fibrosis of the lungs when made to inhale relatively high concen-

110

SILICOSIS

trations of pure silica for periods of one or more years. These lesions
do not necessarily duplicate those of silicosis in man, but they are
specific in that they cannot be induced by any other known irritant.
They are composed of the same elements as the human silicotic nodule,
although manifesting certain differences in arrangement. Their
significance is probably just as great as that of the lesions of tuberculosis
in animals, which also differ from those in the human subject in many
respects.
To produce such reactions within the relatively short lifetime of the
common laboratory rodents it has been customary to expose them to
comparatively high concentrations of dust. The average dust counts
in different experiments have varied from 100,000,000 to 800,000,000
particles per cubic foot of air (3,531 to 28,252 per c.c). While such
concentrations may seem excessive, exposures of even greater intensity
were not uncommon in many industries before control measures were
introduced. For experimental purposes it was discovered long ago
that little was gained by attempting to increase dust concentrations
above the limits of 1,000,000,000 per cubic foot (35,314 per c.c), for the
animals then suffered excessive mortality from endemic pneumonia
and non-specific reactions were produced mechanically by the mere
mass of foreign bodies in the air spaces. The usual schedule of exposure
has been eight hours a day, six days a week, but as short an exposure
as two hours a day will have similar effects in a slightly longer period
of time. To produce such reactions the silica particles must be 3 y.
or less in diameter. The influence of the size factor will be considered
in detail later.
I n the case of mixed dusts that contained from 31 to 76 per cent.
of .free silica, exposure of the same species of animals to three or four
times these optimum atmospheric concentrations has produced surprisingly different results. Although the total concentrations were increased
to levels such that the calculated percentage of silica should have
approximated the quantity necessary to produce nodular fibrosis, such
reaction either failed to develop at all or appeared sporadically after
unusually long periods of exposure. With such mixed dusts the factor
of time appears to have much more significance than that of dosage.
Autopsy material from human beings working in similar mixtures has
likewise exhibited delayed and modified forms of silicotic fibrosis.
The influence of the time factor is harder to evaluate because of the
paucity of autopsies in men employed for short periods of time.
Pertinent data defining the conditions of exposure and the character
of the reaction in different hosts are summarised in table I (p. 111).
In the last column of table I several terms require definition. Pneumonitis is used to describe focal areas in which air spaces are more or less
filled with collections of dust cells and the adjacent alveolar septa are
thickened by infiltration with mononuclear and lymphoid cells. Linear
perilymphatic reaction, casting a shadow on a roentgenographic film
of linear exaggeration, consists of collections of dust cells and other
evidences of chronic inflammation in the areolar tissues about lymphatic
trunks. For the most part such reaction is found in the walls of arteries
and in the interlobular septa. The bronchi and veins are much less
heavily involved. Modified nodular silicosis is characterised by focal
collections of cellular connective tissue, usually deeply pigmented by
heavy accumulations of non-siliceous dust particles. If enough free
silica is also present the centre of such a focus will contain a nodule of
laminated hyaline fibrosis which is relatively free of pigment. The
border of such a nodule is irregular because peripheral strands of the

APPENDIX I I : PROTECTIVE ACTION OP VARIOUS MINERALS

111

TABLE I . — REACTION OP MAN AND ANIMALS TO INHALED PURE SILICA
AND SILICEOUS MIXTURES
Type and source
of dust
Granite, Barre.
Exhaust
collector,
Cutting shed

Free silica
tn
ground rock
Per cent.
35

Millions
per cu. ft.

Particles
per c. c.

Host

Character of
pathologio
reaction

2 % years

Heavy

—

Man

Linear
perilymph.

Heavy

—

Man

Nodular
silicosis

550

19,423

Guinea-pig

Î

—

Man

27 y e a r s
2 y e a r s •+3 months

Ferruginous
c h e r t (iron
oxide a n d
quartz)

Iron mine

Gypsum-quartz

Variable

Total dust concentration

period of
exposure

Mn.YÌTnnm

11 y e a r s

Pneumonitis a n d
linear
perilymph.
Modified
nodular
silicosis
Pneumonitis

54.6

1 y2 y e a r s

465

16,704

Guinea-pig

67.5 t o
76.47

3 years

766

27,051

Guinea-pig

2 y2 y e a r s

766

27,051

Rat

Ditto

—

Man

X-Ray
linear
exaggerat.

Man

Lungs —
linear
perilymph.
L. Nodes —
Nodular
silicosis

33

Synthetic
mixtures

10-34 y e a r s E x c e s s i v e

17 y e a r s

Excessive

Early
atypical
nodular
silicosis

49.7

2 y e a r s -+6 months

336

11,866

Guinea-pig

Pneumonitis. Very
rare nodule
4 months
after 2 y e a r s '
exposure

31.40

2 y2 y e a r s

254

8,970

Guinea-pig

Few atypical
silicotic
nodules

112
TABLE I .

SILICOSIS
EEACTION OF MAN AND ANIMALS TO INHALED PUBE SILICA

AND SILICEOUS MIXTURES (continued)
Type and source
of dust
Pure quartz

Free silica
in
ground rock
Per cent.

Maximum
period of
exposure

95-100

18 m o n t h s

99.6

Total dust concentration

Character of
pathologic
reaction

Millions
per cu. ft.

Particles
per c. c.

Host

Excessive

—

Man

Microscopic
nodular
silicosis

10 y e a r s

?

—

Man

Gross
nodular
silicosis

32 y e a r s

1

—

Man

Gross
nodular
silicosis

3 years

120

4,238

Guinea-pig

Early
nodular
silicosis i n
18 m o n t h s

13 m o n t h s

810

28,604

Guinea-pig

Mature
nodular
silicosis

214 y e a r s

120

4,238

Rabbit

Mature
nodular
silicosis

9 months

550

19,423

Bat

Mature
nodular
silicosis

+

5 months

connective tissue radiate into the surrounding septa and often communicate with linear perilymphatic reactions. Nodular silicosis needs no
definition. The sharply defined nodules of hyaline fibrosis uniformly
distributed throughout both lungs are characteristic in both man and
animals.
Special attention is directed to three of the human cases. The
granite cutter who had been exposed to heavy but undetermined concentrations of granite dust for a period of 2 % years died of lymphatic
leukaemia. His lungs showed no more evidence of silicosis than those
of the animals inhaling the same type of dust for similar periods.
The second man had worked for 17 years in a plant where, at times,
the atmospheric dust might theoretically have contained as much as
33 per cent, of quartz mixed with calcined gypsum, although many
of the silica particles were too large to be inhaled. His lungs showed
a minimal amount of hnear perilymphatic reaction and no trace of nodular
fibrosis. Microscopic examination of one or two of the tracheobronchial

APPENDIX H : PROTECTIVE ACTION OF VARIOUS MINERALS

113

lymph nodes revealed a few clusters of typical hyaline fihrous nodules.
Apparently he had inhaled such a small amount of silica that no reaction
occurred in his lungs; the particles that did enter the air spaces were
removed by the lymphatic system. A sufficient concentration was
built up in the nodes to provoke characteristic silicotic tissue changes.
A roentgenograph of this individual's chest, taken one year before
death from heart disease, revealed only very slight exaggeration of
the normal pulmonary markings. His case was not unique; similar
shadows were observed in roentgenographs of 17 other men employed
in the same operation for periods of 10 to 34 years.
The person exposed to high but undefined concentrations of nearly
pure silica for a period of 18 months was a tunnel worker. His lungs
showed no gross evidence of nodular fibrosis, but microscopic sections
revealed great numbers of minute nodules of hyaline fibrosis on a background of more diffuse interstitial fibrosis. The presence of associated
infection was hard to rule out.
With a few exceptions the most obvious difference between the
human and the animal exposures cited in table I is their duration.
Only the granite cutter and the tunnel worker had been inhaling dust
for periods as short as the animals and in them the results were comparable to those in the experiments. The whole subject might be dismissed at this point with the conclusion that experimental inhalations
might be expected to produce nodular fibrosis if they were continued
for a sufficient period of time. But it has been pursued to discover,
if possible, the reasons for the delayed reactions to silica in combination
with other minerals.
The first point to be settled was whether the mixed dusts used
for experiment were actually identical with those that caused disease
in industrial employees.
The data indicate that the dusts may not be identical even though
they are produced from the same kind of rock. Experience with two
lots of ferruginous chert are illustrative.
The first experiment was done with a large sample of rock, ground
and screened at the mines so that a majority of the particles were 10 j*
or less in diameter. Chemical analysis demonstrated approximately
equal parts of iron oxide and silica. Agitation of the fine material
in an experimental hopper produced a dense cloud of dust disseminated
throughout a room 8 feet in each diameter. The average light field1
count of particles under 10 \i. in diameter was 473 million p.p.cf.
(16,704 per c.c). Guinea-pigs and white rats exposed in this room for
18 months failed to develop even a suggestion of silicotic fibrosis.
Only focal areas of pigmented pneumonitis resulted. An investigation
then demonstrated that most of the particles in this ground rock,
which were fine enough to remain suspended in the air, were iron oxide ;
relatively httle of the harder siliceous component had been reduced
to fragments 3 ;•* or less in diameter.
To overcome this difficulty another lot of the same rock was reground in ball mills until practically
all of it had been reduced to particles 5 ¡A or less in diameter 2. Analyses now revealed from 71.1 to
1
2

p.p.c.f. = particles per cubic foot of air.
The first supply of fine material ground in the ball mill was only sufficient to continue the experiment for 2 1/3 years. For the subsequent
exposures a slightly coarser residue from the first milling was reground in
smaller ball mills until the particles were even finer. Chemical analysis
of this last lot demonstrated a silica content of 76.2 per cent.

114

SILICOSIS

.76.47 per cent, of Si0 3 . A second inhalation experiment was performed in the same room during which the total light field count of
particles less than 10 ^ in diameter averaged 766 million p.p.c.f. (27,051
per c.c). For the first two years of exposure the lungs of guineapigs and rats showed the same non-fibrous type of reaction, but at the
beginning of the third year, a few atypical nodules, suggestive of early
silicosis, appeared in some of them.
The effects upon the animals cannot be used as a criterion of the
comparative activity of the two dusts, as the first experiment was
terminated too early owing to lack of experience with inhibitor substances
at the time. However, the data on the composition of the two samples
of ground rock indicate the possibilities of variability due to methods of
grinding. The experiments have demonstrated the necessity for making
a very thorough study of any ground rock to be tested experimentally.
Not only must size-frequency counts be made of all the particles in such
mixtures, but by chemical and microscopic methods the size of the
siliceous components must be accurately determined.
Probably differences in the methods of fracturing rock may also
explain variations in the reaction of workmen to the dust generated
•in various operations. Patterson 1 has pointed out that crusher-house
dust is so coarse that it could not be expected to produce silicosis,
whereas drilling and blasting the same rock underground is a
notorious cause of such reaction.
But other differences than size of particles may result from variations in the method of fracturing rock. For example, microscopic
examination of larger particles of ferruginous chert ground in ball
mills reveals considerable deposits of iron on the surface of the quartz
grains. Some of the iron can be removed by agitation in water, but
much remains. I t is inferred that the force applied in grinding tends
to mash particles of the softer iron on the irregular surfaces of the
quartz. Since it is accepted that the irritating properties of silica
are proportional to the amount of surface exposed to the tissues, it
has been tentatively assumed that the iron forms a more or less complete
coating on the quartz and that this coating may be responsible for the
delayed appearance of silicotic fibrosis with the chert. Experiments
now in progress may verify this hypothesis. Such surface effects
might be partly responsible for the relative immunity of human beings
engaged in crushing rocks containing silica. On the other hand, the
disruptive forces of blasting and drilling, which shatter rocks, would
tend to separate the silica from other minerals so that no surface
film could form. Thus there may be other explanations for the difference in the hazards from crushing and blasting or drilling the same
rock.
The experiments with the two synthetic mixtures of calcined gypsum
and quartz, summarised in table I, disclosed another pitfall due to
physical differences in the component minerals. Both mixtures were
made with the same lot of quartz whose capacity to produce silicosis
was well established. For the one containing equal parts of the two
minerals a fine calcined gypsum ground in a ball mill was employed;
for the 2 : 1 mixture a slightly coarser product ground by buhr stones
was inadvertently substituted. Surprisingly enough silicosis developed
earlier and was somewhat more frequent on exposure to the mixture
containing less silica. With the higher proportion of silica (49.7 per
cent.) nodular fibrosis did not begin to appear until after completion
1

See Appendix VIII, p. 169.

APPENDIX I I : PROTECTIVE ACTION OF VARIOUS MINERALS

115

of a two-year exposure, when occasional nodules were found in the
lungs of 6 of 16 animals that had been removed to a normal atmosphere and were killed during the ensuing 6 months. With the 2 : 1
gypsum-quartz, containing only 31.4 per cent, silica, two out of five
animals surviving more than 18 months of exposure showed occasional
nodules.
The paradoxical result here is due not to the relative quantity of
quartz nor to the size of the quartz particles, which was the same in
both cases. It was apparently due to a slight difference in the size
of the gypsum component. If, as assumed, the inhibition of the silica
was partly due to surface phenomena, the finer gypsum particles should
have afforded a more effective coating for the quartz grains. As
will be shown later, atmospheric flocculation materially influences
the amounts of silica inhaled from mixtures of gypsum and quartz;
the size of the gypsum particles governs to some extent the amount
of flocculation.
In view of the probability that the ground rocks used for experiment
may have differed from the rock dusts inhaled in industry and in view
of the infrequent and long-delayed atypical silicotic reactions observed
in exposed animals, it then became necessary to discover whether the
particular pulverised rocks were capable of causing fibrosis.
To answer this question injection techniques were employed, as a
wide experience with these methods had shown that free silica in pure
form would invariably produce a progressive fibrosis of characteristic
type and that silicates and non-siliceous particles would not. For this
purpose a standard quantity of particles ground to a standard size
was suspended in physiologic salt solution and injected into animals.
Chief reliance was placed upon fractional injections, totalling 1 gramme
of particles 1 to 3 \i. in diameter, into the ear veins of rabbits. As
corroborative tests, 0.2 c.c. of a 10 per cent, suspension of the same
sized particles were injected into the peritoneal cavities of guineapigs, as recommended by Miller and Sayers.
TABLE II.
REACTIONS TO INTRAVENOUS INJECTIONS OF FREE SILICA
IN PURE FORM. STANDARD DOSE 1 GRAMME. PARTICLE SIZE 1 TO 3 ¡J.
Crystalline silica

Cryptocrystalline silica

Normal quartz 5 +

Chalcedony

Tridymite

7 +

Tripoli
5 +
(Seneca, Mo.)

Crystobalite

6 +

Vitreous silica 5 +

Flint

5 +

5 +

Amorphous silica

Colloidal silica
8 +
(dispersed phase)
Colloidal silica
_+
(gel phase)
Opal 1
Diatomite

4 +
5 -j-

1
Recently petrographers have agreed that opal should be classified as a crystalline form oí
silica.

Most forms of free silica injected in pure state produce silicotic
fibrosis' of the liver, spleen, lymph nodes and bone marrow. The results
are summarised in table II, in which the plus signs indicate the intensity
and severity of reaction. It will be noted that the standard reaction to
normal quartz (5 + ) is exceeded in the cases of the inversion forms,
tridymite and crystobalite, and in that of colloidal silica in dispersed

116

SILICOSIS

phase. Reaction to the latter is unique and consists of necrosis with
acute inflammation followed by little or no fibrosis. Colloidal silica
in gel phase provokes minimal reaction consisting of phagocytosis
without evidences of inflammation.
Reaction to silica is inversely proportional to the size of the particles.
Large splinters of quartz, embedded in the subcutaneous tissue, have
very little more effect than similar fragments of silicon carbide. Quartz
particles 10 to 12 i¿ ia diameter, injected by ear vein, provoke the
formation of foreign body tubercles, which do not progress to a stage
of fibrosis aftel- two years' contact with the tissues. Particles 1 to
3 I* in diameter produce the progressive fibrosis just described, while
those 1 n and less (lower limits undetermined) cause rapid necrosis
of tissue and even death of many animals within three or four months.
TABLE m .

INJECTION OF MIXTURES CONTAINING FBEE SILICA AND
OTHEE MINERALS
Babbits (intravenous)

Mineral and its free silica content

Maximum
period
observed
(months)

Tissue
reaction

Guinea Pigs
\ U 1 W *»" ¿"J 1.1 VU Vi^íVl}

Maximum
period
observed
(months)

Tissue
reaction

12

±_

12

4-

12

_i-

12

+

12

2 +

11

2 +

is y2

3 +

12

2 +

12

15

3 +

+

12
22

2 +

Ferruginous chert
(50 per cent. Si0 2 )
Usual dose = 0.5 g. SiO a . .

12

4 +

19

5 +

2 X usual dose = 1 g. Si0 2 .

12

5 +

South African mine dust
(70 per cent. Si0 2 )

16

4 +

Haematite 1
(6 per cent. SiO a )

(10.13 per cent. SiOa)

(10.39 per cent. SiO¡¡)

(35 per cent. Si0 2 )
Usual dose = 0.3 g. Si0 2
3 X usual dose = 1 g. Si0 2 .

1

. .

3

.+

This figure may include a small amount of combined silica.

—
—

12
—
12

3

+,
—

3 +

APPENDIX I I : PROTECTIVE ACTION OF VARIOUS MINERALS

117

Similar injections of 23 different silicates have shown t h a t none
of t h e m provokes comparable reactions after periods of observation
varying from five months t o two years.
W i t h these findings as a background t h e results of injecting finely
ground rock containing increasing proportions of free silica were reported. The s t a n d a r d particle size of 1 t o 3 ¡A was maintained a n d t h e
standard dose of 1 gramme was injected into all animals. I n addition
multiples of t h e standard dose were sometimes administered so t h a t
a full gramme of silica was introduced in combination with other minerals.
From table I I I , i t can be seen t h a t t h e a m o u n t of tissue reaction
t o mixtures of free silica a n d other minerals does not necessarily correspond with t h e q u a n t i t y of silica injected. Only when t h e mixtures
contain 50 per cent, or more of silica does t h e amount of fibrosis begin
t o approximate t h a t provoked b y silica alone and even t h e n abnormally
long periods of contact are required. W i t h a q u a n t i t y of silica as
small as t h a t in h a e m a t i t e (6 per cent.) there is little indication t h a t
fibrosis would ever develop, as t h e tissue reaction remained unchanged
for twelve successive months. W i t h 10 per cent, silica mixtures there
was a slight tendency toward progression in t h e case of t h e crude fluorite, b u t none in t h a t of anthracite. The reaction to t h e granite with
its 35 per cent, quartz content was particularly instructive. On injecting
t h e standard dose (1 gramme) of this mineral only localised cellular
proliferation w i t h some chronic inflammatory reaction developed
about phagocytes filled with t h e mineral particles. The reaction
progressed a little between t h e twelfth a n d t h e fifteenth m o n t h s , b u t it
never became fibrous in character. When the usual dose was tripled
so t h a t a full gramme of quartz was introduced, t h e character of t h e
tissue changes was n o t much different. E v e n when t h e t i m e factor
was increased a n d t h e material h a d been in contact with t h e tissues
for 22 months no fibrosis had developed.
A standard dose of ferruginous chert (0.5 gramme quartz) produced
only localised immature cellular connective tissue proliferation in 12
months a n d after 19 months t h e nodules, though fibrous, were few
in number a n d atypical in form. However, on doubling t h e dose
so t h a t a full gramme of silica was introduced, m a t u r e sidero-silicotie
nodules developed in one year. A sample of air-floated dust from a
South African gold mine contained 70 per cent, of quartz. Sixteen
months after injection t h e standard 1 gramme dose h a d failed t o excite
quite as much fibrosis as quartz alone usually provokes in a period of
12 months 1.
The injection tests indicate t h a t the natural mixture, ferruginous
chert, is capable of causing a modified silicosis (sidero-silicosis) in
1
Incidentally, this mine dust contained a high percentage of sericite,
but it obviously failed to accelerate reaction, as might be expected if the
Jones hypothesis were true. I n the same connection it is of interest to
recall that Nicol, and later McNally, postulated that, in the presence of
sodium fluoride,' silica would dissolve with unusual rapidity and t h e n react
upon the tissues. No tests upon the sodium salts of this compound have
been made, but the crude fluorite ore cited in table I I I contains about 90 per
cent, of calcium fluoride. While this compound of fluorine is much less
soluble than the sodium salt it does dissolve to a limited degree. If the
hypothesis of the authors mentioned were tenable, one might expect that
crude fluorspar would be unusually irritating. While it did prove to be
more active t h a n anthracite coal contaminated with approximately the
same quantity of silica, the tissue changes were never very marked and a
stage of fibrosis was' not attained during 15 months of observation.

118

SILICOSIS

less than one-third the time required by inhalation, provided that
excessive quantities are introduced into direct contact with the tissues.
But on injecting doses of granite that contained the same quantities
of free silica it has not been possible to produce silicotic fibrosis even
in a period almost as long as that of the inhalation exposure.
The entire series of injection tests with mixtures containing variable
quantities of free silica seems to indicate that a certain undefined
minimum quantity of silica must be present in a mixture to excite any
reaction of a silicotic type and that progression to a mature stage of
fibrosis will be retarded by the other elements in the mixture regardless
of the amount of silica present. Probably at least 10 per cent, of silica
is essential to initiate any significant reaction. The rate of progression
is not the same in all cases, but varies with the character of the other
minerals in the mixture. The nature of these inhibitory actions, which
take place inside the body, is not definitely known. I t has been
suggested that they may be due to the formation of protective films
deposited upon the surface of the grains of quartz. If permanent but
incomplete, such films would merely reduce the surface area of silica
in contact with the tissues (see above); if soluble they would subsequently disappear aud then allow the usual action to occur.
Thus far it has been shown that only under very carefully controlled
conditions is it likely that ground rock containing appreciable quantities of free silica will cause silicosis. When suspensions of such material
containing sufficient quantities of silica in very fine state of subdivision are injected directly into the tissues, reaction develops at a more
rapid rate than when the same kinds of ground rock are inhaled. In
view of the failure to produce any fibrosis by the inhalation of granite
dust and the long-delayed appearance of such reaction to ferruginous
chert, even when the total concentrations of both dusts were maintained
at very high levels, still a third question remains to be answered, i.e.
"Was the silica in the experimental atmospheres being inhaled in adequate quantities to expect even a delayed response in the lungs ?"
A partial answer to this question can be found in the data from the
inhalation experiments with chert and the gypsum-quartz mixtures.
To appreciate their significance more detailed information on the conditions of experimentation must be reviewed. The inhalation experiments
are now conducted in rooms 8 x 8 x 8 feet in diameter, provided with a
hopper situated in one corner to set up dust clouds. Fresh air enters
the rooms from a vent at the floor level under the hopper; two other
openings in the ceiling at the opposite corners of the room communicate
with outside ventilators. The draught is not sufficient to create visible
disturbance in the dust cloud. Three walls of these rooms are lined
with animal cages that have solid backs and tops but their fronts and
their sides are covered with % inch mesh wire screening. Fine dust
enters the cages by diffusion but none can settle directly into them.
As a control upon the effects of the mixed dusts a collateral experiment with pure quartz was carried on simultaneously in another room.
The atmospheric concentration, here was based upon the theoretical
number of free silica particles in the atmospheres of the mixed dusts.
I t so happened that under the conditions existing at the outset the
calculated quantities of free silica in the last three mixtures approximated 100 million p.p.c.f. (3,531 per o.e.), although later tests revealed
enough variation, so that the average figures cited in table IV range from
45 to 257 million p.p.c.f. The expense of carrying three separate control
experiments scarcely seemed justified in view of previous experiences,
which seemed to indicate little difference in the rate of reaction to pure

APPENDIX I I : PROTECTIVE ACTION OF VARIOUS MINERALS

119

quartz with variations in concentration of this order. The first experiment, "Chert I" of the tables, was not controlled, as it was performed
merely to demonstrate the potentialities of this dust. It was the one
which raised many of the questions under discussion.
TABLE IV. — CALCULATED CONCENTRATIONS OB' SILICA IN MIXED DUST
ATMOSPHERES
1

2

3

Average total counts
(Light field)
Mixture

Particle size
of
air-borne dust

4

6

5

Calculated No.
SIO,
SiO,
SiO, particles
air-borne
in
just Inside
rock
cages
(6)
(a)
(per cent.) (per cent.) Millions Particles
p . p . c l . per c. c.

(o)
Millions
p.p.c.i.

(6)
Particles
per c. c.

Chert I

473

16,704

95 p e r c e n t .
<3f/.
5 per cent.
3 ¡J. t o 8 ¡A

54.60

10.47

49.5

1,747

Chert I I

766

27,051

95 p e r c e n t . 67.58
<3¡x
5 per cent.
3 ¡J. t o 8 ¡J.

33.50

257.0

9,075

Gypsumquartz

. 336

11,866

98 p e r c e n t . 49.70
<3¡x
2 per cent.
3 ¡J. t o 8 \J.

.28.30

95.0

3,350

254

8,970

88 p e r c e n t . 31.40
<3[J.
12 p e r c e n t .
3 ¡A t o 10 \i-

17.70

45.0

1,590

1 : 1

Gypsum quartz
2:1

Table IV demonstrates the high total concentrations of fine dust maintained throughout the four experiments and the very marked difference
between the percentage of silica in the ground rock and the air-floated
dust collected inside the animal cages. Column 6 (6) indicates the calculated number of silica particles assumed to be present in the atmosphere
(i.e. the percentages shown in column 6, of the fatal counts in column (a)).
The fallacy of such an assumption is obvious in the case of chert I,
for which it has already been demonstrated that many of the quartz
grains were too large to leave the dust hopper. In how far the same is
true of the other dusts is debatable. Chert I I was ground until practically all of the particles were 5 [/. or less in diameter. The gypsum and
quartz used to make the two artificial mixtures were each ground until
the following percentages of all particles were less than 3 y. in diameter :
quartz, 92 per cent.; gypsum I, 95 per cent.; gypsum II, 72 per cent.
The ideal analysis, based upon the percentage of silica in settled samples
of graded particle size, has not been made, owing to the great difficulties
encountered in separating mixtures of several components.

120

SILICOSIS

Detailed investigations of the dust from different parts of these
rooms revealed the peculiarities that have been summarised in tables
V and VI. These figures, which differ slightly from those cited previously, were obtained from analyses of the air in the same experiments,
but made at various times subsequently. I t will be noted that three
different values are cited for the silica content of chert II. As previously
explained, such variations were discovered as different lots of the same
rock were ground or reground and used for experiment. As a basis
for comparison with conditions in the atmosphere, at a particular
time, it is essential to cite each individually. Three separate chert II
determinations have therefore been designated by the subscripts (a),
(b) and (c).
In table V it will be noted that settled dust collected from the floor
and from the tops of the cages six feet above the floor does not contain
the same proportion of silica as the ground rock. Floor dust varied
with the composition of the mixture; in the case of gypsum-quartz
it contained more silica than the rock, while for chert its composition
was the same. Air-floated dust, on the other hand, always showed less
silica than the original -powdered rock. Dust in the air inside the cages
contained the least silica of all, but repeated sampling in different
cages revealed a surprising uniformity of concentration at different
levels above the floor. The figures suggest that the very fine material
remaining in suspension, which fails to settle, diffuses into all parts of
the room.
TABLE V. —

PERCENTAGE OE SILICA IN DUST SAMPLES AT VARIOUS
LOCATIONS IN EXPERIMENTAL ROOMS

Type of dust

Ground
rock
dust

Settled dust

Atmospheric dust
Inside
Middle
Inside
of room top cages bottom
cages

Ploor

Top of
cages

Gypsum-quartz 2 : 1

31.6

27.6

23.0

23.9

37.4

35.2

Chert I I (a) . . . .

68.6

—

33.5

—

—

—

Chert I I (6)

71.1

—

41.8

—

—

61.7

Chert I I (c) . . . .

76.2

71.3

67.0

67.1

76.3

74.7

Table VI contains the data on four rooms and also includes figures
based on the analyses of guinea-pigs' lungs after exposures of 16 to
30 months, which will be discussed later.
Several causes for this reduction in silica content are conceivable.
The influence of particle size has already been discussed. Obviously,
when the grains of silica are much larger than those of the other components in the mixture, as in the case of chert I, they settle rapidly
and leave a preponderance of non-siliceous matter suspended in the
atmosphere. Differences in specific gravity will likewise influence the
picture.

A P P E N D I X I I : PEOTECTTVE ACTION OF VARIOUS MINERALS

TABLE

VI.

—

R E D U C T I O N I N P E R C E N T A G E OF SILICA
ROCK TO P U L M O N A R Y A I R SPACES

Ground rook "Rafter dust"

Dust

FROM

Air-borne
dust

121

GROUND

Lung ash
(average)

Chert I

54.60

38.40

10.47

3.33 !

Chert I I

69.35

61.03

37.26

16.35 !
26.02 a

Gypsum-quartz
1 :1

. . . .

49.70

53.70

29.65

Gypsum-quartz
2:1

. . . .

31.40

36.10

17.10

1

Exposure 23 months.

* Exposure 30 months.

3

—
15.80 a3
18.98

Exposure 18 months.

The mechanical barriers imposed by the cages have an appreciable
influence. As indicated in table V, differences of 4 or 5 per cent.
between the counts inside and outside the cages were observed. With
the very coarse chert I there was three or more times as much dust
in the outside atmosphere. With pure quartz the counts inside the
cages were within a few millions of those in the outside air.
Flocculation of particles of different composition is a common phenomenon and in two of these experiments it has been demonstrated
to play an important role in reducing the amount of inhalable dust.
In both experiments with gypsum-quartz, inspection of the atmosphere
of the room always seemed to reveal more dust than the counts indicated; the "rafters" and floors were covered with unusually heavy deposits.
Examination of clean slides exposed inside the cages revealed very
large clumps of particles which apparently accounted for this appearance.
In the quartz control rooms similar tests demonstrated only a few
clumps of very small size. Table VII presents the results of two individual attempts to compare the ratios of clumps to single particles in
the air inside the cages of the 1 : 1 gypsum-quartz room and the quartz
control room.
TABLE

VII.

ATMOSPHERIC FLOCCULATION OF G Y P S U M - Q U A R T Z
OF P U R E QUARTZ P A R T I C L E S

Number of single particles

Number of clumps . . .
Clumps per
particles
Size of clumps

Gypsum-quartz

AND

Pure quartz

360

321

403

412

28

55

6

5

100 single

2 to 30 ¡x 2 to 40 ¡x

2 t o 8 [j.

2 t o 4 ¡j,

122

SILICOSIS

I t is believed that this marked tendency of fine gypsum and quartz
particles to agglutinate with one another is due either to the imposition
of opposite electrostatic charges or perhaps to the hygroscopic properties
of the calcined gypsum dust. Whatever the cause, the result is a marked
reduction in the total amount of dust remaining in atmospheric suspen^
sion. The heavier clumps settle to the floor quite rapidly. That they
bring down very considerable quantities of silica is indicated by the
high percentages noted in table V. Only the smaller clumps and the
isolated particles remain in suspension. Some of them are caught
in the wire mesh of the cages, but rather constant numbers are wafted
into the breathing zone, so that both total counts and percentages of
free silica remain quite uniform, regardless of the position of the cage
sampled.
DUST INHALATION OF GYPSUM AND QUARTZ MIXTURE

Comparative analysis of lungs (2 series of guinea-pigs)

Tall columns, cross hatched
Short columns, white
I/Ower black portions
Total height of columns

=
=
=
=

exposure to mixture 2: 1.
exposure to fine quartz.
percentage of lung ash.
percentage of silica in the ash.

Finally, the isolated particles and the small flocculi of dust impinge
upon the nostrils and enter the noses of the animals. Again the larger
masses will be more readily retained and a further screening process
ensues. Analyses were made of the ash of the lungs of guinea-pigs
and rats killed immediately after their removal from the dust rooms
to demonstrate the percentage of free silica in the lungs.
Table VI reveals a still further reduction in the-percentage of silica
in the air reaching the lungs. The degree of reduction varies with the
different dust mixtures, but it is always an appreciable one.
The chart demonstrates the results of comparative analyses of the
lungs of two series of guinea-pigs killed during the course of dust exposures. The tall columns with cross-hatched upper portions represent
animals exposed to the 2 : 1 mixture of gypsum and quartz; the shorter
columns with white upper parts, the controls exposed to pure quartz
in an average concentration of 120 million p.p.c.f. (3,531 per c.c). The

APPENDIX II : PROTECTIVE ACTION OF VARIOUS MINERALS 123
lower, black portion of each column represents t h e percentage of lung ash ;
the total height including both t h e upper, open or cross-hatched portions
a n d t h e lower solid parts, t h e percentage of silica i n t h e a s h . This
diagram indicates t h a t t h e total a m o u n t of d u s t inhaled b y t h e controls
considerably exceeded t h a t for t h e group exposed t o t h e mixture. I n
spite of t h e a t t e m p t t o introduce similar quantities of silica i n t o t h e
atmosphere of each room t h e percentage of silica i n t h e ash of guineapigs exposed t o pure quart? was very much higher t h a n t h a t in t h e
ones exposed t o t h e mixture.
F r o m t h e last group of d a t a i t is concluded t h a t physical factors
governing t h e behaviour of mixtures of particles of silica a n d other
minerals in t h e atmosphere tend t o reduce both t h e total a m o u n t of
inhalable dust a n d t h e amount of silica which enters t h e lungs.

BIBLIOGRAPHY

1. PATTERSON : "Some Investigations on Dust, with Special Reference t o
the Thermal Precipitator".
2. GARDNER, L. U., and CUMMINGS, D. E . : "The Reaction to Fine and

Medium Sized Quartz and Aluminium Oxide Particles." Am. J. Path.
9, p p . 751-763, 1933.
3. MILLER, J . W., and SAYERS, R. R. : "The Physiological Response of Peritoneal Tissue to Certain Industrial and Pure Mineral Dusts." U. S.
Public Health Reports, 51, pp. 1677-1688, 4 December 1936.
4. GARDNER, L. U. : "Reaction of the Living Body to Different Types of
Mineral Dusts with and without Complicating Infection." Am Inst.
Min. and Metallurgical Eng. Technical Publication, No. 929, 1938.

APPENDIX III
ANTHRACO-SILICOSIS AMONG HARD-COAL MINERS
By D R . R. R. SAYERS

In 1933 the Division of Industrial Hygiene of the National Institute
of Health, United States Public Health Service, undertook a study
of the nature and prevalence of chronic incapacitating miners' asthma
in the anthracite coalfield of Pennsylvania. This coalfield occupies
an area of 484 square miles and is the only anthracite coalfield of importance in the United States. Two studies were carried out in three
mines, which differ in geological formation and which represent the old,
the semi-modern and the modern methods of mining respectively.
The mining methods employed in these mines are (1) chamber and pillar
mining and (2) pitch mining.
With the exception of very recent improvements in mechanical
loading and the use of wet methods in cleaning coal, conditions have
changed little in the last 20 or 30 years, and the present dust counts
may be accepted as fairly representative of the dust exposure throughout the past working life of the men now at work.
All of the employees in each of the three representative mines
selected for study were examined. In addition 135 ex-anthracite
coal miners, markedly disabled by respiratory embarrassment, who had
never been diagnosed as tuberculous, were observed during a period
of hospitalisation effected for the purpose of the study. Occupational
and medical histories were taken and physical and roentgenological
findings were recorded on 2,711 men. The diagnosis was made after
a critical review of all the clinical and roentgenological findings in
each case, together with a consideration of the occupational and medical
histories. Of the total number of employees 616, or 22.7 per cent.,
were diagnosed as having anthraco-silicosis : 510 (18.8 per cent, of the
number examined) were diagnosed as showing stage 1 ; 82 (3.0 per cent.),
stage 2; 24 (0.9 per cent.), stage 3.
For the purpose of correlating the diagnosis with the nature, intensity and duration of dust exposure the workers were first divided into
groups on the basis of occupation. This grouping corresponded sufficiently well to differences in the free silica content of the atmospheric
dust. Each group was then subdivided, first, according to the general
dustiness of the air inhaled by the men during work and, second, according to the duration in years of the dust exposure. Workers whose dust
exposure averaged less than 5 million particles per cubic foot of air
served as a control group. No cases of anthraco-silicosis were found
in the 361 men in this control group.
Group A, comprising 1,435 men, includes all employees except those
in non-mining occupations underground and except those who had
worked in rock dust for more than two or three years. Miners and
their helpers and breaker-house employees account for the major part

APPENDIX n i : ANTHRACO-SILICOSIS AMONG HARD-COAL MINERS 1 2 5

of this group. The free silica content of the dust to which these men
were exposed was less than 5 per cent. The particle size of the dust
averaged between 0.78 and 0.96 micron. Only seven cases of anthracosilicosis were diagnosed among the 500 men in this group who had
been exposed for less than 15 years. The proportion of cases varied
under different dust concentrations, but was never as great as 2 per cent.
With 15 to 25 years' exposure the percentage of men in this group who
had the disease rose to 14 per cent, with a dust concentration of 100-199
million particles, to 29 per cent, with a concentration of 200-299 million
particles, and to 58 per cent, with a concentration of 300 million particles
or more. When the period of exposure exceeded 25 years, the anthracosilicosis rate in the men of this group was 7 per cent, for a dust concentration between 5 and 100 million particles, 54 per cent, for a concentration
of 100-200 million particles, 71 per cent, for a concentration of 200-300
million particles, 89 per cent, for a concentration of 300 or more million
particles.
As to the severity of the disease, no cases that had advanced beyond
the first stage were found in Group A, when the dust count was less
than 100 million, however long the exposure was; or when the time of
exposure was less than 15 years, however high was the dust concentration. In other words, if these observations hold, it would appear
that a man may work in the anthracite coal-mining industry in Pennsylvania throughout
his life without serious risk of acquiring a disabling
anthraco-silicosis J if he can avoid rock work, and if the dust concentration at his breathing level is kept below 100 million particles per cubic
foot Of air. On the other hand, when the dust concentration exceeded
100 million, but was less than 200 million particles, 6 per cent, of the
workers in Group A developed stage 2 or 3 anthraco-silicosis after 25
years' exposure ; when the dust count was between 200 and 300 million,
12 per cent. ; and when the dust count was above 300 million, 5 per cent.
of this group developed these more advanced stages of the disease after
15-24 years' exposure, and 27 per cent, after 25 years' exposure.
Group B, a subdivision of Group A, is confined to contract miners
and their helpers who had spent most of their working time to date
in the one occupation of mining hard coal, and contained 426 men.
The duties of a contract miner comprise drilling holes either with a
jack hammer or hand drill in the face or breast of the coal and charging
the holes with explosive. After firing, he assists in the removal of the
coal from the face or breast. In addition, he drills, blasts and loads the
rock encountered in the vein. He has some other duties associated
with less dust exposure. In this sub-group, first-stage anthra.co-silicosis
was diagnosed in three of 14 men who had worked 15-24 years in air
with a dust count of 100-200 million; 22 out of 32 showed the disease
after 25 years' exposure to this same concentration; in 19 of these the
condition had not advanced beyond the first stage, in three it was in
stage 2 or 3. None of the eight workers exposed to 200-300 million
dust particles for less than 15 years had developed the disease; 11 of the
37 who had worked in this dust concentration for more than 15 and
less than 25 years had the disease, 10 in stage 1, one in stage 2 or 3;
33 of the 44 who had worked in the same dust concentration for 25 years
or more had contracted the disease, 28 were in the first stage, five in
the second or third stage. 73 of the 115 workers who had been exposed
to this dust concentration for 15-24 years had developed anthraco1
There is, however, evidence that exposure to concentrations greater
than 50 million particles may contribute to respiratory disease.

126

SILICOSIS

silicosis ; 65 were in the first stage, eight in the second or third stage.
85 of the 90 workers exposed to the same dust concentration for 25 years
or more had the disease; 44 in the first stage, 41 in stage 2 or 3.
In other words, on the basis of experience in Pennsylvania, anthracosilicosis need not be expected to appear in contract miners and their
helpers in the hard-coal mining industry with an exposure of less than
15 years with any concentration of dust encountered in the work as
there carried on. With an exposure of 15-24 years, the dust concentration must be 300 or more million particles per cubic foot of air to give
rise to the disease in the second or third stage. But when exposure
is prolonged to 25 years or more, workers inhaling air containing 100-200
million particles of dust per cubic foot will be no longer safe from acquiring the disease and in a certain but probably small proportion it may
advance to the later stages, and after 25 years' exposure to a concentration of 300 million or more, anthraco-silicosis will constitute a
definite danger to the health and working capacity of the miner and
his helper.
Group C, comprising 602 men, is composed of workers in nonmining industries underground, excluding rock workers. These men
were exposed to dust containing approximately 13 per cent, of free
silica, the increase over the quartz content of the dust breathed by the
miners being explainable in large part by the practice of strewing sand
on the tracks in the haulageways, but the bulk of these workers were
not exposed to dust of a higher average concentration than 99 million
particles per cubic foot and less than 2 per cent, worked in air containing
as much as 200 million particles. In Group C the influence of length
of exposure .is very conspicuous : of 450 men working in an atmosphere
containing less than 100 million particles of dust per cubic foot, one
out of 282 developed the condition in less than 15 years, two out of
65 in from 15 to 24 years, and 24 out of 103 after 25 years' exposure.
No case of second or third stage anthraco-silicosis was encountered
in Group C.
Group D comprised the rock workers, i.e. men who had been employed
at rock work for more than two or three years during their total working
life to date. There were 151 men in this group. Rock work is the
dustiest of all occupations in the hard-coal mining industry and the
free silica content of the dust is the highest, showing an average of 35
per cent. The average particle size is approximately 1 micron. In
this group-, 27 men worked in an atmosphere containing more than 99
million and less than 200 million particles of dust per cubic foot ; ; 10
of these showed first-stage anthraco-silicosis, three showed second or
third stage. While the numbers dealt with are small, it is nevertheless
noteworthy how rapidly in this group the ratio of cases to number of
men exposed increases with length of exposure : with a dust concentration of 200 to 300 million particles, four out of 17 men working less than
15 years showed anthraco-silicosis; 10 out of 14 working from 15 to
24 years; seven out of eight working 25 years or more. There were,
however, but two cases in the second or third stage in the men exposed
to less than 300 million particles ; the remaining cases were in the first
stage. With 300 or more million particles in the dust count, three out
of 34 rock workers who had worked less than 15 years showed the
condition, nine out of 16 who had worked between 15 and 24 years,
23 out of 24 who had worked 25 years or longer. Ten of the 35 cases
which developed in rock workers from exposure to this concentration
of dust had advanced to the second or third stage; these 10 cases were
all in men who had worked 25 years or more. Altogether, 66 cases of

APPENDIX I I I : ANTHBACO-SmCOSIS AMONG HARD-COAL MINERS 1 2 7

anthraco-silicosis were diagnosed in 151 rock workers, a percentage of
43.7. Of these cases 52 were in stage 1, 14 were in stage 2 or 3. Thus,
9 per cent, of the men who had worked in rock more than two or three
years during their working life to date showed evidence of anthracosilicosis of the more advanced stages, and nine out of ten of these men
who remained in the industry for more than 25 years developed the
disease.
An important characteristic of anthraco-silicosis appears from
this study to be a slow development of the lung pathology, regardless
of the intensity of the exposure. Among rock workers, however, cases
developed more rapidly; about 13 per cent, of the men who had worked
in rock dust for more than two or three years during employment in
the hard-coal mining industry totalling less than 15 years were found
in stage 1 anthraco-silicosis.
Age fer se appeared to play a minor role in the development of
anthraco-silicosis.
Physical disability. — Physical impairment was rated on a clinical
estimate of the employee's decreased capacity to perform strenuous
work. Of the total number of 616 men diagnosed as anthraco-silicotics,
63 per cent, showed some degree of physical impairment, 21 per cent.
showed moderate and 6 per cent, marked impairment. Of the 510
first-stage anthraco-silicotics, 56 per cent, showed some degree of
impairment, 10 per cent, moderate or marked, and 2 per cent, marked.
Of the 82 second-stage cases there was some degree of impairment
in 96 per cent., which was moderate or marked in 67 per cent, and
marked in 11 per cent. Of the 24 cases in the third stage, there was
some degree of impairment in 100 per cent., moderate or marked in
92 per cent, and marked in 71 per cent. Disability sufficient to handicap the workers was, with few exceptions, found only in the second
or third stage, and in all of these handicapped workers there was a
complicating pulmonary infection.
There was shown a marked tendency toward increase in the proportion of men showing physical impairment with increase in the
number of years of employment in the anthracite coal-mining industry.
This tendency is manifested whether one considers disability of any
kind, disability to which pulmonary infection was contributory, or
disability due largely to tuberculosis. The occupations most affected
by each of the above types of disability were found to be, in order
of decreasing frequency : (1) rock workers, (2) regular miners, (3) all
others exposed to more than 100 million dust particles per cubic foot
of air, and (4) men in haulageways except rock workers.
With less than 10 years' work in anthracite there was little disability
in any group. When the time of employment was less than 20 years,
no group with the exception of rock workers showed moderate or
marked disability appreciably in excess of that among the controls.
However, an excess in the prevalence of slight impairment was found
among the regular miners and among others in Group A who had worked from 10 to 20 years in atmospheres containing more than 100 million
dust particles per cubic foot. The rates mount sharply thereafter,
especially among the rock workers and the regular miners at the face.
The percentage showing physical impairment increased with length
of service more slowly among the men in the non-mining occupations
underground, but their rates, nevertheless, were definitely greater
than those of the control group. Although in all groups physical
impairment increased markedly with age, the effect of dust becomes

128

SILICOSIS

evident for certain groups when comparison is made between the dustexposed groups and the control group.
Symptoms. — The cardinal symptom in the men diagnosed as
having anthraco-silicosis was shortness of breath, often associated
with productive cough. In the more advanced cases there were frequent complaints of weakness, chest pain, gastric disturbances, and
haemoptysis. Fever and night sweats were seldom mentioned. The
objective sign most frequently observed was dyspnoea; other signs
observed were prolonged expiration, change of contour of the chest,
decreased chest expansion, clubbing of the fingers, change in breath
sounds, altered fremitus, and impaired resonance.
Roentgen findings. — Definite lung changes were demonstrated
roentgenologically in all cases diagnosed as anthraco-silicosis. Occasionally these changes were slight among persons definitely disabled,
and sometimes marked changes were exhibited in workers showing only
slight disability. As a rule, however, the lung changes revealed by
X-ray examination corresponded with the amount ofNphysical impairment
found.
Decrease in the motility of the diaphragm as demonstrated by
fluoroscopy was frequently observed.
In every case diagnosed anthraco-silicosis, film examination indicated generalised fibrosis, and frequently concomitant emphysema.
The fibrosis was manifested by an increase in the usual linear lung
markings to granular, nodular, coalescing nodular, and conglomerate
shadows. The evidence of emphysema increased with the density
of the shadows denoting fibrosis. It was usually more pronounced in the
lower lung fields, but it could often be demonstrated in other areas.
While evidence of fibrosis was, as a rule, more readily interpreted than
that of emphysema, the latter was occasionally more prominent than
the indications of fibrosis would lead one to expect.
In practically all cases of anthraco-silicosis the hilar shadows were
abnormal as to size and density. Changes in position and size of the
heart, and traction on the trachea with other evidence of mediastinal
distortion were not infrequently noted in the more advanced cases
(stages 2 and 3).
Cases of anthraco-silicosis in which infection was diagnosed usually
showed less symmetrical lung markings. In many cases this change
was similar to that commonly regarded as due to pulmonary tuberculosis.
The individual shadows were usually less discrete or defined in those
cases of anthraco-silicosis in which other signs of infection were noted.
Non-tuberculous respiratory complications. — Non-tuberculous respiratory disease was found to be about three times more prevalent in
dust-exposed workers in the industry than in a control group.
Tuberculosis. — Clinical pulmonary tuberculosis was diagnosed
provisionally in 124, or more than 20 per cent, of the 616 anthracosilicotics, as against less than 1 per cent, of the control group and
approximately 6 per cent, of the total number of employees examined.
About 15 per cent, of the men in the first stage and 43 per cent, of the
men in the second and third stages of anthraco-silicosis appeared to
have tuberculosis as a complication.
The clinical tuberculosis rate was below the normal rate in the
young adult ages, but at 35 years of age it was 5 per cent., which

APPENDIX I H : ANTHRACO-SILICOSIS AMONG HARD-COAL MUSTERS 1 2 9

is approximately two and a half times the average prevalence rate
of pulmonary tuberculosis in the general adult male population of the
country. At ages 45 to 54, it was 10 per cent., and at ages 55 to 64,
20 per cent. No such rise with age occurred in any general population
group for which comparable data are available.
The prevalence of tuberculosis was greatest among the rock workers.
The next highest rate was found among anthracite workers who had
changed more than five years previously from very dusty to relatively
non-dusty occupations in the industry. The third highest rate was
exhibited among persons who had had appreciable exposure to harmful
dusts in other industries. Among the regular miners working at the
face the rate was definitely higher than in the control group.
When employment exceeded 20 years, more than two or three of
which involved exposure to heavy concentrations of rock dust, about
37 per cent, of the workers showed evidence of tuberculosis. Employment of 25 to 34 years was associated with a tuberculosis rate of 8
per cent, among non-rock workers employed in the haulageways, of
14 per cent, among the regular miners, but for men exposed to less than
5 million dust particles per cubic foot of air the rate was less than 2
per cent.
Of the 135 ex-miners disabled with anthraco-silicotic respiratory
embarrassment and believed to be non-tuberculous, 13, or 10 per cent.,
proved positive for tuberculosis on guinea-pig test.
In a state sanatorium for tuberculosis in Pennsylvania it was possible
to make comparative observations on the clinical picture of pneumoconiosis plus tuberculosis and tuberculosis without pneumoconiosis.
Shortness of breath was the initial symptom in every case of tuberculosis
with pneumoconiosis, but was a late secondary symptom in tuberculosis
without pneumoconiosis. Productive cough, chest pains and loss of
weight were the initial symptoms in cases without pneumoconiosis
but were much later complaints in cases with pneumoconiosis. Haemoptysis appeared early without pneumoconiosis, later with pneumoconiosis. The roentgen picture showed more massive conglomerate
shadows with pneumoconiosis present than in pure tuberculosis. Cavity
formation appeared more marked in pure tuberculosis. About the
same percentage were sputum-positi ve in tuberculosis complicating
pneumoconiosis as in tuberculosis alone. Pneumoconiosis appeared
to hasten the progress of the tuberculosis.
Pathology. — Specimens for pathological study were furnished
through the courtesy of physicians practising in the anthracite coal
region of Pennsylvania. None of the cases used in the description of
lung pathology was complicated by tuberculosis.
The pathology of anthraco-silicosis as observed in these cases is
essentially a deposition of coal dust in the lungs, accompanied by an
extensive fibrosis, both diffuse and nodular, with associated functional
and degenerative changes.
Grossly, the lungs are dark grey in colour, and are usually heavy.
The coal dust appears to collect up to the subpleural connective tissue.
' In early cases a fine, black, linear network appears on the surface of
the lungs where the dust has been deposited. The external surface
may appear spotted with light grey areas where the pleura has thickened.
In advanced cases thickening of the pleura is usually encountered.
Scattered tough, fibrous adhesions, rarely diffuse, attaching the visceral
to the parietal pleura, are seen in varying numbers. The interlobar
pleurae are often fused. The lobes involved are distorted and con5

130

SILICOSIS

tracted, the degree depending on the amount of fibrosis present. The
lungs are firm, and nodules of varying sizes may be felt. Palpable
crepitus occurs in scattered areas or may be entirely absent. Emphysematous blebs, often of large size, are seen in advanced cases.
On section the appearance of the cut surface varies with the extent
of the process. In early cases irregular, stellate, black, fibrous nodules,
and linear black markings are noted throughout the lungs. The nodules
are most numerous in the upper portions of each lobe, and the upper
lobes usually are the more involved. Between the black nodules the
lung tissue appears more or less normal. As the condition progresses,
the nodules become larger. The increase in size is not uniform, as
nodules of varying sizes and shapes occur. These nodules become
confluent, presenting large, black-to-grey areas of consolidation, most
frequent in the upper portions of the lobes. The apex of the lung
is often free of nodules. These coalesced nodules have the appearance
and consistency of black rubber. Strands and whorls of grey fibrous
connective tissue are noted on their dryish cut surfaces. The appearance may be laminated, but the outer margins are irregular. Occasionally a central area of pasty, non-caseous necrosis, often with cavitation.
is present in these masses. The walls of the cavities are rough and
irregular, with protruding strands of fibrous tissue. In advanced cases
the lung tissue between the black fibrous masses may be congested,
oedematous, or emphysematous. A compensatory emphysema, often
involving large areas of a lobe, is usually present. Very large emphysematous blebs ranging from 0.5 to 4.0 centimetres in cross section are
occasionally seen. The walls of the bronchi and blood vessels appear
thickened in advanced cases. The peritracheal and pulmonary lymph
glands are large, firm, and black. Interlacing strands of grey fibrous
tissue occur in their matrices. Matting of the lymph glands is rarely
noted in cases uncomplicated by infection.
Black particles of coal dust are seen in macrophages (large mononuclear leucocytes) in the alveoli. Free dust particles in the alveoli
and particles in the alveolar epithelial cells are rarely seen. Collections
of dust-bearing macrophages are noted in the perivascular and peribronchial lymphatics and the adjacent interstitial connective tissue.
A fibrous hyperplasia is present along the lymph channels, which in
portions so increases as to form nodules that appear as concentric
masses and irregular diffuse areas. In the centres of some of the smaller
nodules are noted compressed and hyalinised blood vessel walls. The
centres of the larger nodules are often free of appreciable amounts of
coal dust, and are of well-formed, white fibrous connective tissue.
Hyaline degeneration is frequently seen. Collections of dust-laden
macrophages are present at the periphery of these nodules, but free
extracellular coal dust in large quantities is noted in the adjacent
underlying area of dense connective tissue. When the nodules become
confluent, the peripheral depositions of coal dust become enclosed in
the larger nodule so formed. Areas of central, amorphous, finely
granular necrosis are sometimes seen. The peritracheal and pulmonary
lymph glands show a fibrous hyperplasia and dense depositions of coal
dust, often so marked as to obscure the histology of the organ.
Chemical examination of pathological material. — Ash and total
silica determinations were made on six specimens of lung tissue received
from the anthracite coal region. One hundred grammes of dried tissue,
selected from representative portions of the lungs, were used in making
the analyses.

APPENDIX I I I : ANTHRACO-SILICOSIS AMONG HARD-COAL MINERS 1 3 1

No definite conclusions have been drawn from this limited number
of cases. However, the variations shown in the ash and silica determinations on anthraco-silicotic lung tissue examined from Pennsylvania
hard-coal miners are within the limits found by other investigators,
and the pathological changes in the lungs are associated with the presence of excessive amounts of silica.
Recommendations. — As outcome of this series of investigations
into conditions existing in the hard-coal mining industry, certain recommendations have been formulated. It is felt that the dust hazard may
be greatly lessened and in some instances adequately controlled by
extension of certain dust-control measures already partially employed
in the mines studied, viz., by :
(a) Provision for adequate ventilation of all workplaces.
(6) Employment of wet methods in all mechanical drilling operations.
(c) Thorough wetting of all coal and rock before loading..
(d) Substitution of mechanical loading for hand-loading methods
wherever practicable.
(e) Insistence upon arrangements permitting the lapse of a period
of time sufficient to reduce the dust concentration to a safe
limit after firing charges.
(/) Use of wet methods in processing coal.
In addition to the methods of dust abatement already employed
in some of the anthracite coal mines, certain other preventive methods
have been found of practical value in other industries in which dust
control is an important problem. For the sake of completeness a list
of recommendations is given below, regardless of whether some of them
are already being carried out in certain mines.
1. Dust should be controlled at its point of generation so as to
prevent it from reaching the breathing zone of the workers or contaminating the general air. Thorough wetting by water is a general
method of dust control at the point of origin. Material abatement
of dust in the working environment may be obtained by wet methods
in almost all coal mining and processing operations.
Another method of controlling dust at its point of origin is by means
of local exhaust ventilation. Dust-removal devices have been successfully employed in rock drilling operations in open excavations, and may
prove feasible in drilling operations in anthracite mines. If this type
of dust-removal device is employed, wet methods would still be necessary to allay the dust produced in the loading of coal and rock.
2. Adequate ventilation of all workplaces in the mines would tend
to replace dusty air with clean air. Although satisfactory standards
of air velocity in anthracite mines have not yet been established, an air
movement of at least 50 feet per minute seems desirable from the standpoint of eliminating "dead-ends".
3. The more general use of mechanical methods of loading coal
would contribute substantially to the solution of the dust problem.
A dust count of less than 30 million particles per cubic foot of air was
found associated with mechanical loading operations in one of the
mines surveyed. This concentration is within the limits of toleration
for the coal dust found in the mines studied.
4. Since blasting operations produce a large amount of dust, especially in dry mines, the firing of shots should be done only at the end
of the shift.

132

SILICOSIS

5. An important source of silica dust in the haulageways was found
to be sand used to prevent slipping of the transport motors. Thorough
wetting of the roadbed would minimise the dust hazard in the haulageways.
6. Periodic studies of the condition of the working environment
appear necessary to determine whether the control methods adopted
are really adequate. The work of inspection and review should be
performed by persons trained for such duties.
7. Tn order to prevent the spread of respiratory infection, workers
having active pulmonary tuberculosis should not be permitted to work
underground or in dusty occupations above ground.
8. For the purpose of detecting cases of pulmonary tuberculosis and
of anthraco-silicosis which have progressed to the point where further
exposure to dust would jeopardise future working capacity, physical
examinations, including X-ray of the chest, are necessary, not only of
applicants for work, but also of all anthracite coal-mining employees
annually. The periodic examination of all employees is required also
for determining whether the preventive measures instituted for the
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are needed to measure the degree of anthraco-silicosis and the extent
of impairment of working capacity among the employees from year
to year. Comparable records will not be obtained unless the periodic
examinations are conducted in a standardised way, preferably by a
permanent medical board composed of physicians specially trained and
having adequate experience in the diagnosis of anthraco-silicosis and
other diseases of the respiratory system.
9. I t is recommended that consideration be given to the methods of
medical control which have been found practical and effective in other
industrial fields in which dust is a health problem, as in the Union
of South Africa, Australia and Ontario. Those recommended by the
Special Industrial Disease Commission of Massachusetts in February
1934 are also worthy of study.
10. In the control of dust hazards there is no single measure applicable to all dusty operations and processes. All the means of prevention
mentioned must be practised to insure success in the solution of the
problem.
Investigation by Charr and Riddle. — Mention should be made
here of research recently conducted by Charr and Riddle, of White
Haven Sanatorium and the Department of Pathology of the University
of Pennsylvania, in respect of the pulmonary circulation in anthracosilicosis. These investigators tested the pulmonary circulation time
by the method of injecting a bitter-tasting substance into the antecubital
vein. The test was made in 15 cases of pure anthraco-silicosis and in
10 cases of anthraco-silicosis with accompanying pulmonary tuberculosis.
The pulmonary circulation time was, with few exceptions, prolonged
in anthraco-silicosis, with or without pulmonary tuberculosis, owing
largely to cardiac weakness. They suggest that regardless of the
normal circulation time which they found in cases of artificial
pneumothorax and in a few of the silicotics, the volume of blood flow
will be less through narrowed and shortened vessels, although, in the
absence of myocardial weakness, the velocity of blood flow may be
practically normal. Dyspnoea was marked and constant when the
circulation time was above 17 seconds. Cyanosis and oedema of the
ankles became manifest when it was longer than 30 seconds.

APPENDIX I I I : ANTHRACO-SILICOSIS AMONG HABD-COAL MINERS

133

Bituminous coal-mining industry. — No investigation comparable
to that of the anthracite industry has been made on the bituminous
coal-mining industry, but there are a few data which may be cited from
an unpublished study of the latter industry, conducted by the Division
of Industrial Hygiene of the National Institute of Health, United States
Public Health Service, in conjunction with the Bureau of Mines of the
United States Department of the Interior. About 800 men employed
at all types of underground work were examined. Nineteen cases of nonnodular pulmonary fibrosis were found. As visualised on the X-ray
film, the pathological changes were similar to the typical early stages
of the fibrosis that is known to be due to exposure to siliceous dust.
All of the men showing these pulmonary changes had worked many
years in the transportation department and had in fact been exposed to
siliceous dust by reason of the sand used on the tracks.
From some bituminous coal-mining areas an excessive incidence
of miners' asthma has been reported. In an almshouse in the region
of the Pennsylvanian bituminous fields, among 257 male inmates past
40 years of age, 55 diagnoses of miners' asthma had been made by the
medical officer of the institution. Of these 55, the number, if any,
who were also suffering from pulmonary tuberculosis cannot be stated.

APPENDIX IV
THREE COMMUNICATIONS ON THE EFFECT OF MIXED DUSTS
By D B . E. L. MIDDLETON

I should explain that I have taken the part only of collecting these
data; all the scientific work is due to others whom I will mention and
it may well be that some of that work will be published by them. The
results, inconclusive and incomplete though they may be, give point
to the claim for the essential co-operation of physicists, geologists and
chemists with medical workers on the problems of the pneumoconioses.
I. — PULLER'S EARTH

Fuller's earth is a mineral substance of indefinite composition which
is found in deposits, the most important in England being in a welldefined area in the CQunty of Surrey. This substance has the property
of adsorbing impurities from cloth, which gives it its name, and from
oils and other commercial products, for which it is now largely used.
The material as quarried from the deposits occurs in lumps with
a varying amount of moisture. It is dried over coke fires and the lumps
are then crushed and the material sieved and bagged. It is an old
industry and although considerable quantities of dust are produced
in the processes of grinding, sieving and bagging the material, the
occurrence of pulmonary disease amongst the workers as a result
of exposure to dust had never been suspected until these cases arose.
I t is probable t h a t conditions have been altered by the extended use
of machinery and a speeding-up of output, and that the time required
for producing disabling disease has thereby been shortened and now
falls within the span of a working lifetime.
My attention was first called to the condition of workers in fuller's
earth by Dr. Campbell, the Tuberculosis Officer, who produced chest
radiographs of two men suspected of tuberculosis. In one of these
the film showed irregular patchy shadows throughout both lung fields
and the sputum was negative for T.B. In the other case two radiographs
showed rather a uniform distribution of shadows, suggesting nodular
fibrosis, especially in one of the films of his chest; in another film,
taken on the same day with higher penetration, the nodule-like shadows
were not so characteristic.
In order to follow this up I selected five men, out of the 14 men
continuing at work on the grinding and sieving processes, for radiographic examination. The selected men had been exposed to the dust
for 4, 5, 19, 35 and 39 years respectively. Those of the first three men
might be considered to fall within normal limits; the last two show
definite changes. A.T. T., with 35 years' exposure, shows fine punctate

APPENDIX IV : THE EFFECT OF MIXED DUSTS

135

nodulation with some aggregation of shadows ; the appearances resemble
those found in some clay workers and some cases of miliary tuberculosis.
J. W., with 39 years' exposure, shows shadows suggesting nodulation
with a linear arrangement ; the film resembles those met with in haematite iron-ore miners. One of the men reported by Dr. Campbell died
at the age of 56, a year after the radiographs referred to were taken.
He had been employed on fuller's earth since the age of 18 and had worked
continuously on the mill from 1908 to 1919. He had suffered from cough
and dyspnoea during the last few years and was incapacitated 18 months
before his death. An autopsy was made by Dr. S. R. Gloyne. The
body was well developed and well nourished; there was no clubbing
of the fingers or toes. Recent pleural adhesions, parietal and interlobar,
were present ; and on the surface of the visceral pleura there were a few
scars which were hard and seemed to indicate consolidation of the subjacent lung; there were also white spots surrounded with a dark area
suggesting nodules. The lungs were bulky, dark in colour, and palpation yielded a resistance as of nodules, and emphysematous bullae were
present at the apices and less marked along the margins of the lungs.
On section of the lungs numerous dark coloured nodules were seen;
these were firm to touch, as were also dark masses in the upper parts
of the lungs. The trachea and main bronchi were not altered. The
heart was slightly enlarged and the right ventricle was dilated; the
muscle was lacking in tone. There was no valvular lesion. The coronary
arteries were patent and there was a moderate degree of atheroma
of the aorta. The liver showed very early fatty change. The other
organs were normal.
A piece of lung tissue weighing 205 grammes, evidently representing
the anterior part of right lower lobe, and a small portion of the middle
lobe attached, was examined by Dr. T. H. Belt. The specimen had
been fixed in formalin and was somewhat leathery, but floated in water.
The pleura was of a dark bluish black colour, with some slight milky
opacity superimposed on the pigmentation; the surface was wrinkled
and along the margin there were some grape-like emphysematous bullae.
The cut surface showed excessive black pigmentation. The lung
tissue was considerably tougher to cut than normal lung, but it was
partially air-containing throughout and peppered with numerous
dilated air sacs about 1 mm. in diameter. The whole spongy parenchyma was diffusely and rather heavily studded with little black
rubbery lesions, each of which seemed to represent the cut end of a
small blood vessel or bronchus. On the eut surface they appeared like
nodules, but in their depth they were linear and cord-like, sometimes
beaded, representing thickened blood vessels and bronchioles. They
were more deeply pigmented than the surrounding lung tissue, but
none was calcified. The interstitial tissues stood out prominently and
interlobular septa were in many places clearly visible as greyish or black
streaks. There was no gross evidence of tuberculosis. The impression
from macroscopic examination was that of a purely lymphatic pneumoconiosis without silicotic nodulation, massive fibrosis or tuberculosis.
The microscopic examination showed extensive patchy, dustladen fibrosis affecting all parts of the lungs more or less uniformly.
This fibrosis occurred in small linear masses along the blood vessels,
bronchioles, interlobular septa and sub-pleural connective tissue, and
brought into prominence all the connective tissue structures which
house the lymphatic channels of the lung by a marked fibrous thickening
and dust impregnation. The fibrosis consisted of compact masses of
elongated reticular cells containing dust in their bodies and connected

136

SILICOSIS

together by a dense mesh-work of fine reticulum or stout collagenous
bundles of hyalinised, acellular connective tissue. I t was in the main
of a cellular-reticular t y p e a n d did not form t h e dense acellular whorls
and nodules commonly found in full-blown silicosis. All parts were
heavily impregnated with dust a n d there seemed no more fibrous proliferation t h a n was necessary t o incarcerate the dust collections. The
reaction was like a simple, foreign-body fibrosis with little evidence of
redundancy. There was no evidence of tuberculosis or other specific
inflammatory change. The dust consisted of a mixture of pigmented
and non-pigmented particles. The former were evidently carbon, while
the latter were visible only with Nicol prisms, when t h e y showed u p as
a h e a v y deposit of very fine, doubly-refracting particles. Dr. Belt's
comment is t h a t this was a case of advanced pneumoconiosis. There
was extensive visible a n d palpable dust fibrosis throughout t h e specimen,
but t h e usual t y p e of silicotic nodule was not seen a n d there was no
evidence of tuberculosis.
Dr. E . J . K i n g made an analysis of portions of t h e lung for silica
and alumina. H e found an average of 6.1 per cent, of silica and 3.6
per cent, of alumina in t h e dried lung tissue.
A report on t h e composition of a sample of fuller's earth from the
works a t which these men were employed was made by Dr. A. Brammall
of t h e Imperial College of Science and Technology, London, for the
Committee on Industrial Pulmonary Disease, and the following notes
have been t a k e n from Dr. BrammaU's report.
The raw material is composed of two distinct fractions : (1) a granular fraction equivalent to 79.9 per cent, and (2) a fine fraction ; divided
into finest, 3.1 per cent, a n d t h e rest 17.2 per cent. Comparative
analytical d a t a (Geochemical Laboratory, London) show the total
silica content of t h e raw material t o be 56.89 per cent., t h a t of the "finest"
grade 51.30 per cent, and the rest 52.58 per cent.
"Silica in some readily responsive form (not quartz) computed in
relation t o t h e a b u n d a n t d a t a for well-established clay-species is of
the order 8.5 per cent, (as a minimum). This silica is most readily
'responsive' in t h e fine grade b u t is not confined to t h a t grade; it is
deemed t o be t h a t silica excess which cannot be accommodated even
in beidellite or montmorillonite. I t is best regarded as amorphous :
Si02.nHaO".
Certain minerals with particle-sizes from 10 \>. t o 40 \i. have been
identified, which together amount t o about 13.3 per cent, of the raw
material. These were, in order of q u a n t i t y : feldspars (NaKCa—aluminosilicates) 3.8 per cent.; hydrated ferric oxide (Fe 2 0 3 . aq.) 3.5 per
cent. ; calcite (CaC0 3 ) 2.7 per cent. ; muscovite [K 2 A1 4 (Si6Al2) O 20 (OH.F) J
1.4 per cent.; sphene (CaO.Ti0 2 .Si0 2 ) 0.9 per cent.; kyanite, fibrolite
(Al a 0 3 .Si0 2 ) 0.3 per cent, with smaller proportions of rutile; anatase;
a p a t i t e ; zoisite; b a r y t e s ; zircon; tourmaline a n d quartz (very rare).

II. — HEARTHSTONE

This substance, known as hearthstone, is so called from its use
for rubbing on t h e hearthstone doorsteps and window-sills of houses.
The fashion is old and is dying out a n d t h e industry is dying with it.
The raw material, described as calcareous firestone in The Geological
Memoir on Cretaceous Bocks of Britain, p . 327, is a rotten-stone, a
friable rock from which much of the calcareous bonding material has

APPENDIX IV : THE EFFECT OF MIXED DUSTS

137

been removed by the action of water. It is mined in a part of Surrey
near to that from which fuller's earth is obtained. The mines, approached by an adit or level, are very wet and the material is brought out in
lumps. The lumps are chopped by hand into pieces about the size of
a fist, for retail as hearthstone. The waste material is crushed
in a disintegrator and the powder is filled into cartons for sale or is
mixed with a small proportion of cement and pressed into blocks.
Although a considerable quantity of dust is given off in handling the dry
material, no ill-health has been observed to result from exposure to
the dust. The number of persons employed in the industry is very
small and both employers and employed tend to follow the industry
as a hereditary one. In this way the observation that the industry
enjoys freedom from occupational disease is probably well based on
experience. To test this, I selected for radiological examination the
two men longest employed in the industry, aged 55 and 54 years respectively and employed 40 and 35 years. The films show a slight change
from normal in the form of an increase in thickness and prominence of
the linear shadows and, in one of them, some faint small mottling is
seen in the upper and outer areas. The men were in good health and
had no respiratory symptoms.
Chemical analyses (Government Laboratory, London) of samples
of hearthstone from three sources in the district gave comparable
results. Total silica was 66.5, 68.7 and 70.8 per cent, (free silica 28.0
22.7 and 31.4 per cent, for the corresponding samples). Alumina
5.8, 5.8 and 6.4 per cent.; calcium oxide 7.7, 8.7 and 9.2 per cent.;
iron oxide 2.2, 2.8 and 2.9 per cent.; alkalies (by difference) 1.4, 1.6 and
3.0 per cent. Dr. W. R. Jones kindly examined three similar samples
of rotten-stone and the following notes are taken from his report. The
presence of numerous grains of glauconite suggest the probable origin
from the green sands. The amount of free silica, as determined from
one thin section of each, is B. 24 per cent. ; G. 30 per cent. ; R. 27 per
cent. Much of the silica shown in the chemical analysis is present in
the form of glauconite. In addition to quartz and glauconite there
are present also a number of other minerals such as sericite, chlorite,
calcite and, rarely, kyanite, staurolite and tourmaline. The amount
of sericite is from 3 to 5 per cent. This proportion would increase very
rapidly with the degree of fineness of the dust, and as the quartz grains
are relatively coarse they would settle quickly when freed. By a simple
experiment he obtained a dust from the R. rock in which the proportion
of free silica was only 7 per cent, and that of sericite about 35 per cent.
This dust contained many particles too coarse to be respirable. With
finer dusts the ratio of sericite to quartz would be greater.

III. — ALKALI

The effect of admixed non-siliceous dusts concerns the presence of
free alkali in a mixture containing silica, sodium carbonate and powdered
soap. These are the usual ingredients in domestic scouring powders.
Reference has been made elsewhere J to a number of fatal cases of pulmonary disease occurring in a factory where such a scouring powder
was made and packed, and the opinion was expressed by some observers
at the time of the occurrence that the acute form of the disease was,
1

MiDDLETON, E. L.

The Lancet, 1936, ii, 3.

138

SILICOSIS

or might have been, d u e t o t h e presence of free alkali in t h e d u s t 1 .
This view was n o t supported b y some other observers. A t t h e time of
the occurrence t h e r e was n o opportunity t o make scientific investigation
of t h e conditions of exposure t o dust of t h e affected persons, owing t o
the cessation of work on t h e process. When a fatality occurred in
another factory where a similar process was being carried on advantage
was t a k e n t o m a k e some detailed investigation. I n t h e first series
of cases one prominent fact was t h a t of 14 known deaths, 13 occurred
in persons who weve first exposed t o t h e d u s t between t h e ages of 14
and 16 years. T h e single case now t o be mentioned was t h a t of a man
aged 58 years. F o r a period of 5 years he was employed on mixing,
packing a n d weighing scouring powder which contained 56 parts of
ground silica, 10 parts each of anhydrous carbonate of sodium a n d
powdered soap. All his previous employment was in other industries
which involved n o unusual exposure t o dust. A n autopsy was made b y
Dr. Denton Guest. T h e body was very wasted; there were dense
pleural adhesions over b o t h lungs, especially a t t h e apices; t h e lungs
were shrunken a n d showed large areas of silicosis, particularly in t h e
upper lobes over which t h e pleura was thickened. There was no sign
of active tuberculosis. T h e right side of t h e heart was hypertrophied
and dilated.
Portions of t h e lungs were examined microscopically b y Dr. S. R .
Gloyne. H e found confluent silicotic nodules almost entirely obliterating lung structure. There was no sign of tuberculosis. There were
no definitely distinctive characteristics which would distinguish these
sections from those of a n advanced chronic silicosis.
A sample of t h e ground silica was analysed (Government Laboratory,
London) a n d found t o contain 98 per cent, silica, 1.3 per cent, alumina,
0.3 per cent, calcium oxide a n d 0.1 per cent, iron oxide.
The late Dr. H . H . Thomas (Geological Survey, London) made a
pétrographie examination of a sample of t h e ground silica. H e found
t h a t the material was almost entirely pure quartz. There was a fraction
of 1 per cent, of feldspar, which although turbid, did not appear t o give
rise t o sericite.
A portion of t h e lung was sent t o Dr. W . R . Jones, London, a n d he
recovered the minerals b y his well-known method. Dr. Jones found t h e
bulk of t h e mineral particles t o be of t h e usual size found in such lungs
and particles of quartz outnumbered very greatly those of a n y other'
mineral. There were a few fibres of sericite. Dr. A. W . Groves (Imperial College of Science, London) made a chemical analysis of 0.5 gramme
of minerals extracted from t h e lung a n d found t h e following percentage
composition : sibca 73.25, A1 2 0 3 6.19, CaO 10.91, P 2 0 5 2.33, MgO 1.32,
T i 0 2 0.46, F e 2 0 3 0.39, alkalis, b y difference 5.09.
The particles size of t h e ground silica was determined b y Mr. H . L .
Green (Portón). T h e material was shaken vigorously in distilled water
and a representative sample transferred by means of a pipette t o a coverslip a n d t h e w a t e r slowly evaporated in a desiccator. T h e smooth
size distribution curve was as follows : 14 per cent. < 0.5 [A, 34 per
cent. < 1 p., 58 p e r cent. < 2 i>., 86 per cent. < 5 ['•. Median size of all
particles 1.6 \i. or, neglecting all those over 8.0 \t,, t h e median size was
1.45 [i.. This size range is somewhat above t h a t found in most industrial
dust clouds.
Mr. H . H . W a t s o n (Portón) examined particles recovered from a
thick section of t h e lung a n d found t h a t t h e proportion of t h e smallest
1

MACDONALD, G. and others.

The Lancet, 1930, ii, 846.

APPENDIX IV : THE EFFECT OF MIXED DUSTS

139

particle sizes was somewhat lower than that in the raw material, the
median size being 1.8 \J., 8 per cent. < 0.5 \J-, 58 per cent. < 2 \i.,
97 per cent. < 5 \>.. He found too that the particles of quartz in
the lung had lost their birefringence. He assumed from the latter observation that the body fluids had changed the silica into an isotropic
form. With regard to increase in particle size distribution in the lung,
Mr. Watson inclined to the view that the small particles had been more
readily reduced to colloidal size by the action of the alkaline body
fluids and thus had escaped detection (see Heffernan P., and Green,
A. T., J. Ind. Hyg. 10, 272).

APPENDIX V
BELGIAN MINERS' SILICOSIS
B y D B . A.

LANGELEZ

Since the International Labour Conference of 1934, which passed
a resolution expressing t h e desire t h a t "special investigations should be
made in each country regarding the possible incidence of silicosis and
tuberculosis in t h e coal-mining industries, regarding which knowledge
is a t present far from complete", we have succeeded in Belgium in
assembling a series of facts and d a t a which we now wish to submit to
the Conference. The information in question is confined exclusively
to clinical a n d statistical data. We are n o t able, for t h e moment,
to furnish a contribution of a scientific or experimental character contributing t o t h e elucidation of the problem of the aetiology or pathogenesis
of silicosis.
I. ^— R E S C U E W O R K E R S I N M I N E S

A category of workers exists in Belgium subject t o special selection,
whose mission consists in fulfilling the role of rescue workers in the
mines on the occasion of explosions, fires, emissions of firedamp, etc.
These workers are chosen with all possible care from young, intelligent
and particularly well-developed individuals. They are subject to severe
examination on engagement and annually thereafter are examined by
medical factory inspectors. These rescue workers constitute shifts
constantly on t h e alert. They are regularly trained in rescue work and
are especially required t o carry on their work in smoky surroundings
wearing various breathing apparatus to which t h e y must learn to become
accustomed. On account of the special conditions attending their work,
it was decided in 1936 t o complete t h e medical examination of these
rescue workers b y pulmonary radiography. I t was considered in fact
t h a t complete integrity of the respiratory system was indispensable
for men obliged t o carry out rescue work, while wearing masks in a n
irrespirable atmosphere.
The following are t h e results of radiographic examinations of the
whole shift of rescue workers in the Borinage examined in 1936 : 20
workers were examined, t h a t is t o say the whole of t h e 1936 staff of the
Central Rescue Organisation of the Borinage.
The men were aged 25 t o 42 years, with t h e following distribution
of age groups :
25
30
35
40

to
to
to
to

30
35
40
42

years
yeara
years
years

3
4
9
4

APPENDIX

V : BELGIAN MINERS'

SILICOSIS

141

They h a d a working experience of underground work varying from
6 t o 17 years.
Fourteen of t h e m worked on the coal seam; t h r e e were tunnellers
a n d one was employed as a mason on maintenance of t h e underground
workings.
The result of the radiographic examinations was as follows :
Tunnellers
normal
fibrosis
nodular condition
Workers on the coal seam
normal
fibrosis
nodular condition

0
1
2
6
1
10

None of these workers presented any perceptible clinical symptom.
Age

Occupation

Length
of service

34
41
35

Tunneller
Tunneller
Tunneller

13
7
11

38

17

40
35
27
38
32
29
37
38
38

15
15
6
13
11
8
13
16
16

34
40
29
35
32
38
42

• W o r k e r on coal s e a m <

14
17
14
13
14
16
Mason

..

Kadiography

H i g h l y d e v e l o p e d fibrosis
Nodular aspect
Nodular aspect — well
developed fibrosis
H i l u m filled i n — v e r y
fine n o d u l a r c o n d i t i o n
Fine nodulation
Normal
Normal
F i b r o s i s . H i l u m filled in
Normal
Normal
Snowstorm picture
Nodular elements
Nodular state — beginning
of coalescence
Nodular state
Pseudo-tumoral state
Normal
Nodular state
Nodular state
Nodular state
Normal

II. — GENEBAL ENQUIRY IN THE M I N E S

I n 1935, t h e Belgian Government requested t h e F a c t o r y Medical
Service t o undertake an enquiry t o discover those industries in which
silicosis occurs, a.nd as far as possible t o determine its extent a n d t h e
number of cases involved.
An enquiry was effected by t h e F a c t o r y Medical Service in industries
other t h a n mines and an enquiry in coal mines was undertaken conjointly
with 15 particularly competent medical men, including several university
professors.

142

SILICOSIS

The medical examiners adopted for their enquiry in mines the same
methods of examination as those utilised for the enquiry carried out by
the Factory Medical Service.
A total of 1,000 working miners, chosen by the Mining Engineers'
Organisation in agreement with the workers' groups, were submitted to
examination : 900 workers who had worked exclusively on rock (tunnellers) and who were on this account probably exposed to inhalation of
mineral dust, and 100 workers chosen as controls, who had been occupied
exclusively on coal (workers on the coal seam). They had never worked
on. rock.
Both in the case of the rock workers and coal miners the men were
selected from miners not on the sick list, regularly presenting themselves
for underground work and having completed at least 10 years' work
in dusty surroundings.
The workers were selected in a given proportion from the different
coalfields of the country : the Borinage, Central, Charleroi, Liege and
Campine coalfields.
All the radiographs obtained during examination were submitted
for examination to all the doctors who took part in the enquiry into
silicosis. For the enquiry in the mines the doctors adopted the terms
of the so-called Geneva classification, taken from the Report of the
Johannesburg Conference of 1930, comprising classification into eight
types : normal thorax, rather more fibrosis than usual, more fibrosis
than usual, commencing generalised fibrosis, marked generalised fibrosis,
well-marked fibrosis, very well-marked fibrosis and gross fibrosis.
In the two categories of workers already referred to—rescue
workers in mines and workers who were the subject of the 1935 enquiry—
the workers were healthy subjects with no complaints and quite normally
fit for work, some of them even presenting an appearance of robust
health.
Radiographic examination proved however that among those
workers who had completed at least 10 years' work in the mine, normal
subjects were the exception. A considerable number of them were
suffering from extensive or highly developed fibrosis or even from nodular
and pseudo-tumoral forms. I t was surprising to discover in workers
employed on the coal seam, that is to say, extracting the coal and not
in contact, or at least in direct contact, with rock dust, just as high
a proportion of subjects affected as among the rock workers. The
reason for this is doubtless that the coal workers are subject to the
action of siliceous dust coming from sandstone strata (rarely) or more
frequently from schist encountered in the coal cutting or again from
silica contained in the coal itself.
Coal schist in fact contains silica in extremely variable proportions.
The composition of this rock varies from one coalfield to another, from
one pit to another and even from one level to another. Generally
speaking, argillaceous schist, known as coal schist, is—as stated in
the geological treatise by Cornet, a Belgian scientist whose work is
universally known—a coherent argillaceous rock, though not a very
hard one, with dull, earthy fracture laid down in parallel detached
strata or readily laminated strata, the laminae being more or less parallel
to the stratification. It has the same microscopic composition as clay.
Alongside silicate of alumina quartz in impalpable grains, mica and
a series of very thin plastic elements are found : rutile in short needles,
tourmaline, apatite, zircon, etc. Calcite is also seen at times in microscopic grains. The colouring matters are the same as those found in
clays. Iron is sometimes present in the state of carbonate (coal schist).

APPENDIX V : BELGIAN M I N E E S ' SILICOSIS

TABLE I.

143

1936 ENQUIRY INTO SILICOSIS IN COAL MINES

Total
Mons
Central
Liege
Campine
Charlerolj
Total number of workers
examined
963
295
150
162
200
156
(on rock work
= 864) 1 „.,„
(on the coal seam = 99) | 0* Rock Seam Rock Seam
Rock Seam Rock Seam Rock Seam Rock Seam
142
864
99
14
22
263
32
17
145
17
136
178
Radiographs of type I I . . .
per cent.

10
5.62

2
9.09

3
2.11

0

13
4.95

2
6.25

4
2.94

0

37
4.28

6
6

1 204
S.88 23.61

15
15

2
7
4.83 11.7

Radiographs of type I I I . . .
per cent.

55
4
2
5
64
45
24
3
16
30.89 18.18 45.07 14.28 17.12 15.62 17.65 21.43 11.03

Radiographs of type IV . . .
per cent.

55
6
50
4
54
30.89 27.27 35.22 28.57 20.54

1
38
4 246
49
5
3.12 36.03 35.71 26.22 23.53 28.47

20
20

Radiographs of type V . . .
per cent.

40
3
20
4
4
2
35
4 179
51
33
22.47 13.64 14.08 28.57 19.39 12.51 24.26 14.29 24.14 23.53 20.72

17
17

Radiographs of type VI . . .
per cent.

9
5.06

Radiographs of type VII . .
per cent.

4
4
2.25 18.19

Radiographs of type VIII . .
per cent.

5
2.81

Totals

. . . .

178

2
9.09

1
4.54
22

3
3
56
9
2.11 21.44 21.28 28.12

2
17
4
97
12
8.82 14.29 11.72 23.53 11.23

2
1.41

1
27
7
7.14 10.27 21.87

8
5.88

1
20
7.14 13.79

1
5.88

61
7.06

14
14

4
17
6.45 12.51

6
4.42

1
7.14

12
8.27

1
5.88

40
4.64

7
7

136

14

145

17

864

99

0

0

142

14

263

32

The most frequently encountered colour is bluish, brownish or blackish
grey. Other schists are greenish red, yellow, brown or even black.
Grains of sand, carbonate of iron, pyrites, limestone fossils, etc.,
are encountered as accessory, voluminous elements in the schists.
The schist may be highly micaceous and arenaceous, passing finally
into psammite. Other schists are calcareous and pass into limestone
schist. Others again show a state of transition passing towards chert
(siliceous schist).
On the other hand the following information has been obtained
from Professor Marlier of the Mons College of Mines : "There is no
single coal 'schist' but rather coal 'schists' showing infinite variety.
The various aspects and compositions are due chiefly to the relative
proportions of the constituents and between schist and sandstone
are to be found as many intermediate varieties as there are shades of
grey between black and white. This means that Belgian coal schist
entirely exempt from free silica (quartz) is the exception. The rocks
designated 'schists' all or almost all contain detrital quartz in more
or less fine grains and in varying proportions. I do not possess any
analysis of coal schist. Geologists have devoted scarcely any attention
to this study".
Renier, a Belgian geologist, stated in 1914 that the argillaceous
or claystone soils known as "schists" formed, in the opinion of statisticians, 77 to 60 per cent, of the Belgian coalfields, while "sandstone",
including psammites and quartzites, accounted for 20 to 38 per cent.,
and coals for 3 to 2 per cent. Knowledge of sterile rocks is obtained

20
20

144
TABLE II.

SILICOSIS

—

CLASSIFICATION

O F R A D I O G R A P H I C T Y P E S I N ACCORDANCE W I T H AGE
Liege

Years

Campine

Charleroi

Central

Mons

Total

Rock Seam Rock Seam Rock Seam Rock Seam Rock Seam Rock Seam
22
142
14
263
32
136
14
864
99
178
145
17

Type II

from 30 to 40 . .
from 40 to 50 . .
from 50 to 60 . .

3
4
3

1
21
0

0
0
1

0
0
0

8
6
0

1
0
1

1
3
0

0
0
0

3
2
2

0
1
1

15
15
6

2
2
2

Type III

from 30 to 40 . .
from 40 to 50 . .
from 50 to 60 . .

24
21
10

0
3
1

47
19
2

1
1
0

14
22
9

1
3
1

5
16
3

1
2
0

5
10
1

1
0
0

95
88
25

4
9
2

Tvpe IV

from 30 to 40 . .
from 40 to 50 . .
from 50 to 60 . .

7
31
17

2
2
2

25
17
5

2
3
0

15
28
10

1
0
0

12
24
13

1
2
2

7
19
12

2
' 2
0

66
119
57

8
9
4

Type V

from 30 to 40 . .
from 40 to 50 . .
from 50 to 6 A

11
23
G

0
2
1

9
10
2

4
0
u

16
23
VI

2
1
1

1
15
17

0
1
1

6
17
12

0
1
3

433
88
49

6
5
6

Type VI

from 30 to 40 . .
from 40 to 50 . .
from 50 to 60 . .

6
3
0

1
0
1

1
1
1

1
1
0

6
31
19

2
5
2

1
4
7 .

0
2
0

0
11
5

0
3
1

14
50
32

4
11
4

Type VII

from 30 to 40 . .
from 40 to 50 . .
from 50 to 60 . .

0
0
4

0
2
2

1
1
0

1
0
0

5
14
8

3
3
1

0
4
4

0
0
1

3
7
10

0
0
1

9
26
26

4
5
5

from 30 to 40 . .
Type VIII from 40 to 50 . .
from 50 to 60 . .

0
3
2

0
0
1

0
0
0

0
0
0

1
8
8

0
4
0

1
2
3

0
1
0

0
8
4

0
1
0

2
21
17

0
6
1

178

22

142

!.*

263

32

136

14

145

17

864

99

Totals

. . . .

exclusively from macroscopic study. Chemical analyses are infrequent
and often limited to the determination of only one or only a few elements :
limestone, chromium, vanadium, etc.
I t is therefore proved by current knowledge of the subject that
underground workers in the mines are exposed to the action of mineral
dust.
As regards the rock workers (tunnellers) the fact is neither open
to denial nor is it denied. I t suffices to recall the great complexity
of the Belgian deposits, thrown up and contorted by numerous fissures
and overthrowing, folding, bending backwards, etc., in order to understand the necessity for opening and maintaining numerous galleries in
the rock. On the other hand the thinness of the seams involves very
deep cutting through the enclosing rock, the wall or the roof to permit
of the passage of men, animals, engines, etc. Finally, the extension
of the use of explosives, of drilling with pneumatic hammers or blasting
by electricity has involved as a consequence greater daily advance
and has resulted in the production of considerable quantities of mineral
dust, the nature of which depends on the type of the rock pierced.
I t should also be noted that the explosives at present utilised, being
more powerful and causing greater fragmentation, augment the quantity
of dust and, by the concussion which they cause, raise and drive into

APPENDIX

TABLE H I . —

V : BELGIAN M I N E R S ' SILICOSIS

CLASSIFICATION O F RADIOGRAPHIC T Y P E S I N ACCORDANCE
W I T H T H E Y E A R S O F SERVICE
Campine

Liege
Years of service

.
.
.
.

2
S
3
0

from
from
from
over

10 to 20
20 to 30
30 to 40
40 . . .

.
.
.
.

14
25
10
1

from 10 to 20
from 20 to 30
from 30 to 40
over 40 . . .

.
.
.
.

6
26
22
1

from 10 to 20
from 20 to 30
from 30 to 40
over 40 . . .

.
.
.
.

8
20
11
1

from 10 to 20
from 20 to 30
from 30 to 40
over 40 . . .

.
.
.
.

0
4
5
0

from 10 to 20
from 20 to 30
from 30 to 40
over 40 . . .

.
.
.
.

1
0
2
1

?

from 10 to 20
Type VIII from 20 to 30
from 30 to 40
over 40 . . .

.
.
.
.

1
0
3
1

?

Totals . . .

178

Type I I I

Type IV

Type V

Type VI

Type VII

Charleroi

Central

Mons

Rock Seam Rock Seam Rock Seam Rock Seam Rock Seam
14
14
17
142
32
136
145
22
263
178

from 10 to 20
from 20 to 30
from 30 to 40
over 40 . . .

Type I I

145

?

?

?

?

•>

2
1
0
0

0
0
0
0

13
0
0
0

1
1
0
0

2
2
0
0

0
0
0
0

5
2
0
0

2 .
0
0
0

40
20
1
2

1
1
0
0

36
7
2
0

3
1
1
0

16
8
0
0

1
2
0
0

7
4
3
0

1
0
0
0

26
15
8
0

3
2
0
0

41
10
3
0

0
1
0
0

39
10
0
0

0
2
2
1

16
13
6
0

3
1
0
0

12
5
5
0

4
0
0
0

35
15
1
0

3
0
1
0

29
4
0
0

1
1
0
0

17
11
7
0

1
0
2
0

0
2
1
0

1
1
0
0

41
12
3
0

2
4
3
0

7
4
1
0

0
2
0
0

4
11
3
0

0
4
0
0

1
1
0
0

1
0
0
0

24
3
0
0

1
3
3
0

6
2
0
0

0
0
0
1

3
11
5
1

0
2
0
0

0
0
0
0

0
0
0
0

8
6
3
0

0
1
3
0

6
0
0
0

0
0
1
0

4
8
2
0

0
1
0
0

142

14

263

32

136

14

143

17

Note : There is an error of 2 units in the Mons statistics. In order to rectify it, it would have
been necessary to recommence the whole study of the cards and it would not in any case have
altered the general view of the situation.

suspension the dust which settles on the walls, pit props, piping, etc.
A final source of dust results from removal of fragments of broken
rock either by shovelling or by the action of gravity.
The work of the hewers properly so-called, also involves exposure
to mineral dust for the following reason : the workings have at times
to be bored partly through the enclosing rock (wall or roof) in order
to obtain a section permitting of the required amount of traffic.
Mechanical cutting machines used at present attack not only the
coal, but the rock, schist or sandstone, while, on the other hand, the coal
is often mixed with the schist (French "limet"). Account must also be
taken of the dust coming from shotfiring in the mines, dust resulting
from removal of rock and, finally, the dust expressly introduced into
underground workplaces for purposes of safety to combat explosions
of fire damp and coal dust.

146

SILICOSIS

I I I . — I N F O R M A T I O N F U R N I S H E D B Y T H E ANTI-TUBERCULAR
D I S P E N S A R I E S AND SANATORIA

The fact t h a t t h e majority of working miners show radiographic
signs of silicosis h a s led t o interpretation of these signs b y some authorities as indicating t h e presence of cold miliary tuberculous lesions.
I n reply t o this interpretation we provide t h e following d a t a obtained
from Dr. Courtois, Director of t h e Marcinelle Sanatorium, who has
undertaken a thorough s t u d y of coal miners' silicosis.
F r o m information furnished b y the various Belgian sanatoria
(Alsemberg, Eufen, Buysinghen a n d L a Hulpe-Waterloo) it is seen t h a t
out of a t o t a l of 522 patients representing t h e total population of these
sanatoria a t t h e time of t h e enquiry, 11 showed a more or less typical
granular picture, while of these 11 cases 9 h a d bacillary sputa. Of t h e
11 cases in question 3 were coal miners. N o case showed a pseudotumoral picture.
On t h e other hand, in t h e sanatorium under t h e direction of Dr.
Courtois, situated in a coal-mining district, o u t of t h e 137 tubercular
coal miners first treated, 120 showed a characteristic radiological
background. Among these, 68 showed a state of pure granulation
and 52 a pseudo-tumoral state, while among t h e 17 remaining patients
9 were very y o u n g coal miners a n d 5 showed hybrid pictures, which
were not easy t o interpret. Similarly, a t the Charleroi dispensary,
where Dr. Courtois was t h e medical m a n in charge of pulmonary diseases,
95 o u t of 100 tubercular coal miners showed t h e characteristic radiological background of silicosis.
Pursuing his enquiry over a more extensive field, Dr. Courtois
thereafter requested information from various m u t u a l aid organisations in t h e country : catholic, socialist, liberal, non-political, occupational. The figures obtained do n o t perhaps represent the total number
of tubercular subjects in t h e districts concerned, b u t t h e advantages
granted t o these patients b y t h e mutual aid organisations are such
t h a t i t m a y be said t h a t almost t h e t o t a l number of these cases is covered.
The various m u t u a l aid organisations were asked t o furnish t h e following information :
Number of coal miners inscribed on the books; number of adults
other than coal miners; for each of these categories, the number of
tubercular workers treated in the sanatorium, or in hospital or
recognised as incurable and treated at home.
F o r t h e years 1932, 1933 and 1934 t h e number of members of t h e
various m u t u a l a i d organisations was :
Year

coal miners

Coal miners

1932
1933
1934

212,588
216,156
219,071

70,171
70,828
70,630

147

APPENDIX V : BELGIAN MINERS' SILICOSIS

These figures may be divided as follows as regards tubercular morbidity :
District

Charleroi

Occupations

Tubercular morbidity recorded by
Registered
the
mutual aid organisations : rate
members :
per 10,000 registered adults
average for the
1934
1933
1932
three years

N.M.
M.

128,812
27,128

27.43
17.27

26.73
24.10

28.61
30.72

N.M.
M.

62,321
17,143

18.66
29.94

19.53
41.72

19.22
31.58

N.M.
M.

25,106
26,223

11.24
34.87

6.36
34.39

13.74
33.83

N.M. = Non-miner. M. = Miner.

By taking the three-year average this table may be more graphically
presented by the following diagram :
Per cent.
27.59

Liege

20.69
14.00

Charleroi

34.41
10.44

Borinage

34.36

Tuberculosis rate among non-miners.
Tuberculosis rate among miners.

These figures, taken in conjunction with those obtained during
the general enquiry conducted in mines, reveal the fact that among
Belgian coal miners a great number of workers show definite radiographic pictures of advanced silicosis; among Belgian coal miners
tuberculosis is definitely more frequent than in other occupations.
A final enquiry covering patients regularly treated at the Charleroi
PolycUnic in the mining district by Dr. Courtois, all of whom were
radiographed, has shown that tuberculosis is a very frequent consequence of coal miners' silicosis.
From 1932 to 1937, 461 coal miners were regularly under observation and among them :
196 showed a granular or pseudo-tumoral picture of silicotic type,
184 showed no silicotic picture,
81 were "doubtful" cases.

148

SILICOSIS

Tuberculosis was distributed as follows :
Totals

Cases of bacillary
tuberculosis

Cases of tuberculosis

\
P i c t u r e s of silicotic t y p e :
196 cases . . .

95

49.74 p e r c e n t .

54

28.27 p e r cent.

5

2.72 p e r cent.

1

0.54 p e r cent.

Ordinary pictures :

Data relative to the Belgian

coal-mining

area : The

Borinage

A p a r t from t h e above d a t a relating to the Charleroi coalfields, we
have obtained equally suggestive d a t a affecting another coal-mining
district : t h e Borinage.
Dr. Baudry, doctor to the anti-tubercular dispensaries in the Borinage,
the Borinage polyclinic a n d medical inspector of the anti-tubercuTar
dispensaries in H a i n a u t , engaged in an enquiry t o determine the extent
of silicosis, not a m o n g healthy miners a t work, b u t among those miners
who, believing themselves to be suffering from a pulmonary disease,
had consulted a medical specialist.
The figures given below concern men who sought medical advice
for t h e first time i n 1937 a t the anti-tubercular dispensary of Frameries
(Borinage). These workers all belonged t o neighbouring districts. They
were all subjected t o radioscopy a n d often t o radiography and analyses
of t h e s p u t u m were carried out wherever possible. The diagnosis
of "nodular form" was only made where it h a d been established by radiography, with t h e result t h a t numerous cases of nodular forms are on
this account included among t h e cases of more or less extensive
fibrosis.
The statistics cover a district of 31,700 inhabitants. The new
patients seeking advice in 1937 were divided into two groups :
1.

Coal miners (workers on t h e coal seam, tunnellers, t o p rippers,
stone drifters engaged in cutting ways), b u t excluding barrow
men, pit m e n , underground fitters.
2. All other m e n seeking medical advice.
I.
Negative examinations (negativepictures)
I I . Tubercular pleurisy
I I I . Fibrous, non-progressive tuberculosis
or stationary tuberculosis
IV. Progressive tuberculosis with presence of
Koch bacillus
V. Clearly defined pulmonary fibrosis . . .
VI. Pseudo-tumoral masses :
non-ulcerative
(30) . . . .
ulcerative with K. B. . . ( 8) . . . .

Coal miners Non-miners
29
25
3
7
2
6
20

5
5
10

38

102
48
These statistics show, therefore, t h a t out of 102 coal miners who
sought advice a t t h e Frameries dispensary in 1937, 38, or 40 per cent.,
showed pseudo-tumoral pictures a n d 20, or 20 per cent., clearly defined

APPENDIX V : BELGIAN MINERS' SILICOSIS

149

pulmonary fibrosis, giving a total of 58 per cent, showing definite
pictures of pneumoconiosis.
In the case of non-miners there was no pseudo-tumoral picture
observed and 10 cases of fibrosis were reported.
Among the 150 patients examined, in 48 cases of non-miners 5 cases
with bacilli were discovered aged 34, 28, 22, 17 and 24. Among 102
coal miners there were 14 bacillary cases, aged respectively 36, 36,
38, 40, 43, 45, 46, 47, 48, 52, 53, 54, 57 and 65.
These ages indicate clearly the impression of the occurrence of
"retarded" tuberculosis in the miners.

In order to complete this information Dr. Baudry also collected
observations made regarding all the coal miners engaged on underground
work and examined during two years, either at the Frameries dispensary
or at the polyclinic. A distinction was made between the patients
seeking advice at the polyclinic and those visiting the dispensary.
All the patients were subjected to radioscopy and the majority to
radiography. The author of this report concludes that radioscopy
gives sufficient information for detection of "pseudo-tumoral masses"
but cannot be considered sufficient for research in the case of purely
nodxilar conditions.
The enquiry covered 464 persons seeking advice. They were distributed in accordance with their occupational qualifications. Each
of the groups was so subdivided as to distinguish between workers engaged
over 10 years and those with a shorter working experience.
1. Negative pictures showing a normal state of slight or ordinary
fibrosis, simple emphysematous lesions, pictures with calcification,
pleural sequelae, etc.
2. Pictures of actual sero-fibrous pleurisy, pachypleuritic pictures
showing a recent pleural accident.
3. Pictures of true non-progressive fibrous tuberculosis.
4. Pictures of common, fibro-caseous, ulcero-caseous or ulcerofibro-caseous tuberculosis.
5. Pictures of interstitial nodular fibrosis, showing general distribution but occurring en bloc with the exclusion of hilar fibrosis.
6. Pseudo-tumoral pictures, non-ulcerative and ulcerative with
positive sputum.
This second statistical return shows that out of 464 working miners
who had sought advice for two years back, either at the dispensary
or at the polyclinic, 30 per cent, showed pseudo-tumoral images and
20 per cent, of these tubercular infection.
The age of the coal miners with bacillary infection was a3 follows :
25 to
30 „
35 „
40 „
45 „
50 „
55 „
60 „
65 „

30 years
35 „
40
45
50
55
60
65
70

1
1
7
11
8
9
6
1
3

SILICOSIS

150

Tunnellers
only

Coal seam
workers
only

Day
labourers
and barrow
men

Mixed

Total

Less
Less
Less
Less
10
10
10
10
years than 10 years than 10 years than 10 years than 10
years
years
years
years
I. Negative

pio-

l i . Tubercular pleu-

6

67

6

54

3

3

3

9

18

7

1

12

6

26

5

2

6

i

26

19

5

66

67

149

14

53

51

118

2

13

16

31

1

risy
I I I . Non-progressive
or s t a t i o n a r y fibrous tubercuIV. Progressive tuberculosis w i t h
K o c h bacillus .
V. Well defined p u l m o n a r y fibrosis
VI. Pseudo - t u m o 16
ral masses . .
Non-ulcerative.
Ulcerative w i t h
K. B.
. . .

22

1

2

174

12

161

3

9

54

10

1
X

1 O

54

4

73

201

11

464

Data furnished by the Warocqué Sanatorium at Obourg
The final information given below was sent me by Dr. Denet-Kravitz,
who was the first to study silicosis in Belgium and has carried out
remarkable work in this connection. He is at present in charge of the
sanatoria in the Borinage. He collected data relating to 127 miners
who had been under treatment at the Warocqué Sanatorium during
the year 1937. Among these 127 sick coal miners, 73 showed bacillary
infection and 54 were free from bacilli.
Considered from the standpoint of the radiographic picture, the
figures are as follows :
Nodular non-bacillary condition
16 or 12 per cent.
Nodular bacillary condition
26 or 22 „ „
Pseudo-tumoral, non-bacillary condition
19 or 15 „ „
Pseudo-tumoral bacillary condition
33 or 26 „ „
Pseudo-tumoral and bacillary nodular condition . .
3 or 2 „ „
Atypical pictures
30 or 23 „ >,
The figures for subjects with a pseudo-tumoral bacillary condition
is higher in Dr. Denet's statistics than in those of Dr. Baudry. This is
explained by the fact that in this case the subjects were in advanced
stages of the disease, having no hope or no resources other than the sanatorium. I t is also to be explained by the fact that research for bacilli
is obviously more thorough when effected in the sanatorium than in
dispensaries or at the polyclinic.

APPENDIX VI
NOTES ON SILICOSIS
By D B . L. G. IRVINE

1
1. — General 'pathological conception of silicosis. Since the meeting
of the International Silicosis Conference at- Johannesburg in 1930
substantial agreement has been reached regarding the general
pathological conception of silicosis. In this conception there are
three distinctive features :
(1) That an uncomplicated silicosis in the sense of a simple pathological reaction to the effective occupation of the lungs by siliceous
dust is a distinct pathological entity consisting of the development of
a more or less generalised miliary fibroid nodulation of the lung, with
associated fibrotic changes. There is always an accompanying bronchitis, and in established cases varying degrees of emphysema. In many
cases this is, throughout, the sole condition present.
(2) That the effective occupation of the lungs by siliceous dust not
only leads to fibrosis but facilitates the development of tuberculosis,
either at sites of pre-existing dormant foci of tuberculosis, or at sites
where silicotic lesions are developing or have developed, and at which
tubercle bacilli may arrive and become arrested.
(3) That the conjunction of a limited tuberculous infection with
developing or developed silicotic lesions does not, however, in general,
lead to the development of an active spreading tuberculosis (although
this may and does happen in particularly susceptible individuals),
but to a mutual modification of the infective and the silicotic process,
resulting in the production of chronic indurated lesions of mixed silicotic
and tuberculous origin, marked by an excessive fibroid reaction, with
the result that the extension of the infective process in the lung and to
other organs is limited and retarded and not infrequently becomes
virtually arrested for lengthy or indefinite periods. Commonly, however,
these mixed lesions are slowly progressive and tend ultimately to break
down and to initiate a terminal active tuberculosis. Ultimately,
either from this source or by dissemination of the infection from some
less conspicuous focus within the respiratory organs or by re-infection
from a source outside the lungs an active tuberculosis is present at death
in perhaps a majority of cases of silicosis who die from their disease.
The distinctive mixed lesions described are seen in no other disease ;
but in silicosis they are so common that it is practically true to say
that an unmodified tuberculosis of the famihar forms is not met with
in silicosis, except as a terminal phenomenon or in cases in which an
active spreading tuberculosis is associated at the outset, with a slight
amount .of silicotic fibrosis.

152

SILICOSIS

Hence t h e relation of silicosis and tuberculosis tends t o have a dual
character—(1) a n initial facilitation, and (2) a subsequent limitation
and retardation of the infection associated with a n excessive fibrosis.
2. — Note on terminology.
I t is desirable to adopt a uniform terminology
for a single condition.
The distinctive modification which a limited tuberculous infection
produces and undergoes in the silicotic lung was first described in South
Africa in 1916 a n d was then designated "tubérculo -silicosis".
This
term has since been accepted in England. American writers apply
the term "silico-tuberculosis"
to precisely the same condition. I t is
suggested t h a t , since t h e condition is one of occupational origin, inasmuch
as a silicotic element is present in all such lesions, it is desirable to retain
the original t e r m , and t o regard the condition as a modification of the
occupational disease, silicosis, i.e. as a "tubérculo-silicosis",
and not as
a modification of tuberculosis, as the term "silico-tuberculosis" implies.
This is in Une w i t h the designation of other accepted variants of silicosis,
e.g. "axúhx&co-silicosis" and
"sidero-silicosis".
There are valid medico-legal reasons also for regarding cases of
"tubérculo-silicosis", unaccompanied b y evidence of active tuberculosis,
as cases of modified silicosis, reserving the term "tuberculosis with
silicosis" or "silicosis with tuberculosis" as is done in South Africa, t o
denote a condition in which silicosis is associated with a clinically
detectable active a n d physically deteriorating tuberculosis.
The t e r m "silico-tuberculosis" is redundant a n d is not now used
in South Africa. I t was a t one time employed in a clinical rather t h a n
a pathological sense b y Watkins-Pitchford.
3. — Relation of pathological conception to the classification of clinical,
radiographic and medico-legal types of silicosis.
I n accordance
with the above considerations three clinical a n d radiographic types
of silicosis are distinguished in South Africa, namely—
(1) "Silicosis of simple type", i.e. a simple miliary silicotic nodulation,
uncomplicated (apart from the accompanying bronchitis) by infection;
(2) "Silicosis
of infective type", in which tuberculo-silicotic, or
possibly other similarly modified infective lesions, are an evident feature,
but in which there is no local or constitutional evidence of active tuberculosis; and
(3) "Tuberculosis with silicosis", i.e. a condition in which silicosis
is associated with a n overt active tuberculosis.
I n South African medico-legal practice three conditions are compensable.
(1) "Silicosis",
i.e. a silicotic fibrosis uncomplicated b y active
tuberculosis. T h e clinical types (1) and (2) are not distinguished for
medico-legal purposes.
Both are graded as cases of silicosis in three stages, according to the
degree of silicosis present, as determined b y clinical and radiographic
evidence, and t o t h e degree of disability, which in the earhest stage m a y
be absent or inconspicuous, in the second is moderate, and in the third
is serious or total.
Cases of "silicosis" are not prohibited from continuing in underground
employment.

APPENDIX VI : NOTES ON SILICOSIS

153

(2) "Tuberculosis with silicosis", i.e. a condition in which silicosis is
associated with an active or physically deteriorating tuberculosis.
Medico-legally all cases of this conjunction are graded in the stage
of serious or total incapacitation.
(3) "Tuberculosis", i.e. a simple active tuberculosis, uncomplicated
by silicosis. Simple tuberculosis is not regarded as an occupational
disease, but an award is granted to these cases in return for loss of
occupation, since all cases of tuberculosis or tuberculosis with silicosis
are required to relinquish underground work forthwith. It is found
that a considerable proportion of cases, which on first detection presented
signs of a simple tuberculosis only, later on develop evidence of the
presence of a silicotic element also; these are then transferred to the
class of "tuberculosis with silicosis".
The above considerations are submitted as contributions towards
an agreed nomenclature of types and varieties of silicosis, should this
matter be discussed by the Sub-Committee.
4. — Relation of clinico-pathological types of silicosis to variations in
the aetiological factors. The general conception of silicosis previously
outlined explains the variations in type met with in individual
cases of the disease and in different occupational groups of persons
exposed to siliceous dust.
These appear to depend upon :
(1) variations in the dust factor in respect of—
the amount of free silica contained in the particular dust, with
possible modification by other constituents ;
the number concentration, and
the "size" distribution of the particles inhaled, and
the duration of exposure;
(2) variations in the infective factor in respect of individual susceptibility to tuberculous infection and opportunity of exposure to it.
So far as mining and tunnelling are concerned, the modern history
of silicosis is associated with the application to rock-breaking of rockdrilling machines driven by compressed air and of high power nitroglycerine explosives, in the latter part of last century.
Illustrations of effect of variations in the concentration of dust.
On the Witwatersrand one has had experience of all gradations
in the type of silicosis in association with respective degrees of concentration of dust.
(i) A group of cases of a rapidly developing "acute silicosis" in sewer
miners engaged in tunnelling for sewer construction in the Witwatersrand formation under conditions of exposure to very high concentrations
of drilling and blasting dust and to the fumes from blasting.
The disease was contracted after exposures as short as from nine
to eighteen months and was of a very severe and rapidly developing
type and the after-life was correspondingly short. Death occurred
from pneumonia or tuberculosis or directly from the disease.
Simson and Strachan have reported on the pathological conditions
found in these cases, and Gardner has described a similar rapidly developing type of silicosis associated with tunnelling operations in the United
States.

154

. SILICOSIS

(ii) The silicosis of the Rand gold miner has been of a progressively
more chronic and more slowly developing type.
It is unlikely that a very significant number of cases of an equally
rapid development to that described occurred amongst gold miners
on the Witwatersrand even in the earlier years of the industry. But
in the first decade of this century the prevalent type was a very severe
one, with heavy bulky lungs, much massive fibrosis, and marked marginal emphysema. Death occurred typically either from congestive
heart failure with ascites and anasarca without clinical evidence of
tuberculosis, or from a terminal tuberculosis.
The disease was particularly prevalent amongst rock-drill miners,
who were at that time much exposed to drilling dust and the dust and
fumes from blasting. No systematic survey of the dust in mine air
was then carried out, but a number of quantitative observations of the
total dust in the mine air in working places made in 1902-1903 by the
sugar tube method showed that in development ends over 100 mg.
per cubic metre might be present.
The average age of those found to be affected was 3514 years;
the average duration of work prior to contraction of the disease amongst
rock-drill miners was from 4% to 6 years.
(iii) The subsequent progressive development of preventive measures
directed particularly to the control of blasting dust and fumes and
drilling dust, and to improvement in ventilation, and the consequent
progressive reduction of the average dust content in mine air was
followed by a gradual amelioration in the type of silicosis produced.
Death from heart failure with general anasarca became infrequent,
and death from a terminal tuberculosis became relatively more prominent. In the cases of silicosis detected from 1917 to 1920 the average
duration of previous service was Qy2 years and the average age at first
detection was 42 years.
(iv) During recent years a further amelioration in type has occurred,
with prolongation of the average effective duration of previous exposure
in those affected, which is now 17 years, and with a corresponding
increase in the average age at first detection, which is now 50 years
or over. The cases of silicosis now arise on the average amongst men
in late middle life, and in an increasing proportion the clinical type is
one of a disabling emphysema and bronchitis associated with comparatively lesser degrees of silicotic fibrosis.
This feature is in accordance with the two outstanding statistical
features amongst those engaged in occupations with silica risk elsewhere, namely—
(1) An excess mortality from pulmonary tuberculosis in middle life,
and,
(2) An excess mortality from conditions associated with dyspnoea
and cough, and commonly generically labelled as "bronchitis",
in the later decades of life.
We appear on the Rand to be moving towards the latter group
as the probable prevalent type in the future.
Although the sugar tube by no means tells the whole story regarding
the dust danger, it is clearly of significance that, while in 1909 the
estimated average content of dust in mine air was of the order of 20
mg. per cubic metre, the first systematic survey carried out in 19141915 gave an average figure of 5.4 mg. and that the average figure
for the past ten years has been consistently below 1 mg.

155

APPENDIX VI : NOTES ON SILICOSIS

It is of interest to note that observations by Patterson on the dust
retained in the lungs in cases of "acute" silicosis in the group of sewer
miners who had been exposed to extremely high concentrations of
dust, the observations on the lungs of gold miners of McCrae in 1913,
and Patterson's similar observations on lung residues prepared by
Simson from recent cases in gold miners show that the size distribution
of lung residue dust has been of a similar order throughout. It would
appear therefore that the progressive amelioration in the type of silicosis described is to be associated with reduction in the number concentration of the fine particles and the abolition of danger from nitrous
fumes rather than with any significant change in the size distribution
of the particles which become lodged in the lungs.
Illustration of the effect of differences in susceptibility to tuberculosis
is supplied by a comparison of the type-incidence of silicosis and
tuberculosis amongst European miners and Native labourers on thymines. These differences depend in part upon a lower average duration
of exposure to dust and in part upon a higher susceptibility to tuberculosis amongst Native labourers than among European miners.
The subjoined data provide a summary illustration of the differences in question :
COMPAEISON OF INCIDENCE OF SILICOSIS, TUBERCULOSIS WITH SILICOSIS,
AND TUBERCULOSIS AMONGST EUROPEAN MINERS AND NATIVE LABOURERS

(Bates per 1,000 per annum)

(«)

Tuberculosis
with silicosis
(b)

1927-28
1936-37

19.41
8.04

0.00
0.08

1927-28
1936-37

1.08
1.42

Years

Silicosis

Tuberculosis
(c)

European

miners
|
1

Native mine
2.04
1.08

Tuberculosis
in both forms
(6 and c)

2.19
0.84

2.19
0.92

labourers
3.85
2.16

5.89
3.24

It will be seen that the incidence of silicosis is very much higher
amongst European miners than amongst mine Natives; but that, while
the general incidence rate amongst the former has fallen by 59 per cent.
in the past nine years, it has remained of much the same order amongst
the mine Natives.
The incidence of cases of "tuberculosis with silicosis", i.e. of cases
in which both conditions are present at the first detection of either, is
negligible amongst European miners, but is considerable amongst
mine Natives, who also show a higher incidence of simple tuberculosis.
The incidence of cases of simple tuberculosis has always been low
amongst European miners, in whom indeed it would appear that the
factor of tuberculosis does not become significant until the lungs have
become occupied by siliceous dust to an extent which has produced
or which is capable of producing detectable silicotic lesions, and that
it then in general operates rather in the production of cases of tubérculo
silicotic type than of cases of active unmodified tuberculosis. It

156

SILICOSIS

may be that in the more susceptible Native the threshold of facilitation
of tuberculosis owing to the presence of siliceous dust is lower than in
the European.
I t is satisfactory to note that in these nine years the general incidence of tuberculosis in both forms amongst European miners has
fallen by 58 per cent., and amongst mine Natives by 45 per cent.
While this result is presumably due in part to improvement in occupational conditions, it is probably also in large measure due to the systematic measures adopted for the early detection and removal from underground work of cases of active tuberculosis. Judged by European
standards, the incidence of tuberculosis in both forms amongst mine
Natives does not appear to be excessive.
2

"Anthraco-silicosis" well recognised.
What evidence exists for or against a simple "anthracosis" ?
Result of examination of mine Natives at Wankie Colliery, Southern
Rhodesia.
"Sidero-silicosis".
Can show radiographs of early results of retention of dust in ironstone
miners, but have no confirmatory post-mortem evidence.
3
Asbestosis
Result of investigation of asbestos quarries and mines and crushing
and grading plants in Southern Rhodesia.
Will communicate these observations verbally.
4

(a) Diagnosis of Silicosis
Pathological. — The pathological diagnosis of silicosis rests essentially
upon the presence on the cut surface of the lung and under the pleura
of definitely palpable circumscribed nodular or massive fibrotic lesions
accompanied by similar lesions in the root glands.
Diagnosis during life. — Three conditions are essential :
1. A definite history of a significant degree and duration of
occupational exposure to siliceous dust;
2. A reliable radiograph of the chest ; and
3. A thorough clinical examination.
The first condition is obviously necessary ; in the other two conditions
the radiograph forms much the most important single factor in diagnosis, since it alone presents permanent objective evidence of the precise
degree and distribution of fibrotic or other observable changes in the
lungs at all stages and in all varieties of the disease. Clinical examination alone, although suggestive, is indecisive in the diagnosis of early
silicosis and, although the complex of symptoms and physical signs
is much more definite in well-marked cases, even in these a purely
clinical diagnosis should be regarded as provisional and should always

APPENDIX VI : NOTES ON SILICOSIS

157

be confirmed by radiographic evidence. Nevertheless, a thorough
clinical examination is always necessary, since it alone can supply
information as to the functional condition of the lungs, the degree of
disablement, if any, which is present, and the presence or absence of
active pulmonary infection, or of concurrent disease of other organs.
Observation of these matters is obviously essential to the final summing
up of the case.
On the other hand, reliable interpretation of the radiograph must
in turn depend upon extensive experience of the correlation found to
exist between the radiographic appearances and the underlying pathological condition. Such a correlation has now been carried out by the
Miners' Phthisis Medical Bureau in Johannesburg in the case of some
1,500 deceased miners. But taken together and properly interpreted
by observers experienced in both methods of examination, the radiograph
and the clinical examination will enable a diagnosis of the presence or
absence, the degree and the type of silicosis to be made with substantial
accuracy.
Early Diagnosis of Silicosis. — The clinical symptoms' and signs
of an early silicosis were described in a paper on "The Radiology and
Symptomatology of Silicosis" contributed to the Records of the International Conference on Silicosis in 1930, and need not be repeated in
detail.
The slight shortness of breath on exertion, with some amount of
irritative cough, the occasional slight pains in the chest, the somewhat
reduced elasticity of the chest in the movements of respiration, the
general reduction in air entry, the pronounced or harshened and somewhat shortened breath sounds and the other alterations there described
form a characteristic complex which is almost constantly present in
early cases. Such signs, however, although suggestive, are not sufficiently distinctive to be decisive, and no substantive diagnosis of early
silicosis should be attempted without confirmation by radiological
examination.
The most important single matter for discussion in the early diagnosis
of silicosis is radiographic diagnosis.
Radiographic Diagnosis of Silicosis. — The abnormal radiographic
appearances which are of significance in considering the possible presence
of silicosis are :
(1) The appearance of a well-marked or very well-marked increase
in "linear striatum" visible in the lung fields, culminating in what has
been called the "leafless tree" appearance;
(2) the appearance of small discrete and generally rounded shadows
in the lung fields cast by individual fibrous "nodules". This appearance,
the so-called "silicotic mottling", is the distinctive radiographic sign
of a definitely established nodular silicosis ; and
(3) the appearance, along with either of the above-mentioned conditions, of larger areas of diffuse opacity, indicative of pulmonary consolidation. The significant types of consolidation are in particular those
due to "massive areas" of latent or arrested tubérculo-silicosis, or to
active tuberculosis. Although the radiograph frequently permits of
a provisional distinction between these, the final decision in this respect
will commonly rest upon the clinical evidence.
The evidence supplied by the second and third types of appearance
can readily be appraised, and need not be discussed further.

158

SILICOSIS

The appearances which present difficulty are those of the first group,
namely, those which show a well-marked and abnormal increase in the
linear striation visible in the lung fields, without definite significant
indications of the presence of nodular fibrosis. It appears, therefore,
to be desirable to discuss in more detail the available evidence regarding
the relation of appearances of this order to the development of pneumoconiotic changes in persons exposed to the inhalation of harmful
siliceous dust.
Slight increases, or a moderate simple increase in linear striation
over the amount visible in normal radiographs, may for the present
purpose be regarded as being "within practical normal limits", inasmuch
as the correlation studies of the Bureau already referred to have shown
that they are not associated with detectable silicotic fibrosis. Some
may show indications of a slight arrested or latent primary tuberculous
infection, which are common enough in quite healthy people who have
never been underground, and in perhaps a majority of whom a permanent arrest appears to have been established.
On the other hand, a well-marked or very well-marked increase
in linear striation, associated as it practically invariably is with increase
in the hilar shadows, is of greater significance, and is definitely abnormal.
Such appearances, however, are not necessarily due to pneumoconiotic
changes. They may be produced in cases of chronic cardiac disease
with a chronic congestive condition of the lungs, or in well-marked
chronic bronchitis, or from other causes. In such cases the appearances
show a well-marked simple increase in the linear and dendritic striati ons
radiating from the hiius to the periphery of the lungs. The appearance
may arise also from simple retention of dust unassociated with detectable
fibrosis.
In many instances, however, the abnormal increase in striation is
associated with the stigmata of localised arrested or latent tuberculous
infection, namely, the "asthenic" type of heart shadow, the "peribronchial thickening", and the presence of appearances indicative of limited
and circumscribed foci of infection in the root glands or here and there
in the lung fields. Such cases are of significance, since it has been shown
experimentally by Gardner and .by direct case observation that the
retention of siliceous dust in the lung may lead to the reactivation
of dormant foci of tuberculosis, with the consequent development
of tuberculo-silicotic lesions in and around these.
American experience of silicosis amongst certain granite workers
has shown that cases presenting an abnormal increase in striation,
without obvious evidence of nodulation, and without obvious disablement
are nevertheless liable to develop tuberculosis, and the same sequence
appears, to a considerable extent, to occur in Native mine workers on
the Rand.
In addition to these considerations, the correlation studies of the
Medical Bureau at Johannesburg have supplied definite objective
evidence regarding the interpretation of films showing an "abnormal
increase of striation", when this is found to have developed in persons
exposed to the inhalation of siliceous dust, and of their relation to the
development of silicotic fibrosis. A careful analysis of this relation
is contained in the last Report of the Medical Bureau and is available
for the Sub-Committee.
These studies have shown that in many hundreds of instances, in
which successive radiographs have been taken of underground workers
over a period of years, the cases which have ultimately developed the
distinctive characters of an established nodular silicosis have previously

APPENDIX VI : NOTES ON SILICOSIS

159

passed, with practical uniformity, through the stages of abnormal
increase in linear striation, and have shown, further, that in rather
over 40 per cent, of the cases, which during life exhibited appearances
of the types of abnormal increase in striation described in the Report
of the Bureau, a slight but significant degree of simple silicotic or of
tuberculo-silicotic nodulation was found to be present by pathological
examination after death.
Accepting, therefore, the existence of a significant degree of silicotic
nodulation as the standard of pathological diagnosis of the presence
of silicosis, the experience of the Witwatersrand suggests that for general
statistical purposes it may be assumed that, amongst underground
workers, 40 per cent, of cases which during life presented the appearance
of "abnormal increase in striation" may be regarded as cases of "incipient silicosis".
These circumstances are accordingly taken into account by the
Medical Bureau on the Witwatersrand in making its decisions upon
individual cases in which appearances of this character have developed
during the course of underground work.
How far the general formula for the interpretation of these types of
radiograph, which Witwatersrand experience suggests, may be applicable to other bodies of underground workers Avili depend upon the
relative proportions in which the different varieties of "abnormal
striation"—whether well-marked, or very well-marked, or whether
simple or infective in type—may be present.
A radiographic examination during the present year of 1,007 mine
Natives employed in the gold mines of Southern Rhodesia showed a
steady increase in the prevalence of cases showing an abnormal increase
in linear striation with successive increased durations of underground
service, in strong contrast with the absence of any such increase in a
non-siliceous coal mine. The run of the figures was as under :
Duration of underground service, in years
13 gold -mines :
5 to 9 10 to H 15 to 19 20 and over
Prevalence per cent, of cases of
"abnormal increase in striation"
9
15
31
20
Prevalence per cent, of cases of
definite silicosis
1
10
13
26
The Wankie Coal Mine :
Prevalence per cent, of cases of
"abnormal increase in striation"
—
5
5
—
The conclusion suggested by this observation is that the respective
amounts of increase in the prevalence rate of cases of "abnormal increase
in striation" may be taken as a measure of the respective risk of ultimate
liability to the contraction of silicosis or of tuberculosis produced by
the occupational conditions existing in different dusty occupations.
These considerations cannot be overlooked in administrative decisions regarding the presence or absence of silicosis. Although the Medical
Bureau accepts the appearance of a significant amount of "definite
mottling" as the earnest unequivocal radiographic evidence of the
presence of established nodular silicosis, it has from the foregoing
evidence been led to take into account cases which show well-marked
increase in linear striation when these appearances are associated with
definite evidence of respiratory disability.
Films will be submitted showing in a representative individual
case the progression through, the several degrees of increased striation

160

SILICOSIS

to a definite nodular silicosis; also representative films of the three
clinico-pathological types of silicosis.
(b) Effect of Early Removal from Occupation on the Progression of
Silicosis
Silicosis is intrinsically a progressive disease, for two reasons :
1. Even a simple silicosis is progressive up to a practical limit
determined by the amount of siliceous du3t retained in the lungs.
When that limit has been reached, whether it be at an early or moderate
or well-marked stage of the disease, a substantial degree of arrest
takes place, provided that the case does not become complicated by
active infection. Since, however, apart from infection, the important
cause of disablement in silicosis is the degree of emphysema which may
have been produced, arrest of progression can in general mean merely
a stabilisation rather than an amelioration of symptoms. The Bureau
has had experience of very many cases in which such a practical stabilisation has been reached at each of the several stages of the disease.
2. Apart from this primary tendency, the most important single
factor in producing progression to a stage of grave or total incapacitation
is the secondary factor of increased predisposition to complication by
tuberculous infection, in the absence of which a degree of grave incapacitation would be a much less common end result of silicosis.
For these reasons it cannot be expected that, granted that the
lung has already been effectively occupied by siliceous dust to an extent
which is capable of producing a definite silicosis, early removal will
prevent the ultimate progression of pulmonary fibrosis at least to a
degree proportionate to the amount of fine dust which has become
lodged in the lungs at the time of removal. I t will in general only
mitigate the rate and ultimate amount of progression in proportion
to the extent to which it secures a reduction in the amount of retained
dust.
The following data are submitted in illustration in respect of three
groups of cases of silicosis, namely :
A. A group of 895 "primary stage" cases detected from 1917 to
1920. These were cases of well-marked silicosis; their average
age at detection was 40 years, and the average duration of
previous exposure 9% years.
B. A group of 1,235 new cases of "ante-primary" stage silicosis
detected from 1923 to 1926. These were cases of a slight to
moderate degree of silicosis; their average age at detection was
42 years, and the average duration of previous service was
10% years.
G. A group of 917 new cases of "ante-primary" stage silicosis detected
from 1926 to 1929. These were cases of a slight degree of silicosis
detected in a rather earlier condition than group B ; their average
age at detection was 43 years, and the average duration of previous service was 11 years and 11 months.
The further history of Group A has now been followed for 17 years,
that of Group B for 11 years, and that of Group C for 8 years. The
following data represent the situation at the end of the eighth year
after first detection in the three respective groups :

APPENDIX

TABLE I .

VI : NOTES

OK SILICOSIS

161

COMPARATIVE P O S I T I O N AT T H E E N D O F T H E 8 T H YEAK A P T E K
DETECTION O F T H R E E GROUPS OF CASES O F SILICOSIS

1. Total transfers from outset to "secondary" stage of grave incapacitation as
a percentage of the original number .
2. Average annual rate of transfer to
secondary stage during eight years .
3. Average annual mortality rate of cases
transferred to secondary stage . .
4. Average annual mortality rate, all cases
5. Percentage of original number who

A

B

C

51

37

27

9.0

6.1

4.2

17.3
6.1

13.5
4.25

14.4
3.6

40

29

26

TABLE I I . — COMPARATIVE DATA O F MORTALITY A N D P R O G R E S S I O N
I N T H R E E GROUPS O F CASES O F SILICOSIS l

At end of years
subsequent
to detection

l

2

Total transfers to
secondary stage
from outset as
percentages of
original number

Annual rates of
transfer to
secondary stage

A

B

C

A

1
2
3
4
5
6

9
15
21
26
32
39

5
9
15
21
26
29

3
5
9
12
17
20

9
6
8
7
9
12

7
8
9
10
11
12

45
51
53
55
56
58

34
37
40
43
46

24
27

11
13
7
5
4
5

13
14
15
16
17
18

59
61
62
63
64

6
6
6
4
8

B

3

5

4

Annual mortality Annual mortality Total deaths from
outset as
rates of secondary rates per cent.
percentage of
stage cases
in whole group
original number

C

A

B

C

A

5
5
7
8
7
4

3
3
4
4
6
4

13
20
18
25
18
18

12
24
IS
20
11
11

25
21
19
14
14
15

5
5

8
5
4
4
5

6
5

16
12
10
9
14
12

14
8
6
11
10

9
6

8
S
7
5
7
6

15
10
13
8
7

0

7
6
8

8
5
8
5
5

B

0

A

B

C

4
4
4
5
4
4

4
2
4
3
3
4

5
9
14
19
24
30

4
8
11
16
20
.23

4
6
10
13
16
19

5
4
4
5
5

3
6

35
40
44
47
51
54

28
29
32
36
38

22
26

58
60
63
65
66

1
A. 895 new cases of primary silicosis, 1917-20, average age at outset 40. — B. 1,235 new cases of anteprimary silicosis, 1923-26, average age at outset 42. — C. 917 new cases of ante-primary silicosis, 1926-29,
average age at outset 43.

The most reliable measure of the comparative progression in these
groups is the respective percentage amount of progression to the stage
of grave incapacitation (column 1). It is apparent that earMer removal
has not secured an arrest of progression. It has, however, definitely
retarded it and may reduce its ultimate amount.
6

162

SILICOSIS

The rates of transfer are higher in all groups in the earlier years
up to about the eighth (table II, column 2). Thereafter they continue
at a lower and more uniform level. The cases of early transfer are
those in which an active infective element is present at the outset,
or develops shortly thereafter, and it is significant that the mortality
rate of these early transfers is almost equally high in all three groups,
being at a maximum in the first four years and declining thereafter
(table II, column 3).
There is indeed a striking parallelism in the percentage of transfers
of the original number in Groups A and B in the earlier years, the
progression in the latter group showing for the first five years a lag
of one year only behind that of the former. Thereafter the lag increases
so that, whereas by the end of the seventh year, 45 per cent, of the original
number in Group A have progressed to the secondary stage, that
percentage is not reached by Group B until 3 % years later (see table II,
column 1).
In Group C the transfers appear to be about to follow the same
course, with a lag of two years, but the lag definitely increases after
the third year.
The general result is a gradual increase in longevity in the later
groups (table II, column 5).
These data appear to show that all cases of silicosis, once it is contracted, are progressive up to a limit which may be regarded as being
in general determined by the amount of fine dust which has become
lodged in the lungs. Earlier removal may reduce the limit, but will
not in general secure arrest below that reduced limit.
The only way to secure entire absence of progression is to reduce
exposure to dust to the critical point at which definite silicosis is no
longer produced.
7
The Causes and Estimation

of Functional Disability

in

Silicosis

Although in cases of silicosis there is an average general relation
between the amount of nodular development indicated by the radiograph
and the degree of respiratory disability, the relation is by no means
precise and very considerable variations are shown in individual cases.
In minor degrees of silicosis disability may be absent and is at most
slight, and cases with appearances of well-marked nodulation are in
general fit for moderate work and some for practically ordinary work.
One finds a good many cases of simple type who remain in the same
condition for many years or with at most a slow deterioration in condition. But there always remains the average tendency to progression
which has been demonstrated under Item 4 (a). In advanced cases
there is very definite disability.
The characteristic respiratory disability in silicosis is "exertion
dyspnoea", going on in advanced cases to a "resting dyspnoea". Progressive dyspnoea in silicotic cases is due to progressive impairment
of the functional capacity of the lungs and is associated, along with
other factors, with a progressive condition of emphysema which is
in part compensatory to the effects produced by the development and
subsequent retraction of pathological fibrous tissue, in part to the strain
of recurrent cough from the associated bronchitis, and in elderly men
may be also in part of simple atrophic type. Emphysema tends to be
more marked in cases of infective type with extensive areas of massive
fibrosis.

APPENDIX VI : NOTES OK SILICOSIS

163

Bronchitis and emphysema are not accidental complications of silicosis; the former is an integral p a r t of the process and, a p a r t from
complication b y active infection, the degree of emphysema which m a y
develop is probably the most important single factor in producing
respiratory disability. The general clinical type of chest disability
in a well-marked uncomplicated silicosis is one of chronic emphysema
with a dry bronchitis. A simple silicosis is not often the direct cause
of death, but, as in all well-marked instances of chronic bronchitis
and emphysema, it tends in the more marked cases t o become associated
with degenerative myocardial changes and so to predispose and contribute t o death, and it m a y do so also in death from acute respiratory
infections. The characteristic conformation of the chest in cases of
well-marked silicosis is one of partial emphysema suggesting a balance
between retraction and distension; b u t the full "barrel-shaped" chest
of marked simple hypertrophic emphysema is not frequently m e t with
except in cases of well-marked "pycnic" physical t y p e .
The results of physical examination of silicotic cases bear out these
observations.
I n an examination of over 500 miners ranging from normal cases
(200) to cases of early (100) and medium (100), with a few advanced
cases of silicosis, carried out a t the Medical Bureau, Johannesburg,
in 1929, the following average changes were found in the silicotic cases :
There is a progressive diminution in chest expansion. The
vital capacity is progressively reduced, only slightly so in early
cases, by about 15 per cent, in intermediate cases, a n d b y about
25 per cent, in more advanced cases.
The volume of tidal air is progressively reduced and the breathing becomes proportionately shallower. I n compensation t h e respiratory rate is increased and the lung ventilation per m i n u t e a n d
per kilo of body weight and the ratio of the former t o t h e vital
capacity are increased.
The pulse rate is not markedly increased, but the t e m p o r a r y
increase induced by exercise is prolonged, and the latter observation
is true also of the respiratory rate.
No significant changes were noted in the haemoglobin percentage
or in the systolic or pulse pressure.
The mean standard weight was not affected.
These observations have been supplemented b y others. A paper
in the Journal of Industrial Hygiene, April 1937, by Kaltreider, F r a y
and van Zile Hyde, shows similar results in cases of pulmonary fibrosis,
namely a progressive reduction in the total lung capacity a n d t h è vital
capacity and a relative increase in the residual air as the degree of fibrosis
increases.
I n minimal fibrosis (associated apparently with simple abnormal
increase in striation) there were no significant deviations from the normal.
I n typical nodular silicosis there was reduction in t o t a l a n d vital
capacity and the ratio of residual air t o total capacity was increased.
W h e n pulmonary fibrosis was complicated by emphysema V. C. was
markedly reduced and the ratio -¿T-TT increased. These changes are
proportional to the degree of dyspnoea. There is a reduction in ability
to expand the chest in pulmonary fibrosis.
I t is concluded t h a t individuals with minimal fibrosis are not disabled
and those with nodular fibrosis are partially so. W h e n t h e condition

164

SILICOSIS

is complicated b y pulmonary infection or cardiac failure the pulmonary
reserve is greatly reduced and total disability is t h e rule.
See, however, regarding t h e relationship of "abnormal increase in
striation" t o symptomatology : "A Clinical S t u d y of Silicosis", by
Moore and Kelly, Canadian Medical Association Jul., April 1937,
which in this a n d other respects is in close agreement with R a n d experience. The observation made in this contribution t h a t t h e kidney
is n o t affected by t h e presence of silica in the body and t h a t liver damage
from silica is negligible is in conformity with South African observations,
A s t u d y of "Lung Capacities of Pottery Workers suffering from
Silicosis", by Dr. Torsten Bruce (Trans. Cer. Soc, X X X V , Nov. 1936)
provides additional objective d a t a in relation t o the question of disablement.
The results of the general observations cited t h u s show general
agreement regarding t h e causes of respiratory disability in cases of
silicosis.
Estimation of Disability in Silicosis
I n the estimation of disability in silicotic cases for purposes of
compensation regard should be paid only t o disablement directly or
secondarily induced b y t h e lung condition.
I n t h e series of objective measurements carried out by the Bureau,
which were first cited, it was found t h a t great variations were shown
in individual cases in each group. I t was felt also t h a t , because of the
element of u n c e r t a i n t y which might be introduced by variations in the
subjective factor in tests of respiratory capacity, t h e application of
m a n y of these tests would be unrehable for t h e routine diagnosis of
respiratory disability, t h e more so as t h e y occupy too much time foiroutine application in the daily examination of large numbers of men.
The respiratory a n d circulatory reaction to a simple effort test, e.g.
"chair stepping" or "weight lifting", gives more unequivocal objective
results, and this is employed on occasion and should be considered.
B u t much reliance must be placed on the trained clinical judgment,
based on routine clinical examination and observation of the physical
condition.
I n South African practice, cases of silicosis are graded in three stages :
(1) The "ante-primary stage", i.e a condition presenting the "earliest
detectable specific physical signs of silicosis", in which the estimated
disability is either nil or does n o t exceed 25 per cent.
(2) The "primary stage", in which t h e estimated disability does not
exceed 50 per cent, a n d is not "serious a n d permanent".
(3) The "secondary stage" of grave or total incapacitation, with
an estimated disability from 60 to 100 per cent.
B y "disability" is meant disability for physical work.

I n addition to t h e consideration of disablement it might be desirable
for t h e Sub-Committee to consider whether agreement could be reached
regarding : .
(a) the medico-legal definition of t h e earliest detectable degree of
silicosis, a n d
(b) whether cases of uncomplicated silicosis should be permitted
to continue their occupation and, if so, on what conditions.

APPENDIX VI : NOTES ON SILICOSIS

165

8

(a) Notes on Requirements of Candidates at the Initial Examination at
the Medical Bureau, Johannesburg, in respect of Fitness for Underground Work (February 1937).
1. The general object of the initial examination of the Bureau is to
debar from underground work, in their own interest, candidates who
are in general not likely to be able to stand up satisfactorily to the
ordinary hazards of underground work as a permanent employment,
and in particular to debar those amongst that number who appear on
examination to have more than an average liability to contract tuberculosis or silicosis, or who are actually suffering from the former.
2. The general requirements which the Bureau considers essential are :
1. Good constitutional stamina, i.e. a robust constitution;
2. Good general physical conformation;
3. Good respiratory physique;
4. A reasonable grade of mental alertness and intelligence; and
5. A range of age (with few exceptions) of from 18 to 40 years.
3. In recent years an almost overwhelming number of examinees
of nearly all ages and almost all grades of physical condition have presented themselves. There were over 18,000 in 1936. The majority
are young adults of from 18 to 25, mainly country-bred, and many
of indifferent development. In 1936, the Bureau permanently rejected
as unsuitable for mining a little over one-third of the candidates. The
remaining two-thirds (approximately) were divided practically equally
between cases deferred for further examination and cases passed at
once. Those deferred for further examination were mostly youths
who were not yet fully developed, or who had defects which the Bureau
considered might be remedied by training and who, in either case, might
later on reach a suitable standard.
4. In respect of physical and constitutional make-up, human beings
have been distributed into three broad physical types : "asthenic",
"intermediate" or "muscular", and "pycnic", the intermediate class
numbering about one half, or more. The Bureau has found this typedivision of much practical value.
(1) The "asthenic" type is under standard weight, tall in proportion
to breadth, spare in face and body, and languid-eyed, flat-, narrowand shallow-chested and with poor posture, but usually with good
expansion.
Well-marked cases of this type have poor constitutional stamina,
are more prone to develop tuberculosis or may be in part the result
of childhood tuberculosis, and are quite unsuitable for underground
work. Individuals over 6 feet in height are frequently asthenic and,
unless well-developed and robust, are unsuitable.
(2) At the other extreme is the man who is unduly broad for his
height (called the "pycnic" type), typically shortish and short-necked, overweight, deep- and broad-chested, but with imperfect expansion, and with
a well-marked tendency to early and increasing obesity. "The man who
laughs with his belly" is the typical "pycnic". This type has good
resistance to tuberculosis, but tends to early bronchitis and emphysema
and later to cardiac and renal degeneration. Well-defined cases are
therefore unsuitable for mining. The flabby, obese "pycnic" is a
particularly bad subject.

166

SILICOSIS

(3) Between these extremes is the normally proportioned "intermediate" or "muscular" type, of good muscular and chest development.
These, provided their constitutional stamina is good, form the best
material. They range on either side towards the two former types.
The "intermediate to asthenic" type, rather underweight, small-boned
and spare but hardy, wiry and of good constitutional stamina, is also
suitable.
Although these physical types are readily distinguishable by simple
observation, the radiograph is a most valuable adjunct. The "asthenic"
type has a characteristic radiograph, with narrow heart-shadow and
prominent hilus and peribronchial markings, and frequently with
evidence of limited foci of old healed infection in the root glands or
in the lung fields, appearances which correlate closely with the physical
type.
5. Particular points to be noted in addition to the general features
mentioned above are :
1. Age, as already noted;
2. Height (in socks). As stated, men of over 6 feet are unsuitable,
unless robust and well-developed;
3. Weight (in socks and trousers only) should be up to standard
but in general not much more than 10 per cent, over or 5 per
cent, (or very exceptionally 10 per cent.) under standard;
4. Eyesight. D.V.6/24 in one eye, without glasses, and not less
than 6/60 in the other. Total blindness of one eye excludes.
Exception is allowed for technically trained men whose vision
is corrected by glasses and who habitually wear them;
5. Hearing of normal acuteness for ordinary conversation;
6. Posture, erect. Good chest conformation and expansion;
the latter not less than 2 % inches measured from the resting
position at the end of a normal expiration, up to full inspiration,
with the shoulders in normal position and the arms hanging
loosely at the sides. Mouth breathers are unsuitable;
7. Obvious disabling diseases — cardiac; renal; nervous (e.g.
epilepsy); arthritic, etc., exclude;
8. Obvious disabling physical defects (e.g. loss of an eye or of portion
of a limb, hernia, etc.), which do not permit of full muscular
work, also exclude;
9. Two photographs, preferably with bathing slip only, and taken
one in profile, the other from the front, should accompany the
application ;
10. A technically good radiograph of the chest should also be
supplied.
6. The physique of ill-developed youths with poor chest conformation and indifferent posture may be greatly improved by a course of
general physical' culture, with special emphasis on chest development
and the attainment of a habitual erect posture. But even a course
of physical training will not eliminate the evidence of a radical lack
of constitutional stamina, obvious in the languid-eyed, spare-faced
type. Misdirected physical culture, however, is apt to develop a
"muscle-bound" chest.
(6) Information can be submitted in regard to the preventive measures
against silicosis adopted on the Witwatersrand and their result in the
reduction of incidence and modification of type of the disease.

APPENDIX VII
SILICA IN BODY FLUIDS IN RELATION TO SILICOSIS
By D E . E. J. Knre

The discovery that silica is a natural constituent of body fluids, and
that its concentration in some of them can be influenced by the administration of silica, led to the hope that determinations of the amount
of silica in a body fluid might be of use in the study of silicosis and
perhaps even in the diagnosis of silicosis. Several years' experience
with the analysis of blood, urine, faeces and sputum, has led us to
believe that not a great deal of use can be made of the results of most of
these determinations. They furnish interesting data when correlated
with silica intake in man and experimental animals; and they have
led to the interesting conclusion that silicic acid is a physiological
substance having a normal metabolism like several other mineral substances, which are absorbed, circulated, built into the tissue substances,
released when they are degraded, and excreted in the urine and faeces.
The study of silica in the blood is a difficult one. There is so little
silica there that the most exacting microtechnique is required to gain
a measure of its concentration. The amount is something of the order
of a little less than 1 mg. per 100 c.c. of whole blood and results from
several laboratories—gravimetric, colorimetrie and spectrographic
data—are in agreement in this regard. Only one set of workers (Kraut
and Boehme) are at variance in their findings. By a micro-gravimetric
technique, which made no attempt to separate the silica and which
depended on the loss in weight of the "sulphate ash" of blood when
treated with hydrofluoric acid, these workers found the silica content
of normal blood to be from 10 to 30 mg. Si0 2 per 100 c.c. with an average
of 16. This figure is considerably in excess of the maximum solubility
of silica in blood. Miners with and without silicosis, and tuberculosis
patients, had considerably more. So great was the divergence of the
pathological from the normal figures that the probability of the
diagnostic use of the method was foreseen.
In our experience the use of this method gives completely erroneous
results, and the figures obtained have no relation either to the absolute
amount of silica in the blood, nor to the different silica intakes or exposures which the subjects may have suffered. The matter is .worth
stressing because to our knowledge this method of blood analysis is
being used or is being proposed for use in different parts of the world
with the hope of obtaining data of diagnostic value in the study of
industrial siliceous dust hazard disease.
A prolonged study of the effect of silicic acid administration—by
ingestion, by inhalation, and by intraperitoneal and even intravenous
inoculation, has convinced us that it is not possible appreciably to alter
the silica concentration of the blood. The renal mechanism for the
elimination of this substance is so efficient that silicic acid is transferred from the blood into the urine almost as fast as it gains access to

168

SILICOSIS

the general circulation. Very large amounts of silicic acid m a y be
found in t h e urine without any measurable increase in the blood.
The silica concentration of the urine varies enormously, and
seems t o depend o n the a m o u n t of absorption of silicic acid which
takes place either in the gut or in t h e lung. Ingestion of gelatinous
silica, or of foods or drink rich in silica, or of such preparations as the
magnesium trisilicate of Mutch (used as an anti-acid), m a y greatly
increase t h e urinary output. The inhalation of amorphous silica m a y
lead to a p r o m p t increase in the silicic acid of the urine, b u t inhalation
of powdered crystalline silica has led to only minor increases. Only
by giving massive doses in intra-tracheal insufflation experiments with
animals has it been possible to demonstrate a n excretion of extra silicic
acid. The average excretion of a group of gold miners was found
to be higher t h a n t h a t of a non-mining population, a n d Dr. Sayers
has shown a very interesting relation between t h e silica dust exposures
of miners, in his anthraco-silicosis studies, a n d the amount of silica in
their urines. B u t in our experience, in England and Wales, no definite
parallelism could be seen between excretion and exposure. Dietary
silica so readily causes variations in the -urinary silica., t h a t the food and
drink of silicotics would have t o be very rigidly controlled before any
significant changes due t o the dissolution of silica in the lungs could
be looked for. Two or three glasses of beer on a Saturday night might
well result in t h e excretion of more silica t h a n would result from a whole
day's exposure t o siliceous dust.
The s t u d y of t h e u r i n a r y output of silica m a y yield highly interesting
results in such well-controlled groups of investigation as those of Dr.
Sayers. Certainly such results are of great physiological interest,
and t h e y m a y sometimes be of pathological importance. B u t when
used in a h a p h a z a r d fashion to see if a n individual is excreting abnormal
amounts of silica, t h e investigation can have no value in the study of
silicosis.
The excretion of silica in the faeces has not been thoroughly studied.
Since only a small p a r t of the ingested silica is absorbed, it must vary
enormously with t h e dietary silica. Most of the silica experimentally
administered t o h u m a n s or animals is recovered in the faeces. B u t
there is also some evidence t h a t parenterally administered silica is
excreted in p a r t b y t h e gut, possibly in a similar way to calcium. As
far as I a m aware there are no d a t a on faecal excretion of silica in
relation t o inhalation.
The determination of the silica in the sputum is probably t h e best
analytical m e a n s of demonstrating t h a t silica dust has been breathed.
If such dust is inhaled, some of it, perhaps most of it, is inevitably
coughed u p . B u t this is not only a n immediate affair. We have found
by analysis a b n o r m a l amounts of silica in the sputa of old T.B.s years
after their exposure t o the dust h a d ceased. And Burke has demonst r a t e d , b y pétrographie a n d X-ray analysis, particles of quartz a n d other
minerals in t h e s p u t a of silicotic patients. This sort of determination
can be of use in proving a previous exposure t o dust. We have such d a t a
for several Welsh coal miners. B u t the demonstration of silica in the
s p u t u m can only be considered as evidence of exposure to dust., The
presence of siliceous d u s t in the lungs is not, per se, evidence of silicosis.
The determinations of the silica concentrations of body fluids m a y
be interesting, p e r h a p s even useful, adjuncts to the study of silicosis,
b u t t h e y cannot be considered t o have, of themselves, any value in t h é
diagnosis of t h e disease.

APPENDIX VIII
SOME INVESTIGATIONS ON DUST, WITH SPECIAL REFERENCE
TO THE THERMAL PRECIPITATOR
By H. S. PATTERSON, Physico-chemical Investigation Department,
Transvaal Chamber of Mines

The thermal precipitator is an instrument introduced by the British
Government some eighteen months ago for the accurate sampling of
dusts between certain size limits. This instrument was developed by
Whytlaw-Gray and Lomax, and was later perfected by Green and Watson.
By the courtesy of the British Government three of these instruments
were loaned to me about three and a half years ago, and have since
been used continuously in this department for sampling the dusts
found in the gold mines of the Witwatersrand.
A full description of the latest form of the instrument has been
given by Watson (Bulletin Inst. Min. and Met., London, No. 386,
November 1936), whilst details of the methods used for investigating
the dusts in the gold mines by use of this instrument have been described in a recent report (Patterson, The Thermal Precipitator, Report
on the Investigations of the Miners' Phthisis Prevention Committee,
page 74, 1937).
Mine Sampling with the Thermal Precipitator
In certain cases, for example in sampling the air issuing from a
dust filter or the air supplied by a pipe to some working place, the
thermal precipitator is used in a fixed position on a tripod, according
to the method described by Watson. In most cases it is desirable to
obtain a sample with the instrument in motion, as the dust conditions
in the air breathed by a mine worker are generally not uniform, but
vary as he moves about, so that the average dust concentration which
he breathes is often difficult to measure if the instrument is used in a
fixed position.
In order to obtain moving samples, a small holder has been devised
for carrying the thermal precipitator, battery, etc., which can be fastened
by means of a harness on to a man's back or, if needs be, his chest.
This carrier has been extensively used in obtaining samples in every
type of working place in the gold mines. Recently O. J. Go war of this
department has designed a more compact form of carrier in which the
instrument is actually built into the carrier, thus making the outfit
even more portable.
In obtaining moving samples, the man carrying the thermal precipitator moves about the place which is being sampled, whilst a second
man with him sees that the flow of water from the aspirator is uniform.
The water is collected in a small measuring cylinder which is emptied

170

SILICOSIS

from time to time. In this way a very good estimate can be formed
of the average conditions in a working place. In many places, e.g.
dead ends, in which comparatively dust-free air currents may be present, due to ventilation or the exhaust from the drill, it is exceedingly
difficult to obtain an idea of the average dust conditions if the instrument is used in a fixed position, but if used in the way described over a
reasonable period of time, there is likely to be little error in the estimation of the average dust concentration.
This method of sampling may also be used to obtain so-called occupational dust records. I n this case the thermal precipitator is started
off at the bottom of the shaft and the man carrying the instrument
keeps as far as possible throughout the working shift with the operative
whose dust risk is being investigated, until the operative again returns
to the bottom of the shaft. As above, a second man walks behind the
thermal precipitator to ensure uniformity of flow from the aspirator.
One record may be taken to cover a shift, or the dust may be collected
in a number of sections by changing the cover-glasses at intervals.
These occupational records are of great value in estimating the average
dust exposure of different operatives. It frequently happens that
the dust in some working place varies largely in concentration throughout
the shift, or that the operative moves about in fresh air from one working
place to another. By this method his average dust exposure may
easily be found and this is very difficult to estimate in any other way.
Method of Examination of Thermal Precipitator Records
The records as obtained from the thermal precipitator are often
visible to the naked eye as faint white lines across the middle of the
cover-glasses. The covers are first ignited to remove the carbon particles due to the acetylene lamps used in the mines. Before ignition
two small ink dots are made opposite to each other and at right angles
to the record, as after ignition the records may be almost invisible and
difficult to mount vertically. The covers are ignited on an electrically
heated plate for about a quarter of an hour. That this heating is effective
may be shown by placing a cover-glass specially coated with carbon on
the heater. In practice the heating is so great that the cover-glasses
on cooling have a glazed appearance.
The ignited cover-glasses are now mounted on 3 X 1 in. glass
microscope slips and in such a way that the records are perpendicular
to the length of the slips. To do this a shallow circular cell is made
on the slips using a microscope turntable and a solution of pure cellulose
acetate in acetone as medium. The cover-glass is then placed on the
cell and pegged down with the medium. This medium has the advantage
that it dries quickly and enables permanent preparations to be made
and examined almost immediately. J. B. Ebden in this department
suggested the use of thin paper rings about 0.25 mm. in thickness and
gummed on one side, instead of cellulose acetate rings. This procedure
appears to be perfectly satisfactory provided the covers are not pegged
to the ring while the gum is moist. In photographic work this method
has the advantage that the covers are mounted exactly parallel to the
slip, whilst the cellulose acetate ring may not be of uniform height
throughout.
The process of heating and mounting must be carried out in an
atmosphere of humidity not greater than about 45 to 50 per cent.
In damp weather the humidity can easily be reduced to this limit

APPENDIX VIII : SOME INVESTIGATIONS ON DUST

171

by fitting the evaporator unit of a refrigerator into a small room. If
the atmosphere is too damp, the covers after mounting show imperfections, due, presumably, to the condensation of moisture.
The slides are next examined microscopically using a microscope
with a mechanical stage and an achromatic substage condenser of aperture approximating to N.A. 1.0 into which is fitted a substage lamp
containing a blue screen. Three objectives are used, namely Beck
16 mm. and 4 mm. achromatics, and a Beck 2 mm. apochromatic oil
immersion N.A. 1.30, together with a Beck X 11 compensating eyepiece. The record is found by using the 16 mm. objective and a stop
beneath the substage condenser to produce dark field illumination.
The 4 mm. objective is then turned into position and the condenser
stop swung out, after which the 2 mm. objective is oiled on. To increase
the visibility of the finest particles the condenser is adjusted to give
about a third cone of light, i.e. to work at a N.A. of about 0.40.
The eyepiece of the microscope contains a special graticule described
by Patterson and Cawood [Trans. Far. Soc. 32, 1084, 1936). This
consists of a rectangle containing nine divisions above and below
which are 10 disks and circles respectively, of various sizes, which are
numbered from 1 up to 25. In counting and obtaining the numbers
of particles included in the various size ranges, the record is moved
from one side to the other perpendicular to its length across the rectangle
by means of the mechanical stage of the microscope and the particles
situated within a given width counted and matched as regards size
with the circles or disks. For an objective with an initial magnification at 160 mm. of about 90, the graticule was so devised that the
numbers on it multiplied by 2 and divided by 10 give in microns the
diameter of the particles equal to the graticule sizes. The particles
are thus divided into size groups of 0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 2.5, 3.0, 4.0
and 5.0 microns. Accordingly, knowing the width of the rectangle
in the graticule used, the length of the dust record and the number of
cubic centimetres of dust-laden air from which it was produced, the
number of particles per cubic centimetre and the size number relationship can be deduced.
Occasionally, in order to obtain greater accuracy on a thin deposit,
the record may be traversed a number of times. When a number of
records are being studied, the whole process of investigation, including
ignition, mounting, counting and size estimation, numerical calculations, curve plotting and a very detailed system of indexing and crossindexing takes, on the average, about three-quarters of an hour for one
complete double record.
If samples of suitable volume are taken, it is easy to obtain satisfactory results for numbers of particles per cubic centimetre ranging
from 100 to 10,000, and comparatively easy for numbers from about
10 to 100,000. Outside these limits, however, the measurements become the more difficult and the less accurate, the greater the divergence
from these limits.
It may be mentioned that before a deposit is counted, a blank
portion of the cover away from the record is studied in order to ensure
that there are no visible imperfections on the glass which might affect
the results. The cover-glasses also, before they are used in the thermal
precipitator, are treated with dilute hydrochloric acid and examined
microscopically to see that the surfaces are not imperfect for the conditions used.
It may be noticed that the records are not washed with hydrochloric acid either before or after ignition. This method has been

172

SILICOSIS

suggested as a means of removing acid soluble particles which may
be formed by the evaporation of spray. In my experience, mine water,
in addition to containing dissolved salts, such as calcium sulphate, etc.,
also contains suspended and dissolved siliceous material, and accordingly
spray particles may on evaporation give rise to siliceous particles coated
or mixed with the dissolved salts. Acid treatment would be very
likely to result in such particles being washed off the slides. Quite
apart from this, as far as my knowledge goes, the principle of washing
the record is open to grave doubt.
The, Visibility of Particles with Light Ground Illumination
The visibility of particles under the microscope depends upon a
number of factors. Amongst the most important of these are the
particular optical appliances used, the glare due to the illuminating
system, the contrast of the particles and the personal equation of the
observer.
Using the optical appliances mentioned above, it is easy to separate
the particles into groups down to a size of about 0.2 micron. In most
thermal precipitator records there are in addition a number of particles
which appear to be much smaller than 0.2 micron. These particles
we separate into a group of < 0.2 micron, but the line of demarcation
between these two sizes is not constant but varies somewhat with the
observer. Whether these very fine particles on the limit of visibility
are more properly classified as size 0.2 or 0.1 micron is unknown, but
experiments are being made by D. G. Beadle in this department in
order to endeavour to determine their size. Many of these particles
may be glared out by using a wide cone of light from the condenser,
but their visibility does not seem to be increased by using a cone
narrower than about one-third of the aperture of the objective.
The visibility of the very fine particles may also depend upon the
degree of contrast exhibited by the particular material, but it seems
unlikely that the small differences of refractive index between quartz
and ordinary silicates will render any particular siliceous material visible
down to appreciably smaller limits than another.
The number of these very fine particles which will be seen also
depends upon the observer. Many observers see about the same number,
but with some a considerable fraction of the finest particles is missed.
D. G. Beadle has recently made a detailed study of various of the
factors which may influence visual counting. Whilst any figures for
visual counting must depend upon the observer, his results certainly
indicate factors which may influence the counts. Generally speaking,
his data appear to indicate that for an observer with a high degree of
visual acuity, the exact conditions of observation may be of less importance than for an observer of lesser visual acuity.
On the Witwatersrand, in order to endeavour to keep to an even
standard of counting, men are selected, as far as possible, who count to about the same standard. As a rule, in my experience, young observers
obtain satisfactory counts, whilst older men are less likely to do so.
The Investigation „ of Thermal Precipitator Records by Dark Ground
Illumination
The counting of thermal precipitator records by dark ground illumination has two disadvantages. In the first place the size distribution of the particles cannot be obtained with even approximate accuracy.

APPENDIX V i l i : SOME INVESTIGATIONS ON DUST

173

In the second place the corners of large particles may show up as separate
entities, so that the total number of particles enumerated may be too
large; for fine dusts, however, this error does not appear likely to be of
important significance. On the other hand, using dark ground appliances, the particles should be visible down to a smaller size, so that
fine particles beyond the limit of vision, using light field illumination,
should be able to be enumerated.
A few preliminary comparative experiments have been made using
respectively the ordinary method of light field illumination which has
been described for the examination of thermal precipitator records
and dark ground illumination. These experiments have shown that
with high power dark ground illumination, there is a decided increase
in the number of particles counted. I t appears likely that by a combination of light ground and dark ground illumination, another size
group of particles will be able to be enumerated.
Photographie Investigations of Thermal Precipitator Records
Detailed investigations are being made by D. G. Beadle on the
photography of thermal precipitator records using light ground illumination. Numerous experiments have been made in order to determine
the best conditions for showing the maximum number of particles on
these records. It has already been found that about the same number
of particles can be photographed with a Beck 8 mm. apochromatic
objective N.A. 0.65, as can be seen visually by a good observer with the
2 mm. oil immersion N.A. 1.30. When photographs are taken of a
drilling dust with the 2 mm. N.A. 1.30, more than twice as many particles are photographed as can be seen visually.
Some Other Instruments for Investigating Dusts
. A comparison of the results with the konimeter and the thermal
precipitator indicates that the dust numbers found with the konimeter
are far too small where fine dusts are concerned. In the Report for
1936 of the Committee on Mine Air of the Transvaal Chamber of Mines,
there is a table giving the total number of particles per cubic centimetre
obtained using the konimeter by Inspectors of the Chamber of Mines
in various positions in a number of mines. The samples were treated
with acid and ignited to remove carbon, and were counted using dark
ground illumination.
The table is as follows :
TOTAL PARTICLES PER CUBIC CENTIMETRE

Development .
Machine stopes
H a m m e r stopes
Ore-bins
. . .
General a v e r a g e

.
.
.
.
.

63rd survey
October 1934
to May 1936

62nd survey
January 1933
to August 1934

1932

231
164
140
189
177

196
125
80
165
143

130
97
76
170
115

For comparison with these figures occupational dust numbers,
obtained in the Witwatersrand gold mines in 1936-1937, using the thermal

174

SILICOSIS

precipitator in the way described above, may be used. These figures
give the average number of dust particles per cubic centimetre breathed
by various workers throughout an 8-hour shift. Actually these thermal
precipitator occupational figures are not strictly comparable, but are
too low, as a developer for example does not spend all "his shift in a
development end, but may spend quite an appreciable fraction in air
which is but slightly contaminated. The comparison is however sufficiently striking. Thus the occupational figure for developers is 4,500
particles per cubic centimetre, and for machine stopers 1,300 particles
per cubic centimetre. In the neighbourhood of ore-bins the dust
numbers in my experience are often quite low, less than 100 per cubic
centimetre. I t may be mentioned that developers and stopers are
exposed to the fine dust produced by machine drills, whilst in the
neighbourhood of ore-bins the dust is generally much coarser.
The cause of this discrepancy has been investigated by taking simultaneous samples of mine dust with both the thermal precipitator and
konimeter and finding the numbers of particles of various sizes obtained
with both instruments. It appears that the same number of particles
of sizes 2 microns and greater are given by both instruments, but with
decrease of size below 2 microns, a decreasing fraction is found with
the konimeter until at a size of about % micron the fraction obtained
is very small.
The average velocity with which the air passes through the orifice of
an impinger may be very high, though in the case of the konimeter
it is only of the order of 50 to 100 metres per second (Miners' Phthisis
Prevention Committee, Final Report, p. 11, 1919), and experiments
have been made t o see whether siliceous particles are shattered on
impingement by this instrument. The preliminary results so far obtained indicate that for a dust consisting mainly of particles of sizes
between 1 and 10 microns, shattering certainly does take place, but
how far this factor is likely to influence the counts with an ordinary
mine dust is more doubtful. It is possible however, that shattering
might affect appreciably the figures obtained with some of the coarser
mine dusts.
In the Owens dust counter the velocity of impingement is much
higher than in the konimeter, and is said to be of the order of 200 metres
per second. Experiments which have been made to ascertain whether
shattering of siliceous particles occurs with this impinger, indicate
that under comparable conditions, this effect is much greater with
the Owens dust counter than with the konimeter, and appears to be
likely to increase the number of fine particles enumerated, particularly
if many coarse particles are present in the original dust.
An instrument in which the light scattered or reflected by a particulate cloud is used to measure dust concentrations is known as a tyndallmeter. When a beam of light is passed through a column of a particulate cloud, the amount of scattered and reflected light depends not
only upon the number of particles in unit volume, but also upon their
size and the material from which they are formed 1 . In the more
important dusts encountered in the gold mines, the sizes of the siliceous
particles differ for different dusts, and in addition a considerable fraction
of the particles generally consists of carbon, etc., and not of silica or
silicates. In these circumstances it is apparent that a tyndallmeter
would not give information of value regarding siliceous mine dusts.
1

WHYTLAW-GEAY, R. and PATTEBSON, H. S. : Smoke : A Study of Aerial

Disperse Systems. Edward Arnold and Co., London, 1932. See Chapter XIII.

APPENDIX Vili : SOME INVESTIGATIONS ON DUST

175

The Dusts Produced by Various Mining Processes
The actual number of dust particles produced by any mining process and also the size distribution of the particles depends upon the
method of formation of the dust, and the rock from which the dust
was formed.
Processes in which large amounts of energy are expended upon a
small amount of rock produce much fine dust, whilst generally speaking
for a given expenditure of energy, the larger the quantity of rock upon
which the energy is expended, the coarser the dust. The total number
of particles produced depends of course upon the total expenditure of
energy.
In drilling into rock with a machine drill, the energy of the blow
is largely concentrated in the cutting edges of the bit of the drill steel,
and accordingly a great deal of energy is expended upon a small amount of
rock. In these circumstances with a hard rock very fine dust is produced,
whilst with a soft rock the energy is expended upon a larger amount
of rock (the drilling rate is higher) and the dust obtained is not so fine.
On the other hand, if pieces of rock are thrown against each other as
in emptying rock into an ore-bin, the blow is, in general, distributed
over a much larger rock surface, and the dust produced is coarser, but
here again with a hard rock a finer type of dust is produced than with
a soft rock. As is well known, a piece of soft rock when dropped is much
more likely to break into large fragments than a piece of hard rock
which may merely be chipped.
In blasting, the dust is probably mainly produced in a similar way,
and tends, as far as percentage composition is concerned, not to be
of a very fine type, though finer than that produced in an ore-bin, as
the concentration of energy is greater. Very high dust numbers of all
grades of size are however obtained in blasting, as there is a large expenditure of energy over a short time period.
In the following table are set out experimentally determined figures
for the dust produced by tipping ore and by blasting. The actual
numbers of particles per cubic centimetre given are, in my experience,
characteristic respectively of those found inside an ore-bin and in a
development end soon after blasting.
Ore-bin dust
Size of
particles,
microns

<0.2
0.2
0.4
0.8
1.2
1,6
2.0
2.5
3.0
4.0
5.0 1
> 5.0 j

Blasting dust

Number
per
c.c.

Per
cent.
number

Number
per
c.c.

Per
cent.
number

182
315
285
262
225
214
164
141
134
146

8.0
13.8
12.6
11.5
9.9
9.4
7.2
6.2
5.9
6.4
9.1

13,020
19,140
17,170
13,240
9,080
6,460
7,110
6,560
6,460
5,690
5,470

11.9
17.5
15.7
12.1
8.3
5.9
6.5
6.0
5.9
5.2
5.0

207

176

SILICOSIS

I t will be seen that the number of the smallest particles in each
case is less than those of size 0.2 micron, and the numbers of the different
sizes from about one quarter to one micron are similar. In neither of
these processes therefore does the finest dust form a large percentage
of the total dust. In the case of the blasting dust, however, it forms
a decidedly greater percentage than in the case of ore-bin dust, since
the energy of impact of the rocks is very much greater in the case of
blasting. A rough calculation may also be made of the number of
particles per cubic centimetre produced per unit mass of rock moved in
the two processes. Such a calculation indicates that on the average
very much more dust is produced in blasting than in tipping ore. In
blasting in my experience the dust is similar to ore-bin dust, but is of
a coarser type and the number concentration is smaller.
An example of the type of dust formed when large amounts of
energy are expended upon a small amount of rock is furnished by drilling
dust. I n the following table are given figures for the dust produced
during wet drilling with a central water-feed machine drill in ä hard
quartzite type of rock, drinking water being used for the water-feed
of the machine.
WET DKILLING DUST
Size of
particles,
microns

<0.2
0.2
0.4
0.8
1.2
1.6
2.0
2.5
3.0
4.0
5.0

Number
per
c.c.

Per
cent.
number

1266
483
234
111
61
47
54
35
•28
28
14

53.6
20.4
9.9
4.7
2.6
2.0
2.3
1.5
1.2
1.2
0.6

This type of dust is entirely different from that due to tipping or
blasting. The percentage of the finest particles is very high and the
percentage number decreases rapidly with increase of size. With
softer rocks the percentage of the finest particles decreases considerably
and with a very soft rock may be quite small.
Dry drilling with a machine drill gives rise to dusts of entirely
different type which does not appear to depend, to the same extent as
in wet drilling, upon whether a hard or soft rock is being drilled. Actually very similar size distributions were obtained by drilling into
a quartzite and a very much softer schist. The percentage size distribution of a typical dry drilling dust is given below. I t will be seen that
the percentage of the finer particles present is small. The irregularities in the percentages of the largest particles are due to experimental
error.

APPENDIX VIII : SOME INVESTIGATIONS ON DUST

Af^fV

F I G . I. — Thermal precipitator as mounted for use
on Witwatersrand.

»

20
•

W

•

W

li

,„

B

*

l5

•

10 8 6 421
•

•

"

BS 10 8 6 4 2 1

Fie. 2. — Graticule.

SILICOSIS

«% « I "

'• #
<*•

*

^

»

¿»

9
««te

*

^

«
*

~

,.

0

• •
*

*
i «.

« %

« *

.

%
8

*

<9

«

*

9

%

'

T I G . 3. — Blasting ( x 1,000). < 3 microns, 82 per cent.; < 1 micron, 56 per cent.;
< 0.25 micron, 27 per cent.

*

J

% • y*

i%

•*

. t

•

%f

*5t,

•& ' < b ....

F r c 4. — Ore bin {x 1,000). < 8 microns, 78 per cent.; < 1 micron, 46 per cent.;
< 0.25 micron, 20 per cent.

APPENDIX

V I I I : SOME I N V E S T I G A T I O N S

ON

DUST

9 '* *

a* #

y ••*

#-V
"71'

" ¡f-Jf

I» .

• ft

a.;

h

b_

<* -

''

*
PIG.

5

•

©

.*

0

«

\

'

O

«

PIG.

6

P I G . 5-6. — Dry pom-pom machine. Chain formation — high electrification ( x 1,000).
< 3 microns, 73 per cent.; < 1 micron, 35 per cent.; < 0.25 micron, 10 per cent.

SILICOSIS

'Iß*

A

V

•••

ß
•%

9
» _
•

»

*

FIG. 7. — Wet drilling machine : "Jack hammer" ( x 1,000). < 3 microns, 06 per cent.;
•i 1 micron, 88 per cent.; < 0.25 micron, 74 per cent.

0%

i

Cr»

o

•ft- •

%j

,,'C?
'*-••«

¿G?-

#-.

#
FTG. 8. — Lung dust. Digested tissue ( x 1,000). < 3 microns, 92 per cent.; < 1 micron,
82 per cent.; < 0.25 micron, 66 per cent.

APPENDIX VIII : SOME INVESTIGATIONS ON DUST

177

DRY DRILLING DUST
Size of particles,
microns

<0.2
0.2
0.4
0.8
1.2
1.6
2.0
2.5
3.0
4.0
5.0

Per cent.
number

3.1
6.4
10.6
16.0
12.3
9.0
8.9
7.5
8.6
10.0
7.6

Microscopic examination of these dusts shows the presence of large
numbers of "chain"-like aggregates of particles. It is a characteristic
of highly electrified dusts that on coagulation they form structures
something like strings of beads, which are generally called "chains".
It appears probable that in dry drilling, owing to the friction of
the drill steel in the drill hole, the particles become highly electrified,
and that the chains which are formed contain a considerable fraction
of the fine particles produced.
Mineral Dusts Found in Outside Air
Dusts which are blown up by an air current tend to consist of large
particles rather than small. This is due to the cohesive forces acting
at any surface. If two surfaces are brought very close together, such
forces tend to make the surfaces stick together, but only become of
importance when the surfaces are very close to each other. With
large particles the surfaces are on the average too far from another
surface for cohesive forces to be appreciably effective, but with small
particles the cohesive force becomes the more effective the smaller
the particles. It must not, of course, be supposed that it is impossible
to stir up fine particles, but the finer the particles the more difficult
it is to do so.
On a still day the number of particles of a siliceous type suspended
in the air is very small. In the city of Johannesburg, even in a dust
storm, the number of such particles is only of the order of 10 per cubic
centimetre and about half of these are smaller than 2 microns. The
number of dust particles in a thick dust cloud stirred up by a car on
an untarred road is rather higher, about 200 per cubic centimetre,
but this dust again is of a coarse type.
In the neighbourhood of mine sand dumps in a thick dust cloud on
a windy day, the numbers are also of the order of 200 per cubic centimetre, but the dust is somewhat finer, the greater number of particles
being of a size of about one micron.
The Dust Produced by Machine Drills
In drilling into any substance it is necessary to arrange that the
disintegrated material is removed, otherwise little progress will be made
with the drilling. There are two methods in general use in rock drilling
for the purpose, both of which depend upon the use of a hollow drilling
steel for the machine; in the first case air is passed down the steel,
in the second water mixed with air.

178

SILICOSIS

When air alone is passed down the drill steel, the process, dry drilling,
leads to the dissemination into the atmosphere of large quantities of
visible dust.
As already mentioned, this dust is not of a very fine type, since,
owing to the electrification occurring in the hole, much of the finest
dust is probably removed by coagulation. A considerable amount
of fine dust however escapes into the air, in addition to large amounts
of coarse dust, and the method is certainly one to be avoided.
If water alone is passed down the drill steel, the chips of rock and
dust which are produced are removed as sludge. If it could be arranged
that the drill steel was always immersed in water, wherever it came into
contact with the rock, and in addition if any atomisation of the water
by the machine could be avoided, no dust should escape into the air.
When, however, air, in addition to water, passes down the drill
steel, much fine dust escapes, since there will be no tendency for the
dust to be removed by electrification, but it will remain suspended in
the bubbles of air in the water coming from the hole, and so pass out
into the atmosphere.
Atomisation of water by the machine will also lead on evaporation
to the formation of siliceous dust particles, if, as frequently happens
in the mining of siliceous rocks, the water used for the machine contains
suspended or dissolved silica.
In practice there is considerable difficulty in ensuring that water
alone passes down the drill steel, and that the machine does not atomise
the water. In ordinary wet drilling, a central water-feed machine
drill is used. With machines of this type, a stream of water is fed
through a small tube passing through a cylindrical, hole in the piston
of the machine to the top of the drilling steel, which also contains a
cylindrical hole through its length. Water is thereby fed through the
drilling steel to the bit end of the steel, and it might be supposed that any
large escape of dust would be prevented. Actually this is not the case
for the following reason. Owing to the reciprocating motion of the
drill steel, there is a gap between the end of the drilling steel and the
end of the tube through which water is fed to the steel. The compressed
air used to drive the machine reaches this gap either by passing between
the water tube and the piston, or along the sides of the piston; part
of this compressed air passes down the drilling steel, whilst part escapes
through the release ports of the machine and atomises some of the water
which also passes through them. The result is that the drilling does not
take place entirely under water ; in addition the atomised water particles
on evaporation give rise to particles of siliceous type, if the water used for
the machine contains appreciable quantities of dissolved or suspended
silica. The bubbles of compressed air which escape from the drill hole are
filled with suspended dust ; most of the larger particles settle on the sides
of the bubbles, and are carried off with the water, whilst many of the
finer particles escape into the air. If coagulation of the smaller particles to form chains occurred as in dry drilling, most of the dust would
be removed in this way, but in the presence of water, marked electrification probably does not take place, and a large amount of fine dust
escapes into the air. The central water-feed machine drill thus removes
the larger particles in a much greater proportion than the finer.
A number of experiments have been made by A.M. van Wijk of
this department, with a central water-feed machine drill, in order to
obtain an estimate of the dust which escapes from the hole, when
different pressures of compressed air are used for the machine, other
variables being kept as constant as possible. These tests were carried

APPENDIX V i l i : SOME INVESTIGATIONS ON DUST

179

out in an underground experimental chamber at Nourse Mines, which
has been very kindly made for me by Mr. Cr. Hildick-Smith, General
Manager of the mine. This experimental chamber consists of a length
of drive, which has been partitioned off by brattice to form a reasonably
airtight enclosure, with a door at each end. Downcast air is blown
in through a pipe at one end, across the face at which drilling is taking
place, and the dust samples are taken at the air exit pipe at the other end.
A rigged machine was used for the experiments. Horizontal holes
about four
feet long were drilled into a hard quartzite, the bit diameter
being 1 3 / 16 inch. The gap between the ends of the water tube and the
drill steel respectively was about % inch. Drinking water was used for the
feed of the machine, as this water has been found to give rise to relatively
few particles on atomisation, so that certainly most of the dust comes
from the hole. Other conditions being constant, the amount of dust
produced did not appear to vary greatly with the amount of water
used for the machine, between the limits tested, namely 1 % to 3 % pints
per minute actually passing down the steel during drilling. Slightly
less dust was produced with the greater water flow.
The numbers of particles per cubic centimetre were corrected to an
airflow through the chamber of 1,000 cubic feet per minute and to
continuous drilling. They were also corrected for any dust in the intake
air. At air pressures of 90 lbs. per square inch, corresponding to a
drilling rate of about 9 inches per minute, about 1,600 particles per
cubic centimetre were obtained. At air pressures of 50 lbs. per square
inch, the drilling rate was about 3 % inches per minute, and the corresponding dust number 700 particles per cubic centimetre. Between
these two limits the drilling rate and the amount of dust produced
appear to be proportional to the air pressure used for the machine.
Various modifications of central water-feed machine drills have
been designed with a view to preventing the escape of compressed air
down the drilling steel and the atomisation of water, but none of them
have been entirely satisfactory.
In machine drills in which water is fed into the side of the drilling
steel instead of the end, the escape of compressed air down the drilling
steel and the atomisation of water can be largely avoided. In machine
drills of this type, often called external water-feed machines, the cylindrical hole does not extend through the full length of the drilling steel,
but finishes a few inches from that end of the steel which fits into the
machine. The water is fed through a side hole leading into the axial
hole. In these machines the water is kept quite apart from the compressed air. Consequently compressed air does not pass down the drilling
steel and water is not atomised by compressed air, so that both the
escape of dust into the air and the atomisation of water are each largely
eliminated. These machines produce relatively little dust, but they
are still in an experimental stage, and are not yet suitable for ordinary
use in mines.
The Size Distribution of the Particles Found in Lung Dust
In 1913 McCrae extracted the siliceous dust from the lungs of deceased
silicotic miners by digesting the lung tissue with hydrochloric acid and
potassium chlorate until all the organic matter was destroyed. The
white deposit obtained was washed by a combination of centrifuging
and décantation, and dried at a low temperature. He found on microscopic examination that this dust contained 70 per cent, of particles

180

SILICOSIS

smaller in size than one micron, whilst particles of 5 microns were
relatively few and those of 10 practically absent.
The method of extraction used by McCrae has been criticised on the
ground that particles of silicates might be altered by the treatment,
and various other methods have been suggested. Recently Dr. F. W.
Simson, of the South African Institute of Medical Research, has prepared
lung dust by a method which should diminish the possibility of alteration of the mineral constituents. The minced lung tissue is digested
with papain for ten days in an incubator at a temperature of 54° F.
The opaque liquid is diluted, put through a Sharpies centrifuge, and
washed by a combination of centrifuging and décantation. To remove
the phosphates, it is then warmed for half an hour with hydrochloric
acid and washed in the same way as above.
Dr. F. W. Simson has kindly extracted for me a specimen of siliceous
dust from the lung of a deceased silicotic miner by this method in order
to enable the size distribution to be determined. The specimen was
in the form of a suspension which was diluted with distilled water.
A drop of the resulting suspension was put on to a microscope coverglass and allowed to evaporate. The cover was then ignited and the
deposit examined with a Beck 2 mm. apochromatic objective N.A.
1.30, using light field illumination. The eyepiece of the microscope
contained the graticule for the size distribution determinations previously
described. The measurements were made by D. G. Beadle and the
percentage distribution is given below.
LIWG
Size of particles,
microns

< 0.2
0.2
0.4
0.8
1.2
1.6
2.0
2.5
3.0
4.0
5.0 t
> 5.0 J

DUST
Per cent.
number

48.1
17.6
11.6
5.2
2.5
2.8
2.2
2.1
2.8
1.8
3 3
àà

Very few particles greater than 5 microns were present and these
are included with those of 5 microns. It will be seen that in general
agreement with the results of McCrae, a very large percentage of particles smaller than a size of 1 micron are present. Actually, owing
probably to finer dust having been inhaled and to the use of highly
perfect modern optical appliances, rather more fine particles are seen.
The percentage of fine particles down to the limit seen may be greater
than measured, as some coagulation of particles may have occurred,
leading to too small a percentage of fine particles.
The Particles Bemoved from Dust-laden Air by Breathing
A number of experiments have been made in order to determine
whether dust particles of various sizes are removed to an equal extent
during breathing, or whether the percentage removed is dependent
on size.
To investigate this question it is necessary to determine the size

APPENDIX v r n

: SOME INVESTIGATIONS ON DUST

181

and number of the particles in the inhaled and exhaled air. For these
experiments two thermal precipitators have been used, one of which
collects a sample of the inhaled air at the breathing level of the observer.
The other instrument is fixed into the bottom of a specially designed
air-tight box, through which the exhaled àir passes. The observer
wears a face piece, covering the nose and mouth, fitted with two valve3
which permit respectively the inhalation of outside air and the exhalation of air into the box.
Experiments with this apparatus have been carried out underground
by A. M. van Wijk, P. H. Kitto, H. H. Harvey, and O.J. Gowar of this
department, the dust being produced by a central water-feed machine
drill with drinking water for the water-feed. Whilst the samples were
being taken the observer moved about near the apparatus, his average
breathing rate being of the order of 12 to 16 litres per minute. The
results indicated that the particles of size 2.5 microns and greater
were practically completely removed from the air by breathing. For
particles smaller than 2 microns, the percentage removed decreased,
until for sizes of < 0.2 and 0.2 microns the removal was in the neighbourhood of 25 per cent.
It is clear therefore that the selective removal of particles of different
sizes is very marked. The size distribution of the dust in the inhaled
air was similar to that of the most important dusts ordinarily encountered
in these mines at the present day. The percentage size distribution
of the dusts removed in these experiments was approximately the same
as that of the lung dust already given; this obviously would not necessarily be the case, as if a coarse dust was inhaled very few fine particles
would be removed. The overall percentage removal of dust particles
by breathing was in the neighbourhood of 35 per cent.
The reason for the selective removal of particles of different sizes
must probably be sought in their physical properties. The most
important physical factors which might influence the removal of dust
from the inhaled air appear to be the momentum of the particles, their
rate of fall and their Brownian motion. Momentum does not appear
likely to have any appreciable effect on the deposition of particles in
the lungs, since the air velocities involved, as pointed out by Drinker
and Hatch {Industrial Dust, McGraw Hill Book Co., New York, 1936,
p. 22) are small and vary gradually. I n order to be able to form some
estimate of the effect of rate of fall and Brownian motion, the following table has been computed for silica particles of various sizes :
Diameter of
particles,
microna

Distance fallen
in one second,
microns

Brownian displacement
in one second,
microns

0.1
0.2
0.4
0.6
0.8
1.2
1.6
2.0
2.5
3.0
4.0
5.0

2.1
5.7
•17.4
35.1
59
88
189
327
500
720
1,250
1,940

36.3
21.1
13.1
10.1
8.5
7.5
5.9
5.1
4.6
4.1
3.5
3.1

182

SILICOSIS

Consider first the "values for the distance fallen in 1 second. The
table shows that, whereas for 5 micron particles, this distance is about
2 mm., for sizes of a micron and smaller it is less than 0.1 mm. The
effect of falling is however counterbalanced to some extent by the
turbulence of the air, and'on this account it appears likely that sedimentation will not bring about the deposition of the smaller particles
in the lungs effectively. In the case of the larger particles, however,
the rate of fall should be of considerable importance, since the distance
fallen in 1 second is large compared with the diameter of the smaller
air passages. Thus the alveoli when fully distended have a diameter
of about 0.5 mm., and it is accordingly very unlikely that particles of
a greater size than 2 or 3 microns will reach these passages before deposition. With decrease of size, there will be a greater chance of their
reaching the smaller passages, but sedimentation will then become less
effective in bringing about deposition. It would appear then, that owing
to sedimentation the larger particles will tend to be removed in the
larger air passages, including the nasal passages, but with decrease
of size, removal will take place in smaller passages, until the rate of fall
becomes too small to be of significance. A complete separation of
particles of various sizes will not, of course, be effected in this way, as
some of the smaller particles will always be deposited along with the
larger.
The action of Brownian motion is different, since it is not unidirectional, and is also independent of turbulence. Owing to molecular
bombardment, particles move on the average in all directions about
their initial position, and at the end of 1 second their average (actually
the root mean square) distance from their initial positions will be that
given in the table. I t is apparent then, that from the point of view
of Brownian motion, the smaller particles have a decidedly greater
chance of being caught in the lungs. Moreover, these are the particles
which will obtain entry into the smallest air passages, since they will
not be removed by sedimentation.
These two factors, rate of fall and Brownian motion, tend therefore
to act on particles of different sizes and to bring about deposition in
different positions. Sedimentation will cause the larger particles to be
deposited in the larger air passages, and Brownian motion the smaller
particles to be caught, probably mainly in the smallest air passages.
The table shows, however, that the smallest particles will not be as effectively removed owing to Brownian motion as are the larger due to their
rate of fall. Generally speaking, these properties appear to give a
satisfactory qualitative explanation of the experimental values of the
percentage removal of dust of different sizes during breathing.
The Particles Retained in the Lungs
It does not follow that if a particle is deposited in the air passages
it will remain there or be taken into the lungs, since several physiological mechanisms come into play. In the whole of the respiratory
passages which constitute the bronchial tree, down to the bronchioles,
there is a lining of ciliated cells, and these tend to propel particles
which are deposited on the wet mucous surfaces of the smaller air
passages into the larger air passages, where they are expelled with
the sputum. This action is aided by the wavelike contraction of
the musculature of the smaller air passages. In the terminal air
sacs there are no such mechanisms, and a certain number of the

APPENDIX v r n

: SOME INVESTIGATIONS ON DUST

183

particles which have entered the air sacs are taken up by free mobile
cells and carried into the interstitial and lymphoid tissues of the lung,
where in due course they may give rise to silicotic "nodules". I t must
be borne in mind that at each normal inspiration and expiration only
a small portion of the total air content of the lung is changed so that
the diffusion of successive doses of dust throughout the lung is a gradual
process and this circumstance should facilitate deposition. Accordingly the larger particles, which have been deposited in the larger air
passages, will tend to be removed with some of the smaller ones by the
expulsive mechanisms of the ciliated epithelium and musculature of
the respiratory passages. The particles which are deposited in the air
sacs and which will consist mainly of the smaller particles, are likely,
on the other hand, to remain permanently lodged in the lungs.
I t is impossible to estimate the exact effect that the selective removal
of various sizes of dust particles will have on the size distribution of
the dust ultimately remaining in the lungs, but it will obviously make
the larger particles less numerous. On this account it may well be
that the smaller removal of the finer particles in the air breathed may
be so counterbalanced by the subsequent removal from the lungs and
nose of the larger particles that, in effect, the size distribution of the
dust suspended in the inhaled air may be some measure of the size
distribution of the dust which ultimately remains in the lungs.
I wish to thank the Gold Producers' Committee of the Chamber of
Mines for permission to publish this communication.

APPENDIX I X
SILICOSIS IN SWITZERLAND
By D E . Fritz LAKG, Deputy of the Chief Medical Officer, Swiss National
Accident Insurance Fund

Until 1931 not much was heard of silicosis in Switzerland. The first
detailed study by a Swiss writer of the question of dust in factories was
the work of a factory inspector named Wegmann, which was published
in 1894. In 1906 Zangger drew attention to various cases of pneumoconiosis among sandstone grinders, who were at first treated for tuberculosis. In 1912, Zangger placed two new patients in a sanatorium.
Staub, examining these two cases in 1916, decided that they represented
a specific form of disease, and carried out clinical and radiological
work constituting a classical presentation of silicosis. Even at this
early date, Zangger indicated the most appropriate preventive measures : the replacement of sandstone by emery wheels, and provision
for adequate dust removal by exhaust and ventilation. At his express
demand, the undertaking compensated the victims—probably the
first two cases of silicosis in Europe to be placed to the charge of the
employer.Unfortunately, however, the precedent thus established was
forgotten, and apart from cases of chalicosis, found among stone
cutters, pneumoconiosis ceased to be diagnosed in Switzerland. In
the preparation of the 1911 Act and of later measures concerning
sickness and accident insurance, the inclusion of silicosis among insured
occupational diseases, or of the toxic agent which causes it in the
"schedule of toxic substances" (commonly referred to as the "list
of poisons") appended to the Act, was not even thought of.
Suddenly, in the course of 1931, numerous cases of silicosis occurred
among the sand blasters employed in a large steel works and among
the workers in the manufacture of scouring powders. The problem of
silicosis, however, was but little known in Switzerland at the time;
no one had any experience of the disease, and an enquiry among the
pathological institutions led to no positive result. The National Fund
was not in a position, therefore, to propose to the Government that
pneumoconiosis should be considered as an occupational disease
entitling the victim to the benefits fixed by the law. Nevertheless,
the Administrative Council of the Fund, on the proposal of the management, adopted the following resolution amending a decision which
it had taken in 1918 concerning certain occupational injuries for which
benefit was to be paid at discretion :
"(1) The decisions of 16 October 1918, concerning the discretionary
grant of benefit in cases of injury occurring in employment but not
considered as accidents under Section 68 of the Act, shall be amended
by the addition of the following words :

APPENDIX IX : SILICOSIS TN SWITZERLAND

185

"The management is authorised at its discretion to grant the benefits
provided for by the Insurance Act to insured persons suffering from
pneumoconiosis, this term being interpreted as applying solely to
specific diseases caused by dust inhaled in the course of work in undertakings subject to insurance, and not to common diseases which,
though not caused directly by such dust, may be aggravated by it.
"(2) The date of the coming into force of the authorisation contained
in paragraph 1 shall be 1 January 1932. Nevertheless, benefits may
also be granted, though only as from that date, to insured persons who
are at that date undergoing treatment for pneumoconiosis, suffering
from total invalidity, or totally and permanently incapacitated for
work, and to the surviving dependants of deceased insured persons,
on condition that the cessation of work in the former case, or death
in the latter case, did not occur before 1 January 1931."
In virtue of this decision the National Fund was thenceforth able
to grant the same benefits for silicosis as for other occupational diseases
covered by the Act. Unemployment benefit, the cost of medical
treatment, and invalidity or survivors' pensions, were granted in full
whenever necessary. The only right which the insured person did not
possess was that of instituting legal proceedings against decisions
regarding him taken by the Fund.
Finally, the experience gained in the course of the last few years,
the Swiss medical profession's improved knowledge of the disease,
and the place allotted to the subject in university teaching, enabled
the Government, at the request of the National Fund, to declare
silicosis an occupational disease subject to compulsory insurance.
The text of the Order on this subject, issued on 14 April 1938, is
as follows :
"The Swiss Federal Council,
"In pursuance of Sections 68 and 131 of the Act of 13 June 1911
relating to sickness and accident insurance :
"And in partial amendment of Order I of 25 March 1916 relating to
accident insurance,
"Hereby makes the following Order :
"Section 1. The following substance shall be added to the schedule,
contained in Section 47 of Order I relating to accident insurance,
of substances the production or use of which causes certain serious
diseases : silica or quartz (silicosis).
"Section 2, This Order shall come into force on 1 May 1938."
The advantages of insurance are not limited to certain trades, as
in Germany, for instance. Cases of silicosis give rise to compensation
in whatever undertaking they occur, provided only that it is an undertaking subject to insurance.
About 200 cases of silicosis have been notified since the National
Fund began to pay compensation in respect of this disease. They
occurred in the following occupations and industries : sand-blasting,
cleaning of castings (chipping and trimming), grinding on sandstone
wheels, construction of tunnels and galleries (mining), quarrying and
cutting of stone containing quartz, the pottery industry (china and
earthenware), and manufacture of scouring powders.
All the victims had spent periods of varying length in an atmosphere
containing quartz. We did not find a single case in which a pneumoconiotic complaint was not due to silicon dioxide. Our limited, and'

186

SILICOSIS

hence inconclusive, observations do not permit us to confirm the
opinion expressed by Jones, Policard, Seiter, Weiland, and others,
regarding the importance of the silicates, and in particular of sericite.
It is true that we have never had occasion to examine a worker exposed
to sericite dust entirely free from quartz. Our findings do agree,
however, with those of many foreign writers on the subject (in particular Böhme, Reichmann, Kcelsch, Cooke, Teleky, Jötten, Kästle,
and others) in showing that the degree of danger to the worker is in
proportion to the quartz content of the dust, allowance being made,
of course, for the influence of other factors such as size and distribution
of particles, the duration of exposure to the dust, the solubility of the
dust, predisposition and pre-existing morbid conditions such as faulty
nasal function, cardiac and pulmonary affections, etc.
The fundamental importance of the silica content of dust as the
cause of silicosis is proved beyond doubt by observations made in
Switzerland, in which the duration of exposure was considered concurrently with the percentage of silicon dioxide in the dust inhaled.
Wherever workers are subject to the influence of dust with a large
silica content, silicosis makes its appearance even within a few years.
This occurs, for instance, among sand-blasters, workers employed in
the manufacture of scouring powders (of which the quartz content
may be anything up to 90 per cent.), workers employed in certain
types of quarry (quartz content up to 70 per cent.), and miners working
on rock containing a high proportion of silica. On the other hand,
where workers are exposed to dust with a small percentage of quartz
only, the disease does not appear for 10, 20 or 30 years, or more. This
is the case with pottery workers (quartz content of dust from 10 to
20 per cent.), chippers, trimmers, polishers, stone cutters, certain
categories of miners, slate workers, etc.
The small number of cases in each occupation makes it impossible
to indicate for each the average duration of exposure to harmful
dust. But it may safety be asserted that the considerable difference
to be observed between the two groups of trades is not the result of
mere chance. The case of the shortest exposure recorded resulting
in death from silicosis is that of a sand-blaster, the duration of whose
exposure was one year. The autopsy confirmed the diagnosis.
Special attention has been given to the action of sericite upon
slate workers. In some quarries the slate contains from 5 to 20 per cent.
of quartz and 20 per cent, of carbonate of lime, the remainder being a
mineral with a sericite basis. If sericite were really the chief cause of
pneumoconiosis, as is sometimes affirmed, workers constantly exposed
to dust containing such a large proportion of this mineral would contract the disease fairly quickly. In fact, however, silicosis does not
occur among slate workers until they have been at work for 20 or
30 years. The length of this period can be accounted for, in our experience, only by the small proportion of silicon dioxide contained in
slate dust. Various writers (including Smith, Collis, and Jötten)
affirm that the addition of CaO or A1302 to dust containing SiOa would
render it less harmful. In support of this view they point to the long
period during which pottery workers, who are exposed to dust containing from 10 to 30 per cent, of Si0 2 and from 50 to 70 per cent.
of clay, remain unaffected. We do not wish to take a definite stand
on this question, in view of our limited experience, but we consider
that in this case, as in that of the slate worker, the very tardy appearance of the disease is due chiefly to the low quartz content of the
dust inhaled.

APPENDIX I X : SILICOSIS I N SWITZERLAND

187

We have found np crises of pneumoconiosis due to dust without
any quartz content. Cement workers' lungs sometimes show on their
surface a more marked network, of honeycomb structure, probably
a sign of slight sclerosis. We have never come across serious anthracotic
changes like those described by Lochtkemper and Hollemann, for
example; nor have we encountered a case of pure siderosis, or of pneumoconiosis due to aluminium, so. much discussed in recent times
(Sutherland, Meiklejohn, Price, Doese, Filipo). Finally, we have not
encountered a single case of silicosis among workers occupied, even
for tens of years, in the use of emery wheels, which contain only one
or two per cent, of silicon dioxide.
In the light of this experience we have adopted a policy of studying
the occupational case history of the insured person—a policy which
has given good results. This practice is not so obvious, under the conditions to which we are subject, as it seems at first sight. Several
cases described to us by well-known doctors as clear cases of silicosis
or silico-tuberculosis have been found, on further examination, to
show no sign whatever of silicosis. The radiographs of these cases
indicated a marked strengthening of the pulmonary network, which
was strewn with mottling, and a very pronounced hilus ; and the functioning of the heart and lungs was very reduced. The workers in question (emery-wheel men and smelters) had worked for many years in
an atmosphere filled with dust containing metal and quartz particles,
the latter in very small quantities. Some cases were shown by postmortem examination to be cases of pure tuberculosis, and the others
included one case of bronchial carcinoma and one of lymphangioendothelioma of the hilus glands. The radiological changes recorded
indicated in the case of the hilus tumour serious anthracosis with a
hardening of the connective cells due to congestive disturbances of the
lymphatic system, and in the case of the bronchial carcinoma to a
proliferation of dust cells in a sidero-anthracotic subject.
On the question of the relation between silicosis and tuberculosis,
opinions are very divided. Our own experience, and more particularly
the results obtained by Uehlinger, in his anatomo-pathological studies
at the Anatomo-Pathological Institute of Zurich, lead us to the conclusion that silicosis of medium gravity (second degree), and especially
serious silicosis (third degree), may favour the outbreak of tuberculosis.
Silicosis may also hasten the development of a pre-existing tubercular
condition, or revive and stimulate pre-existing foci, particularly postprimary tubercular foci. In this respect pneumoconiosis due to mixed
dusts is probably more dangerous than pure silicosis. Uehlinger was
able, in one of these cases, to prove irrefutably that silicosis due to
mixed dusts, though so slight that it could not even be diagnosed
clinically, had revived former post-primary foci by means of very
active dust granulomas. The development of superimposed tuberculosis
appears to vary according to the worker's occupation.
It is to-day admitted that cases of pure silicosis, in which postmortem examination revealed no trace at all of tuberculosis, have
undoubtedly occurred. Nearly half the autopsies carried out in Switzerland have revealed an entire absence of tuberculosis, even in cases
of serious silicosis. These facts contradict, among other views, those
of the French doctors, Croizier, Martin, and Pohcard, who even in
their most recent writings (1938) throw doubt upon the existence of
pure silicosis.
We have no special findings to record as regards the radiological
and clinical symptoms of silicosis and silico-tuberculosis.
Our

188

SILICOSIS

experience in this field confirms the statements—for the most part
concordant—of the numerous foreign writers on the subject. It may
be remarked, however, that the contrast which, as is well known, is
often found between the results of clinical and radiological examination is in many cases very surprising indeed. Generally, while the
radiograph reveals numerous opaque spots representing nodular
formations or "lead shot" lungs (Schrotkornlunge), clinical examination reveals little or no change in the normal condition of the lungs.
We have recently begun to look for silicon dioxide in the blood, but
research along this line has not yet led to results in which confidence
can be placed for the purposes of differential diagnosis. The examination of series of cases is, as yet, but little practised.
On the other hand, our radiographic examinations of silicosis
cases by occupations do permit us, within the limits imposed by the
range of the data at our disposal, to point to special symptoms which
are not connected with the several stages of the disease. This question
is still the subject of considerable discussion among writers. In the
case of sand-blasters, for instance, the mottling of the lungs has been
the symptom most in evidence; the mottling, which forms the "snowstorm" (Schneegestöberlunge) design, is finer than in any other group
of cases examined, and rarely contains nodules even in fatal cases.
The large nodular formations are more common among miners than
among workers of any other occupation. The lungs of stone cutters and
slate workers suffering from pneumoconiosis are generally marked with
fairly large mottling and clearly outlined shadows, the characteristics
of the "lead shot" lungs. Pneumoconiosis due to mixed dusts produces
more blurred shadows, probably as a consequence of the cellular
proliferations occurring in this type of silicosis.
We have encountered pneumothorax formations on several, occasions, and in three cases autopsy has revealed traction-diverticulum
of the oesophagus—a phenomenon already mentioned by various
authors as a complication of silicosis.
To sum up, it may be said that we can guard effectively against
error in this field only by a critical approach to occupational anamnesis,
conditions of work, the history of the disease, and all clinical and
radiological findings.
In about one-third of the cases the disease does not make its appearance until after the cessation of the work which has caused it, the
interval varying between a few months and several years. We have
known many cases in which silicosis developed even after the
worker had changed his occupation.
Here, too the observations
made in Switzerland confirm those made on a far larger scale in other
countries.
Little need be said about the therapy of silicosis. A few months'
rest at a certain altitude, or even in the atmosphere to which the
patient is accustomed, has resulted in effective arrest in cases of silicosis of slight or medium gravity with and without tuberculosis, and
has made possible the worker's return to partial or full employment
for a certain length of time. Bronchitis and circulatory complaints
may be treated in the ordinary way. We have no experience of the
dietetic treatment proposed by Wiesinger, who regards the whole
problem as one of acidosis, or of the thyroxin treatment recommended
by Moore and Kelly.
The aspect of the evolution of the disease that has struck us most
forcibly is the amount of variation found between the different trades.
We have noticed that in the occupations in which the interval between

APPENDIX IX : SILICOSIS IN SWITZERLAND

189

exposure to dust and the appearance of silicosis is shortest (sandblasting, the manufacture of scouring powders, and certain types of
mining and quarrying), the development of the complaint is generally
more rapid and the mortality rate higher than in occupations in which
the disease is slower in revealing itself (stone cutting, polishing, pottery,
and slate work). Quite a number of cases in which the disease was
detected at the outset have remained stationary for long periods,
and usually the patients have recovered their working capacity in
full or in part.
As far as possible, every patient is placed for a few days in a
hospital in Lucerne for examination. In nearly one-third of the cases
notified the silicosis diagnosis has had to be abandoned (according
to Teleky only one-fifth of the cases notified in Germany from 1929
to 1932 were admitted to insurance benefit). As soon as a case of
silicosis coming from an undertaking subject to insurance is recognised,
the National Fund must grant the victim all the statutory benefits.
There are no clauses limiting the grant of benefits to serious cases, or
imposing other restrictions.
Section 68 of the Act, however, prescribes that benefit shall be
payable only if the disease is due exclusively or essentially to the action
of one of the substances included in the schedule (or "list of poisons")
drawn up by the Federal Council. Thus when a worker is suffering
from uncompensated cardiac deficiency, or from serious pulmonary
tuberculosis and contracts also slight silicosis which would alone
have caused little invalidity, if any, the Fund does not grant him
benefit. Cases of this kind are very rare.
Tuberculosis following upon silicosis, on the other hand, entitles
a patient to full benefit, the former being regarded in such cases as a
regular consequence of the latter.
'
When pre-existing tuberculosis is aggravated by silicosis, Section 91
of the Act authorises the Fund to reduce its cash benefit but not the
expenditure on curative treatment.
The Section in question is as
follows :
" The cash benefit granted by the National Fund shall be reduced
proportionally if the sickness, invalidity, or death is only in part the
result of an accident (in this case an occupational disease) covered by
insurance."
As may well be imagined, the application of this provision sometimes presents serious difficulties. The decision involved is particularly
difficult in the case of aged persons (stone-cutters, for instance), for the
determination of the part of the incapacity due to each factor—
silicosis, arteriosclerosis, normal physical wear and tear, etc.—is usually
a matter of personal opinion.
As long as the condition of the patient requires treatment, the
National Fund pays the costs of the cure and sickness benefit. If no
further improvement is to be expected from the continuation of the
treatment, and permanent invalidity seems probable, the insured
person receives an invalidity pension under Section 76 of the Act.
The invalidity pension for silicosis varies between 25 per cent, of wages
and full wages. In case of death the statutory pension is paid to the
surviving dependants.
Finally, the National Fund has adopted certain measures to
combat silicosis. The inspectors of the accident prevention service

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SILICOSIS

visit regularly all undertakings in which the workers are exposed
to the risk of contracting silicosis, whether cases have actually occurred
in them or not. The heads of the undertakings are instructed to have
all workers who are to be employed at work involving such exposure
examined clinically and radiologically before engagement. Further,
workers who are employed in an atmosphere containing harmful dust
are subject to periodic medical examination. In order to ensure
a certain uniformity in these preventive examinations, the doctors
responsible for them are given a standard form to be adhered to in
the presentation of their findings. We examine carefully the reports
and radiographs, investigating doubtful cases separately and deciding,
for the others, the type of work to which the subject is to be put in
future and the date of his next medical examination. A pamphlet
setting forth the problem of silicosis in simple and clear terms has been
issued and widely distributed with a view to informing heads of undertakings and workers' organisations on the subject. The cost of the
preventive medical examinations is levied on the undertakings under
Section 65, subsections (1) and (2), of the Act, which are worded as
follows :
"In all undertakings specified under Section 60, the employer or his
representative shall adopt, with a view to preventing sickness and
accidents, all measures which experience has shown to be necessary
and which circumstances and the progress of science make practicable.
"The National Fund may order the adoption of any useful measure
after consulting the parties concerned; the said parties may appeal
against such orders to the Federal Council within 20 days."
I t is in virtue of these provisions that the National Fund issues
its orders regarding technical preventive measures. We cannot here go
into the subject of these measures. I t may be said, however, that in
undertakings containing departments in which the danger is great
(sand-blasting shops and certain types of quarries) the workers exposed
to the danger are transferred after a certain time to a department where
there is less or no risk, and that in general the systematic application
of medical and technical prophylactic measures in combination has
resulted in a decline of the disease. The problem of effective protection
for miners and stone-cutters, however, remains to be solved.

APPENDIX X
EARLY DIAGNOSIS OF SILICOSIS
By D R . R. R. SAYBKS

In the present state of our knowledge, the possibility of an early
diagnosis of silicosis rests on the experience of the examining clinician
and co-operating roentgenologist, the completeness of the occupational
and medical history-taking, and on the care and skill with which the
various findings are correlated and interpreted.
The Committee on the Prevention of Silicosis through Medical Control,
which reported to the Conference on Silicosis convened in Washington
early in 1937 at the invitation of the Secretary of Labor, has summarised
the symptoms associated with silicosis both in the presence and in the
absence of infection. But the Committee was careful to point out that
each of these symptoms occurs also in the course of other diseases.
Subjective Symptoms
Shortness of breath. —- The complaint most frequently mentioned
by those having silicosis is shortness of breath. This varies with the
degree of development of the condition, from that exhibited only
following exertion, to marked dyspnoea upon the slightest exertion,
or shortness of breath even when at rest. The shortness of breath
noted in silicotics presents one peculiarity, in that unlike the shortness
òf breath shown in other pathological changes, it is seldom accompanied
by orthopnoea, the individual being no more short of breath lying down
than when in an upright position. This peculiarity is not noted, however,
when silicosis is complicated by cardiac affections or true asthma.
Silicosis
Noted as a rule only after sudden or extra
exertion. Seldom so marked as to interfere
with routine duties. However, in cases
with extensive pulmonary fibrosis, this
reaction may limit the individual's activities.

Silicosis with Infection
If complicating infection is not widespread,
may be no more marked than in cases of
simple silicosis, but as infection and fibrosis
increase, it becomes a disabling factor.

Cough. — Many individuals with silicosis complain of a troublesome
cough. This cough is different from that resulting from simple irritation due to inhaled dust particles. The latter clears up upon removal
from exposure. When present, it is more pronounced in the morning or
upon beginning work after a rest period.
Silicosis
The typical silicotic cough is dry and nonproductive. I t usually parallels shortness
of breath in degree, and may be a contributing
factor toward disability.

Silicosis with Infectimi
If the silicotic condition is complicated by
infection, the cough usually becomes more
troublesome and is productive in type. The
sputum raised varies from thick, tenacious,
mucous material to that of a foul, purulent

192

SILICOSIS
or puro-haemorrhagic consistency. Microscopic examination and animal inoculation
frequently reveal tubercle bacilli or, in some
cases, organisms of the fusiform spirochaetal
group, i n advanced cases, productive coughing attacks are common following periods
of rest and are often of such severity that they
leave the individual exhausted,

Chest pain. — This subjective symptom is probably complained of
by more than half of the individuals suffering from silicosis. It varies
from the feeling of tightness in the chest to the sharp, excruciating
pain typical of pleurisy. (Since chest pain is so commonly offered as a
physical complaint due to various common conditions, it cannot be
stressed as especially characteristic of silicosis.)
Silicosi»
A late symptom in cases of simple silicosis.
Then seldom more than a sense of tightness
or feeling of substernal pressure.

Silicosis vpth Infection
Pleuritic pain is suggestive of a complicating infection. I t is increased by exercise,
in coughing, and may be a most distressing
factor in advanced cases with extensive.
infection as a complication.

Haemoptysis. — True haemoptysis is seldom offered as a complaint
by the silicotic. Frequently, however, the sputum may appear bloodstreaked following a severe coughing attack produced by excessive dust
exposure or exertion. This complication must always be considered
as suggesting tuberculosis as a complication.
Silicosis
Occasional blood-streaked sputum, not
noted regularly. May result from alveolar
rupture following sudden exertion in advanced
cases.

Silicosis with Infection
Often the first sign of tuberculous complies
ation. Usually present upon the development
of the pneumothorax. May occasionally
be excessive, in the nature of pulmonary
haemorrhage, if vomicae are present.

General complaints. — Weakness, loss of weight, digestive disturbances, night sweats, insomnia, dizziness and oedema of the extremities
are not characteristic of silicosis.
Silicosis
Where extensive fibrosis has developed,
individuals with simple silicosis sometimes
complain of tiring due to their shortness of
breath.

Silicosis with Infection
The chief significance of this group of
symptoms is that they may usually he
considered as suggesting that the silicotic
• condition is complicated by pulmonary
infection. However, they are seldom mentioned early in cases of slight or moderate
infection. Persistence and increase of these
symptoms depend upon the progress of the
infection complicating silicosis.

Objective Symptoms
General appearance. — Usually there are no changes in the general
appearance which may be considered indicative of silicosis per se.
Such changes as are manifested are due to complicating conditions,
except those which may be evident in individuals with extensive pulmonary fibrosis.
Silicosis
°
Early, these workers appear strong and
robust, and unless they are closely observed,
nothing about their general appearance
suggests the presence of a pulmonary fibrosis.
I n fact, it is common to find the individual
with limited pulmonary fibrosis due to silicosis showing a slight increase in weight,
possibly due to the fact that he is less active.
As the case progresses, respiratory embarrassment is noticeable and there is a general
loss of muscle tone.

Silicosis with Infection
If the condition is complicated by pulmonary infection, the appearance sooner-or later
becomes that of chronic phthisis. The bony
landmarks of the thorax become prominent,
and there is an increase in the anteriorposterior diameter of the chest, possible hypertrophy of the accessory respiratory muscles
of the chest, and in the final stages, retraction
of the supra- and infra-clavicular spaces.
Cyanosis and clubbing of the fingers are not
prominent signs, except in those cases of
long standing, with cardiac disturbances and
pulmonary infection evident.

APPENDIX X : EARLY DIAGNOSIS OF SILICOSIS

193

Chest expansion. — Decrease in the expansion of the chest may
frequently be demonstrated, especially in those cases with extensive
pulmonary fibrosis. This, of course, is more readily observed in the
advanced cases.
Silicosi» with Infection
Decrease in expansion may not be noted
in early silicosis with slight active infection
but, as the condition progresses, a definite
decrease is readily observed and this may
progress to a point where chest expansion may
only be demonstrated by the most careful
observation. When infection is more pronounced in one area of the lung, expansion
may be more markedly decreased on the affected side, particularly if
there is pleural involvement. Pleuritic pain may account for loss of
expansion, more so than fibrosis or decrease in lung function in cases
where any disability is due predominantly to the complicating infection.

Silicosis
While in early cases it is usually not possible
to demonstrate any decrease, by the time the
condition has progressed to a degree where
the activities are restricted, chest expansion
may be lessened by 20 to 30 per cent. The
decrease In expansion is equal on both sides.

Prolonged expiration. — In most cases, decreased chest expansion
is preceded and later accompanied by a definite change in respiratory
rhythm. Close observation may reveal that even at rest there is a
distinct tendency to prolongation of the expiratory phase. As silicosis advances, this becomes more marked and in the later stages the
time required for expiration may be two or three times that required
for inspiration. When this is the case, the individual, following exercise,
will be found to breathe less rapidly than the normal person under
similar conditions, for the simple reason that the lungs cannot be emptied
rapidly enough to permit more rapid respiration and, although the respiratory rate is not so rapid, an increase in the rate will persist for a
correspondingly longer period following exercise.
Silicosis
Early in the development of simple silicosis,
prolonged expiration may only be evident
following exertion. Later the degree of
this change usually parallels the increase In
pulmonary fibrosis..

Silicosi» with Infection
In cases of early silicosis with slight infection, prolongation of the expiratory phase may
be no greater than in the case of simple silicosis.
As the condition progresses and if the silicotic fibrosis increases, this phenomenon is
marked and may simulate characteristic
asthmatic respiratory rhythm; but corresponding increase in pulse rate will accompany
a persistent high respiratory rate following
exercise, evidenced by silicotics with active
infection.

Signs revealed by palpation of the chest. Except in the late stages
of silicosis complicated by infection, little is revealed by palpation of
the chest. However, in those cases where there is a measurable decrease
in expansion, one will note by palpation that, upon forced inspiration;
the anterior chest wall tends to be lifted forward by the accessory
muscles of respiration.
Silicosis
No change noted until extensive fibrosis
has taken place, at which time, by palpation,
the examining physician is more conscious
of decreased chest expansion and slight
increase in tactile fremitus generally.

Silicosi» with Infection
Slight or beginning complicating infections
do not influence the findings upon palpation,
but when the infection is extensive, tactile
fremitus is increased and occasionally friction
rubs may be elicited by palpation. Extensive
thickening of the pleura or pneumothorax
may result in a decrease or absence of tactile
fremitus over affected areas.

Percussion. — While there is generally a definite impairment of
the percussion noted, this is usually revealed over all areas of the chest
and, unless one is particularly observant, the change will not be noted.
With care, one is usually able to demonstrate a decrease in diaphragmatic excursion by palpation.
7

194

SILICOSIS

Silicosis
Since impairment in resonance is general
over all lung areas, It is difficult to demonstrate until advanced fibrotlc changes have
developed. Decrease in diaphragmatic excursion may be revealed by percussion
methods.

Silicosis with Infection
Increase in loss of normal resonance. When
massive areas of fibrosis have developed, this
impairment may amount to absolute dullness
over areas involved. Amphoric resonance
may be elicited over areas of pneumothorax.
Decreased diaphragmatic excursion may
readily be shown by percussion methods
in cases of advanced silicosis with Infection.

Findings wpon auscultation. — Definite changes can usually be
noted in the character of the breath sounds. There is usually a decrease
in intensity. The characteristic prolongation of the expiratory phase
of respiration is readily noted by auscultation.
Silicosis
Decrease in breath sounds is general and
more marked as the condition progresses.
Subcrepitant rales, which clear up following
coughing, are occasionally heard.

Silicosis with Infection
Usually heard, some of the following:
Persistent post-tussic crepitant and subcrepitant rales, coarse rhonchi associated
with productive coughing, wheezing or
musical rales increased by exertion and
coughing, amphoric breath sounds over
cavities and areas of pneumothorax. Pleural
friction rubs are occasionally heard.

Roentgenological Findings
Definite lung changes can be demonstrated roentgenologically
in all cases of silicosis. Although it is not possible to make a diagnosis
of silicosis based upon X-ray findings alone, nevertheless the chest
conditions revealed by fluoroscopic and roentgenogram examinations
of the chest furnish the most important evidence of pulmonary fibrosis
which we may obtain.
Fluoroscopic examinations. — Fluoroscopic examination is of distinct
advantage. Particularly is it possible, by such procedure, to study
diaphragmatic movements and note the presence of areas of increased
density which, of course, are better shown in detail by film examination.
(The general information regarding pulmonary density furnished by
fluoroscopic examination is of benefit in deciding the particular technique
to use to secure the best roentgenogram of the lungs.)
Silicosis
Enlargement of the hilar shadow, generalised diffuse increase in density of the lung
fields noted. Later the hilar shadow may
appear more dense and less increased in size,
and actual modulation over the lung fields
may be shown by clear fluoroscopic examination. A decrease in diaphragmatic excursion
with flattening of the domes, can often be
shown by fluoroscopic methods. Beginning
emphysematous changes will usually be
noticed in cases evidencing reactions contributing toward disability.

Silicosis with Infection
Irregular increase in density of the lung
fields, coalescing of nodular areas and massive
conglomerate dense areas may be demonstrated. The increase in anterior-posterior
diameter ol the chest may be noted, thus, by
lateral examination, revealing the increased
depth of the substernal and retrocardiac
spaces. Cavity formation and arcas of
pneumothorax may be likewise interpreted.
Adhesions between the visceral and parietal
pleurae and mediastinal distortion may be
shown. Emphysematous changes, particularly at the bases, and adhesive bands between
the diaphragmatic and visceral pleurae are
commonly shown.

Film examination. — Before the development of specific nodulation,
the film of the presilicotic subject may exhibit excessive linear markings,
but since other factors such as age, circulatory disturbances and infection
may produce similar changes, such findings cannot be considered pathognomonic of silicosis. Early in the development of these nodular
densities, the field presents a diffuse grainy appearance (beading along
the trunk shadows) but it is usually agreed that definite roentgeno-

APPENDIX X : EARLY DIAGNOSIS OP SILICOSIS

195

logical evidence of silicosis is not established until nodular shadows
are present. Usually, as the condition progresses, nodular fibrosis
development is revealed in the X-ray film as a coalescing of the smaller
nodular shadows. Good films will usually reveal at this time scattered
areas of increased radiographic transparency, denoting emphysema.
Roentgenograms of cases of silicosis complicated by infection vary
SCHEME OP X - R A Y INTEBPBETATIONS

Lung Field Appearance
X-Ray Interpretation
Usual
fibrosis

Commencing
generalised
fibrosis

. Generalised
fibrosis 3+
Conglomerate phase

(1)
(2)
(3)
(4)
(5)
(6)
(7)

Normal lung markings or first-degree exaggeration of linear pulmonic markings.
Second-degree exaggeration of linear pulmonic markings, with or without beading.
First-degree diffuse ground glass or grainy appearance, not obliterating linear markings.
Second-degree diffuse ground glass or grainy appearance, obliterating linear markings.
First-degree disseminated nodules up to size of miliary tubercles.
Second-degree disseminated nodules exceeding 1 mm. in size. Emphysema.
Conglomerate with any of the above manifestations (e.g. nodulo-conglomerate). Moderate
or marked emphysema usually present.
Asymmetrical distribution of shadows, unilateral Increase of markings, and less discrete or
coalescing shadows (mottling), imply complicating pulmonary infection and modify any of the
phases illustrated above.

according to the silicotic development and the severity and extent of
the infective process. Where extensive pulmonary infection has
antedated the pulmonary fibrosis, an asymmetrical distribution of
shadows will be noted. Evidence of pleural adhesions, mediastinal
distortion, and localised areas of pneumothorax, calcification and cavity
formation may also be demonstrated by the X-ray film. When emphysema becomes marked, it is usually more prominent at the bases.
Occasionally, large pleural blebs may be demonstrated.

196

SILICOSIS

The roentgenological appearances due t o changes caused by silicosis
are graphically illustrated on page 195, a n d descriptive terms are
suggested.
W h e n these terms are used, the final X - r a y report expresses t h e
extent and character of t h e shadows observed a n d while such information,
taken alone, is inadequate for a diagnosis of silicosis, it could prove
highly significant in a case with a history of exposure to dust, negative
medical history for respiratory infection and absence of clinical evidence
of p u l m o n a r y infection. T h e figure shows t h e overlapping of the
linear, granular, nodular a n d conglomerate phases. Exaggerated
linear markings alone are indicative of changes n o t infrequently found
in healthy lungs. Age and the respiratory infections experienced
b y t h e average adult in some cases m a y cause even a beginning decrease
in linear marking and t h e appearance of beading or granular shadows.
Early generalised pulmonary fibrosis (the border-line case) is generally represented by t h e granular phase, with a beginning decrease
in t h e usual linear markings. Likewise, t h e border-line case between
general fibrosis 2 and general fibrosis 3 is generally represented b y t h e
coalescing nodular phase in which evidence of emphysema is not marked.
I t will b e noted t h a t asymmetrical markings, unilateral increase
in shadows, and shadows with irregular a n d less discrete edges imply
infection, a n d modify a n y of t h e four phases. The extent t o which
infection is responsible for changes in X-ray film markings can be
judged only through careful consideration of all pertinent clinical and
historical d a t a .
A sub-committee of t h e Committee ön t h e Prevention of Silicosis
through Medical Control 1 prepared a tabulation of t h e various lesions
of silicosis together with terms t h a t a t t e m p t t o depict the character
of t h e shadows cast on t h e X-ray film by these lesions. The tabulation
applies only t o silicosis, other forms of pneumoconiosis, such as asbestosis, being excluded because their pathology is essentially different
from t h a t of silicosis. The tabulation contains three columns : on
the left are t h e roentgenological appearances, in t h e middle t h e corresponding pathological lesions, a n d on t h e right t h e subjective symptoms,
if any, which m a y be caused b y these lesions. There is further subdivision to describe the appearances of (1) healthy lungs, (2) the uncomplicated silicotic lung a n d (3) lung of silicosis with infection. The
changes described under t h e first division are those compatible with a
state of good health and, while t h e y m a y be produced by t h e inhalation
of relatively small a m o u n t s of silica dust, t h e y are not sufficiently
characteristic or advanced t o substantiate a diagnosis of silicosis.
Similar or identical appearances m a y also result from the inhalation of
1
The members of the Committee on the Prevention of Silicosi? through
Medical Control were : Dr. R. R. Sayers, Chairman, U.S. Public Health
Service, Washington; Dr. L. U. Gardner, Saranac Laboratories, Saranac
Lake, N.Y. ; Mr. W. M. Graff, National Bureau of Casualty and Surety
Underwriters, New York City; Mr. Thomas Kennedy, United Mine Workers
of America, Harrisburg, Pennsylvania; Dr. A. J. Lanza, Metropolitan Life
Insurance Company, New York City; Dr. W. S. McCann, Strong Memorial
Hospital, Rochester, N.Y. ; Dr. E. P . Pendergraps, University of Pennsylvania, Philadelphia; Dr. H. F . Ringo, Montreal, Wisconsin; Dr. H . L . Sampson,
Trudeau Sanatorium, Saranac Lake, N.Y. ; Dr. O. A. Sander, Milwaukee,
Wisconsin; Dr. B. L. Vosburgh, National Electrical Manufacturers Association, Schenectady, N.Y. ; Dr. C. H . Watson, National Safety Council,
New York City; Dr. J. N. White, Scranton, Pennsylvania; and Dr. R. R.
Jones, U.S. Public Health Service, Washington.

A P P E N D I X X : EARLY

D I A G N O S I S OP SILICOSIS

197

non-siliceous dusts, from certain infections, from cardio-vascular disease,
and from certain other rare conditions. The changes involved are for
the most part confined to the lymphatics and perilymphatic connective
tissues and do not affect the parenchyma of the lung. Since, by definition, silicosis is a disease characterised by nodular fibrosis in the parenchyma of the lung, these alterations, even when they may have been
caused by inhaled silica, do not constitute a basis for a diagnosis of
silicosis. The second group covers the discrete and conglomerate
nodular fibrotic reactions of simple silicosis. The last group deals
with silicosis complicated by infection. In the majority of instances
the infecting organism is the tubercle bacillus, but the classification
is sufficiently broad to include other types of infection.
Roentgenological
appearances

Histological
appearances

Subjective
symptoms

Healthy Lungs and Adnexa
1. Healthy lungs. — As defined
by the N.T.A. Committee
report.
2. Irregular exaggeration of the
linear markings, with possibly
some beading confined to the
trunks.

3. Increased root shadow.

1. Essentially the normal tissues
of the vascular tree, the
mediastinum, the bronchi,
and trachea.
2. Cellular connective tissue proliferation about lymphatic
trunks in the walls of vessels
and bronchi. Beading may
be due to various causes, as
blood vessels seen end on,
arteriosclerosis, minute areas
of fibrosis in lymphoid tissues
along the trunks.
3. Cellular reaction in the tracheobronchial lymph nodes
with extensions along afferent
lymphatic trunks.

1. None.

2. None.

3. None.

Thcsc changes come within normal variations when not accompanied by recognised organic disease.

Simple Silicosis
4. Modulation. — Discrete shadows not exceeding 6 mm. in
diameter, tending to uniformity- in size, density, and
bilateral distribution, with
well-defined borders surrounded by apparently normal
lung shadow. The outer and
lower lung fields characteristically show fewer nodules.

4. Circumscribed nodules of hyaline fibrosis located in the
parenchyma of the lung.
Occasionally some of these
nodules may show microscopic foci of central necrosis.

4. Barely symptoms except with
advanced degrees of nodulation with associated emphysema, when a mild dyspnoea
on exertion may become
evident. Seldom, however,
is this sufficient t o cause any
marked decreased capacity
for work. At which stage
of development of the fibrosis
dyspnoea will become conscious varies considerably
with different individuals.
With an unimpaired cardio' vascular system in an individual of average weight,
with no definite emphysema,
a conscious dyspnoea may
never develop in this stage.
On the other hand, dyspnoea
will become evident earlier
in obese persons, in those
with definite cardio-vascular
degenerative changes, and
in those in the later decades
in life.

5. Conglomerate shadows that
appear to result from a
combination or consolidation
of nodulation usually with
associated emphysema manifested by—

5. The result of coalescence of
discrete nodules; an area in
which the nodules are closely
packed and most of the intervening lung is replaced by
more or less hyaline fibrous

5. But few show symptoms if
conglomerate shadows are
small and localised and no
emphysema has developed.
With the development of
larger areas of conglomera-

198
Roentgenological
appearances
(a) Localised Increased transparency of the lung with loss
of fine detail.
(6) Intensification of the
trunk shadows by contrast.

SILICOSIS
Histological
appearances
tissue. The lung architecture is partially obscured.
No demonstrable evidence of
infection. Emphysema Is a
compensatory dilatation of
the air spaces with or without
thickening of the septa.

(c) Depression ol the domes
with possible tendency toward
Individualisation of the C03tal
components of the diaphragm.
(d) Lateral view : Increase In
the preaortic and retrocardiac space with exaggerated backward bowing of the
spine. Widening of the spaces
between the ribs may or may
not be present.

Subjective
symptoms
tlon and definite emphysema,
dyspnoea on exertion Is apt
to be marked. As with
simple nodulatlon, onset of
dyspnoea Is partially dependent on cardio-vascular,
weight and age factors.
Dry cough, chest pain, and
bloodstreaked sputum may
bo present. A considerable
decreased capacity for work
Is frequently seen in these
cases with more advanced
fibrosis. Very marked disability, however, is suggestive
of complicating infection.

¡Silicosis with Infection
The characteristic appearances described under simple silicosis are modified by infection as follows :
6. Localised discrete densities
and/or string-like shadows
accompanying those of simple
silicosis described above.

6. Strands o! fibrous tissue,
often along trunks and septa,
with or without areas of
calcification : indicative of
"healed" infection.

Symptoms seldom present
except with advanced degree
of nodulatlon with associated
emphysema, exactly as with
groups I and 5. The small
healed infection scars are no
factor in the onset of
symptoms or disability.

7. Mottling.—Shadows varying
in size with ill-defined borders
and lacking uniformity in
density and distribution,
accompanying simple silicosis.

(a) Areas of broncho-pneumonia with or without
caseation, i.e. acute infection.

7. (a) Symptoms of acute respiratory Infection, such as
fever, productive cough, dyspnoea, pain in chest, haemoptysis, weight-loss—some or
all of which may be present.
The degree in which they
are manifested is dependent
upon the extent and severity
of the complicating Infection
and the silicotic fibrosis.
The degree of disability
obviously is temporarily total; whether or not it becomes permanent (total or
partial). depends upon the
healing of the Infection, the
extent of the silicotic fibrosis,
circulatory efficiency, age,
or other systematic changes.

(6) Lobular areas of proliferative reaction with or
without caseation, i.e. chronic
infection.

(¡>) Symptoms are similar to
those with acute respiratory
infection, usually varying only
in severity. Tuberculosis is
the usual complicating infection, disability being permanent and slowly progressive
in most cases. Exceptions to
this are those few early cases
of silico-tuberculosls which
may respond to treatment.

8. Permodular cellular reaction
either exudative or proliferative In character.

Symptoms and resulting disability differ only from
those under 7 In that they
may be more severe, due to
the fact that the tuberculous
changes are more rapidly
progressive.

8. Soft nodulatlon. — The
nodular shadows described
under simple silicosis, 4, have
now assumed fuzzy borders
and/or irregularities In distribution. This change may
or may.not accompany the
simple mottling of 7.

APPENDIX X : EAKLY DIAGNOSIS OF SILICOSIS
^Roentgenological
appearances
9. Massive shadows of homogeneous density, not of pleural
origin symmetrically or asymmetrically distributed.

Histological
appearances
Extensive areas of fibrosis
probably due to organised
pneumonia of tuberculous or
non-tuberculous origin superimposed upon a coexistent
silicotic process. Outlines of
normal structures may be
partially destroyed.

199

Subjective
symptoms
9. Dyspnoea or orthopnoea, productive or non-productive
cough, chest pain most often
in form of sense of compression; II fever, haemoptysis,
or weight-loss occur, active
infection within the dense
fibrosis must be suspected.
The symptoms manifested by
this group until the terminal
stages are often lesa pronounced than would be
expected from the changes
that are visualised. All in
this group, however, are
permanently and totally disabled as far as work is concerned which requires any
physical exertion.

The fallibility of accentuated linear markings for establishing
an early diagnosis of silicosis was well brought out by two investigations
made by the Saranac Laboratory and included in the report of the
afore-mentioned committee. Roentgen evidence of so-called "linear
fibrosis" was present in 22 per cent, and 82 per cent, respectively of
groups of workers exposed over long periods to high concentrations of
dust so low in free silica content as not to present a true dust hazard
from the standpoint of silicosis. "There are, however", it is added,
"certain mixtures of silicates with free silica whose dusts first produce
this type of reaction, but it is followed by more advanced changes when
the exposures are prolonged. As the amount of uncombined silica
in the mixture increases, so does the probability of developing nodular
fibrosis. Even where there is no typical nodulation, the reaction
may be associated with clinical symptoms and excessive incidence of
tuberculosis." It is suggested that this may possibly be the case in the
Barre (Vermont) granite industry, where "the dust may contain about
35 per cent, of quartz and" yet linear fibrosis rather than nodulation
characterises the reaction seen in a pulmonary roentgenogram." While
the time required for development of "this equivocal roentgenological
finding" in a given concentration of dust may be valuable in determining
the standard of safety for the industrial process under consideration,
this standard cannot be applied to another industry where the concentration of silica and the other materials involved are different. In
nodular fibrosis, on the other hand, we have a more specific reaction,
which is produced by inhalation of free silica and of no other dust.
Laboratory Examinations
The Committee, reporting briefly on the laboratory findings in
silicosis, said : "There are no blood changes characteristic of simple
silicosis. . . . There are no findings that might be called pathological
observed in the examination of the urine excreted by the simple silicotic. . . . There is no evidence that the absorption or excretion of silica
exerts any deleterious effect upon the kidney. . . . All (investigators)
agree that quantitative measurements of silica in the urine will vary
with silica absorption. Diets rich in siuca result in increased silica
elimination and individuals exposed to unusual atmospheric concentrations of silica will likewise show an increase in elimination of silica.
To date such information has proven of no specific value in determining

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SILICOSIS

the degree of exposure to silica-bearing atmosphere. Probably because
it has been impossible to estimate the amount of silica taken in with food
and drink. . . . The chief value of sputum examination is to determine
the absence or presence of tubercle bacilli. As in the case of chronic
fibroid phthisis, too much dependence must not be placed upon microscopic examinations of the sputum."
Burke, of the New York State Hospital for Incipient Tuberculosis,
feels inadequate attention has been paid to the demonstration of mineral
particles in the sputum in the diagnosis of silicosis. In 25 patients
who gave no history of exposure to dust, minute doubly refractive
particles were not found in the sputum in significant numbers, whereas
in 10 of 15 patients who had been exposed to dust for from 2 to 15
years, such particles were present in the sputum in large numbers.
In one of these 10 patients, necropsy revealed advanced silicosis together
with pulmonary tuberculosis. Some of the doubly refractive particles
were identified as quartz by means of immersion liquids of known
refractive indices. These patients had been away from their dusty
occupations for periods ranging from 3 months to 15 years.
The results obtained bTr Q-oldwater of the New York TJniversitv
College of Medicine, from measurements of the urinary excretion of
silica in five non-silicotic human subjects under controlled conditions,
apparently indicate (1) that urinary silica concentration may show
wide variations depending on the specific gravity of the urine; (2) that
the same individual on a constant diet may show wide fluctuations in
daily output of silica and in urinary silica concentration; and (3) that
different individuals on similar diets show great differences in urinary
silica output and concentration.
Differential Diagnosis
Siderosis in electric arc welders. In 1936 two English investigators,
Doig and McLaughlin, described X-ray appearances of fine pulmonary
nodulation in electric arc welders who were.otherwise in normal health.
Enzer and Sander in 1938 described similar discrete shadows uniformly
distributed in both lungs, with individual apparent nodules ranging in
size from less than 2 to 4 mm., no conglomerate shadows and no calcification, in chest films of five electric arc welders in good health, who had
done much work inside tanks where the fumes are especially dense,
and exaggerated trunks shadows in five other electric arc welders.
These investigators have been able to clear up the question of the
origin of these nodular shadows by the opportunity for post-mortem
examination in the case of one of the men, who died from extraneous
cause. They found no reactive phenomenon that could be described
as fibrosis, but a deposit of apparently inert foreign pigment of extremely
low silica content, uniformly distributed throughout the lung fields and
accumulating in the lymphatics. The extent of the pigment deposit
was by far insufficient to occlude significantly the lymphatics. Its
striking distribution around the blood vessels affords adequate explanation for the peculiarly discrete roentgenographic shadows. Prom
this observation it appears that the X-ray appearances of nodules may
result from deposition of iron in the perivascular lymphatics. The
suggestion is made that even in individuals with silica exposure and some
degree of pulmonary silicosis, the extent of the shadows may not be a
true picture of the pathological processes, particularly if there has been
exposure to iron oxide dust. That deposit of particulate iron in the
lungs may be responsible for X-ray shadows of a nodular character has

APPENDIX X : EARLY DIAGNOSIS OF SILICOSIS

201

been suggested a number of times. I t appears, however, that the present
report for the first time demonstrates this to be a fact. The term
"siderosis", these investigators insist, should exclude the concept of
fibrous tissue reaction. The men studied by Enzer and Sander had
been working with bare metal electrodes. Welding with coated electrodes has almost completely replaced the bare rod welding. What
additional effects the fumes from the many varieties of coatings will
have cannot be determined with assurance until more time has elapsed.
This research emphasises the care that is necessary for making an
accurate differential diagnosis of the pneumoconioses.
Pancoast and Pendergrass have described a number of non-pneumoconiotic conditions which may closely simulate the roentgen appearances
found in early, simple silicosis, with early nodulation :,
"Passive congestion of the lungs as a result of cardiac decompensation.
In this condition, the enlargement of the heart shadow, the abnormalities of the cardiac silhouette, especially in cases of mitral stenosis,
and the clinical picture serve to establish the presence or absence of
passive congestion. If pneumoconiosis is absent, the appearances are
likely to disappear with restoration of compensation, but not altogether
in long-standing cases. Marked emphysema is usually associated with
pneumoconiosis and is apt to cause a certain amount of rotation of the
heart, so that the sagittal roentgenogram does not convey a direct
impression of cardiac size or shape. The lateral view is always necessary
to clear up the diagnosis. Any phase of pneumoconiosis is likely to
have its roentgenograph«; appearance greatly accentuated by passive
congestion.
"Passive congestion associated with coronary thrombosis. Following
the incidence of coronary thrombosis, passive congestion with its
roentgen appearances of prominent hilar and trunk shadows is commonly
found for from three to six weeks after the attack, and is especially
noticeable in the right lung. This appearance and the clinical evidences
of decompensation will clear up when compensation is established.
If the thrombosed vessel is in the proper location, a very marked localised dilatation of the left ventricular curve. will be noted where the
muscle has undergone atrophy and has been largely replaced by fibrous
tissue. At least one case of this kind has been mistaken for pneumoconiosis.
"Advanced bilateral bronchiectasis. The dilated bronchi, retained
secretions and associated tracheo-bronchitis, characteristic of this
condition, will produce the appearance of greatly accentuated trunk
shadows, which may simulate that of early silicosis with nodulation.
"Asthma. This condition will produce various alterations from the
normal roentgenographic appearances of the lungs. There is very
apt to be an intensification of the hilar and trunk shadows and very
often an associated coarse mottling of the lung fields. The latter
appearance may appear or become accentuated during and immediately
following an asthmatic attack. The two conditions may be associated,
and what might be a very moderate degree of incapacitating pneumoconiosis would have its appearance greatly accentuated by the asthma.
"Infiltrating or permeating malignant metastases. Quite frequently
metastatic malignant processes in the lungs are of such a character as

202

SILICOSIS

to take on the appearance of greatly accentuated hilar and trunk
shadows which closely simulate the appearance of pneumoconiosis.
Naturally, the individual is incapacitated and often in a way similar
to the silicotic subject.
"Polycythemia or erythemia. This condition may be regarded from
two points of view. As an independent disease, it usually has
associated with it an increased prominence of the ( hilar and trunk
shadows due to the engorged pulmonary vessels. On the other hand,
high red cell counts, even sufficient to suggest polycythemia, are very
frequently associated with pneumoconiosis. There seems to be reason
to believe that lung fibrosis and pulmonary arteriosclerosis can produce
a condition perhaps akin to polycythemia and presumably due to
impaired interchange of oxygen and carbon dioxide. . . .
"Mycotic infections. Most of the mycotic (fungus) infections of the
lungs have produced appearances simulating the nodular phase of
silicosis. The differentiation may be difficult. . . .
"The nodular phase is productive of the most characteristic roentgenographic appearances of silicosis. . . . The appearance may be
simulated by a number of conditions, including tuberculosis of the
perinodular silico-tuberculosis type, tuberculous broncho-pneumonia
and miliary tuberculosis. The appearance is simulated also by the
following conditions :
"Actinomycosis. This may occasionally present an appearance like
that of nodular silicosis, although the nodules are usually larger and
less numerous in comparison with it and with similar infections to be
mentioned later.
" ¡Sporotrichosis. Pancoast and Pendergrass have examined one
case of this infection in which the appearance was so identical with that
of the nodular phase of silicosis that they could not be convinced that
the condition was not the latter, except by the lack of exposure and by
subsequent observations at autopsy. Miliary tuberculosis was also
considered as a possibility and, in fact, the alternative.
"Leptothrix infection. They have encountered one case of this
kind presenting an appearance quite similar to nodular silicosis, but
the diagnosis was readily made by culture from the sputum and the
absence of occupational history.
"Nodular metastatic malignant conditions of the lungs. Most cases
of this kind are readily differentiated from nodular silicosis, but occasionally one encounters a case in which appearances are very similar,
because the metastatic nodules are small, unusually numerous, and
widely and symmetrically scattered.
"The advanced diffuse or terminal fibrosis stage of silicosis is to be
differentiated mainly from chronic diffuse tuberculosis, or an accompanying tuberculosis is to be determined. Occasionally, a case showing
massive fibrotic areas medially located may present an appearance
resembling a mediastinal tumour."
As to the question of the value of an early diagnosis from the standpoint of removal of the affected worker from a dusty environment,
the Division of Industrial Hygiene of the National Institute of Health,

APPENDIX X : EARLY DIAGNOSIS OF SILICOSIS

203

United States Public H e a l t h Service, holds t h e view t h a t no worker
should be removed from work t o which he is accustomed a n d a t which
he is able t o earn his living, merely because a diagnosis of simple silicosis
has been made. W e hold, rather, t h a t t h e atmospheric d u s t i n which
he works can and should be brought within safe limits. If this is done,
his silicosis will not, we feel, advance appreciably more rapidly t h a n if
he were made a clerk or given outdoor employment. And if i t is n o t
done, removing him a n d putting a fresh m a n in his place will expose
another worker t o t h e same hazard. W e urge t h a t t h e m a n be left
and t h e dust removed, rather t h a n t h a t t h e dust be left a n d t h e m a n
removed.
W h e n active tuberculosis exists, i t is another m a t t e r . E v e r y
tuberculous worker should be removed from a dusty industry, p u t
under t r e a t m e n t a n d n o t permitted t o return. A n y further exposure
to silica will be harmful t o him a n d he constitutes a danger t o his fellow
workers. Since infection is t h e main cause of serious progression of
silicotic fibrosis, t h e adoption of those preventive measures which will
materially reduce t h e possibility of contact with pulmonary infection
m a y serve t o postpone, if n o t actually stop, t h e progress of early changes
(fibrosis) in t h e lung tissue. W e feel t h a t such preventive measures
m a y a n d should be so efficient t h a t no disability due t o silicosis will
develop in the usual working lifetime. I t appears quite possible t h a t
the worker with a non-disabling simple silicosis will be safer continuing
in a so-called dusty industry where t h e d u s t is properly controlled a n d
from which tuberculous workers are carefully excluded, t h a n if h e were
transferred t o non-dusty work with less attention paid t o contact w i t h
fellow workers having active pulmonary tuberculosis.

BIBLIOGRAPHY

BURKE, Hugh E. : "The Detection of Mineral Particles in the Sputum in
Silicosis", J. Indust. Hyg., Vol. 17, J a n . 1935, p . 27.
GOLDWATEB, Leonard J . : "The Urinary Excretion of Silica in Non-Silicotic
Humans", J. Indust. Hyg. and Toxicol., Vol. 18, No. 3, March 1936,
p. 163.
DOIG, A. T. and MCLAUGHLIN, A. I. G. : "X-ray Appearances of the Lungs
of Electric Arc Welders", Lancet, Vol. 1, 1936, p . 771.
ENZEB, Norbert and SANDER, O. A. : "Chronic Lung Changes in Electric
Arc Welders", J. Indust. Hyg. and Toxicol., Vol. 20, No. 5, May 1938,
pp. 333-350.
PANCOAST, H. K. and PENDEBGRASS, E. P . : "Roentgenologic Aspects of

Pneumoconiosis and I t s Differential Diagnosis", J.A.M.A.,
1933, pp. 587-591.

Vol. 101,

APPENDIX XI
, MEASURES OP PREVENTION OF SILICOSIS IN DUTCH COAL MINES
By D R . A. H. VOSSENAAB

The campaign against silicosis cannot be successfully achieved by
attention to one aspect only. Nevertheless, the utility of effective
measures, given adequate collaboration, is no longer a matter for doubt.
Where the methods employed are all directed to a single aim, they may
form part of a coherent plan, even where they each present aspects
of an entirely different nature. Technical experts and medical men
must join forces and succeed in convincing the workers also of the
importance of the measures to be taken.
Only continual collaboration on the part of technical services, the
medical service and the workers will enable favourable results to be
anticipated.
Health measures are essential in view of the relation now recognised to
exist—though so far not wholly understood—between certain pulmonary
diseases and silicosis. No worker should therefore be admitted to
underground work if he shows symptoms of tuberculosis in any form
whatsoever. This disease may be present in individuals with a perfectly
healthy appearance and may persist throughout the whole of their
lives in a latent form. It, however, not infrequently happens that the
disease, for unknown reasons, undergoes fresh evolution and becomes
active, providing in this new form a source of infection of a highly
dangerous character.
All applicants showing former pleuritic adhesions should also be
excluded, since these adhesions point to a probable connection with
tubercular infection. However, limited chest expansion is in itself
sufficient to favour prejudicial consequences as a result of dust inhalation.
Further, attention should be directed to the fact that the capacity for
dust fixation, together with its consequences, differs considerably in
different individuals. Experiments have provided useful indications
in explanation of this fact. Ordinary diseases of the lungs and of the
upper respiratory passages are also to be feared, since the auto-cleansing
properties of these passages against dust, which constitute an important
factor, might be impaired.
I t cannot at the moment be established with sufficient certainty
up to what point the filtering power of the nasal mucous membrane
may constitute an element of importance in effecting good selection
of workers for rock work. In my opinion it cannot be included among
methods of examination, on the basis of which definite selection can be
effected, and which can henceforth be considered as a reliable guide.
Some time ago Professor Reichmann (Bochum) published a detailed
enquiry in which he demonstrated with certainty that no connection
exists between the aspect of serious siUcosis and an insufficient filtering
capacity of the nasal mucous membrane. In Holland we have not at

APPENDIX X I : MEASURES OF PREVENTION

205

our disposal the means enabling us to effect an independent investigation
into this question. I t has however always seemed doubtful to us that
a relative filtering capacity should constitute a basis for the selection
of miners not showing any predisposition, or merely very reduced predisposition, to silicosis. Thus, as has already been shown by a certain
parallelism between the length of the working experience on rock
work and the consequences of dust inhalation, absolute quantities
appear in general to be decisive in the intensity of the changes to be
expected as the result of such inhalation.
It is generally recognised that the action of a prejudicial effect
does not become manifest until after a long lapse of time. Nevertheless
very great individual variations occur in this connection. It may be
assumed that the dust so far liberated from the adjacent rock in the
Dutch coal mines is relatively of a slightly harmful character in the
general sense of the term. Duration of work for decades, however,
almost always leaves certain traces, when the worker reaches an advanced
age, and it would seem advisable to insist on limitation of the time
spent on rock work, for instance, to about 15 years. The workers at
the end of that time still retain sufficient physical strength to perform
useful underground work of another type. The line of action recommended is in perfect accord with the general principle followed in industry
of refraining from exceeding the limits of tolerance in detrimental
causes. The proposal that rock work should be given up after 15 years
is one which, of course, need not necessarily be rigidly enforced, since
periodical medical examination will indicate the proper limit to apply,
a period of 15 years being perhaps even too long, in the case of signs of
abnormal dust fixation. On the other hand, during the examinations
carried out in 1934 and in 1938 a certain number of workers still
showed a perfectly normal picture after 20 years of rock work. Yet
caution is essential. Advancing age involves such handicaps as dyspnoea, etc., most usually without other apparent cause. I t is not
without importance to avoid giving rise to subjective complaints, which
may be made the basis of demands finding no justification in the
real state of matters, as far as the influence of rock work is concerned.
In general, rock work should therefore be discontinued before subjective
symptoms become manifest.
There is every reason to hope for particularly favourable results
from the combined technical measures such as those described under
Nos. 5 - 1 1 inclusive. It is possible on this account that the duration
of rock work may in future be extended beyond that in force at present
and that provisions concerning duration of work may be made more
elastic. Nevertheless advancing age will always constitute a serious
handicap, while a change-over in occupation at an advanced age is
invariably attended with difficulty.
In consequence, as far as medical collaboration is concerned, in regulation of rock work the following four principles may be laid down :
1. No worker should be admitted to underground work unless radiographic examination shows his lungs to be free from tubercular infection
in any form whatsoever (pleuritic adhesions, etc.)
2. Special radiographic examination is necessary before admission
to rock work. In effecting it particular attention should be paid to
the breathing passages (broncho-vascular shadows). In the case of the
meatus constituting an insufficient passage, of super-elevation of the
palate, etc., rock work should be contra-indicated.

206

SILICOSIS

3. Periodical examination every three years should suffice, but
examination should include a through investigation to ascertain whether
predisposition to dust fixation and intercurrent diseases are not such
as to constitute an obstacle to continued rock work.
4. Rock work for a duration of 12 to 15 years should as a general
rule be followed by change of work. If it is announced on engagement
for rock work that this limit will probably be applied, it will be possible
to do so without awakening suspicion of the existence of consequences,
fears of which are unfounded.
Suppression of causes falls within the competence of the technical
service, the task of which is to prevent, by appropriate means for combating dust, certain types of dust—among which, for the purposes of this
report, rock dust is exclusively considered—from causing injury by
penetrating the respiratory tract.
The type of measures to be adopted depends on the kind of rock
work undertaken and also on the various possibilities which can be
realised to render the rock dust harmless. The methods of accurate
counting available in laboratories for dust research enable the efficacy
of the methods employed to be assessed, both in regard to the number
of particles and to their size frequency.
In applying preventive measures, special preference should always
be given in the anti-dust campaign to removal at the point of origin.
Only as a secondary resort should individual prophylaxis be resorted to,
whether it is eventually found necessary to substitute it for efforts
made to remove the cause or only to make it supplementary to these.
Certain methods of carrying this into effect in the case of rock
are worthy of consideration :
5. Boring and blowing out of sockets represent considerable sources
of dust production of particular intensity, which lasts for a fairly long
time. Various means of combating exposure to this dust have proved
effective. Boring with a dust-aspirating device on the boring tool has
the considerable advantage of rendering a great quantity of the dust
entirely harmless. In boring by wet methods preference should be
given to the system which provides for agglutination of the dust at
the moment of its formation (Atlas-Diessel boring tool).
Watering of the orifice of the boring hole'by the application of a
fine water spray does not possess the advantage just referred to. Dispersion of fine droplets is not free from risk of the diffusion of disease
germs.
Both in using the dry method and the wet method it would be essential to combine the means applied at the source of production, in such
a way that they cannot be used except in conjunction.
Only where liberation and dispersion of dust cannot be prevented
by one or other method is it permissible to have recourse to masks, as
a protection against dust during boring or blowing out.
6. The formation of clouds of dust during shot-firing and the raising
of the dust by the resulting concussion may be avoided by pulverisation
of dust agglutinating substances (Hollmann Pulveriser). In this
case castor oil might probably be replaced by a more practical medium.
7. In order to approach the cutting face after shot-firing it is necessary to penetrate an atmosphere containing for the time being large
quantities of floating dust and irritating explosion fumes. The large
dust particles irritate the respiratory passages and at times give rise
to violent attacks of coughing, further encouraged by the products

APPENDIX XI : MEASURES OF PREVENTION

207

liberated by the explosive and thus exert an unfavourable effect on the
bronchi. The fine dust which penetrates into the deeper parts of the lung
does not cause coughing (analogous to certain gases which for that very
reason are principally used as war gas : phosgen).
The wearing of a good mask, which must satisfy certain conditions
to merit this designation, is in such circumstances particularly advisable,
for conditions are then very favourable to its use.
A mask may not be recommended unless it possesses a strong and
effective filtering capacity for particles of extremely fine dust and as
low a resistance as possible to respiration. It should moreover have a
very small "dead space", a mechanical escape valve which cannot
become obstructed, and a very good face fit. I t should be easily
cleaned and should have a well-selected centre of gravity. Attention
should be paid to the band fixing it round the head, which must be
properly fitted on.
8. Various researches carried out have demonstrated the important
part played by good ventilation. I t should be applied up to the point
at which it begins to cause discomfort. The duration and intensity
of inhalation of dust are influenced in the highest degree by ventilation.I t is advisable to provide a fixed minimum amount of ventilation ensuring
an uninterrupted supply of fresh air per unit of time at the working
post or rock face.
9. The rock broken by the shot-firing should be wetted before removal.
I t is true that wetting involves several disadvantages, but its advantages
cannot be denied.
10. A solution of the problem of organising mechanical loading and
transport of schists is urgently awaited. The ideal method would be
achieved if one worker, provided with a mask, could direct the movement of the barrows and if his work were limited to doing that. At the
present time preference should be given to methods of loading which
do not demand too much physical effort on the part of the workers,
among other means by arranging merely slight inclines for the fall
of rock, where this does not involve acceleration of the working rhythm.
' 11. Efforts should be made to adopt methods of shot-firing in which
both the cut and the round are effected at the same time at the end
of the shift. It is nevertheless difficult to carry this out in mines subject to firedamp.
We further attach special value to provision of hygienic living
conditions for our workers. Under this heading the importance is
first of all recognised of providing separate dwellings for each family,
open to light and air on every side, surrounded by a garden and having
a pleasant aspect. An active campaign should be engaged in against
the abuse of alcohol. A good system of social insurance also contributes towards maintaining, during periods of unemployment due to
disease or accident, the relative standard of welfare enjoyed by the
Dutch miner.

APPENDIX XII
PREVENTION OF SILICOSIS BY METALLIC ALUMINIUM
Progress Report covering the Work from the Preliminary Paper,
July 1937 to July 1938
By J. J. DENNY, M . S C , Metallurgical Engineer, W. D. ROBSON,
M.B., Chief Surgeon, Mclntyre Mine, Schumacher, Ont., and Dudley
A. IBWIN, M.B., Associate Professor, Department of Medical Research,
University of Toronto.

(1) Action of Aluminium is Local and not Systemic
The action of metallic aluminium on inhaled quartz is a local one
in the. lungs and is not systemic, as the inhalation of quartz -f- 1 per
cent, metallic aluminium does not produce fibrosis, while the inhalation
of quartz dust produces fibrosis in the lungs of rabbits that have been
fed metallic aluminium or had it injected intraperitoneally or intramuscularly.
(2) Mechanism of Action of Aluminium
We are of the opinion that in the lung metallic aluminium inactivates
quartz not by dilution but by reacting with water to form positively
charged aluminium hydroxide which is adsorbed to the surface of the
negatively charged quartz particles. The evidence of adsorption is
(1) an aqueous suspension of quartz (all particles negatively charged)
is flocculated by the addition of metallic aluminium powder. After
flocculation the quartz particles carry no charge. (2) The addition
of small amounts of metallic aluminium powder to a solution of silicic
acid (25 p.p.m.) results in only a 50 per cent, decrease in the concentration
of the silicic acid in a period of a month. (3) Metallic aluminium
lowers the solubility of aluminium silicate. (4) The surface of ordinary
quartz does not adsorb alizarin, but quartz treated with an aqueous
suspension of metallic aluminium will adsorb alizarin.
(3) Administration — Admixture or Separately
Metallic aluminium will inactivate quartz when inhaled as a mixture
with quartz. Metallic aluminium will also inactivate quartz if inhaled
separately before or after exposure to quartz. A daily 40-minute
exposure of 10,000 p.c.c. of aluminium has protected rabbits for 10
months against an exposure of 30,000 p.c.c. of quartz for 1 1 % hours
daily.

APPENDIX X I I : PREVENTION BY METALLIC ALUMINIUM

209

(4) Stability of Aluminium — Quartz Bond
In vitro, the bond between the adsorbed film of aluminium hydroxide
and the quartz particles is stable between pH 4 and 11, as shown in
the accompanying charts.

pH of buffer
FIG. 2

pH of solution
FIG. 1

Asbestos
Asbestos + aluminium

Granite
Granite + aluminium

12

pH of buffer
FIG. 3
Quartz
Quartz + aluminium

li

t

5

l

7

8

1

10

It

U

pH of buffer
FIG. 4

-.

Talc
Talc + aluminium

(5) Duration of Inaetivation of Quartz by Aluminium in Rabbits
The inaetivation of quartz by 1 - 3 per cent, metallic aluminium
persists for at least a" year in rabbits. Control rabbits exposed to
quartz dust for one year developed fibrosis. Rabbits dusted with a
similar concentration of quartz + 1 per cent, metallic aluminium dust
did not develop fibrosis.

IS

210

SILICOSIS

(6) Relation between Solubility of Quartz and Tissue Reaction
The relation between the solubility of quartz in water, the amount
of metallic aluminium needed to inactivate quartz, the tissue response
and the stability of the quartz-aluminium hydroxide bond is shown
by the following experiments :
A hatch of < 5 i* quartz was prepared by grinding quartz on quartz
and separating by air sedimentation. To samples of this batch varying
amounts of metallic aluminium powder were added and the solubility
of the mixtures determined in water after 20 hours' agitation. 10 mg.
amounts of all these samples were injected subcutaneously into each
of several rabbits and cross sections of the injected areas examined
microscopically at two and seven months showed the same type of
response. The histological preparations made of cross sections of the
injected areas show a nodular fibrosis with metallic aluminium concentrations up to 0.33 per cent, and a foreign body reaction with 0.85 per
cent, of metallic aluminium and greater.
(7) Minimum Dosage of AluminiumExperimental work on the minimum dosage of metallic aluminium
necessary t o inactivate quartz in the lung has been under way for
some time. The evidence obtained to date strongly suggests that
1 per cent, metallic aluminium is a safe minimum. However, definite
conclusions cannot be drawn until further information is obtained
regarding the period of time that metallic aluminium will continue to
inactivate quartz in the lung.

APPENDIX XIII
THE DEPRESSION OP SILICA SOLUBILITY IN BODY FLUIDS
BY MINERAL DUSTS
By

DES.

E. J. KING, W. ROMAN and M. MCGEORGE, British
Postgraduate Medical School, London

The presence of silica in blood, and the much larger amounts sometimes found in urine, indicate that silica is appreciably soluble in body
fluids. The determination of such solubility, however, is a matter of
considerable difficulty and different results have been obtained by

O

2

«f

fc

8

10

13.

IH

Days

Effect of concentration of solid on the dissolution of silica. Percentage
figures represent grammes of rock crystal dust suspended in 100 ml. of
ascitic fluid.
various workers, owing to the operation of such variable factors as
particle size, concentration of suspension, method of separation of solid
from liquid phase and method of estimation of silica in solution. In
addition, soluble silica is present partly in true molecular form and
partly as a colloidal dispersion; and higher figures for solubility will

212

SILICOSIS

be obtained if colloidal silica be included in addition to that present
in true molecular solution. I t may be questioned whether this colloidal
silicic acid should be included in a solubility figure. I t reacts, however,
with certain chemicals, e.g. molybdic acid, as if it were in true solution,
and from the biologist's standpoint it is probably significant, since it
is biologically available, e.g. in natural waters for the nutrition of phytoplankton.
TABLE

I.

DISSOLUTION

OF SILICIC ACID

FBOM MINERAL

DUSTS

(2 g. dust suspended in 100 c.c. ascitic fluid at 37 degrees)
(mg. Si0 2 per 100 c.c.)

Mineral
Dusts
Quartz

.

lday

3 days

6 days

4.4

5.4
1.7
1.1
1.0
0.1
1.0

7.0
1.6
1.4
1.2
0.1

0.5
0.6
1.4

0.6
0.9
1.5

0.6
1.5
1.3

C h l o r i t e (Mass.)
0.1
Stone
Dusts
D . a n d C. S . 1 9 4 1 ( 3 9 % free
Si02)
C . a n d W . S.201(7%free Si02)
P.andP.S.311(21%freeSiOa)
S o m e r s e t G r e y s 2 ( 4 9 % free
Si02)
P e n n a n t B o c k 2 ( 4 2 % free
Si02)

2^2

2.1

2.0

2.4

2.5

2.2

Air-borne
Am. D.
Am. E.
Am. I.
Am. J.

1.2
1.5
1.2
0.8

1.4
1.4
1.4
1.3

1.4
1.1
1.8
1.4

0.9
1.5
1.7
1.5

1.5
2.1
1.6
1.2

2.0
2.1
1.8
1.4

Goal

Coal-mine
(16 % a s h )
(39 % ash)
( 1 8 % ash)
(25 % a s h )

Dusts

Dusts

C y . b o t t o m coal
Yn. anthracite
1
!

Shale dusts used for dusting underground.
Coal measure sandstones, produced by drilling.

The amount of silicic acid which a mineral form of siHca such as
quartz will yield in solution is exceedingly small when the mineral
is present as crystals of ordinary dimensions, or as coarse fragments.
But if it be reduced to a particle size of the order of the dust in the air,
i.e. < 1 - 10 p., then the rate at which siHca appears in solution becomes
appreciable. While the evidence does not warrant the statement
that the smaller the size of the particles the more quickly they dissolve,
it is true that with "sized" particles more siHca is yielded in solution
in a given time by small particles than is obtained with the same weight
of particles of a larger size. The amount dissolved also varies with
the total amount of dust suspended in the Hquid : at sufficiently low
concentrations of dust the siHca in solution is roughly proportional

APPENDIX X u ! : THE DEPRESSION OE SILICA SOLUBILITY

213

to the amount of the solid phase. This simple proportionality is not
maintained at higher concentrations of the solid, where a condition
of apparent saturation may be approached.
The depression of solubility of silica through the agency of various
metal and metallic oxides, notably iron and aluminium, was briefly
referred to in a former communication (King and McGeorge, 1938). I t
was suggested that the apparent decrease in solubility of quartz and
amorphous silica was due to precipitation of the dissolved silicic acid
as the insoluble silicate of the metal. The phenomenon has now been
explored further and the effects of various mineral dusts on the solution
of silica from quartz have been investigated.
TABLE II. v - DISSOLUTION OF SILICIC ACID FROM MIXTURES OF QUARTZ
AND MINERAL DUSTS

(mg. SiOj per 100 c.c. ascitic fluid)

2 p e r cent. J q u a r t z (alone) . .
2 p e r cent. D . a n d C. s t o n e d u s t

1 day

3 days

6 days

4.4

5.4

7.0

0.5

0.6

0.6

3.2

3.4

4.6

1.8

1.7

2.4

1.6
1.5

1.4
1.2

1.7
1.4

2.9

2.6

3.7

2.2
0.1

1.8
0.1

2.2
0.1

2.7

4.6

4.5

1.8

2.3

2.4

0.4

0.3

0.3

2 p e r cent, q u a r t z + 1 p e r c e n t .
2 p e r cent, q u a r t z -f- 2 p e r c e n t .
2 p e r cent, q u a r t z + 4 p e r c e n t .
2 p e r cent. Y n . coal d u s t (alone)
2 p e r cent, q u a r t z + 2 p e r c e n t .
2 p e r cent, q u a r t z + 4 p e r c e n t .
2 p e r cent, kaolin (alone) . . .
2 p e r cent, q u a r t z + 2 p e r cent.
2 p e r cent, q u a r t z + 4 p e r c e n t .
2 p e r cent, q u a r t z + 1 per cent.
ALO,

1
The percentage figures (e.g. 2 per cent, quartz) denote the number of grammes oi dust per
100 c.c. of ascitic fluid.

Most of the mineral silicates are much less soluble in blood serum
and ascitic fluid than are the free forms of silica. Kaolin is particularly insoluble. In a mixture of kaolin and quartz powder suspended
in a liquid it might be expected that the amount of silicic acid dissolved
would be conditioned by the most active member of the mixture and that
in the presence of an excess of the mixed powder the solubility would
approach that of quartz powder taken alone. Naturally occurring
kaolins, such as those represented in shales, contain variable, sometimes
large, amounts of free silica and yet these natural stone dusts havean exceedingly low solubility. The amount of free silica represented
in a sample of stone dust would lead to much more solution of silicic
acid if suspended alone in the same volume of liquid. This fact was
brought out in the previous communication where it was shown that

214

SILICOSIS

a stone d u s t (shale—used for mine dusting) h a d a solubility of less t h a n
1 mg./100 m l . despite its free silica content of about 40 per cent., whereas
the 0.8 g. of free silica present in t h e 2 g. of stone dust t a k e n for 100 m l .
of ascitic fluid would lead t o a solubility of 9 mg./100 m l . if taken
separately. The figures set out in table I demonstrate these points
more fully 1 . The silicate minerals are much less soluble t h a n quartz
in t h e ascitic fluid used 2 and t h e stone dusts are only slightly soluble,
TABLE III.

EFFECT OF MINERAL BUSTS ON THE DISSOLUTION OF QUAETZ

(Reduction in amount of silica dissolved from 2 g¡ quartz in 100 ml. of ascitic
fluid expressed as percentage)

Sericite (2 g.)
Biotite (2 g.)
Chlorite (Mass.) (2 g.) . .
Chlorite (Saxony) (2 g.)
Kaolin (1 g.)
Kaolin (4 g.)
A1 2 0, (1 g.)
Al2Os(4g.)
D. and C. stone dust (1 g.)
D. and C. stone dust (2 g.)
D. and C. stone dust (4 g.)
C. and W. stone dust (1 g.)
C. and W. stone dust (2 g.)
P. and P. stone dust (1 g.)
P. and P . stone dust (2 g.)
Somerset Greys (1 g.) . .
Somerset Greys (2 g.) . .
Pennant Bock (1 g.) . . .
Pennant Bock (2 g.) . . .

1 day
Per cent.

3 days
Per cent.

6 days
Per cent.

3
59
90
96
28
58
63
28
39
44
55
23
28
37
42

49
79
80
81
0
62
96
98
38
69
75
20
38
37
61
36
44
44
44

61
»1
78
81
18
64
96
97
27
66
74
32
40
50
64
37
57
56
65

despite their free silica contents, which in some cases are large. The
silicic acid dissolved from the coal a n d coal-mine dusts is, of course,
related t o t h e silicate minerals contained in them a n d is of the same
order as t h a t from t h e stone dusts.
T h e a p p a r e n t depression of t h e solution of the free silica contained
in the stone dusts suggested t h a t t h e effect might be extended t o quartz
admixed with t h e stone dust. This proved to be the case, as is shown in
table I I .
1
The methods used in this investigation—preparation and examination of dusts, solution experiments, separation of solid from liquid phase,
determination of dissolved silica, etc.—were those described in the previous
communication (King and McGeorge, 1938).
2
This ascitic fluid was of a more alkaline reaction (pH 8.0) than that
used in the previous investigation, and the figures for solubility of the
various dusts are hence not identical with those recorded in the previous
paper.
The mineral dust—sericite, chlorite, biotite and kaolin—were kindly
given us by Dr. A. Brammall, the stone dust and coal dust by Dr. A. G. B .
Whitehouse, a n d the air-borne coal-mine dusts by Professor H. V. A. Briscoe.

APPENDIX X H I : THE DEPRESSION OF SILICA SOLUBILITY

215

The addition of increasing amounts of stone dust to 2 g. of quartz
suspended in 100 m l . of ascitic fluid led to a progressive decrease, in
the silicic acid dissolved, from 7 mg. SiOa/100 ml. to 1.7 mg. when twice
as much stone dust as quartz was present. The phenomenon, moreover,
seemed to be a general one, as a similar depression was obtained with
several stone dusts and silicate minerals (table III).
Coal dust, and the air-borne dusts from coal mines, appear to depress
silica solubility in proportion to their silicate mineral content. Pure
carbon (Kalbaum's "carbo activ") and charcoal of low ash content
have no effect on the solution of quartz. The depressions seen in table
IV are probably dependent on the presence of stone dust in the coal.
TABLE IV. — EFFECT OF COAL DTJSTS ON THE DISSOLUTION OF QTJABTZ

(A decrease in the amount of silica dissolved is denoted by a negative
percentage figure : an increase by a positive percentage figure)
1 day

3 days

6 days

Per cent.

Per cent.

Per cent.

Am. D. air-bome dust
(16 per cent, ash)
lg.)
[2 g.)
4g-)
Am. E . air-borne dust
(39 per cent, ash)
lg.)

•
.
•

+

23
0
— 23

+ 30
+ 19
— 6

+
1
— 7
— 21

•

2g.)

•

4g.)
Am. I . air-borne dust
(18 per cent, ash)
lg-)
2g.)
*g-)
Am. J . air-borne dust
(25 per cent, ash)
lg-)

•

— 21
— 43
— 53

— 38
— 61
— 63

— 47
— 70
— 75

•
•
•

+ 25
— 9
— 33

+ 1
— 22
— 50

— 4
— 11
— 54

•

'2 g . )

•

4g-)

•

— 39
— 64
— 71

— 9
— 51
— 64

— 9
— 57
— 71

lg-)
2g.)
'4 g-)

•
•
•

+
5
+
5
— 40

+ 22
+ 24
— 30

— 2
+
3
— 35

•
•
•

+ 36
+ 29
+ 11

+ 30
+ 47
+ 34

+
+
+

•
•

— 25
— 47
— 54

— 33
— 39
— 55

— 40
— 45
— 62

•
•

+ 3— 34
— 51

+
8
— 52
— 69

+
8
— 47
— 69

Am. anthracite (powdered)

Cy. bottom coal (powdered)
lg-)
2g.)
4g-)
Ba. soft coal (powdered)
lg-)
2g.)
4g-)
Yn. anthracite (powdered)
lg-)
2g.)
4g.)

7
7
5

Depression of silica solubility may be dependent on adsorption of
the dissolving silicic acid, on its precipitation as an insoluble silicate,
or by the coating of the silica particles by some constituent of the
depressor. In table V are shown the results of adding silicate and mixed
stone dusts to a saturated solution of silica in ascitic fluid, which had
been previously filtered free of all suspended particles.

216
TABLE

SILICOSIS

V.

P R E C I P I T A T I O N O F DISSOLVED SPLICA B Y M I N E R A L

DUSTS

(Decrease of dissolved silica in a "saturated solution" of quartz in ascitic
fluid, 9 mg./100 ml., due to added dust)

Sericite
Biotite . . . . . . .
Chlorite (Mass.) . . .
Chlorite (Saxony) . .
Kaolin
Kaolin
Al a 0 3
Al a 0 3
.
D. and C. stone dust .
D. and C. stone dust .
O. and W. stone dust
C. and W. stone dust
P. and P. stone dust .
P. and P. stone dust .
Somerset Greys . . .
Somerset Greys . . .
Pennant Rock . . .
Pennant Bock . . .

g. oí dust
added per
100 ml.

Per cent, decrease In dissolved silica
3 days

6 days

2
2
2
2
2
4
2
4
2
4

24
33
76
69
29
26
94
95
31
44
53
50
25
21
10
20
5
14

17

4
2
4
2
4
2
4

1 day

32
38
94
95
24
36
40
46
27
37
11
22

75
66
26
32
94
96
38
54
18
27
24
30
12
30

The depression of the amount of dissolved silica by the various mineral
dusts is here clearly a case of precipitation. The silicic acid dissolved
in the liquid probably reacts with some constituent of the mineral dust
to form an insoluble precipitate. Further work on the composition of
the solid phase of these mixtures should reveal an enrichment of the
dust in terms of Si0 2 and may possibly reveal the nature of the change
taking place. Most mineral silicates are not of rigidly uniform composition and it is possible that many of them may be capable of reacting with
excess free silica with a consequent formation of some sort of a complex
of higher silica content than the original.
The precipitation of silica from its solution by the coal dusts and
air-borne dusts from coal mines seems again to be related to the ash
content, i.e. to the amount of silicate mineral contaminating the coal
(table VI).
Those coal dusts with a high silicate content cause a considerable
precipitation of the dissolved silica, those with a small silicate content
cause less silica to be removed from solution. It would seem that
adsorption of silicic acid by the carbonaceous particles of the coal dust
plays a negligible role in the retardation of solution of silica and in the
precipitation of silica already existing in solution. The coating of
the solid particles of silica by a layer of the "depressor", e.g. of aluminium hydroxide, would account for the depression of solubility shown
in table III, but this sort of mechanism cannot furnish a satisfactory
explanation of the precipitation of dissolved silica shown in tables V
and VI.

APPENDIX X u ! : THE DEPRESSION OP SILICA SOLUBILITY
TABLE VI.

217

PRECIPITATION OP DISSOLVED SILICA BY COAL DUSTS

(Decrease of dissolved silica in a "saturated solution" of quartz in ascitic
fluid, 9 mg./100 ml., due to added dust)
g. of dust
added per
100 ml.

Am. D. air-borne dust . . . .
Am. D. air-borne dust .
Am. E . air-borne dust .
Am. B . air-borne dust .
Am. I . air-borne dust .
Am. I. air-borne dust .
Am. J . air-borne dust .
Am. J . air-borne dust .
Am. anthracite (powdered)
Am. anthracite (powdered)
Cy. bottom coal (powdered)
Cy. bottom coal (powdered)
Ba. soft coal (powdered) .
Ba. soft coal (powdered) .
Yn. anthracite (powdered)
Yn. anthracite (powdered)
1

Per cent decrease in dissolved silica
1 day

3 days

6 days

6
9
4
14
8
16
20
27
11
23
10
12
4
16
(8)
(6)

1
4
21
18
12
23

0
4
20
21

2
4
2
4
2
4
2
4
2
4
2
4
2
4
2
4

—

21
8
23
0
10
22
24
(8)
(6)

—
12
7
21
3
29

•(2)1

9
15
(1)
(5)
(4)

Figures in parentheses denote an increase.

Discussion
T h e stone dusts used i n this s t u d y 1 were samples of dust which h a v e
long been used for t h e stone-dusting of coal mines in Great Britain
with t h e purpose of diminishing t h e hazard of explosion. Industrial
experience during m a n y years h a s led t o the belief t h a t stone-dusting
in coal mines has n o t introduced or increased t h e silicosis hazard.
Indeed the belief h a s slowly evolved t h a t stone-dusting actually diminishes the risk of silicosis in mines (cf. Haldane, 1917, 1931), a n d t h a t
certain mineral silicates m a y offset t h e action of quartz in its relation
t o t h e disease.
Recent experimental work on t h e effect of mineral dusts on t h e lungs
and other tissues of animals (Gardner, 1938) has appeared t o indicate
t h a t m a n y mineral silicates m a y modify and even retard t h e action
of free silica. The finding t h a t stone dusts and mineral silicates depress
the solution of quartz is n o t advanced as an explanation of t h e clinical
a n d experimental experience, b u t as a n interesting parallel which merits
further investigation.
REFEBENCBS

GARDNEB, L. U. (1938). 2nd International Silicosis Conference, Geneva.
BALDASTE, J . (1917). Trans Inst. Min. Eng. 55, 264.
(1931). Ibid., 80, 415.
K I N G , E . J . and MCGEOBGE, M. (1938).

Biochem. J. 32, 417.

WHITEHOUSE, A. G. R. (1938). J. Indus. Hyg.a.

Tox., Nov. 1938, p . 556.

1
They were furnished by Dr. A. G. R. Whitehouse, of the Safety in
Mines Research Laboratory, who finds them t o have a depressing action
on the solution of free silica in sodium carbonate solution (1938).

APPENDIX XIV
DETERMINATION OF THE PROPERTIES WHICH MASKS SHOULD
POSSESS
By D R . A. H. VOSSBNAAK

I n examining the properties of masks careful reproduction of the
special peculiarities encountered in practice should be given first consideration.
The respiratory volume of the men at work should be observed during
the passage of air for a given unit of time.
Secondly, it is essential to determine the dust catching capacity for
particles of dust of the dimensions of 2 ;JI and under.
Thirdly, the resistance to breathing should be determined at the same
time as the dust-catching capacity.
Further, a fundamental condition is that examination of the abovementioned properties should be continued for several hours.
I t is also advisable to choose those types of dust against which it is
intended to use the mask as a means of protection.
Reliable methods of measurement should enable the composition
of the dust to be determined both as regards quantity and dimension
of the particles.
I n the dust laboratory attached to the Dutch coal fields we have
constructed an apparatus which corresponds to these conditions and the
construction of which may be described as follows :
The principle part is the dust chamber (B) in which the mask to
be examined is fixed by means of perfectly fitting attachments.
•The production and maintenance of an atmosphere with an unvarying dust content are obtained in the following manner :
A compressed-air pipe supplies compressed air, which is reduced to
a slight surplus pressure. The air passes through several parts to
purify and dry it and then enters a pressure regulator in which a variable
quantity of the excess air is allowed to escape and thence passes into a
hydrostatic speedometer calibrated by means of a gas meter in such a
way that about 400 - 600 litres of air per hour pass through the apparatus.
A dust-distributing apparatus is then set in motion. The dust is supplied
regularly by means of a small bucket chain. To avoid, as a result of
escape of the finest particles of the dust, a dust content relatively poorer
in small particles, a quantity of dust equal to that furnished by the
bucket chain is regularly withdrawn by means of an endless screw.
In addition to this, a mixer is placed in this part of the apparatus in
order to prevent agglomeration of the dust.
The compressed air leaves this part of the apparatus by way of a
ring furnished with several very fine holes.
The finest particles of the dust are caught up in a cyclone and carried
away by the air current towards a sedimentation chamber, the dimensions

APPENDIX XIV : PROPERTIES O F MASKS

219

of which are chosen in such a way that only dust particles of about
2 ¡A reach the pipe at the top, while the largest particles fall down.
The air charged with floating dust particles then reaches the dust
chamber. In the dust chamber only particles of 2 ,J. and under can
be measured.
In order that it may be possible to obtain and maintain in the dust
chamber any desired particle concentration of the floating dust, the air
may be rarefied by a second compressed-air pipe possessing the same
measuring and purifying accessories as the dust-distributing pipe.
The latter is further supplied with a device by means of which a
fraction of the dust may be allowed to escape.
The dust concentrations are measured by means of a thermal precipitator introduced into the dust chamber through an aperture.
Two kinds of dust are used, sandstone and pulverised clay dust,
both after screening (normal screen No. 80—6,400 squares to the cm2).
Masks are examined in two types of dust concentration :
A. About 100,000 particles per cm3.
B. About 30,000 particles per cm3.
Concentrations such as those in A occur in mines as quasi-maximum
concentrations during shot-firing. Concentrations of the. order of B
are comparable to the average concentration to which rock hewers are
exposed after blasting for the two minutes during which they return
to the cutting face through a cloud of dust.
The methods of measurement are similar to those used in the Dutch
coal fields, consequently the optic method is preferable, permitting of
rapid and continual determination of the dust concentration by means
of the Tyndall beam effect obtained.
The air which leaves the dust chamber is carried away by the tube L
and passes through the half of the photometer in which the Tyndall
effect obtained is determined.
The intensity is proportionate to the dust concentration.
The air is aspirated in two ways through the mask. Where the mask
to be examined does not possess an escape valve and the air must in consequence pass through the mask and then return through it, an artificial
lung is set in action, the number and amplitude of the movements of
which may be regulated at will. In the case of examination of masks
with valves, in which the air always passes through the mask in the same
direction, an injector is set in motion and the quantity of air aspirated
by it per unit of time is likewise determined by means of a speedometer.
In both cases the air aspirated through the mask passes into the pipe
in which the other half of the Tyndallmeter is introduced into R.
The Tyndall effect obtained in R may be immediately compared to that
obtained in L. The ratio found represents, after slight adjustment,
the percentage of dust which the mask allows to pass.
In order to measure the respiratory resistance of the mask, we use
pressure meters and recorders, which by means of tubes and a stopcock with three orifices may be placed in communication with the space
in which the mask is placed and with the point P behind the mask.
The difference in pressure between P and B, less the difference in pressure
between these two points when there is no mask between them, indicates
the resistance of the mask. It is obvious that certain further particularities must also be observed in order that the measurements may
be rendered reliable. I have restricted mention here to a description
of the principle of the method adopted.

220

SILICOSIS

In conclusion, I give below the results of measurements of two dust
masks, the Willson flat mask and the Degea hose mask.
Surface
in cm'

Mask

Flat Willson A
Model
B
Hose mask

To start
with

After
2 mins.

5i/4

20

5i/2

sy*
ey2

2%

3i/4

4y 4

2

3

200

A
1,300
B

Particles of dust passed
through after :

Resistance in mm. of water

1

After
30 mins.

2 mins.

15 mins.

30 mins.

—
10%

4

1

10

—

A <= Dust concentration of about 100,000 particles per cm3.
B = Dust concentration of about 30,000 particles per cm3.

I t should be stated here that the Willson mask apparently becomes
quickly obstructed and on this account presents greater resistance.
I t may be restored to its original condition by shaking.
The hose mask allows a certain quantity of dust to pass through
at the outset. It would therefore be advisable to substitute another
type of material for that at present used.
This apparatus for testing of masks obviously permits of determination of their physical properties only. Other points, such as good
face fit, dead space, etc., should be made the subject of a special test
in accordance with their importance.

INDEX
A
Age factor, 23.
Agenda of the Conference, 5.
Alkalis-accelerating action, 9, 10,
28, 137.
Aluminium dust, 9 et seq.
, for prevention, 88, 207.
Analysis, chemical, 33.
—-, regional, 34.
ANDEBSSON, G., 4, 94.

Anthracite, 33, 35, 123.
Anthraco-silicosis, 33, 123.
Anti-dust campaign, 90, 101.
Arc welders, 39.
Assessment, 65, 99, 102, 164.
Australia, 3, 4.
B
BADHAM, Ch.,

3,

17,

24,

28,

30,

34, 40, 46, 53, 56, 61, 62, 74,
81, 90.
Belgium, 3, 4, 31, 60, 69, 140.
Blood, see Silica.
BOHBBN, 64.

Bronchitis, 72.
BUTLEB, H., 7, 94.

C
Canada, 3, 9, 68, 69, 71, 75, 80,
84, 88, 211.
CABOZZI, L., 91.

Coal, bituminous mines, see Mines.
— dust, 11, 21, 28, 96.
— mines, see Mines.
Compensation, 64, 69.
Constitution, 23, 63.
Cryolite, 36.
—, artificial, 38.
D
Denmark, 3, 22, 36.
D E N N Y , J . J.,

209.

Diagnosis, early, 40, 41, 42, 97,
156, 191.
Disability, 65, 73, 96, 162.
—, clinical, 66.
— tests, 67, 68.
DUBKAN, 108.
Dust concentration, 49, 52..
—
content, 52, 60.
—, admixed with silica, 95.
—
estimation, 96.

Dust, exhaust ventilation, 78.
—
exposure, 57, 98.
—, in lung tissue, 57.
—, in sputum, 60.
—
instruments, 53 et seq., 60,
65, 169.
—
investigation, 52, 97, 169.
—, non-siliceous, 8, 95.
—, pathogenicity, 8, 99.
—
sampling, 52, 64, 97.
—, size frequency, 52, 55, 97.
E
Electric welders, 39, 200.
Emery grinders, 39.
— paper making, 39.
Emphysema, 50, 72.
Examination, initial, 45, 74, 100,
165.
—, periodical, 45, 75, 100.
F
Faeces, see Silica.
Fibrosis, pulmonary, 8, 19.
Film "Stop Silicosis", 28.
Fluorides,, 38.
Fluorosis, 36.
Fluospar, 39.
Foundry, 40.
France, 3, 4, 3 1 , 39, 82.
Fuller's earth, 25, 134.
G
GABDNEB,

L. II.,

3,

8,

15,

16,

18, 23, 38, 40, 58, 60, 61, 89, 108.
GEOBGE, W. E., 4, 43, 65,

74.

Gold mines, see Mines.
Granite, 9.
Great Britain, 3, 21, 35, 52, 62,
68, 69, 78, 81, 86, 134.
GBEENBTTBG, L., 4, 18, 35, 40,

59,

64, 67, 79, 82, 89.
GUDJONSSON,

S.,

3,

22,

34,

36,

31,

32,

39,

39, 83.
H
Haematite, 25, 26, 40.
HAXJSSEB, G.,

4,

18,

45, 49, 77, 82, 86, 91.
Heart failure, 18, 50, 201.
Hearthstone, 28, 136.

222

SILICOSIS

I
Infection, 96, 97.
—, superimposed, 15, 17.
Inoculation, intracutaneous,
IRVINE, L. G.,

3,

12,

20,

26,

24.
30,

40, 41, 46, 49, 57, 59, 66, 67,
70, 71, 72, 76, 82, 87, 151.
IRWIN, D. A.,

Japan,

208.

J
4, 32, 51, 57, 75, 81, 94.
K

Kieselguhr, 22.
KING,

E. J.,

3,

10-23,

26,

33,

52, 61, 71, 87, 169, 211.
KlTAOKA, J., 4, 94.
L

3, 9, 39,

ROBSON, W. D.,

184.

M
MACGEORGE, M.,

68,

69,

MIDDLETON, E. L., 3, 7, 8, 21,

25,

30, 35, 52, 54, 62, 68, 69, 73,
78, 82, 86, 92-134.
Mine-, coal, 28, 60, 96, 123, 139,
204.
—, dust prevention in, 78, 80.
—, gold, 63, 151.
—, ventilation, 81.
N
Nasal filtration, 71.
Netherlands, 4, 31, 41, 56, 60, 7 1 ,
76, 77, 81, 85, 204.
New South Wales, 3, 28, 43, 65, 74.
New York, see United States.
O
Ontario, see Canada.
A. J.,

3,

7,

33,

39,

42, 54, 58, 62, 63, 73, 78, 80,
85, 90, 92, 93, 94.
P
PATTERSON, H. S.,

169.

Peritoneal reaction, 20.
Permutite, 27.
Pneumoconiosis, early diagnosis, 43.
3,

8,

9,

13,

16,

19, 21, 25, 27, 29, 34, 41, 49,
57, 59, 60, 61, 62, 63, 69, 87, 92.

208.

«OMAN, W., 211.

s
Sandblasting, 18, 39.
SAYERS, R.

211.

Masks, 84 et seq., 218.
Members of the Conference, 3.
Mica, 61.
Microscope dissecting, 29.

POLICARD, A.,

R
Racial factor, 63.
Radiography, 26, 40, 41, 43, 47,
66, 157, 194.
Removal from dusty occupation,
44, 46, 71, 97, 160, 204.
Report of the Conference, 95.
—, discussion, 93.
Rhodesia, 48.
71, 72, 75, 80, 83, 88.

LANGELEZ, A., 3, 15, 18, 30,
32, 38, 41, 69, 73, 77, 140.
Lung tissue (dissociation), 57.

OBENSTEIN,

Q
QuiNET, 4.

R I D D E I A , A. R.,

Land work, 44.
LANG, F., 64,

Preventive measures, 78, 100, 204,
208.
, by personal protection, 84,
100, 218.
, by aluminium dust, 88, 208.

R-,

3,

20,

33,

35,

50, 67, 71, 72, 86, 92, 124, 191.
Scouring powder, 137.
Sericite, 10, 23, 60.
Siderosis, 2C0.
Silica, action on tissues, 19.
—, alkalis, 9.
—, body fluids, 52, 167.
—, depression of solubility in body
fluids, 88, 211.
—, free, 95, 108.
—, in blood, 43, 97.
—, in faeces, 43, 97.
—, in urine, 43, 64, 97, 199, 200.
—, protective action of various
minerals against free, 108.
Silicates, 3, 11, 24, 60, 95, 96.
Silicosis, acute, 3, 10, 12, 18, 21, 96.
—, clinical types of, 46, 152.
—, biochemistry of, 105.
—, diagnosis of, 40, 41, 42, 97,
156, 191.
—, experimental, 8, 13, 15, 16,
17, 18,20,22,24.
—', human and animal, 20.
— in mines, see Mines.
— in Switzerland, 184.
—, pathology of, 8, 21, 46, 95, 151.
—, pseudo-tumoral form, 40.
— symptoms, 191.
Silicotic nodule, 18, 19, 21.
Sodium fluoride, 38.
South Africa (Union of), 3, 12, 40,
42, 49, 54, 57, 63, 66, 70, 76, 78,
151, 169.
Specific therapy, 87, 101.
Sputum, 60, 97, 200.

223

INDEX
Swiss National Accident Insurance
Fund, 64.
Switzerland, 64, 184.
T
Talc, 39, 40.
Team work, 62.
Thermal precipitator, 62, 169.
Transvaal, see South Africa.
TSUKATA, J., 3, 10, 32, 51, 57,

V
VAN
75,

81.
Tuberculosis, 34, 35, 44, 47 (note),
69, 95.
TZAUT, Ch., 4, 91,

94.

IT
United States, 3, 4, 9, 33, 38, 40,
67, 71, 83, 89, 108, 191.
Urine, see Silica.
BBKEDEN,

J.,

4,

30,

70.

Ventilation, local exhaust, 71, 79.
—, general, 80.
Vitamin factor, 63.
VOSSENAAB, A. H.,

4, 10, 32,

41,

56, 60, 72, 76, 77, 81, 85, 105,
• 205, 218.