Dust exposure and pneumoconiosis in a South African pottery. 1. Study objectives and dust exposure

ABSTRACT

Dust exposure and pneumoconiosis were investigated in a South African pottery that manufactured wall tiles and bathroom fittings. This paper describes the objectives of the investigation and presents dust measurement data. x Ray diffraction showed that the clays used by the pottery had a high quartz content (range 58%-23%, mean 38%). Exposure to respirable dust was measured for 43 workers and was highest (6.6 mg/m3) in a bathroom fitting fettler. Quartz concentrations in excess of 0.1 mg/m3 were found in all sections of the manufacturing process from slip production to biscuit firing and sorting. The proportion of quartz in the respirable dust of these sections was 24% to 33%. This is higher than is usually reported in English potteries. Four hundred and six (80%) of the 509 workers employed at the pottery were potentially at risk of occupational lung disease. The finding of large numbers of pottery workers exposed to unacceptable dust concentrations is not surprising as poor dust control was found in all six wall tile and sanitary ware factories surveyed by the National Centre for Occupational Health between 1973 and 1989. Dust related occupational disease can be expected in potters for many years to come.

MATERIALS AND METHODS

THE PRODUCTION PROCESS

The factory in which the survey was conducted manufactured both wall tiles and small bathroom fittings (such as soap dishes). The general manufacturing process for these articles was as follows: the raw materials were crushed and milled, and converted into a “slip” by the addition of water. After removal of impurities the slip was pumped through a press where “dewatering” occurred and the contained air was expelled. The resultant material is known as the ceramic body. The method adopted for shaping the body into the required product (ware) depended upon the complexity of the article. The more complex bathroom fittings were slip cast by liquifying the body (blunging) and pouring it into a plaster of Paris mould. The wall tiles were shaped by first drying the body to a “dust” (spray drying) and then pressing the dust in tile shaped dies (dry pressing). Once shaped, the blemishes and rough surfaces of the ware were removed by fettling or sponging.

The articles were fired before and after glazing. After the first firing the ware is known as “biscuit”. The biscuit ware was glazed with a liquid glaze and fired. This second firing produces a ware known as “glost”. The application of glaze to biscuit was by spraying for the bathroom fittings and by machine dipping for the wall tiles. The final process before packing and despatch was inspection to identify poor quality ware.

Besides the manufacture and glazing of wall tiles and bathroom fittings, the factory also produced floor tiles and refractory pieces for separating and supporting ware during firing, as well as manufacturing the glaze. The raw materials and production processes used in manufacture of glaze, floor tiles and refractory products were different from those used for wall tiles and bathroom fittings and are not considered in this paper. Wares were manufactured predominantly during one shift whereas firing and maintenance took place over a number of shifts. The work process was uniform and did not vary significantly from week to week. Housekeeping activity such as floor cleaning was done daily during the routine workshift. A personal protective equipment programme was not in place in the factory; consequently workers did not wear respirators during the performance of routine work.

RAW MATERIALS

The rough general formula for the wall tile and sanitary ware body used by this factory was clays 55%, silica sand (quartz) 35%, and calcium carbonate (calcite) 10-12%. Broken biscuit was added to these raw materials.

Six or seven clays were used. Bulk samples of the clays were analysed at the National Centre for Occupational Health (NCOH) using x ray diffraction. The methods of Klug and Alexander and Bradley were used for this determination.

To confirm the NCOH findings from x ray diffraction this investigation was repeated by an independent institution, the Council for Mineral Technology (MINTER). A sample of production dust was analysed as well. The MINTEK x ray diffraction was semiquantitative: minerals were categorised as major, minor, or trace constituents of the dust analysed. If a mineral constituted about 40% or more of the dust it was classified as a major constituent.

DUST CONCENTRATIONS

Exposure to dust was assessed as follows: (1) low dust areas were identified by walk through inspections conducted by a doctor experienced in occupational medicine, an industrial hygienist, and a factory management representative. Sections were classified low dust only if they were not directly involved in the production process and the doctor, the industrial hygienist, and the management representative found that they had low dust on inspection. To test this subjective evaluation personal breathing zone samples of respirable dust were collected from five workers in two of the low dust areas—namely, despatch and die shop. Quartz content of the samples was determined using x ray diffraction. The rest of the factory was classified as dusty and further assessed by collecting personal breathing zone samples of respirable dust from 38 workers and determining the quartz content of respirable dust for 12 of these samples using x ray diffraction. Measurement of dust was conducted over two weeks during a period of usual production. An experienced industrial hygienist selected individual workers to wear personal dust samplers. The hygienist inspected each manufacturing section and selected workers thought to be representative of all the important procedures or jobs (tasks) performed in the section (random sampling such as that proposed by the US National Institute for Occupational Safety and Health, was not adopted). Supervisors and cleaners who worked in more than one manufacturing section were categorised as general factory workers and were not selected to wear dust samplers.

Respirable dust concentrations in air in the workplace were determined by drawing a measured volume of air through a 25 or 37 mm diameter, 8 µm pore size membrane or glass fibre filter. The filters were in Casella or BCIRA cyclone heads placed in the breathing zone of workers selected to participate and sampling times ranged from 228 to 293 minutes. The respirable <7 µm dust thus collected was measured by weighing the filters before and after sampling. The procedure was as recommended by the UK Health and Safety Executive. Quartz content of respirable dust was determined using x ray diffraction for 12 dust samples collected on 25 mm diameter membrane filters.

RESULTS

MINERALOGICAL COMPOSITION OF CLAY AND PRODUCTION DUST

The quartz content of six clays, as determined by x ray diffraction at the NCOH, was 58%, 42%, 35% 35%, 34% and 23% respectively. Table 1 shows the detailed composition of three of the clays. MINTEK x ray diffraction confirmed that kaolinite and quartz formed major parts of all the clays and of the tile production dust (table 2).

RESPIRABLE DUST AND QUARTZ CONCENTRATIONS IN WORKPLACE AIR

Five personal dust samples were collected from the despatch and die shop areas; both of which were categorised as low dust during walk through inspections. Concentrations of respirable dust ranged from 0.02-0.4 mg/m3 with a median of 0.2 mg/m3. Quartz concentrations determined for these samples ranged from no quartz (undetected) to 0.03 mg/m3. These
are below generally recommended upper limits for quartz exposure—for example, the threshold limit value (TLV) is 0.1 mg/m3.

A further 38 personal respirable dust samples were collected from workers in seven dusty manufacturing sections. Table 3 shows these manufacturing sections by respirable dust and quartz concentrations in air in the workplace.

Average respirable dust concentrations were similar in the seven dusty sections sampled with the median dust concentrations ranging from 0/9 mg/m3 (biscuit firing and sorting) to 2.1 mg/m3 (bathroom fitting glazing). The range for individual workers was larger ranging from 0.2 mg/m3 (blunging in bathroom fitting section) to 6.6 mg/m3 (fettling of bathroom fittings). Concentrations varied within a single job category—for example, 1.2 mg/m3 to 6.6 mg/m3 for the bathroom fitting fettlers. As shown in table 3, median dust concentrations were a conservative measure of average exposure to respirable dust as the mean dust concentrations were generally higher than median concentrations.

Quartz concentrations were determined for 12 of these 38 samples of respirable dust. In the sections glazing tiles and bathroom fittings the proportion of quartz in respirable dust was low (7% and 8% respectively). In all other sections the percentage of quartz in the respirable dust was between 24% and 33% and the TLV of 0.1 mg/m3 was exceeded (table 3).

Table 4 shows the manufacturing sections of the pottery by degree of dustiness. Also shown are the number of workers employed in each section and the number who participated in the radiological component of the survey. Of the 509 workers, 173 (34%) were in high dust sections. Maintenance and electrical section workers spent about half of their working day in the low dust workshops and the rest in high dust sections; they were, therefore, classified as intermittent high exposure. The glazing and glost sorting sections were classified as high dust/low quartz sections based on dust and quartz concentrations shown in table 3. These 233 workers (46%) in intermittent high or high dust/low quartz sections were potentially at risk of excessive dust exposure. Sixty four workers were employed in the sections of the factory in which no assessment of exposure to dust was performed.
