Respirable Dust and Respirable Silica Concentrations from Construction Activities

ABSTRACT

Measurements of respirable dust (RD) and respirable crystalline silica (RCS) concentrations are reported for a range of activities, materials and workplaces in the construction industry. Some activities leading to high dust exposures are identified, such as work in confined spaces, the use of hand-held Stihl saws and angle grinders without suppression or extraction, and dry sweeping and removal of rubble. RD and RCS concentrations are related to conditions on site including such factors as the softness, friability and silica content of the material; the weather conditions (dampness) and whether indoors or out; the proximity of the source of the dust to the worker’s breathing zone, and the use of water suppression or extraction and general ventilation.

MONITORING

Sampling and Measurement

Sampling and gravimetric determination of respirable airborne dust was carried out following the procedures and recommendations of MDHS 14/2, using the sampling  convention in force at the time (see below). Direct on-filter analysis for quartz and, where appropriate cristobalite, was carried out in accordance with MDHS 51/2 and MDHS 76. The percentage of quartz or cristobalite in the respirable dust was calculated from the mass of quartz or cristobalite and the total mass of dust on the filter. Bulk samples of the material(s) being worked on were taken whenever practicable and their quartz and cristobalite content determined using standard Health and Safety Laboratory (HSL) operating procedures, based on Taylor.

The Measurements in Context

The survey reported in this paper took place between spring 1996 and spring 1997. The harmful effects of exposure to silica, principally silicosis, and possibly also the potential to produce lung cancer, result from inhalation and partial retention of crystalline silica in the lung. The biologically relevant measure for occupational health is therefore believed to be the concentration of RCS. Up to the end of 1996, the BMRC convention reference curve was used to define respirable dust and the UK exposure limits were 5 mg.m-3 (OES) for RD and 0.4 mg.m-3 (MEL) for RCS. From January 1997, the ISO/CEN convention came into effect. The mass of dust sampled under the new convention is about 20% less than under the old and to allow for this the exposure limits were revised to 4 mg.m-3 for RD and 0.3 mg.m-3 for RCS. Depending on when the sampling was carried out, some results reported here use the old convention and some the newer although for practical purposes, this does not greatly matter in the present survey: Sampling variations are notoriously large in measurement of airborne dust concentrations and where interpretation is attempted it is based on gross differences in concentrations.

Exposure limits apply to the 8-hour time-weighted average (TWA) concentrations. All the airborne concentrations in this paper are from personal monitoring except where noted as being from static samplers (results from static sampling should be viewed with caution as they are strongly dependent on the exact position of the sampler in relation to the work). The results are the prevailing concentrations averaged over the sampling period and are not TWAs. The TWA concentration will be lower than the values given if the particular job is done for only part of the working day. But in the majority of cases, the worker does the job for most of his working day and the measured concentrations quoted should be a fair indication of his TWA exposure.

It should also be noted that the control measures or type of Respiratory Protective Equipment (RPE) to be used are determined by the peak airborne concentration (worst case) irrespective of its duration. No short-term exposure limit has been specified for RCS but HSE guidance indicates that it should not normally exceed 3 times the 8-hour TWA, i.e. a maximum of 0.9 mg.m-3 over 15 min. Health surveillance is advised for workers regularly exposed to respirable crystalline silica levels exceeding 0.1 mg.m-3, 8-hour TWA.

RESULTS

RD and RCS Concentrations Associated with Hand-Held Power Saws

Comparison of Different Materials

Table 1a shows details of the work, the RD and RCS concentrations and the crystalline silica content of the material and the respirable dust measured for the same worker cutting paving kerb, blue brick and concrete breeze block with a petrol-driven Stihl saw without dust suppression or extraction in the same place on the same site on the same day in a trial which the site manager kindly helped HSL to carry out (fig. 1). Data on similar cutting of facia block on another site are also included in table 1b. The density of the visible dust cloud (fig. 1) varies markedly with position and direction relative to the Stihl saw and the worker, whose exposure will be strongly dependent on his exact working position and technique.

High RD and RCS concentrations are indeed found, generally in excess of the exposure limits (except for the RCS concentration from sawing the facia block). The RD concentrations are highest for the softest and most friable materials and lowest for the hardest. The RCS concentrations appear to be determined by the crystalline silica content of the material as well as its softness or friability.

The percentage of crystalline silica in the respirable dust is in every instance lower than that of the bulk material being cut. Partly this may be the case because quartz and cristobalite are harder than the other components present and therefore less easily comminuted into particles of respirable size. But also cements are normally finely ground when manufactured and the cement component of the paving kerb, breeze block and facia block may have more particles in the respirable size range to begin with. The same may apply to the mullite component of the brick produced when the fine-grained kaolinite clay is fired.

Wet Suppression

Table 2 shows measured RD and and RCS concentrations for sawing under wet conditions, in one case (table 2a) with a hand-held Stihl saw outdoors in wet weather and in the others (table 2b, c) indoors with much larger wall and floor saws fitted with water sprays, whose main purpose is to reduce wear on the saw blade but which incidentally help to suppress dust.

The results in table 2a are roughly comparable with those for the paving kerb containing 33% quartz in table 1. As expected, the RD and RCS concentrations are lower in wet conditions even without any kind of suppression. The other results in table 2b, c show that with water sprays in use on the saw much lower dust concentrations, well within the exposure limits, can be achieved even though the saws are much larger and more powerful. A similar though more dramatic reduction in RD and respirable quartz concentrations during concrete cutting has been demonstrated in trials of water suppression systems for hand-held saws.

The set of results in table 2a does not fit the observation that the crystalline silica content of the dust is usually lower than in the material being cut and this is unexplained. In the other sets of results (table 2b, c), the quartz content of the dust is lower than that of the material if comparison is made taking account of the composite nature (surface screed + internal concrete) of the wall and floor.

RD and RCS Concentrations during Drilling and Breaking Out Concrete

When drills are in use outdoors or in well-ventilated indoor areas, the RD and RCS concentrations (table 3) are lower than for saws where conditions are otherwise the same (compare table 3a and table 2a in which the measurements were made on the same site on the same day). Though perhaps higher than desirable, the dust concentrations are within the exposure limits; the amount of dust visible is also sometimes less (fig. 2). The other results in table 3b are for the same day and site as those in table 2b, c but the comparison is not straightforward as the saws had water spray suppression and the drills and jackhammers did not. A better comparison may be with the materials containing about 30% quartz in table 1a, in which the sawing was done without water suppression: this supports the view that RD and RCS concentrations are lower for drilling than sawing under otherwise roughly similar conditions.

Alarmingly high RD and RCS concentrations may occur when drilling is carried out in confined, poorly ventilated spaces (table 4a). This was an exceptional case and work extended over no more than half a day but the conditions were still totally unacceptable. The lower concentrations in table 4b occurred under less extreme conditions (fig. 3) but even so they demonstrate that quite simple extraction properly used can give a marked improvement. RPE was worn, correctly fitted and appropriate for the dust levels measured. The results in table 4b also provide another example of the reduction in dust concentrations in damp weather.

The composition of the respirable dust at the site in table 4b was completely different from that of the dust collected from below the drill hole. The respirable dust sampled contained mainly cristobalite and quartz with a little lime (from cement) and haematite. The respirable dust sampled must have come from the brick lining the arch since cristobalite could only have been produced at high temperature during brick-making. The dust from below the drill hole contained some quartz but is mainly dolomite. It is suspected that dolomitic limestone has been used as fill behind the brick lining of the arch and that the material at the drill hole comes from the drill tip whereas the airborne dust comes from abrasion with the brick at the outer end of the hole.

Measured RD and RCS concentrations for workers breaking out concrete using various types of equipment and clearing away the resulting rubble are shown in table 5. The ’crusher’ breaks up a concrete wall or floor between hydraulically powered jaws.The ’burster’ is inserted into a hole about 20-30 cm in diameter in a concrete wall drilled by auger; its hydraulic rams then push outwards to crack the surrounding concrete. Exposure to RD and RCS is significant for most workers and in some cases exceeds the exposure limit (no RPE was used). But the RD and RCS concentrations are very variable probably because the men did different jobs with different tools as the work required. Significant exposures were found for shovelling and clearing up rubble although such exposures are not obviously related to tools which might generate dust. A similar mean RCS concentration (0.25 mg.m-3) has been reported for workers transporting demolition waste with a wheelbarrow.
RD and RCS Concentrations during Sweeping and Clearing Up

RD and RCS concentrations measured for workers doing this type of work are given in table 6. The concentrations are higher for hand sweeping than for using the mechanical sweeper, which has a cover and vacuum extraction over its rotating brushes. A similar conclusion emerged in the much more comprehensive survey; the RCS concentrations in table 6 are in the ranges reported there. A lower RD and a slightly lower RCS concentration are found for outdoor work (table 6b).

Powered equipment is evidently not an essential requirement to produce RD and RCS concentrations in excess of the exposure limits which can be reached in simple dry sweeping. The design of the mechanical sweeper clearly plays a significant part in reducing dust and silica exposure.

RD and RCS Concentrations during Chasing and Grinding Cement

Table 7 gives details of the working conditions and RD and RCS concentrations for chasing cement with an angle grinder with some additional observations on related work. RD and RCS concentrations are as high or higher than for dry cutting with a Stihl saw (table 1). The results in table 7a are very variable partly because of interruptions to the work and partly because the extraction unit failed on one of the angle grinders. The particular type of extraction used does not appear to be effective since the RD and RCS concentrations without extraction are no lower than those when the extraction was working.

The slightly lower RD and RCS concentrations reported in table 7b may be related to the hardness of the cement floor screed as compared with the friability of the cement being chased out at the first site (table 7a). In wetting down the floor cracks ahead of chasing, water was applied rather sparingly. The worker wetting down was further from the source of the dust and lower airborne concentrations would therefore be expected for him. So too was the breathing zone of the worker grinding the filled cracks level; again the RD and RCS concentrations are lower but still significant.

RD and RCS Concentrations at Concrete Crushers

Following the introduction of landfill tax, large machines have come into use to crush concrete rubble from demolition for use as hard core. The operator normally stands on the upper platform of the crusher and an enclosed cab is often provided as protection against noise and weather. Water jets are sometimes fitted over the conveyor, at the jaws and at the feed point but on only two of the occasions when sampling was carried out were these in use. Work details and RD and RCS concentrations at concrete crushers are given in table 8.

In dry weather these machines look very dusty but it seems that appearances are deceptive and that in this survey the actual respirable dust concentrations around concrete crushers, even without water suppression, are well within the exposure limits. Presumably most of the visible dust consists of particles larger than the respirable size range. Paradoxically, the highest RD concentration was for the one crusher with water jetting in use although the RCS was not particularly high in that case. The anomaly may be related to the composition of the material being crushed. The use of water suppression does not seem to make very much difference to the RD and RCS concentrations but there would almost certainly be dust arising from other work on a demolition site.
