Annals of Occupational Hygiene Advance Access originally published online on January 13, 2005
Annals of Occupational Hygiene 2005 49(4):335-343; doi:10.1093/annhyg/meh099
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© 2005 British Occupational Hygiene Society Published by Oxford University Press;
Original Article |
Assessment of Exposure to Quartz, Cristobalite and Silicon Carbide Fibres (Whiskers) in a Silicon Carbide Plant
1 Institut de recherche Robert-Sauvé en santé et en sécurité du travail, 505 De Maisonneuve Blvd. West, Montreal, Quebec, Canada H3A 1C2; 2 McGill University, 3450 University Street, FDA Building, room 31, Montreal, Quebec, Canada H3A 2A7; 3 Centre de santé et de services sociaux de l'Énergie, 1600 Blvd. Hubert-Biermans, Shawinigan, Quebec, Canada G9N 8L2
* Author to whom correspondence should be addressed. E-mail: dion.chantal{at}irsst.qc.ca
ABSTRACT
The main objective of the present paper is to report on the concentration of silicon carbide (SiC) fibres, crystalline silica and respirable dust in a Canadian SiC production plant and to compare the results with earlier investigations. The second objective is to tentatively explain the differences in concentration of the fibrogenic substances between different countries. The assessment of SiC fibres, dusts, respirable quartz and cristobalite was performed according to standard procedures. The highest 8 h time-weighted average concentrations of fibres were found among the crusher and backhoe attendants and the carboselectors with an arithmetic mean of 0.63 fibres ml–1 for the former group and 0.51 fibres ml–1 for the latter group. The results of respirable SiC fibres in the Canadian plant were lower than in the Norwegian and Italian industries. Most of the 8 h time-weighted average concentrations for quartz were less than or around the limit of detection of 0.01 mg m–3. The maximum 8 h time-weighted average concentration for quartz was found among the carboselectors (0.157 mg m–3), followed by the labourers (0.032 mg m–3). Similarly, most of the 8 h time-weighted average cristobalite measurements were less than the limit of detection of 0.01 mg m–3 except for the carboselectors where it was found to be 0.044 mg m–3. The assessment of the Italian occupational settings exposure demonstrated elevated quartz concentrations, while cristobalite was absent. The authors have concluded that the investigations that were performed in the last two decades in this field by researchers from different countries seem to support that SiC fibres (whiskers) constitute a major airborne health hazard.
Keywords: crystalline silica • silicon carbide (SiC) • SiC fibre
INTRODUCTION
Assessment of exposure to chemical hazards in the industry producing silicon carbide (SiC) abrasive material has been documented in several articles in the last two decades. Concentrations of fibrogenic dusts, such as quartz, cristobalite and SiC fibres, in the studies cited below showed variations from country to country. These differences have not yet been explained.
Smith et al. (1984)
were the first group of researchers to report on the complex mixture of chemicals found in an occupational setting. Eight-hour time-weighted average exposures to respirable particulates ranged from 0.01 to 9.0 mg m–3. The particulates contained varying amounts of quartz, ranging from <1 to 17%, and most exposures were below the threshold limit value (TLV®) of 0.1 mg m–3 recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) at that time. In addition, only traces of cristobalite (<1%) were found in the particulates and were well below the TLV® of 0.05 mg m–3 recommended by the ACGIH at that time.
A year later, the Norwegian researchers (Bye et al., 1985) reported on the concentration of SiC fibres during the industrial production of angular abrasive material. Although it was known that SiC may exist as whiskers and continuous fibres, Bye and colleagues were the first group to report on airborne fibre concentrations and these were in the range of 1 fibre ml–1 but could be as high as 5 fibres ml–1 for the mixing of raw material in Plant C. The TLV® proposed by ACGIH (2003) for fibrous SiC is 0.1 fibres ml–1. From a population of 100 fibres, the authors evaluated that the geometric mean diameter of these fibres was 0.23 µm and the geometric mean length 4.5 µm.
In 1987, the Canadian group (Dufresne et al., 1987a) reported on the concentration of carbonaceous matter, quartz, cristobalite, SiC, respirable dust, benzene soluble matter and 12 common polycyclic aromatic hydrocarbons. The geometric mean concentration of quartz was 0.086 mg m–3 (based on two samples only) from the Acheson furnaces in Plant A for the attendant unloading the old mix with a small diesel powered loader, and 0.112 mg m–3 (based on four samples) in Plant B for the attendant of the mixer/scale work operation. The highest concentration of cristobalite was 0.036 mg m–3 found in Plant B in the work area called the fourth level.
In 1992, the Italian research team (Scansetti et al., 1992) published a meticulous study on the assessment of exposure to quartz and SiC fibres in a single industry using the static sampling approach. The geometric mean concentration of respirable dust ranged from 1.29 to 5.25 mg m–3 and respirable quartz from 0.02 to 0.39 mg m–3. The polymorphs cristobalite and tridymite were not found in the occupational setting. Geometric mean concentrations of respirable fibres ranged from 0.06 to 2.75 fibres ml–1.
Recently, a job-exposure matrix covering the same three Norwegian plants (Romundstad et al., 2001) reported crystalline silica and SiC fibre concentrations among other contaminants. It was not stated whether the samples that were used to construct the job-exposure matrix were static or personal or whether they were actual or time-weighted average concentrations. Nevertheless, the results were estimated to range from 0.04 to 12.0 fibres ml–1 between 1912 and 1996.
As stressed by Scansetti et al. (1992) in their article, exposure assessment in the SiC industry is incomplete and there is a need to know more about exposure to silica polymorphs and SiC fibres as well.
The two remaining companies producing SiC abrasive material in the province of Quebec in Canada stopped their production activity in the last decade. However, before the final shutdown of Plant A in July 2000, a survey was conducted with the objectives of assessing the level of exposure to SiC fibres from different jobs and of re-evaluating the exposure to quartz and cristobalite.
The first objective of this paper is to report on the concentration of SiC fibres, crystalline silica and respirable dust during this last sampling campaign and to compare the results with previous exposure assessments. The second objective is to tentatively explain the exposure pattern among different plants existing in Canada, Norway and Italy. It is hoped that this information will be helpful to researchers involved in risk assessment for fibrogenic material found in the SiC industry.
MATERIALS AND METHODS
A diagram of the process and a description of the work sites in four working areas, material storage, the preparation areas, the furnace area and the product areas have been previously presented (Dufresne et al., 1987a). To facilitate comparison with the exposure assessment carried out in other countries, we are providing a short description of the jobs as they were being carried out during the 1999 sampling campaign. The jobs assessed were the following:
- Assistant operators of station 01 (two workers). Their main task is to connect and disconnect Acheson furnaces to the main power line. In addition, they have to inspect the cooling water channels of the Acheson furnace as well as transformers and capacitors linked to the furnaces. In some areas of the plant, they may be required to wear a self-contained breathing apparatus (SCBA), since CO concentrations may reach levels as high as 1500–1800 p.p.m. They are the only workers with the two operators of station 01 that work 12 h shifts.
- Acheson furnace attendants (loading of mix and graphite into the furnace) (three workers on three shifts). The loading of the Acheson furnace with old and new mix is accomplished in three steps. First, the bottom of the furnace is filled up with the old mix by the overhead crane operator using a clamp shell bucket attached to the crane (note that the furnace attendant is not involved in this first step). Second, the new mix and the central core of graphite are loaded with a large bucket that is moved by the crane operator lengthwise over the Acheson furnace. During this second step, the Acheson furnace attendants walk beside the bucket as it is moved by the crane operator. They open or close the slot located at the bottom of the bucket to control the flow of the new mix delivered into the furnace. Third, the crane operator tops the core of the new mix with a substantial amount of old mix.
- Subproduct attendants (two workers on two shifts). With a large bucket attached in front of the loader, they fill the ‘Revert hopper’ with the old mix removed from the bottom of the Acheson furnaces. From the hopper, this old mix is taken up to giant silos via a bucket elevator and recycled back to the Acheson furnaces as needed.
- Crusher and backhoe operators (three workers on three shifts). Their duties are to crush all grades of SiC produced by the carboselectors, to screen all grades produced by the carboselectors into several sizes, to store the screened material in large silos and to fill cars that will be shipped out to customers. In addition to the previous tasks, they have to break the big lumps lying on the cleaning floor with a backhoe before the lumps are chiselled by the carboselectors.
- Carboselectors (12 workers on three shifts). Workers with pneumatically powered chisels break up large lumps of SiC and sort them into three grades. This is completed in a series of six to eight work periods of
30 min during an 8 h shift. When all the lumps have been sorted and sent to the crusher via a system of conveyers, the platform is cleaned with a front loader.
- Millwrights (11 workers on one shift). They fix all types of mechanical problems in plants. For instance, they may have to repair a conveyor in a confined space (where they may be exposed to high levels of dust), repair the motor of an overhead crane, restore buckets attached to the conveyor of the revert hopper, etc.
- Electricians (five workers on two shifts). Their main duty is troubleshooting in the plant or conducting preventive maintenance on all electrical appliances (Acheson furnaces, overhead cranes, power lines, electrical motors, etc.) around the plant. They regularly use compressed air to clean the overhead cranes or bus bars connected to the Acheson furnaces.
- Labourers (three workers on three shifts). They can do a variety of different jobs. For instance, they shovel dust that has fallen from conveyors, wheelbarrow dust, they clean pits and so on. As in the case of millwrights and electricians, the prediction of their exposure profile can be a difficult task for the industrial hygienist.
- Lift-truck operators (two workers on two shifts). Using the ‘hyster’ loader, their duty is to transfer large metallic boxes of crude SiC lumps or old mix from building 57 to the cleaning floor, which is located in the main building, or to the revert conveyor also located in the main building.
- Operator of loader during Acheson furnace maintenance (one worker). His main task is to transport large cement and calcium carbide blocks used to make the Acheson furnace heads (cathode and anode) from the storeroom to any of the Acheson furnaces located in building 57 or the main building.
- Attendants for Acheson furnace maintenance (four workers on two shifts). Their main duty is to put the Acheson furnace cathodes and anodes in place with a loader. They may also perform maintenance on furnaces such as patching holes in cathodes and anodes with SiC fine dust.
The job performed by the attendant who unloaded furnaces with a ‘bob-cat’, as previously described in 1987, did not exist anymore. The coke dryer attendants (three workers on three shifts), overhead crane operators (eight workers on three shifts), baggers and shipping attendants (six workers on two shifts), mechanics (three workers on one shift) and carpenter (one worker) were not monitored during the present sampling campaign. Due to a limited budget, the investigation strategy aimed at measuring exposures expected to be above the recommended TLV® of quartz, cristobalite or SiC fibres on the job site.
The sampling process was performed as described in Table 1. Measurements were taken for 5 days over two consecutive weeks for logistic consideration and comparability with previous assessments. Workers considered that the production activity was normal during the sampling campaign.
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Procedures to assess fibres, respirable quartz and cristobalite are described in the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) Sampling Guide for Air Contaminants in the Workplace (2000). Analyses were performed according to the IRSST 206-2 method for crystalline silica, which is similar to the National Institute for Occupational Safety and Health (NIOSH) 7500 method, to the IRSST 48-1 method for gravimetric analysis, which is similar to the NIOSH 0600 method, and to the IRSST 243-1 method for mineral fibre counting, which is similar to the World Health Organization (WHO) method.
The IRSST 206-2 analytical method calls for an indirect preparation method and requires two 2 h low temperature ashing in order to eliminate the polyvinyl chloride (PVC) membrane and other organic or volatile materials. When we suspect the presence of graphite or carbonaceous material, which is known to interfere positively on the main diffraction plan of quartz (3.34 x 10–8 cm), a longer period of ashing must be used. Also, the method calls for a complete scan around the three main diffraction lines of quartz and for the computation of the ratio of the second (4.26 x 10–8 cm) and the third (1.82 x 10–8 cm) most important diffraction lines to the main diffraction line in order to ascertain that the sample is free of a positive interference such as mineral graphite (3.36 x 10–8 cm). The second and third lines are clearly visible with silver membrane substrates having less than 20 µg of pure quartz deposited on their surface.
The asbestos fibre counting methods do not allow for fibre identification, the speciation of the fibres was previously done by transmission electron microscopy (Dufresne et al., 1987b).
Whenever possible, workers were chosen at random and sampled over two shifts. A single sample (respirable dust, quartz and cristobalite) or successive samples (SiC fibres) were taken to cover the exposure duration which represented at least 88% of the work shift. Several samples were needed for the fibre assessment to cover a single work shift in order to minimize overloading on the filter. The remaining time was considered as without exposure since workers left their occupational setting for an environment where they were not exposed to chemicals.
Sampling for crystalline silica was done for 4 days in the second week of December 1999. The dual sampling head (PVC membrane filter in a plastic cassette and Dorr-Oliver cyclone) was operated at 1.7 l min–1. Sampling periods were between 329 and 457 min except for two samples at 272 and 223 min. The limit of detection (LOD) for the 206-2 method is 6 µg which corresponds to 0.01 mg m–3 for a volume of 800 l. Results below the LOD were given a value of 0.005 mg m–3 which corresponds approximately to half of the LOD.
Sampling for fibres was done for 5 days at the end of November and beginning of December 1999. The sampling rate was 1.0 l min–1. The sampling duration varied from 29 to 157 min, based on the requirements of the analytical method for the optimal density of fibres at the surface of the filter.
Exposure concentrations are presented as arithmetic averages (AAs) with their standard deviation (SD), and geometric means (GMs) with their geometric factor (GSD) when the number of samples is sufficiently large.
RESULTS
Results of the exposure assessment are shown in Table 2.
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Assessment of SiC fibres
The highest 8 h time-weighted average concentrations of fibres were found among the crusher and backhoe attendants and carboselectors with an arithmetic mean of 0.63 fibres ml–1 for the former group and 0.51 fibres ml–1 for the latter group. The GSDs were 1.39 (crusher and backhoe) and 1.55 (carboselectors), showing reasonable homogeneity among these groups of workers.
Assessment of respirable dust
The highest arithmetic mean 8 h time-weighted average concentrations of respirable dust were found in the labourer group (1.16 mg m–3), followed by the millwright (1.15 mg m–3) and carboselector (0.73 mg m–3) groups. However, it should be stressed that the maximum 8 h time-weighted average concentration was found among the carboselectors with a value close to 2 mg m–3.
Assessment of crystalline silica
Most of the jobs shown in Table 2 had 8 h time-weighted average concentrations of crystalline silica less than or around the LOD of 0.005 mg m–3. It should be stressed, however, that the maximum 8 h time-weighted average concentration for quartz was found among the carboselectors (0.157 mg m–3), followed by the labourers (0.032 mg m–3). Similarly, most of the 8 h time-weighted average cristobalite measurements were less than the LOD of 0.005 mg m–3, except for the carboselectors where it was found to be 0.044 mg m–3. A high GSD of 4.58 demonstrates a non-homogeneous distribution of concentrations of quartz among the group of carboselectors.
DISCUSSION
The overall results for respirable dust, quartz and cristobalite for the present exposure assessment in Plant A are quite comparable with the results published from the same plant (Dufresne et al., 1987a). However, we acknowledge that the exposure profiles of some jobs, which comprise many tasks, consisted of a few samples collected in a non-random fashion. The SiC fibre or crystalline silica exposure profiles of the millwrights and electricians that we find in the present study, could probably not be used to predict future exposures since the tasks they performed were not felt to be associated with the highest exposures to these contaminants. For instance, monitoring of electricians during the use of compressed air to clean overhead cranes or bus bars connected to the Acheson furnace was not done and this task creates a very dusty environment. Similarly, millwrights did not work in conveyor pits, which are recognized as being dusty environments by workers. Although the second highest concentration of respirable dust was associated with the millwright's work, it could be related to fumes from welding operations performed during the sampling time, and not to respirable crystalline silica polymorphs or other minerals. It is generally assumed that the exposure situations of maintenance workers normally show a large variation and quite a large number of measurements may be needed to give a reliable description of their exposure profile. Kromhout et al. (1993) and Rappaport et al. (1993) reported that a large number of random measurements must be made on individual workers in order to calculate the within-worker and between-worker components of exposure variability for an accurate definition of the exposure profile of similar exposure groups. It is unfortunate that logistic considerations did not allow us to collect more samples.
Assessment of SiC fibres
This is the first time that our group has reported on the quantitative assessment of SiC fibres in Plant A. The 8 h time-weighted average concentrations ranged from 0.03 (assistant operator of station 01) to 0.89 fibres ml–1 (carboselector). The upper range concentration in the present study is somewhat lower than the upper range concentration (4.9 fibres ml–1) reported by Bye et al. (1985). On the other hand, GM concentrations from static stations reported by Scansetti et al. (1992) seem to be similar to our results, except for the unreacted material removing job which was 2.40 fibres ml–1 in their study.
Designing a sampling strategy that will lead to the true exposure profile will always remain a difficult task for an industrial hygienist. Although a process may be stable from day to day, tasks that are performed by workers can be missed by the industrial hygienist and often these missed tasks will contribute significantly to the exposure dose. In the present study, we found that assistant operators during the night shift performed more tasks than the one described above. For instance, they may clean dust collectors for which we measured concentrations of 1.78 fibres ml–1 (task lasted 46 min) and 2.41 fibres ml–1 (task lasted 34 min) (note that data are not shown in Table 2). It is clear that assistant operators on night and day shifts could not be assigned to the same exposure group.
It has been suggested that clustering all the samples in a short period of sampling when autocorrelation occurs may result in an underestimate of variability in the exposure profile as well as an imprecise estimate of the mean exposure (Francis et al., 1989; Buringh et al., 1991). However, it seems that strict autocorrelation is less likely to occur in consecutive day to day exposure measurements than in shorter periods (George et al., 1995; Rappaport et al., 1995). Nevertheless, we acknowledge that there may have been systemic influence caused by unknown environmental or process parameters on those clustering days that may have affected the estimation of the variability of the exposure measurements and introduced a bias in the results.
In the 2003 edition TLV® booklet, ACGIH proposed a TLV® of 0.1 fibres ml–1 for SiC whiskers based on lung fibrosis and lung cancer critical effects. Four of the six jobs that were assessed showed average concentrations exceeding the TLV® by a factor of up to six (e.g. crusher and backhoe attendant). Romundstad et al. (2001) in their study found an excess risk of lung and stomach cancer in the Norwegian SiC plants, and the research group suggested that exposure to SiC fibres was a plausible cause because strong correlation between the different exposures was present. Published industrial hygiene, toxicology and epidemiology data seem to converge in the same direction as to the exposure to SiC fibres and its association with lung cancer and non-malignant lung disease (Infante-Rivard et al., 1994; Romundstad et al., 2001, 2002).
Assessment of respirable dust
This set of data is the most useful in comparing results from the present study with those published in 1987 by our research group. Although the number of respirable dust samples is limited and special precaution should be taken when interpreting the results, they seem to show that exposure over time was relatively stable in Plant A. For instance, the attendant who loads mix and graphite into the Acheson furnace was exposed to an average concentration of 0.32 mg m–3 in 1987, which was the same value in the present study; the average concentration was 0.63 mg m–3 for the labourer in 1987 and 1.16 mg m–3 in the present study; it was 0.95 mg m–3 for the carboselectors in 1987 and 0.73 mg m–3 in the present study; it was 0.72 mg m–3 for the old mix operator in 1987 and is actually 0.27 mg m–3 for the subproduct attendant; and it was 0.43 mg m–3 for the crusher operator and is currently 0.48 mg m–3 for the crusher and backhoe attendant.
Overall, respirable dust concentrations in Plant A are lower than in the Italian plant (Scansetti et al., 1992), where they range from 1.29 (side opening) to 5.25 mg m–3 (selection). In Canada, the raw material used in SiC plants (quartz or petroleum coke) was rather coarse. The only task where a substantial amount of energy was applied on the material was on the cleaning floor where pneumatic chisels were used and this could generate a substantial amount of dust. Moreover, estimates of total dust exposures based on 121 samples constituting a job-exposure matrix (Infante-Rivard et al., 1994) ranged from 0.1 to 80 mg m–3, which revealed the dusty situation caused by rather large particles in this occupational environment. Also, particle size distributions of total dust measured by Coulter Counter for the carboselectors or Acheson furnace attendant were 
9.8 µm for the former and 9.2 µm for the latter (Dufresne et al., 1987a). Therefore, it is suggested that the difference in concentration between the Italian and Canadian plants could probably be due to a difference in the raw materials and, to some extent, to the variations in the industrial set-up, process and ventilation control measures.
Assessment of crystalline silica
The concentrations of crystalline silica in Plant A in the present survey are somewhat comparable with the concentrations of quartz found in our first assessment (Dufresne et al., 1987a). The highest GM concentration of quartz in Plant A was then found for the attendant unloading Acheson furnaces (0.086 mg m–3), a job that has been mechanized through the use of a large shovel attached to the overhead crane. In the present survey, two samples exceeded the ACGIH TLV® (0.05 mg m–3) and they were all picked up from two carboselectors on the cleaning floor with a maximum concentration of 0.157 mg m–3 followed by 0.083 mg m–3. The third highest concentration was obtained for the labourer group (0.032 mg m–3). Similarly, the highest concentration of cristobalite was detected for a carboselector (0.044 mg m–3). All other measurements of cristobalite were less than or around the analytical method's LOD. The concentrations for this set of crystalline silica samples are, in general, lower than those found in the Italian SiC plant where the environment of the cylinder breaking attendant had a concentration of 0.390 mg m–3 for a static sampling procedure.
The two highest above-mentioned samples measured for the carboselectors did not clearly show the second and the third diffraction lines and this observation may indicate that a positive interference (graphite) was still present on the silver membrane substrates. In fact, in this survey, we did not use the prolonged ashing period. Therefore, the concentration of quartz might have been overestimated in those samples. Even if those results were overestimated, they were much lower than the results from the Norwegian plant. However, during the validation process of the analytical method in 1985, we had observed that an ashing time of 16 h to burn 2 mg of graphite on a PVC membrane filter was sufficient to eliminate this mass of graphite. Therefore, in the first study, we trust that graphite was absent in the carboselectors' samples and that the intensity measured at 3.34 x 10–8 cm was mainly due to the reflection of quartz being in a preferred orientation on the surface of the SiC fibres. This phenomenon was documented from raw material that was used in evaluating the pulmonary clearance of fibrous and angular SiC particulates in the sheep model of pneumoconiosis (Dufresne et al., 1992). This remains a research hypothesis that should be tested.
Potential risks for lung diseases
In the Canadian SiC production plants, two silica polymorphs, quartz and cristobalite, were present as respirable particulates. The two silica polymorphs were also present in the lung tissue of some Canadian workers with long tenures in this occupational setting (Dufresne et al., 1995). However, the concentrations of quartz in the lungs of SiC workers were lower than those found in the lungs of gold miners with fibrosis (Perrault et al., 1998).
In addition, respirable SiC fibres were present in Canadian plants, at least in Plant A. Moreover, these fibres were also found in large numbers in the lungs of workers in both Plants A and B. This type of fibre underwent the ferruginous transformation that is characteristic of asbestos fibres as shown in a previous article (Dufresne et al., 1995). The observation of this phenomenon can be an indicator of occupational exposure to SiC fibres and to a certain extent to disease.
Approximately two decades ago we reported that SiC fibres and isometrics were probably coated with a thin layer of silica material (Dufresne et al., 1987b). Man-made vitreous fibres and refractory ceramic mineral fibres have been shown to be pathogenic in animal inhalation studies (Bunn et al., 1994). Surface-associated free radical activity has been suggested as a mechanism of asbestos-mediated toxicity (Kamp et al., 1992), but man-made fibres appear to have little activity in this respect (Gilmour et al., 1995). In this context, there is a need to estimate the concentration of quartz and cristobalite on the surface of isometric and fibrous SiC particles as well as to document the effects of these two substances on lung parenchyma of workers.
CONCLUSION
In the present survey, the results of concentrations of respirable dust and crystalline silica (quartz and cristobalite), although limited and thus warranting a cautious interpretation, were somewhat comparable with the concentrations found in our first assessment and lower than the results from other countries. In fact, the assessment of the Italian occupational settings exposure demonstrated higher quartz concentrations, while cristobalite was found to be absent.
Respirable SiC fibres were present in Norwegian and Italian occupational settings in a higher concentration than in Canadian industries. The investigations that were performed in the last two decades in this field of research by researchers from different countries seem to support that SiC fibres (whiskers) constitute a major airborne health hazard. The significance of the ACGIH exposure level of SiC fibres (0.1 fibre ml–1) and the classifying of SiC fibres as being carcinogenic to humans (A2) demonstrate the potential health hazard of this substance. Fibres in four of the six jobs measured showed average concentrations exceeding the recommended TLV® by a factor of up to six (e.g. crusher and backhoe attendants). It is our opinion that these occupational environments were uncontrolled, i.e. a large percentage of the exposures exceeded the recommended TLV® for SiC fibres.
ACKNOWLEDGEMENTS
The authors wish to thank Gabrielle Chamberland and Suzanne Paradis for the fibre counting analysis and Claudette M. Dufresne for the silica analysis. The authors also wish to thank Julie McCabe for her help in revising the text.
Received May 14, 2004; in final form October 1, 2004
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  S. Foreland, E. Bye, B. Bakke, and W. Eduard Exposure to Fibres, Crystalline Silica, Silicon Carbide and Sulphur Dioxide in the Norwegian Silicon Carbide Industry Ann. Hyg., July 1, 2008; 52(5): 317 - 336. [Abstract] [Full Text] [PDF]
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