Annals of Occupational Hygiene Advance Access originally published online on July 11, 2007
Annals of Occupational Hygiene 2007 51(6):501-507; doi:10.1093/annhyg/mem034
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Changes in Workplace Concentrations of Airborne Respirable Fibres in the European Ceramic Fibre Industry 1987–1996
1 Institute of Occupational Medicine, Research Avenue North, Riccarton, EH14 4AP, Edinburgh, UK
2 Institut National de Recherche et de Sécurité, Avenue de Bourgogne–B.P. 27, 54501 Vandoeuvre Cedex, France
* Author to whom correspondence should be addressed. Tel: +44 (0) 131 449 8044; fax: +44 (0) 870 850 5132; e-mail: brian.miller{at}iom-world.org
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ABSTRACT |
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As part of a wider epidemiological research programme, an occupational hygiene study was carried out during 1995–1996 to assess workers’ current exposures to airborne materials in six European refractory ceramic fibre (RCF) plants. These plants had also participated in a cross-sectional occupational hygiene survey in 1987. The sampling strategy focussed principally on personal shift-average exposures of workers, by occupation, to respirable fibres. Monitoring was undertaken in two integrated phases: a 1-week cross-sectional survey followed by a prospective, and ongoing, programme by the RCF industry. Statistical (analysis of variance) analyses to identify patterns of variability by plant, occupational group (OG) and occupations within group were based on 464 individual shift samples, the greatest amount of data being available for production occupations.
Concentrations of respirable fibres showed marked differences between plants and between OGs. Average respirable fibre concentrations among Primary and Secondary Production and Ancillary workers ranged from <0.1 f ml–1 to up to 0.4 f ml–1, depending on OG and plant. Individual shift-average measurements were almost invariably <1 f ml–1. Within Secondary Conversion and Finishing, plant-specific averages ranged from 0.3 f ml–1 to 1.25 f ml–1. Respirable fibre concentrations were, in some plants, less than half those found in 1987. In other plants, mainly those where concentrations had been relatively low in 1987, the dust exposure had remained essentially unchanged or increased slightly.
An ongoing programme of sampling is being carried out by the participating companies, generating additional information that could assist research in the long term and in improving control.
Keywords: exposure reduction; refractory ceramic fibres • respirable fibre concentrations
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The refractory ceramic fibre (RCF) industry manufactures a range of products, primarily for insulation in situations where high temperatures are reached, e.g. in metal moulding and around heating elements. RCF is produced by mixing and melting aluminium oxide (Al2O3) and crystalline silica (SiO2) in a furnace at
1925°C and spinning or blowing the melt into fibres. Properties, particularly heat resistance, may be affected by various additives, such as chromic oxide (Cr2O3), zirconia (ZrO2), titanium dioxide (TiO2) and magnesium oxides. The end product varies and includes both loose fibres and loosely woven flat blankets. Production plants may also operate a diverse range of secondary production and conversion processes, including: manufacture of paper, felts and board; vacuum forming of complex shapes from a slurry of ceramic fibre and binders, and working these once dry; die cutting blanket and paper; manufacture of rope; and construction of thick blocks of furnace insulation. Geometric mean diameter of the airborne fibres is typically in the range 1–3 µm, so all operations have the potential to produce airborne dust and fibres in the respirable size range, leading to possible worker exposure by inhalation. In Europe, the first systematic study of workers’ health was in 1987 in seven plants in England, France and Germany. The plants had started RCF production variously between 1965 and 1977. The results suggested some weak associations between respiratory symptoms and indices of exposure to RCFs and other materials (Trethowan et al., 1989; Burge et al., 1995). Preliminary results from similar studies in the US suggested that pleural changes, though infrequent, might also be related to exposure (Lockey et al., 1996; LeMasters et al., 1998). Taken together, these results supported a need for continued studies of health in the RCF industry. The International Agency for Research on Cancer (IARC, 1988, 2001), on the basis of animal experiments, has classified ceramic fibres as ‘possibly carcinogenic to humans (Group 2B)’; currently, there is limited epidemiological data on cancer risks in US workers, providing no evidence of respiratory cancer risk (LeMasters et al., 2003), but no mortality data for European workers.
Exposure estimates for workers in the 1987 European study were based on an occupational hygiene survey of the seven RCF plants, described by Cherrie et al. (1989). Six of these took part in a second health study between 1995 and 1996 (Cowie et al., 2001). The principal aim of this investigation was to provide data on current workplace concentrations of respirable fibres, dust and crystalline silica, for use in the estimation of individual exposures in the health study. A secondary aim was to set up a programme of ongoing routine monitoring in the plants, subject to suitable quality controls.
This paper compares the mean respirable fibre concentrations from the 1995–1996 study with corresponding mean concentrations from the 1987 study.
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METHODS |
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Design of occupational hygiene surveys
The 1995–1996 study was based largely on what was known about the industry from the previous study (Cherrie et al., 1989), and the monitoring surveys followed a similar pattern. Preliminary plant visits established liaison with plant staff and workers’ representatives and collected information to assist with the design of the surveys. Current processes and systems of work were recorded, and a number of changes since 1987 were noted in methods of production and work procedures, most notably in the secondary processes. Each plant now employed between
60 and 190 workers in ceramic fibre production and associated processes, similar to numbers in 1987. A number of additional considerations were important in designing the occupational hygiene sampling strategy for this survey, including:
- all important respiratory hazards should be characterized;
- the job structure would need to link with that of occupational histories collected during the health studies;
- sampling should cover the full range of current occupations;
- the results of the ongoing monitoring programme should be comparable with a programme already running in the US (Maxim et al., 1994) and
- the structure of the results should be linked to that of the data from Cherrie et al. (1989), which would also be used in the construction of individual exposure estimates.
Hazards prioritized for quantification were airborne respirable and non-respirable fibres, respirable dust and respirable crystalline silica. A relatively small number of inhalable dust concentrations were also measured.
Typically, plant records described individuals’ occupations, understood as characterizing normal work activities over a period greater than a month. Within an occupation, a ‘job’ was taken to describe the totality of activities undertaken during a single shift, so that an occupation could include more than one job. In a single shift, a job could comprise many ‘tasks’ of variable duration. We used the term ‘occupational group’ (OG) to denote a collection of occupations, generally in the same broad area of work, that were expected to lead, or have been shown to lead, to similar personal exposures. For the present study, sampling effort was allocated at the level of job so as to ensure representative coverage of the jobs within each occupation.
A unified list of over 100 three-digit job codes was constructed, with standardized job titles and tables of correspondence to local names. These were organized within a broad OG structure, based on knowledge of the industry and expanding on the distinctions made by Cherrie et al. (1989). Subsequent data analysis showed that some of the distinctions between non-production OGs could be ignored, and new Occupational Categories (OCs) were constructed as shown in Table 1. Staff who might be exposed by visiting the production areas were distinguished from staff who did not and were therefore not exposed.
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Allocation of sampling effort
The cross-sectional occupational hygiene surveys were undertaken by the research team. Staff from the RCF plants were trained to undertake the ongoing surveys, using a documented quality system to the requirements of EN45000.
The cross-sectional occupational hygiene surveys were designed to last 1 week at each plant, collecting over 100 samples. Effort was divided in the following proportions: 40% for fibre and inhalable dust, 40% for respirable dust and silica concentrations, 10% for total inhalable dust using the Institute of Occupational Medicine (IOM) head and 10% for fibre sizing by scanning electron microscopy (SEM). Within each plant, lists of persons working in each OG in each shift were supplied in advance, and a plan of sampling was constructed, taking random representatives of each OG and shift. This sampling plan was agreed with plant staff. On site, research hygienists from IOM or Institut National de Recherche et de Sécurité (INRS) conducted the sampling, amending the sampling plan where necessitated by changes in daily work routines. Each sample was labelled with a unique identifier and documented with the worker's identity, job and tasks carried out, sample type and time sampled. Whenever possible, samples were taken over the working periods of a shift. In some cases multiple samples were taken within a shift, e.g. to avoid overloading filters in dusty occupations.
The ongoing phase of the study was designed to provide a prospective extension of the cross-sectional surveys, with the sampling to be carried out by plant staff, following training. Its principal purpose was as a tool for ongoing compliance monitoring, but the data it accumulated and the ensuing characterization of concentrations by occupations would be expected to be useful in epidemiological studies, where it would supplement the cross-sectional results. Allocation of sampling effort was planned within the same stratified OG structure and was initially prioritized to provide concentration data for occupations that were not sampled, or under-sampled, during the cross-sectional surveys. The ongoing surveys measured only respirable and non-respirable fibre concentrations and total inhalable dust.
The concentration estimates presented here and used for the epidemiological study used all the valid data from the cross-sectional surveys in 1995 and from the first 6 months of the ongoing programme, i.e. April to September 1996.
Sampling methods
Personal monitoring of worker exposure to respirable and non-respirable fibres was carried out in accordance with the UK Health and Safety Executive (HSE, 1988) method MDHS 59 and the IOM's ‘Instruction Manual for the Collection of Airborne Particulate Samples'.
Two types of filters were mounted in cowled sampling heads on different occasions, (i) pre-weighed 25-mm gridded mixed cellulose ester membrane filters for determination of the airborne mass concentration of total inhalable dust and of the airborne fibre concentration and (ii) pre-weighed 25-mm Nuclepore polycarbonate filters for determination of the airborne fibre concentration by SEM. In both cases flow rates were set to 1.0 l min–1.
A small number of samples were mounted and analysed on site during the surveys to ensure that the fibre densities collected were within an acceptable range for accurate counting. Where necessary, the information from this analysis was used to modify the sampling times. The detection limit for a sample of 480 l was 0.01 f ml–1.
Separate samples were collected for total inhalable dust, respirable dust and respirable crystalline silica, but are not considered here.
Analytical procedures
Airborne samples collected on membrane filters were mounted using the acetone/triacetin method, for analysis for fibre concentration by phase-contrast optical microscopy (PCOM) in accordance with MDHS 59 (Health and Safety Executive, 1988) and WHO/EURO (1985).
Dust samples collected on 25-mm-diameter, 0.4-µm-pore-size polycarbonate (Nuclepore) filters were analysed by SEM to determine fibre concentration and size of fibres (WHO/EURO, 1985), with the particular quality control systems defined in the relevant IOM internal instruction manual. Further details of this assessment are given elsewhere (Groat et al., 1999).
Quality assurance and auditing
Quality control and instruction manuals were prepared for all sampling and analytical procedures, and training courses were run in Edinburgh, for participants from IOM, INRS and the RCF plants.
The cross-sectional and ongoing surveys were audited at both the sampling and the analytical stages. Mostly, non-compliances were few and mainly of a minor administrative nature and could be quickly corrected. Comparisons between IOM and INRS were carried out for fibre counting by PCOM and SEM. These were found to show agreement that would be acceptable in other proficiency testing schemes.
In the cross-sectional surveys, 10% of samples were chosen for replicate analyses, and all showed satisfactory agreement.
Data processing and statistical analysis
All analytical data were transferred into a secure purpose-built database, constructed using SIRpc Version 3.3 (SIR Pty Ltd, 1993), with coded information about sample identity, type and ancillary data. The database allowed cross-checking and validation of field documentation and laboratory data and linkage between multiple samples spanning the same shift. In the latter case, data were combined into time-weighted, whole-shift summaries.
A separate confidential report was provided to each plant detailing and summarizing all samples taken there, the circumstances of the sampling and the results. Recommendations about control measures for high fibre concentrations were included, where appropriate.
Statistical analysis of fibre concentrations was carried out using standard analysis of variance (ANOVA) methods, with the aim of identifying patterns of variability by plant, OG and occupations within group. After exclusion of duplicate samples and concentrations based on <4 h of sampling, the analyses were based on 464 shift-average samples of respirable and non-respirable fibres. As is often necessary for concentration data (and checked here using residual plots), the ANOVAs were carried out on the log scale, producing tabulations of predicted geometric means and standard deviations. Arithmetic means were estimated subsequently, using a well-known approximation for log-normal distributions, which relates the arithmetic mean µA to the mean µ and variance 
2 on the log scale by the formula
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RESULTS |
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Full results from the study have been reported previously (Groat et al., 1999). Here we consider only respirable fibre concentrations, which were the principal focus of investigations.
The majority of the respirable fibre concentrations lay <0.5 f ml–1, which is the industry's recommended exposure guideline. One sample gave an estimated concentration of over 3.5 f ml–1, and there were a small number of results at each plant in excess of 1.0 f ml–1 (the contemporary British Occupational Exposure Limit). Almost all of these were in the OC ‘Secondary Conversion and Finishing', in which items formed from fibres may be cut, sanded or otherwise finished once dry.
Table 2 shows the ANOVA summary for the analysis (on the logarithmic scale) of respirable fibre shift concentrations. This distinguishes plants, OGs and occupations within OGs as sources of variation and also fits interaction terms to allow for the possibility that any differences between OGs or occupations may be in different ratios at different plants. The first observation is that the mean square for plant x occupation interaction was no greater than the residual, so that this interaction was not significant and may be combined into the residual. The other interaction, plant x OG, was highly significant, indicating that relationships between OGs are not in the same ratio at each plant. The terms for plant and OG had much larger mean squares than the plant x OG interaction and were clearly important sources of variation. (This was true regardless of the order in which the terms were fitted.) Differences between occupations within OGs were statistically significant, but would not greatly inflate the residual if ignored. Overall, the ANOVA table provided support for a summary based on estimating means in a two-way table classified by plant and OG.
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Examination of the means in such a table suggested further that some of the distinctions between non-production OGs could be ignored without loss, and with some gain in numbers of samples in the combined OGs. The OCs were constructed from the OGs as shown in Table 1.
A reanalysis of the regrouped data yielded an ANOVA table similar to Table 2, confirming that the most important distinctions had been maintained (not shown).
Table 3 shows estimates of arithmetic mean concentrations of respirable fibres, by combination of plant and OC. The plants are numbered as in the original report (Cherrie et al., 1989). The table also provides comparisons with estimated arithmetic means from the 1987 surveys, formed in the same way after regrouping occupations to correspond with the OCs; these comparisons are shown in Fig. 1.
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The occupations believed not to be exposed to significant fibre concentrations had indeed very low mean fibre concentrations. The means for the Ancillary OC were also low. In each plant that included the OC Secondary Conversion and Finishing, the highest mean concentrations were found in this OC, but they varied 4-fold across plants. Concentrations in Secondary Production were quite similar across plants. There was more variation in Primary Production, with particularly low means in Plants 1 and 2.
Overall, respirable fibre concentrations were lower than those found in 1987. The degree of improvement, however, differed between plants and between OCs. Within Primary Production there were reductions in respirable fibre concentrations at four plants, with the most marked reductions at Plants 1 and 2 (both located in the UK). The mean concentrations at Plant 6 almost doubled over the period, but the concentrations here were much lower in 1987 than at the other plants and the final concentrations were in line with those at other plants. In Secondary Production, Plants 1 and 2 also showed the greatest reduction in fibre concentrations, and the estimated arithmetic mean concentrations reduced at all plants. Changes within the Secondary Conversion and Finishing group were more variable, the greatest reductions being found at Plants 2 and 6, with a smaller reduction at Plant 5. Plant 7 stayed relatively high: 1995–1996 levels were similar to those of 1987. Levels in Plant 4 doubled over the period.
In the Ancillary OC, levels decreased or stayed low, while in the Not Exposed group there was some overall reduction from a very low starting point.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The primary purpose of this study was to produce estimated concentrations of respirable fibres, to permit calculation of individuals’ exposures in an epidemiological study (Cowie et al., 2001). The programme of deployment of the sampling resources was designed to give a wide and representative coverage of shift-average concentrations across the industry's many occupations. Other aims would have implied different designs. The aim here was to collect sufficient samples within an occupation to average out over typical tasks, and it proved important to explain this rationale to the industry contacts.
Samples were allocated by occupation, both in this study and in the 1987 surveys, maximizing the coverage of typical occupations. Personal samples were taken to inform about the concentrations encountered by individuals while engaged in the occupation being sampled. Considerable effort went into selecting the occupations to ensure coverage, on the basis of hygienists’ observations, industry knowledge and the results of the 1987 survey. Methods and analytical quality were similar between the two cross-sectional surveys.
The 1995 sampling programme needed to take into account changes in circumstances within the industry since 1987. In particular, greater flexibility in working practices had been introduced, and activities that might once have been one worker's full-time occupation often now appeared as work that was done on some shifts but not on others. This greater flexibility in more modern work practices was handled successfully by focussing on the concept of ‘occupation’ that a worker might have for a period of several months, as distinct from the ‘job’ that he did on a given shift, each job or shift including many ‘tasks’ or ‘activities'. A pooled estimate of within-OG geometric standard deviation was 0.75, compared with 0.90 in 1987.
The two-stage (cross-sectional plus ongoing) design of the monitoring programme provided good coverage of occupations, but there were a few occupations where the coverage of specific jobs (and so the representativeness of the occupational average) was less comprehensive than we would have liked. This mainly occurred in areas where processes were carried out infrequently or irregularly and where operations did not coincide with the plant hygienists’ availability to carry out sampling, but was relatively unimportant in terms of the overall objective of grouping occupations appropriately, estimating average concentrations by OG and comparing the levels with the earlier survey.
The individual shift respirable fibre concentrations estimated in the present study were generally low relative to occupational exposure limits for workers in the production and ancillary groups. The range of exposure concentrations was much wider, and the mean concentrations higher, for work in Secondary Conversion and Finishing, particularly where machines were used to shape or finish the products. In compliance with the legal requirement to control exposure, many of these areas or activities were designated respirator zones, and operators were required to wear respiratory protective equipment. The generally well-controlled concentrations, coupled with the respirator use policy, were consistent with the results of the epidemiological studies, which found evidence of mild irritative effects, but no evidence of serious respiratory health effects (Cowie et al., 2001).
The degree of improvement in concentrations from the 1987 study to that in 1995–1996 differed between plants and between OCs. There was general improvement in Primary Production, Secondary Production and in Ancillary occupations. Changes within the Secondary Conversion and Finishing group were more variable: it is perhaps not surprising that it was more difficult to reduce fibre levels much below 1 f ml–1 in the conversion and finishing processes than in other parts of the process because of the amount of handling and manipulation of dry materials. In particular, exposure in the finishing areas was particularly dependent on how the operator performed the tasks, despite substantial investment in control measures.
The changes in concentrations between the surveys almost certainly did not happen in a smooth fashion, but we have no data on the pattern of changes or their specific determinants, which were likely to have been multiple. The very fact of carrying out and publishing the 1987 surveys would have heightened awareness in the plants of the importance of controlling exposure, and discussions at that time about classification of the product as a possible carcinogen would doubtless have had an additional impact on industry practices. In 1992 European Ceramic Fibre Industry Association (ECFIA) issued a guidance book to all member companies outlining recommendations for good technological control technology. In the two UK plants, where the largest reduction in airborne fibre exposures in production were observed, there were new production lines installed with ‘state-of-the-art’ local ventilation controls (at one plant in the late 1980s after our survey and the second in 1990).
The current workplace exposure limit for RCF in the UK is 1 f ml–1 (or 5 mg m–3) and a recent criteria document (NIOSH, 2006) proposes a recommended exposure limit for the US of 0.5 f ml–1. The 1995–1996 results were in line with these limits except in the Secondary Conversion and Finishing category, and it is clear that this category would require most attention to control exposures to such limits. The levels observed in our surveys are quite consistent with average concentrations measured in similar occupations in US plants (Maxim et al., 2000; Rice et al., 2005, and with reductions in concentration through the 1990s (Rice et al., 1996; Maxim et al., 2000).
One of the outputs from this study was the ongoing sampling programme, a component of the harmonized European Controlled and Reduced Exposure programme (CARE), which has monitored fibre concentrations in the industry and among end-users of ceramic fibres. Results from that programme are not available to compare with recent US results, but it is planned that data collected to date will be summarized and analysed soon, when more detailed comparison with US results will be possible.
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European Ceramic Fibres Industry Association.
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ACKNOWLEDGEMENTS |
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The authors thank all personnel at the plants, especially the liaison officers, industrial hygienists and the workers studied, and all colleagues and collaborators involved in the studies for their co-operation, contribution and encouragement.
Received November 13, 2006; in final form June 5, 2007
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REFERENCES |
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Burge PS, Calvert IA, Trethowan WN, et al. Are the respiratory health effects found in manufacturers of ceramic fibres due to the dust rather than the exposure to fibres? Occup Environ Med (1995) 52:105–09.
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