Refractory ceramic fiber exposure and pleural plaques

Pleural Plaques and the Role of Exposure to Mineral Particles in the Asbestos Post-Exposure Survey

BACKGROUND

Previous studies have inconsistently reported associations between refractory ceramic fibers (RCFs) or mineral wool fibers (MWFs) and the presence of pleural plaques (PPs). All these studies were based on chest radiographs, known to be associated with a poor sensitivity for the diagnosis of PP.

RESEARCH QUESTION

Does the risk of PPs increase with cumulative exposure to RCFs, MWFs, and silica? If the risk does increase, do these dose-response relationships depend on the co-exposure to asbestos or, conversely, are the dose-response relationships for asbestos modified by co-exposure to RCFs, MWFs, and silica?

STUDY DESIGN AND METHODS

Volunteer workers were invited to participate in a CT scan screening program for asbestos-related diseases in France. Asbestos exposure was assessed by industrial hygienists, and exposure to RCFs, MWFs, and silica was determined by using job-exposure matrices. A cumulative exposure index (CEI) was then calculated for each subject and separately for each of the four mineral particle exposures. All available CT scans were submitted to randomized, double reading by a panel of radiologists.

RESULTS

In this cohort of 5,457 subjects, significant dose-response relationships were determined after adjustment for asbestos exposure between CEI to RCF or MWF and the risk of PPs (ORs of 1.29 [95% CI, 1.00-1.67] and 1.84 [95% CI, 1.49-2.27] for the highest CEI quartile, respectively). Significant interactions were found between asbestos on one hand and MWF or RCF on the other.

INTERPRETATION

This study suggests the existence of a significant association between exposure to RCFs and MWFs and the presence of PPs in a large population previously exposed to asbestos and screened by using CT scans.

Review of refractory ceramic fiber (RCF) toxicity, epidemiology and occupational exposure

This literature review on refractory ceramic fibers (RCF) summarizes relevant information on manufacturing, processing, applications, occupational exposure, toxicology and epidemiology studies. Rodent toxicology studies conducted in the 1980s showed that RCF caused fibrosis, lung cancer and mesothelioma. Interpretation of these studies was difficult for various reasons (e.g. overload in chronic inhalation bioassays), but spurred the development of a comprehensive product stewardship program under EPA and later OSHA oversight. Epidemiology studies (both morbidity and mortality) were undertaken to learn more about possible health effects resulting from occupational exposure. No chronic animal bioassay studies on RCF have been conducted since the 1980s. The results of the ongoing epidemiology studies confirm that occupational exposure to RCF is associated with the development of pleural plaques and minor decrements in lung function, but no interstitial fibrosis or incremental lung cancer. Evidence supporting a finding that urinary tumors are associated with RCF exposure remains, but is weaker. One reported, but unconfirmed, mesothelioma was found in an individual with prior occupational asbestos exposure. An elevated SMR for leukemia was found, but was absent in the highly exposed group and has not been observed in studies of other mineral fibers. The industry will continue the product stewardship program including the mortality study.

Five years update on relationships between malignant pleural mesothelioma and exposure to asbestos and other elongated mineral particles

Despite the reduction of global asbestos consumption and production due to the ban or restriction of asbestos uses in more than 50 countries since the 1970s, malignant mesothelioma remains a disease of concern. Asbestos is still used, imported, and exported in several countries, and the number of mesothelioma deaths may be expected to increase in the next decades in these countries. Asbestos exposure is the main risk factor for malignant pleural mesothelioma, but other types of exposures are linked to the occurrence of this type of cancer. Although recent treatments improve the quality of life of patients with mesothelioma, malignant pleural mesothelioma remains an aggressive disease. Recent treatments have not resulted in appreciable improvement in survival, and thus development of more efficient therapies is urgently needed. The development of novel therapeutic strategies is dependent on our level of knowledge of the physiopathological and molecular changes that mesothelial cells acquired during the neoplastic process. During the past 5 years, new findings have been published on the etiology, epidemiology, molecular changes, and innovative treatments of malignant pleural mesothelioma. This review aims to update the findings of recent investigations on etiology, epidemiology, and molecular changes with a focus on (1) attributable risk of asbestos exposure in men and women and (2) coexposure to other minerals and other elongated mineral particles or high aspect ratio nanoparticles. Recent data obtained on genomic and gene alterations, pathways deregulations, and predisposing factors are summarized.

Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers

In 2011, SCOEL classified RCF as a secondary genotoxic carcinogen and supported a practical threshold. Inflammation was considered the predominant manifestation of RCF toxicity. Intrapleural and intraperitoneal implantation induced mesotheliomas and sarcomas in laboratory animals. Chronic nose-only inhalation bioassays indicated that RCF exposure in rats increased the incidence of lung cancer and similar exposures resulted in mesothelioma in hamsters, but these studies may have been compromised by overload. Epidemiological studies in the US and Europe showed an association between exposure and prevalence of respiratory symptoms and pleural plaques, but no interstitial fibrosis, mesotheliomas, or increased numbers of lung tumors were observed. As the latency of asbestos induced mesotheliomas can be up to 50 years, the relationship between RCF exposure and respiratory malignances has not been fully determined. Nonetheless, it is possible to offer useful perspectives. RCF and rock wool have similar airborne fiber dimensions and biopersistence. Therefore, it is likely that these fibers have similar toxicology. Traditional rock wool has been the subject of numerous cohort and case control studies. For rock wool, IARC (2002) concluded that the epidemiological studies did not provide evidence of carcinogenicity. Based on analogies with rock wool (read across), it is reasonable to believe that increases in lung cancer or any mesotheliomas are unlikely to be found in the RCF-exposed cohort. RCF producers have developed a product stewardship program to measure and control fiber concentrations and to further understand the health status of their workers.

Refractory ceramic fiber (RCF) toxicity and epidemiology: a review

This paper provides a review of the relevant literature on refractory ceramic fibers (RCFs), summarizing relevant data and information on the manufacture, processing, applications, potential occupational exposure, toxicology, epidemiology, risk analysis, and risk management. RCFs are amorphous fibers used for high-temperature insulation applications. RCFs are less durable/biopersistent than amphibole asbestos, but more durable/biopersistent than many other synthetic vitreous fibers (SVFs). Moreover, as produced/used, some RCFs are respirable. Toxicology studies with rodents using various exposure methods have shown that RCFs can cause fibrosis, lung cancer, and mesothelioma. Interpretation of these animal studies is difficult for various reasons (e.g., overload in chronic inhalation bioassays). Epidemiological studies of occupationally exposed cohorts in Europe and the United States have demonstrated measurable effects (e.g., mild respiratory symptoms and pleural plaques) but no disease (i.e., no interstitial fibrosis, no excess lung cancer, and no mesothelioma) to date. The RCF industry, working cooperatively with various government agencies in the United States, has developed a comprehensive product stewardship program (PSP) to identify and control risks associated with occupational exposure. One provision of the PSP is the adoption of a voluntary recommended exposure guideline (REG) of 0.5 fibers/milliliter (f/ml). Selected on the basis of prudence and demonstrated feasibility, compliance with the REG should reduce risks to levels between 0.073/1000 and 1.2/1000, based on extrapolations from chronic animal inhalation studies.

Occupational causes of chronic obstructive pulmonary disease

The relation between Chronic Obstructive Pulmonary Disease (COPD, including chronic bronchitis and emphysema (CBE), and exposure to coal dust is well established. This paper reviews the evidence relating to other occupational causes of COPD, including industries associated with exposure to fumes, chemical substances, and dusts. A review of key literature has been carried out with a focus on the magnitude of risks and levels of exposure causing disabling health effects. The literature suggests that elevated risks of developing COPD are clearly associated with several occupations, with risk estimates being high in some, even after taking into account the effect of confounders, such as smoking. Of particular concern are agricultural workers who can be exposed to a variety of gases and organic dusts, among whom CBE is clearly elevated, particularly for pig farmers and exposure to endotoxins, with an increased annual decline in lung function. Similarly, cotton textile workers are exposed to a mixture of substances affecting development of atopy, byssinosis, and CBE, and across-shift and long-term decline in lung function. Atopy also has an important role in the development of COPD in flour mill workers and bakers, with those sensitized to bakery allergens having a greater lung function decline than non-sensitized individuals. Welding processes involve a range of potential chemical, physical and radiation hazards. The average reduction in FEV1 associated with welding fumes is similar to that associated with smoking. Challenges in assessing the evidence include variation in diagnostic methods; concurrent exposure to cigarette smoke (direct or second-hand) and multiple work-place irritants; healthy worker selection/survivor effects; poor exposure definition. Raising awareness of occupational causes of COPD among employers, employees, and health service professionals is important.

Exposures to refractory ceramic fibers in manufacturing and related operations: a 10-year update

Refractory ceramic fiber (RCF) is a man-made vitreous fiber used for its insulating properties. Since 1987, the work environment of approximately 800 persons employed in fiber manufacture or production operations directly related to manufacturing has been monitored to evaluate exposure levels. Samples were collected quarterly from the breathing zones of randomly selected workers. The measurements from those working in areas of similar activities and exposure controls (dust zones or homogeneous exposure groups) were used to calculate a mean exposure during identified time periods. Persons who spent all of their work time in one zone/group were assigned this mean exposure; those with responsibilities in more than one area were assigned an exposure based on a time-weighted formula. A total of 3213 measurements were used to estimate exposure for 130 job titles; because of the mobile jobs, many samples contribute to the estimates of exposure for multiple job titles. The majority of exposure estimates (53%) have remained stable over the operational history of the plant reported here. For 32 job titles (25%) exposures have decreased, and for 28 job titles (22%) exposures have increased. Of the 122 job titles active in 2001, 97 (79%) exposures were estimated to be at 0.25 f/cc or lower; 8 (7%) had an exposure exceeding 0.5 f/cc (range 0.51-0.80) and 17 (14%) of these exposure estimates were in the range of > 0.25 f/cc to 0.5 f/cc. The continuing program to measure exposure supports a respiratory health surveillance program in these facilities.

Quantitative risk analyses for RCF: survey and synthesis

Refractory ceramic fiber (RCF) is a high-temperature insulating fiber used principally in industrial applications. Epidemiological studies on occupationally exposed cohorts have not indicated that exposure leads to fibrosis, increased lung cancer, or mesothelioma. However, inhalation bioassays with rats and hamsters have shown that these animals can develop each of these endpoints when exposed to high levels of RCF-particle mixtures. This work summarizes relevant quantitative risk analyses based upon analysis of the rat bioassay studies, which lead to predicted unit risks that range nearly three orders of magnitude. Additionally, we identify key assumptions that affect the risk estimates and provide additional estimates using the benchmark dose methodology favored by the U.S. EPA in cases where mechanistic models are inadequate or not available. We show that a key determinant of risk is how lung burdens are normalized (e.g., in terms of the number of fibers per square centimeter of lung surface or the number of fibers per milligram dry lung) for species conversion. Plausible values of unit potency/risk range from 1.4x10(-4) to 7.2x10(-4), neglecting any allowance for the effects of particulate material in the RCF tested in the bioassay experiments.

Pulmonary deposition modeling with airborne fiber exposure data: a study of workers manufacturing refractory ceramic fibers

Increasing production of refractory ceramic fiber (RCF), a synthetic vitreous material with industrial applications (e.g., kiln insulation), has created interest in potential respiratory effects of exposure to airborne fibers during manufacturing. An ongoing study of RCF manufacturing workers in the United States has indicated an association between cumulative fiber exposure and pleural plaques. Fiber sizing data, obtained from electron microscopy analyses of 118 air samples collected in three independent studies over a 20-year period (1976-1995), were used with a computer deposition model to estimate pulmonary dose of fibers of specified dimensions for 652 former and current RCF production workers. Separate dose correction factors reflecting differences in fiber dimensions in six uniform job title groups were used with data on airborne fiber concentration and employment duration to calculate cumulative dose estimates for each worker. From review of the literature, critical dimensions (diameter <0.4 microm, length <10 microm) were defined for fibers that may translocate to the parietal pleura. Each of three continuous exposure/dose metrics analyzed in separate logistic regression models was significantly related to plaques, even after adjusting for possible past asbestos exposure: cumulative fiber exposure, chi(2) = 15.2 (p < 0.01); cumulative pulmonary dose (all fibers), chi(2) = 14.6 (p < 0.01); cumulative pulmonary dose (critical dimension fibers), chi(2) = 12.4 (p < 0.01). Odds ratios (ORs) were calculated for levels of each metric. Increasing ORs were statistically significant for the two highest dose levels of critical dimension fibers (level three, OR = 11, 95%CI = [1.4, 98]; level four, OR = 25, 95%CI = [3.2, 190]). Similar associations existed for all metrics after adjustment for possible asbestos exposure. It was concluded that development of pleural plaques follows exposure- and dose-response patterns, and that airborne fibers in RCF manufacturing facilities include those with critical dimensions associated with pleural plaque formation. Analysis of additional air samples may improve estimates of the dose-response relationship.

Mortality of workers occupationally exposed to refractory ceramic fibers

This study was prompted by refractory ceramic fibers (RCF) inhalation studies at high dose levels in animals that demonstrated positive effects for lung fibrosis, mesothelioma, and lung cancer. Current and former male workers employed between 1952 and 2000 at two RCF manufacturing facilities were followed to investigate a possible excess in mortality. The mortality analytic methods included: (1) standardized mortality ratios comparing this cohort to the general and state populations, and (2) a proportional hazards model that relates risk of death to the lifetime cumulative fiber-months/cc exposure among the RCF cohort, adjusted for age at hire and for race. There was no excess mortality related to all deaths, all cancers, or malignancies or diseases of the respiratory system including mesothelioma, but there was a statistically significant association with cancers of the urinary organs SMR = 344.8 (95% CL of 111.6, 805.4). The quality of the data for job history, exposure, and smoking history were very high. Although the cohort was relatively small and young with an average age of 51, the mean latency period was over 21 years. Because of these limitations, the preliminary findings warrant the continuation of this mortality registry for future analyses.

A longitudinal study of chest radiographic changes of workers in the refractory ceramic fiber industry

STUDY OBJECTIVE

This industry-wide longitudinal study examines chest radiographic changes of workers manufacturing refractory ceramic fibers (RCF).

DESIGN

Chest radiographs were obtained every 3 years and were interpreted using the 1980 International Labour Organization classification for pneumoconiosis. Three exposure metrics were calculated: duration and latency in a production job, and cumulative exposure (fiber-months per cubic centimeter).

PARTICIPANTS

The radiographic survey included 625 current workers at five manufacturing sites and 383 former workers at two of the five sites.

MEASUREMENTS AND RESULTS

Pleural changes were seen in 27 workers (2.7%). Of workers with > 20 years of latency from initial production job or 20 years of duration in a production job, 16 workers (8.0%) and 5 workers (8.1%) demonstrated pleural changes, respectively. Results from the cumulative exposure analysis (> 135 fiber-months per cubic centimeter) demonstrated a significant elevated odds ratio (OR) of 6.0 (95% confidence interval [CI], 1.4 to 31.0). The incidence of irregular opacities at profusion categories > or = 1/0 was similar to other nonspecified dust-exposed worker populations at 1.0%, and showed a nonsignificant elevated OR in regard to cumulative fiber exposure of 4.7 (95% CI, 0.97 to 23.5).

CONCLUSIONS

RCF are significantly associated with pleural changes that were predominantly pleural plaques, but have not resulted in a statistically significant increase in interstitial changes.

Risk analysis for mortality from respiratory tumors in a cohort of refractory ceramic fiber workers

Although workers in refractory ceramic fibers (RCF) manufacturing facilities have experienced no elevations in lung cancer or mesothelioma rates, the historical experience of asbestos together with animal studies of RCF have led to ongoing studies of the respiratory health of RCF workers. We have compared lung cancer and mesothelioma in the accumulated mortality experience of a cohort of male RCF production workers (Lemasters et al., 2001, submitted for publication) to that which would have been expected if RCF had a carcinogenic potency similar to that of various forms of asbestos. To accomplish this, we used risk models recently formalized by Hodgson and Darnton (2000, Ann. Occup. Hyg. 41, 13-36) for asbestos cohorts together with the RCF exposure measurements and historical reconstructions of Rice and colleagues (1997, Appl. Occup. Environ. Hyg. 12, 54-61). Deaths from lung cancer in the RCF cohort are statistically significantly below that which would be expected if RCF had the potency of either crocidolite or amosite. The mortality is also lower than would be expected if RCF had the potency of chrysotile, but the difference is not statistically significant. For mesothelioma, the anticipated numbers of deaths under hypotheses of asbestos-like potency are too small to be rejected by the zero cases seen in the RCF cohorts. The current epidemiologic studies do not rule out risk, but they clearly do rule out lung cancer risks like those of crocidolite or amosite. The residual uncertainty justifies ongoing workplace surveillance.

An epidemiological study of the respiratory health of workers in the European refractory ceramic fibre industry

OBJECTIVES

To investigate possible relations between respiratory health and past airborne exposure to refractory ceramic fibres (RCFs) and respirable dust in workers at six European factories, studied previously in 1987.

METHODS

The target population comprised all current workers associated with RCF production, plus others who had participated in 1987 "leavers". Information was collected on personal characteristics, chest radiographs, lung function, respiratory symptoms, smoking, and full occupational history. Regression analysis was used to study relations between indices of health of individual workers and of cumulative exposure to airborne dust and fibres, and likely past exposure to asbestos.

RESULTS AND DISCUSSION

774 workers participated (90% of current workers, 37% of leavers). Profusion of small opacities in exposed workers (51% 0/1+; 8% 1/0+) was similar to that among an unexposed control group but higher than in new readings of the 1987 study films (11% 0/1+, 2% 1/0+). The large difference between 1987 and recent films may be, at least in part, a reading artefact associated with film appearance. Small opacities of International Labour Organisation (ILO) category 1/0+ were not associated with exposure. An association of borderline significance overall between 0/1+ opacities and exposure to respirable fibres was found for some exposure periods only, the time related pattern being biologically implausible. Pleural changes were related to age and exposure to asbestos, and findings were consistent with an effect of time since first exposure to RCFs. Among men, forced expired volume in 1 second (FEV(1)) and forced vital capacity (FVC) were inversely related to exposure to fibres, in current smokers only. FEV(1)/ FVC ratio and transfer factor (TL(CO)) were not related to exposures. The estimated restrictive effect was on average mild. Prevalence of respiratory symptoms was low. Chronic bronchitis and its associated symptoms (cough, phlegm) showed some association with recent exposure to respirable fibres. This could be due to an irritant effect of RCFs.

Pseudoasbestos bodies and fibers in bronchoalveolar lavage of refractory ceramic fiber users

Refractory ceramic fibers (RCF) are widely used to replace asbestos in applications requiring high heat resistance. Ferruginous bodies mimicking asbestos bodies (ABs) have been detected in the lungs of RCF production workers. This raises the question about their presence in other occupational groups and whether "typical ABs" still reflect past asbestos exposures in all settings. An AB counting by phase-contrast light microscopy and a screening test by analytical electron microscopy were systematically performed on all bronchoalveolar lavage fluids (BALF) submitted to our laboratory in 1992 through 1997 (n = 1,800). When RCF were detected in electron microscopy, the structures considered as "typical ABs" were marked under light microscopy and prepared for further chemical and structural analysis. Pseudo-ABs on RCF were detected in samples from nine subjects (0.5%). All of them had worked either as foundry workers, steel workers, or welders. In these subjects, alumino-silicate fibers compatible with RCF accounted for 42% of the core fibers analyzed, other nonasbestos fibers for 28%, and asbestos fibers for 30%. ABs thus remain a valid marker of asbestos retention but attention must be paid to a possible occurrence of pseudo-asbestos bodies on RCF and other nonasbestos fibers in end-users of refractory fibers.

Reliability and validity of chest radiograph surveillance programs

STUDY OBJECTIVES

Due to the lack of consensus in the literature in the use of posteroanterior (PA) vs PA with right and left oblique views as the optimum radiograph surveillance methodology to investigate pleural changes, a study was undertaken to evaluate the reliability, sensitivity, and specificity of these two approaches.

DESIGN

Three experienced radiologist B readers used the 1980 International Labor Office classification system for pneumoconiosis to independently read chest radiographs of workers with individual identifiers masked. All radiographs were read first as a PA view only. Unknown to the B readers, each subject's PA was then matched to his or her corresponding right and left oblique views (film triad) and re-read several weeks later.

SETTING AND PARTICIPANTS

The respiratory health of 652 workers exposed to refractory ceramic fiber was assessed as part of cross-sectional and longitudinal surveillance programs.

MEASUREMENTS AND RESULTS

kappa Statistics for interreader and intrareader reliability between the PA view and film triad methods were calculated. Sensitivity, specificity, and positive predictive value were assessed by comparing the initial cross-sectional study to the longitudinal study. The film triad method had considerably higher interreader reliability (kappa = 0.59) compared to the PA-only method (kappa = 0.44). Results from the initial cross-sectional study were then compared to findings evaluated longitudinally. The film triad again was superior, demonstrating a positive predictive value of 73.7% compared to only 47.8% for the PA method.

CONCLUSIONS

It is our recommendation that the film triad method be used in surveillance studies where both parenchymal and pleural changes are anticipated.

Workplace monitoring of refractory ceramic fiber in the United States

This paper summarizes data from a comprehensive workplace exposure monitoring program for refractory ceramic fiber (RCF) conducted since 1990, including data collected under a 5-year consent agreement (1993-1998) between the U.S. Environmental Protection Agency and the Refractory Ceramic Fibers Coalition. It presents additional data and analyses, which complement an earlier article published in this journal (L. D. Maxim et al., 1997, Regul. Toxicol. Pharmacol. 26, 156-171). As part of this exposure monitoring program, data were collected at plants operated by RCF manufacturers and at customer facilities. Beginning in 1993, at least 720 samples (each sample consisting of one or more cassettes) were collected annually, distributed according to a stratified random sampling plan. The strata consisted of workers in eight functional job categories (FJCs) at manufacturing and customer plants. Time-weighted average (TWA) and task length average data were gathered and analyzed using phase contrast optical microscopy and (for a subset of samples) transmission electron microscopy methods. Data on respirator usage (by type) were also collected. Statistical analysis indicates that there are significant differences in TWA fiber concentrations (exposure) among FJCs (installation, finishing, and removal categories have the highest average concentrations), that workplace concentration data are approximately lognormally distributed, and that weighted average fiber concentrations decreased over the period from 1990 to 1998-although trend curves leveled out as exposures have decreased and further improvements became more difficult. Material differences exist in exposure between RCF manufacturers and their customers (largely because the mix of jobs differ), but these differences have narrowed over the years. Respirator usage varies with exposure. Respirator data are used to derive improved estimates of actual worker exposure. Alternative criteria for selecting FJCs for control efforts are defined and illustrated. Lessons learned for future monitoring efforts are summarized.

A retrospective review of the carcinogenicity of refractory ceramic fiber in two chronic fischer 344 rat inhalation studies: an assessment of the MTD and implications for risk assessment

The purpose of this article is to review previous chronic inhalation studies in rats with refractory ceramic fiber (RCF), the mathematical modeling efforts to describe the deposition, clearance, and retention of RCF fiber in the rat and human, and the concept of "overload," and to assess the possibility that the maximum tolerated dose (MTD) was exceeded. Lastly, based on recent biopersistence and pulmonary clearance studies of several investigators with a particulate-free RCF, we examine the potential impact on the chronic RCF rat bioassay of coexposure to both RCF particulate and RCF fibers. The review concludes, inter alia, that RCF particulate coexposure probably had a major impact on the observed chronic adverse effects, that the MTD was probably exceeded at the highest exposure concentration of 30 mg/m(3) in the rat bioassay, and that inclusion of the highest dose in the risk assessment process may overstate human health risk if a linear rather than nonlinear model is used.

Refractory ceramic fiber: toxicology, epidemiology, and risk analyses--a review

Refractory ceramic fiber (RCF) is an energy-efficient, high-temperature insulation, used principally in industrial furnaces, heaters, and reactors. Prior to the 1980s, there were few publications dealing with the potential health effects of this material. However, with the advent of higher energy costs and the need for thermally efficient high-temperature insulating materials, production of RCF grew rapidly, as did interest in its potential health effects. This article provides a comprehensive and integrated review of the toxicology (in vitro and in vivo), epidemiology, and risk analysis literature of RCF. Based on the available literature, we conclude that an occupational exposure of 0.5 fibers per cubic centimeter (cm(3)) [8-h time-weighted average (8-h TWA)] results in an occupational health risk no greater than 9.1 x 10(-5).

Potential significance of airborne fiber dimensions measured in the U.S. refractory ceramic fiber manufacturing industry

BACKGROUND

To determine dimensions of airborne fibers in the U.S. refractory ceramic fiber (RCF) manufacturing industry, fibers collected through personal air sampling for employees at RCF manufacturing and processing operations have been measured.

METHODS

Data were derived from transmission electron microscopy analyses of 118 air samples collected over a 20-year period.

RESULTS

Characteristics of sized fibers include: diameter measurements of <60; 0.19 to 1.0 micron, m of which 75% are less than 0.6 micron and length ranging from < 0.6 to > 20 micron, with 68% of fibers between 2.4 and 20 micron.

CONCLUSIONS

Exposures in RCF manufacturing include airborne fibers with dimensions (diameter < 0.1-0.4 micron, length < 10 micron) historically associated with biological effects in pleural tissues. Air sampling data and a review of studies relating fiber size to pleural effects in animals and humans support the belief that information on fiber dimensions is essential for studies with synthetic vitreous fibers.

Hazard assessment and risk analysis of two new synthetic vitreous fibers

Isofrax and Insulfrax are two new synthetic vitreous fibers (SVFs) developed for high-temperature insulation (1800-2300 degrees F) applications. In an attempt to significantly reduce or eliminate the potential of adverse health effects, these two fibers were specifically designed to have high solubility and, thus, low in vivo biodurability. In this paper, we review the effects of chemical composition on biodurability, in vitro fiber dissolution rates (K(dis)), and the relevance and relationship of K(dis) to pulmonary fibrosis and lung tumors in chronic rat inhalation studies. We also examine the correlations between K(dis) and weighted in vivo half-life (t(0.5)) of long fibers (>20 microm) and their relation to pulmonary effects in chronic rat inhalation bioassays. Predictions for outcomes of inhalation bioassays and development of nonsignificant risk levels of exposure are provided. Additionally, justification for the use of inhalation versus noninhalation animal data is provided as is a brief review of human health effects of SVFs. We conclude, inter alia, that Isofrax and Insulfrax have low biodurability, would not be expected to produce either pulmonary fibrosis or lung tumors in a well-designed animal inhalation bioassay, have weighted half-lives beneath the threshold established by the European Union for classification as a carcinogen, and based on epidemiological data for SVFs would not be expected to result in incremental cancer in human cohorts. Finally, it is estimated that approximately 90% of workplace exposure concentrations of these materials would be beneath 1 f/cc. At a concentration of 1 f/cc, neither fiber would be expected to result in an incremental working lifetime cancer risk greater than 10(-5).

Longitudinal estimates of pulmonary function in refractory ceramic fiber manufacturing workers

Refractory ceramic fibers (RCF) are man-made vitreous fibers (MMVF) used in high-temperature industrial applications. Between 1987 and 1994, a prospective study evaluated pulmonary function of 361 male workers currently employed in RCF manufacturing and related operations for plausibility of a causal relationship between RCF exposure and pulmonary function changes. Workers included in the analysis provided at least five pulmonary function tests. The exposure-response relationship was modeled with two exposure variables: years in a production job, and cumulative fiber exposure (fiber-mo/cc). Comparison groups were nonproduction workers and workers with up to 15 fiber-mo/cc cumulative exposure. A statistically significant decrease in FVC was demonstrated among workers employed in production jobs more than 7 yr prior to initial test. A similar but nonstatistically significant result was demonstrated for FVC in workers with greater than 60 fiber-mo/cc cumulative exposure prior to initial pulmonary function test. Similar but nonstatistically significant results were obtained for FEV1. These findings, which primarily reflect workers employed before 1980, did not persist with analysis of follow-up production years and accumulated RCF exposure from initial pulmonary function test. Since longitudinal analyses are sensitive to influences that continue to affect annual decline during the study period, lower RCF exposure levels since the 1980s may be responsible for eliminating any further effect on pulmonary function.