Fiber Analysis Vignettes An Inconvenient Truth

Transcription

1 Fiber Analysis Vignettes An Inconvenient Truth Victor L. Roggli, M.D. Duke University Medical Center PO Box 3712 DUMC Durham, NC (919)

2 Victor L. Roggli, M.D., is a professor of pathology at Duke University Medical Center in Durham, North Carolina, and the director of the Electron Microscopy Laboratory at the Durham Veterans Administration Medical Center. His research interests include pneumoconiosis, asbestosrelated disease and analytical electron microscopy. Dr. Roggli has published more than 140 articles and 26 chapters in textbooks. He has also authored or coauthored four books, including Pathology of Asbestos-Associated Diseases, 2nd Ed.

3 Fiber Analysis Vignettes An Inconvenient Truth Table of Contents I. Introduction II. Analytical Methodology III. Canadian Chrysotile Miners and Millers IV. Asbestos Textile Workers V. Insulators VI. Railroad Workers VII. Jewelry Industry VIII. Auto Mechanics IX. Summary and Conclusions Endnotes Fiber Analysis Vignettes An Inconvenient Truth Roggli 453

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5 Fiber Analysis Vignettes An Inconvenient Truth I. Introduction Exposure to asbestos fibers has been associated with the development of a variety of neoplastic and non-neoplastic disease processes. 1-3 These diseases have been shown to follow a dose response relationship. It has also been determined that low levels of asbestos are present in lung tissue samples from individuals with no recognizable exposure to asbestos. There is no evidence that these background exposures cause or contribute to disease. 4 Consequently, there has been considerable interest in the exposure doses that do contribute to the various asbestos-associated diseases. There are several types of asbestos, including the serpentine chrysotile and the amphiboles: amosite and crocidolite (commercial amphiboles) and tremolite, actinolite and anthophyllite (non-commercial amphiboles). It has long been known from animal studies that these fiber types do not accumulate in the lungs of experimental animals to the same degree. 5 The amphiboles accumulate progressively with increasing exposure doses, while chrysotile tends to reach a plateau after a certain level of exposure. In addition, epidemiological studies have shown important differences in the contributions of the various fiber types to asbestos-related diseases, with the amphiboles showing a greater degree of potency as compared to chrysotile. 6 The analysis of lung tissue samples for concentrations and types of fibers provides a useful measure of the cumulative levels found in an individual patient. 7 This information may then be correlated with presence or absence of disease and exposure history. Over the past several decades, there have been several examples where fiber analysis produced unexpected results which were important in our understanding of disease-exposure relationships. It is the purpose of this presentation to summarize these fiber analysis vignettes. II. Analytical Methodology The determination of concentrations of fibers in lung tissue samples involves several steps. 8 First is the selection of an appropriate specimen. Second involves the removal of the organic matrix, typically by wet chemical digestion. Third, the residue is collected on the surface of a filter. Fourth, the filter is mounted and prepared for examination by some form of electron microscopy. Some authors prefer to use transmission electron microscopy (TEM) while others have used scanning electron microscopy (SEM). A discussion of the relative advantages and disadvantages of SEM vs. TEM is beyond the scope of this paper but has been dealt with elsewhere. 8,9 Fiber types can be determined by means of energy dispersive x-ray analysis (EDXA). With TEM, additional information regarding crystalline structure may be readily obtained by selected area electron diffraction (SAED). III. Canadian Chrysotile Miners and Millers When extensive epidemiological studies of Quebec chrysotile miners and millers were begun in the 1960s, it was believed that exposures in this cohort were only to chrysotile. 10 TEM studies of lung tissue samples from these miners and millers were conducted by Fred Pooley and the results reported in To the surprise of these investigators, tremolite was found in concentrations similar to or even higher than chrysotile concentrations. Subsequent investigations showed that veins of tremolite occurred in association with chrysotile deposits, and tremolite exposures were greater in the Thetford area mines than in Asbestos. McDon- Fiber Analysis Vignettes An Inconvenient Truth Roggli 455

6 ald and colleagues subsequently showed that tremolite levels were higher in the central as compared to the peripheral mines in Thetford, and the risk of mesothelioma was also greater in these central Thetford mines. 12 Begin and colleagues challenged these conclusions, noting that mesothelioma rates were similar in miners and millers in Thetford as compared to Asbestos. Since tremolite levels were greater in Thetford mines, tremolite could not be the answer and it must be the chrysotile itself that was the main culprit. 13 However, subsequent studies showed that miners and millers from Asbestos had increased levels of crocidolite and/or amosite in their lung samples, whereas Thetford miners and millers had only tremolite and chrysotile. 14 Some crocidolite and amosite imported from South Africa had been used in the mills in Asbestos, accounting for the commercial amphibole exposures and the risk of mesothelioma in this cohort. Studies of other mesothelioma cases in Canada compared to controls similarly showed that mesothelioma risk is related to long (> 8 µm) amphibole fibers, with no apparent additional risk conferred by lung chrysotile content. 15 In summary, these early studies set the groundwork for subsequent investigations showing a marked difference in potency between chrysotile and amphiboles in terms of mesothelioma risk. 6 In South African chrysotile mines where tremolite contamination is not observed, no increased mesothelioma risk has been identified. 16 In circumstances where an increased risk of mesothelioma from pure chrysotile exposure was claimed, SEM studies demonstrated that tremolite was indeed present and that tremolite to chrysotile ratios 17, 18 in lung samples were similar to those of Canadian chrysotile miners and millers. IV. Asbestos Textile Workers In 1996 Allan Smith published a literature review claiming that chrysotile asbestos is the main cause of pleural mesothelioma. 19 Among the studies quoted in support of this proposition was that of Peto et al. regarding the Rochdale textile workers in the United Kingdom. 20 The type of asbestos used in the factory was predominantly chrysotile although small quantities of crocidolite had been historically used. Twelve deaths were due to pleural mesothelioma, and analysis of lung tissue samples for asbestos fiber content was undertaken using TEM. 21 Mineral fiber analysis was consistent with the known exposure to chrysotile, but the crocidolite content was about 300 times that of the general UK population. The finding of substantial crocidolite exposure implied that mesotheliomas that occurred in this textile factory could not be attributed with any certainty to chrysotile alone. A similar situation exists with South Carolina asbestos textile workers, for which it has been claimed that 100 percent of the exposure was to chrysotile asbestos. 19, 22 Green and colleagues performed lung tissue analyses on 39 former asbestos textile workers and 31 controls. 23 Chrysotile was the predominant mineral fiber type identified along with substantial tremolite. However, crocidolite and amosite were also increased in the asbestos factory textile workers compared with controls, and in 28 percent of the workers exceeded one million fibers per gram of dry lung tissue. More recently, Loomis and Dement published a survey of North Carolina asbestos textile workers, including four cases with mesothelioma from the Marshville plant, which according to the authors utilized only chrysotile. 24 The authors suggested that their findings would alter the potency ratios that had been published by Hodgson and Darnton. 6 However, one of the cases from this plant had been previously analyzed in our laboratory by means of SEM. 25 This 58 year old woman was a spinner, winder and weaver of chrysotile asbestos cloth, and as expected, elevated concentrations of chrysotile and tremolite were detected in her lung tissue samples. However, elevated concentrations of amosite were also detected. This patient s husband worked as an insulator and she regularly laundered his clothes. Domestic exposures are well-recognized sources of 4, 25 significant exposure to asbestos. 456 Asbestos Medicine Seminar November 2012

7 V. Insulators In 1965 Selikoff and colleagues reported that insulators had light and intermittent exposures to asbestos. 26 These investigators had previously reported high rates of lung cancer and mesothelioma in this 27, 28 cohort, and Becklake in 1976 reported that insulators were exposed almost exclusively to chrysotile. McDonald found a relative risk of mesothelioma of 46 for insulators, which was the highest of any of the categories studied. 10 These disparate and contradictory observations are clarified by fiber analysis of lung tissue samples from individuals working with insulation products. Churg and Vedal reported fiber analysis results in 144 individuals from the Pacific Northwest, most of whom were insulators or shipyard workers (exposed to insulation products) or allied trades (e.g., pipe fitters). 29 These authors used TEM and observed that amosite was the most common fiber type, often present at orders of magnitude higher levels than chrysotile or tremolite. For example, among 23 cases with asbestosis, the geometric mean amosite fiber count was 10 million fibers per gram of dry lung, whereas the corresponding values for chrysotile and tremolite were and million fibers per gram. The presence of specific diseases correlated with amosite concentrations but not with chrysotile or tremolite. Most asbestos insulation products used in the United States prior to 1972 contained amosite and often chrysotile, so insulators were not exposed to chrysotile alone. The finding of high concentrations of amosite in these workers lungs is at odds with the assessment of light and intermittent exposures to asbestos. The author has analyzed lung tissue samples from 92 insulators, including 34 with mesothelioma and 43 with lung cancer 7, 9 (and unpublished observations). Forty-eight of the cases had asbestosis and 60 had pleural plaques. The median asbestos body count in 89 cases was 26,600 per gram of wet lung tissue (normal range 0-20 AB/gm). The median asbestos fiber count as determined by SEM was 300,000 fibers 5 µm or greater in length per gram of wet lung (median value for 20 controls was <600 fibers/gm). Similar to the findings of Churg and Vedal, the vast majority of fibers analyzed were amosite. Furthermore, insulators have the highest 7, 30 asbestos content of any occupational group that we have analyzed. VI. Railroad Workers Railroad workers had ample opportunity for exposure to asbestos, especially during the steam era (up until approximately 1958) when large amounts of asbestos lagging were applied to and removed from the 31, 32 boilers of the steam engines. Mancuso reported on a cohort of railroad machinists with mesothelioma. These individuals often worked in the roundhouse, where repairs on locomotives were conducted. According to Mancuso, these workers were exposed almost exclusively to chrysotile. 31 The author has had the opportunity to examine lung tissue samples from 33 individuals whose primary occupational exposure to asbestos was as a railroad worker 7-9, 30 (and unpublished observations). These included 12 patients with mesothelioma and 13 with lung cancer. Three patients had asbestosis and 13 had pleural plaques. The median asbestos body count was 68 AB/gm (normal range 0-20 AB/gm). The median asbestos fiber count as determined by SEM was 7670 fibers 5 µm or greater in length per gram of wet lung (median value for 20 controls was <600 fibers/gm). Amosite was identified in increased concentrations in many of these workers lungs, and in two cases with mesothelioma, crocidolite was the only commercial amphibole fiber type identified. 33 Tremolite and chrysotile fibers were also elevated in these latter two cases. VII. Jewelry Industry In 1992, Kern et al reported a case of mesothelioma in an individual working in the jewelry industry. 34 The patient had worked for 35 years making asbestos soldering forms at a costume jewelry production Fiber Analysis Vignettes An Inconvenient Truth Roggli 457

8 facility. Non-neoplastic asbestos-related disease had been described in similar workers, and it was believed that the exposure was to chrysotile asbestos. 35 The patient underwent an extrapleural pneumonectomy for his mesothelioma, and a fiber analysis was performed. There were 13,300 AB/gm of wet lung tissue by light microscopy, and there were 20,900 amosite fibers per gram of wet lung tissue by SEM. No chrysotile or tremolite was detected. On further investigation, it was determined that a distributor had supplied both chrysotile and amosite during the first 25 years that the patient fabricated soldering forms. The patient had also worked for nine months in the 1940s in a local shipyard, cleaning up after welders fabricating new hulls from steel plates. VIII. Auto Mechanics Several investigators have expressed concern that auto mechanics might be at increased risk of asbestos related disease as a result of working with asbestos-containing friction products (brakes and clutches) However, a number of epidemiological studies have failed to demonstrate an increased risk of mesothelioma among auto mechanics Furthermore, epidemiological analyses have concluded that working with friction products does not increase the risk of asbestos-related disease incurred from working with other asbestos products. 43 Some authors have thus concluded that whether or not asbestos exposure from brake and clutch repair work increases one s risk of mesothelioma is controversial. 44 It is therefore useful to examine the results of lung fiber analyses to see if they are informative in this regard. Butnor et al. reported fiber analysis results on 10 cases of mesothelioma among individuals whose only known exposure to asbestos was from auto repair work. 45 In five of these individuals, the tissue asbestos content as determined by SEM was indistinguishable from that of our reference or control population. In five additional cases, there were elevated levels of commercial amphiboles (amosite in four cases, crocidolite in one). In three of these latter cases, either tremolite or chrysotile concentrations were also elevated. Since commercial amphiboles were not used in friction products in the United States, the authors concluded that these latter five individuals had some other unidentified exposure to asbestos. Marsh et al. reanalyzed these ten cases and added five additional cases, four of which had background levels of asbestos and one of which had elevated concentrations of crocidolite. 46 These authors concluded that there was a correlation between commercial amphibole levels and tremolite levels in these 15 cases, and further noted that there was no correlation between tremolite levels and duration of exposure as an auto mechanic. Together these findings added further support to the conclusion that the 6 individuals with elevated asbestos content had exposures in occupational settings other than brake repair work. The author has had the opportunity to examine the tissue asbestos content in 33 individuals whose only known exposure to asbestos was working in the automotive industry, including 30 cases in the service industry, two in manufacturing, and one shade tree mechanic 7, 9, 30, 46 (and unpublished observations). These included 17 with mesothelioma, 5 with lung cancer and 5 with pleural plaques. Ten cases had interstitial lung disease but none met criteria for asbestosis. The median asbestos body count was 12.5 AB/gm (normal range 0-20 AB/gm). The median asbestos fiber count by SEM was 1,040 fibers 5µm or greater in length per gram of wet lung. In addition, we have analyzed lung tissue from five household contacts of auto mechanics, and all five had tissue asbestos contents within the range of our reference population. A number of other investigators using TEM also reported either background levels of asbestos or elevated levels of commercial amphiboles among brake mechanics with mesothelioma Asbestos Medicine Seminar November 2012

9 IX. Summary and Conclusions This report summarizes some of the more prominent and well-documented examples where electron microscopy has corrected misconceptions regarding asbestos and disease. The fiber analysis vignettes presented here are summarized in Table 1. There are numerous additional examples where electron microscopy has provided useful information regarding the causation of asbestos-associated diseases and their relationship to specific industrial or environmental exposures. Endnotes 1 Roggli VL, Oury TD, Sporn TA, eds. Pathology of Asbestos-Associated Diseases, 2 nd Ed. Springer: New York, Churg A. Nonneoplastic Disease Caused by Asbestos, CH 9, In: Pathology of Occupational Lung Disease, 2 nd Ed. (Churg A, Green FHY, eds.), Williams & Wilkins: Baltimore, 1998, pg Churg A. Neoplastic Asbestos-Induced Disease, CH 10, In: Pathology of Occupational Lung Disease, 2 nd Ed. (Churg A, Green FHY, eds.), Williams & Wilkins: Baltimore, 1998, pg Henderson DW, Rantanen J, Barnhart S, Dement JM, De Vuyst P, Hillerdal G, Huuskonen MS, Kivisaari L, Kusaka Y, Lahdensuo A, Langard S, Mowe G, Okubo T, Parker JE, Roggli VL, Rödelsperger K, Rösler J, Tossavainen A, Woitowitz HJ. Asbestos, asbestosis, and cancer: The Helsinki criteria for diagnosis and attribution. A consensus report of an international expert group. Scand J Work Environ Health 23: 311, Wagner JC, Berry G, Skidmore JW, Timbrell V. The effects of the inhalation of asbestos in rats. Br J Cancer 29: 252, Hodgson JT, Darnton A. The quantitative risk of mesothelioma and lung cancer in relation to asbestos exposure. Ann Occup Hyg 44: 565, Roggli VL, Sharma A. Analysis of Tissue Mineral Fiber Content, CH 11, In: Pathology of Asbestos-Associated Diseases, 2 nd Ed. (Roggli VL, Oury TD, Sporn TA, eds.), Springer: New York, 2004, pg Roggli VL. Tissue Digestion Techniques, Appendix, In: Pathology of Asbestos-Associated Diseases, 2 nd Ed. (Roggli VL, Oury TD, Sporn TA, eds.), Springer: New York, 2004, pg Roggli VL, Vollmer RT. Twenty-five years of fiber analysis: What have we learned? Hum Pathol 39: 307, McDonald JC. Epidemiology of malignant mesothelioma An outline. Ann Occup Hyg 54: 851, Pooley FD. An examination of the fibrous mineral content of asbestos in lung tissue from the Canadian chrysotile mining industry. Environ Res 12: 281, McDonald AD, Case BW, Churg A, Dufresne A, Gibbs GW, Sebastien P, McDonald JC. Mesothelioma in Quebec chrysotile miners and millers: Epidemiology and aetiology. Ann Occup Hyg 41: 707, Begin R, Gauthier J-J, Desmeules M, Ostiguy G. Work-related mesothelioma in Quebec, Am J Ind Med 22: 531, Dufresne A, Begin R, Churg A, Masse S. Mineral fiber content of lungs in patients with mesothelioma seeking compensation in Quebec. Am J Respir Crit Care Med 153: 711, McDonald JC, Armstrong B, Case B, Doell B, McCaughey WTE, McDonald AD, Sebastien P. Mesothelioma and asbestos fiber type: Evidence from lung tissue analyses. Cancer 63: 1544, White N, Nelson G, Murray J. South African experience with asbestos related environmental mesothelioma: Is asbestos fiber type important? Reg Toxicol Pharmacol 52 [Suppl 1]: S92, Yano E, Wang Z-M, Wang X-R, Wang M-Z, Lan Y-J. Cancer mortality among workers exposed to amphibole-free chrysotile asbestos. Am J Epidemiol 154: 538, Tossavainen A, Kotilainen M, Takahashi K, Pan G, Vanhala E. Amphibole fibres in Chinese chrysotile asbestos. Ann Occup Hyg 45: 145, Smith AH, Wright CC. Chrysotile asbestos is the main cause of pleural mesothelioma. Am J Ind Med 30: 252, Fiber Analysis Vignettes An Inconvenient Truth Roggli 459

10 20 Peto J, Doll R, Hermon C, Binns W, Clayton R, Goffe T. Relationship of mortality to measures of environmental asbestos pollution in an asbestos textile factory. Ann Occup Hyg 29: 305, Wagner JC, Berry G, Pooley FD. Mesotheliomas and asbestos type in asbestos textile workers: A study of lung contents. Br Med J 285: 603, Dement JM, Brown DP, Okun A. Follow-up study of chrysotile asbestos textile workers: Cohort mortality and casecontrol analyses. Am J Ind Med 26: 431, Green FH, Harley R, Vallyathan V, Althouse R, Fick G, Dement J, Mitha R, Pooley F. Exposure and mineralogical correlates of pulmonary fibrosis in chrysotile asbestos workers. Occup Environ Med 54: 549, Loomis D, Dement JM, Wolf SH, Richardson DB. Lung cancer mortality and fiber exposures among North Carolina asbestos textile workers. Occup Environ Med 66: 535, Roggli VL, Oury TD, Moffatt EJ. Malignant mesothelioma in women. In: Anatomic Pathology, 1997, Vol. 2 (Rosen PP, Fechner RE, eds.), ASCP Press: Chicago, 1998, pg Selikoff IJ, Churg J, Hammond EC. Relation between exposure to asbestos and mesothelioma. N Engl J Med 272: 560, Selikoff IJ, Churg J, Hammond EC. Asbestos exposure and neoplasia. JAMA 188: 22, Becklake MD. Asbestos-related disease of the lungs and other organs: Their epidemiology and implications for clinical practice. Am Rev Respir Dis 114: 187, Churg A, Vedal S. Fiber burden and patterns of asbestos-related disease in workers with heavy mixed amosite and chrysotile exposure. Am J Respir Crit Care Med 150: 663, Roggli VL, Sharma A, Butnor KJ, Sporn T, Vollmer RT. Malignant mesothelioma and occupational exposure to asbestos: A clinicopathological correlation of 1445 cases. Ultrastruct Pathol 26: 55, Mancuso TF. Relative risk of mesothelioma among railroad machinists exposed to chrysotile. Am J Ind Med 13: 639, Mancuso TF, Mesothelioma among machinists in railroad and other industries. Am J Ind Med 4: 501, Schneider F, Sporn TA, Roggli VL. Crocidolite and mesothelioma. Ultrastruct Pathol 32: 171, Kern DG, Hanley KT, Roggli VL. Malignant mesothelioma in the jewelry industry. Am J Ind Med 21: 409, Kern DG, Frumkin H. Asbestos-related disease in the jewelry industry: Report of two cases. Am J Ind Med 13: 407, Rohl AN, Langer AN, Wolff MS, Weisman I. Asbestos exposure during brake lining maintenance and repair. Environ Res 12: 110, Lemen RA. Asbestos in Brakes: Exposure and Risk of Disease. Am J Ind Med 45: 229, Welch LS. Asbestos Exposure Causes Mesothelioma, But Not This Asbestos Exposure: An Amicus Brief to the Michigan Supreme Court. Int J Occup Environ Health 13: 318, Goodman M, Teta MJ, Hessel PA, Garabrant DH, Craven VA, Scrafford CG, Kelsh MA. Mesothelioma and Lung Cancer among Motor Vehicle Mechanics: A Meta-analysis. Ann Occup Hyg 48: 309, Kelsh MA, Craven VA, Teta MJ, Mowat FS, Goodman M. Mesothelioma in vehicle mechanics: is the risk different for Australians? Occup Med 57: 581, Laden F, Stampfer MJ, Walker AM. Lung Cancer and Mesothelioma Among Male Automobile Mechanics: A Review. Rev Environ Health 19: 39, Wong O. Malignant Mesothelioma and Asbestos Exposure among Auto Mechanics: Appraisal of Scientific Evidence. Toxicol Pharmacol 84: 170, Hessel PA, Teta MJ, Goodman M, Lau E. Mesothelioma among Brake Mechanics: An Expanded Analysis of a Case- Control Study. Risk Analysis 24: 547, Hammar SP, Henderson DW, Klebe S, Dodson RF. Neoplasms of the Pleura, CH 43, In: Dail & Hammar s Pulmonary Pathology, 3 rd Ed. (Tomashefski JF, Cagle PT, Farver CF, Fraire AE, eds.), Springer-Verlag: New York, 2008, pg Asbestos Medicine Seminar November 2012

11 45 Butnor KJ, Sporn TA, Roggli VL. Exposure to brake dust and malignant mesothelioma: A study of 10 cases with mineral fiber analyses. Ann Occup Hyg 47: 325, Marsh GM, Youk AO, Roggli VL. Asbestos fiber concentrations in the lungs of brake repair workers: Commercial amphibole levels are predictive of chrysotile levels. Inhal Toxicol 12: 681, Gordon RE, Dikman S. Asbestos fiber burden analysis of lung and lymph nodes in 100 cases of mesothelioma (abstr). Am J Respir Crit Care Med 179: A5892, Case BW. Exposure to brake dust and malignant mesothelioma: Lung-retained fibre analyses using transmission electron microscopy confirm previous findings at lower magnification by scanning electron microscopy (abstr.), presented at the British Occupational Hygiene Society, Stratford upon Avon, UK, April 5-7, Dodson RF, Hammar SP, Poye LW. A technical comparison of evaluating asbestos concentration by phase-contrast microscopy (PCM), scanning electron microscopy (SEM), and analytical transmission electron microscopy (ATEM) as illustrated from data generated from a case report. Inhal Toxicol 20: 723, Dodson RF, Graef R, Shepherd S, O Sullivan M, Levin J. Asbestos burden in cases of mesothelioma from individuals from various regions of the United States. Ultrastruct Pathol 29: 415, Table 1. Examples Where Electron Microscopy Has Improved Understanding or Corrected Misconceptions Regarding Asbestos and Disease Exposure Category Misconception or Belief Electron Microscopy Findings References Canadian chrysotile Exposure only to chrysotile High levels of tremolite found in miners and millers lungs of miners and millers Thetford vs Asbestos Similar mesothelioma rates Miners and millers from Asbestos 13, 14 miners and millers although tremolite exposure with mesothelioma have elevated much higher in Thetford levels of commercial amphiboles miners and millers (amosite and/or crocidolite) Asbestos textile These workers exposed ex- Many of these workers have ele workers clusively or almost exclu- vated levels of commercial amphisively to chrysotile boles (amosite and/or crocidolite) in their lung tissue samples Insulators Insulators had light and Insulators have high concentrations 7, 9, 26, intermittent exposures to of asbestos in their lung tissue sam chrysotile asbestos ples, primarily amosite Railroad workers Railroad machinists are Railroad workers with mesothelioma 7, 9, exposed almost exclu- typically have elevated levels of sively to chrysotile amosite or crocidolite Jewelry industry Jewelry soldering board Jewelry maker with mesothelioma 34, 35 fabricators are exposed had elevated levels of amosite to chrysotile asbestos Soldering boards were fabricated from chrysotile and amosite Auto mechanics Exposure to chrysotile Auto mechanics either have 36-38, from brake and clutch background asbestos contents repair work causes (60%) or elevated levels of mesothelioma commercial amphiboles with or without elevated chrysotile or tremolite Fiber Analysis Vignettes An Inconvenient Truth Roggli 461

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