Chrysotile, tremolite, and malignant mesothelioma in man

Our plain-English summary of Chrysotile, tremolite, and malignant mesothelioma in man, by A. Churg (Chest, 1988).

The study

• Author: A. Churg
• Journal: Chest
• Published: 1988;93(3):621-8
• Type of article: Review
• Cited by: 101 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000202).

Abstract

The question of whether chrysotile asbestos ever causes mesothelioma in man has become a major public and occupational health issue. Review of the literature suggests that only 53 acceptable cases of chrysotile-induced mesothelioma have ever been reported; of these, 41 cases have occurred in individuals exposed to chrysotile mine dust, all of it naturally contaminated with tremolite. Ten cases have occurred in secondary industry workers, but here the suspicion of amosite or crocidolite contamination is high. Analysis of lung asbestos content indicates that induction of mesothelioma by chrysotile requires, on average, as great a lung fiber burden as induction of asbestosis by chrysotile, whereas amphibole (amosite or crocidolite)-induced mesotheliomas appear at a several hundred-fold smaller lung burden. Tremolite alone has definitely produced mesothelioma in man, particularly when exposure has been to long, high aspect ratio, fibers. Analysis of tremolite:chrysotile fiber ratios in human lung suggests that some, but not all tremolite is removed in milling chrysotile ores. The low incidence of mesothelioma in secondary chrysotile users may reflect the small amount of tremolite left in the product. These observations indicate that although chrysotile asbestos can produce mesothelioma in man, the total number of such cases is small and the required doses extremely large. The data are consistent with the idea that mesotheliomas seen in chrysotile miners and some secondary industry workers are produced by the tremolite contained in the chrysotile ore, but that the short length and low aspect ratio of the tremolite make its carcinogenicity quite low. However, these data are very indirect, and a role for the chrysotile fiber itself is still possible.

In plain English

This 1988 article was a literature review by an author in the Department of Pathology at the University of British Columbia. It asked whether chrysotile asbestos causes mesothelioma in humans, and what role is played by tremolite, an amphibole fiber present in many chrysotile ores. Of 142 mesotheliomas in published reports of chrysotile-induced tumors, the author accepted at most 53. Of these, 41 were in people exposed to chrysotile mine dust, ten in workers at factories with known if minimal amphibole use, and two in others. In the lung fiber data presented, chrysotile workers with mesothelioma had a median chrysotile-plus-tremolite burden some 400 times the amosite-plus-crocidolite burden of shipyard and insulation workers with mesothelioma. Lung tremolite-to-chrysotile ratios were lower in workers using processed chrysotile products than in chrysotile production workers. The author took this to suggest that milling removes some tremolite. The author concluded that the data were consistent with tremolite being the actual cause, but called the evidence very indirect and said a role for chrysotile itself was not ruled out. The author also concluded that chrysotile ore is an extremely weak carcinogen requiring extremely high doses.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Literature review (analysis of published case reports, epidemiologic studies and lung fiber analyses)
• Who was studied: Published reports of mesothelioma attributed to chrysotile: 142 mesotheliomas in total, of which the author accepted at most 53 as chrysotile-induced. The reports covered chrysotile miners and millers, their household contacts, nearby residents (Quebec, Cyprus), chrysotile factory workers, and other individual cases.
• Exposure: Chrysotile asbestos, both as mine dust and in processed products (cement, textile, friction products, gas mask factories, brake repair), and its natural tremolite contaminant. These were compared with the amphiboles amosite and crocidolite, and with tremolite exposure without chrysotile (vermiculite mining, environmental exposure in Greece, talc mining).
• Main results:
  • Of the 53 accepted chrysotile-induced mesotheliomas, 41 were in persons exposed to chrysotile mine dust, ten in workers from factories with known if minimal amphibole use, and two in others with no apparent amosite or crocidolite exposure.
  • Among 13 cases from chrysotile mine dust exposure with lung asbestos data, 12 had only chrysotile ore components (chrysotile plus tremolite) in their lungs. The author regarded this as showing that chrysotile ore can produce mesothelioma.
  • In predominantly chrysotile-using factories, most reported mesotheliomas were associated with amphibole exposure. For example, a reviewed study found historic or mineralogic evidence of crocidolite exposure in nine of 11 mesotheliomas in a chrysotile friction plant.
  • In two cement plants with a mean exposure of about 10 fibers/ml, at most one chrysotile-related mesothelioma was found among 2,143 deaths (0.05 percent).
  • Lung fiber data compared nine chrysotile workers with mesothelioma and nine with asbestosis against shipyard and insulation workers matched by years of exposure. The median chrysotile-plus-tremolite burden in chrysotile workers with mesothelioma was some 400 times the amosite-plus-crocidolite burden in shipyard workers with mesothelioma.
  • Assuming residual lung fiber burdens allow a fiber-for-fiber comparison, the author interpreted the data as indicating chrysotile is at least two orders of magnitude less dangerous than amosite.
  • Arithmetic mean tremolite-to-chrysotile ratios in lung were 8 (9 cases) in chrysotile production workers with mesothelioma and 14 (9 cases) in those with asbestosis. They were 0.3 in 21 users of processed chrysotile products, about 0.3 in textile workers and 0.04 in cement workers. The author took this to suggest milling removes a portion of the tremolite.
  • Tremolite without chrysotile: in a group of workers mining tremolite-contaminated vermiculite, four mesotheliomas made up 2.4 percent of deaths.
  • Tremolite fibers in the lungs of chrysotile miners were short. Only 0.8 percent of 2,500 tremolite fibers from the lungs of nine Thetford miners with mesothelioma were longer than 10 microns.
• The authors' conclusion: The author concluded that the limited data are consistent with tremolite being the actual causative agent of chrysotile-induced mesothelioma. The author also concluded that chrysotile ore, crude or processed, does cause mesothelioma in man, but is an extremely weak carcinogen requiring extremely high doses.
• Limitations the authors state:
  • The author stated that all measurements supporting tremolite as the actual carcinogenic agent are very indirect, and none rule out a role for chrysotile itself.
  • The author stated that the count of 53 cases may be overgenerous for the factory workers and those without documented exposure.
  • Lung fiber burden analysis is affected by unknown fiber clearance over time. Chrysotile, unlike amphiboles, does not accumulate readily in lung.
  • The author stated that the fiber size data are sparse and show trends only in very broad outline.
  • The author noted that a study reporting mesothelioma linked to tremolite-containing whitewash and stucco was unintentionally left out of the review.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile, Lung cancer, Mesothelioma, Tremolite.

Medical terms in this study, explained in our Lexicon: Chrysotile, Tremolite, Malignant Mesothelioma, Mesothelioma, Amosite, Crocidolite, Fiber Burden, Amphibole Group.

Related studies

• Tremolite/chrysotile ratios — G. Scansetti, G. Piolatto (1987)
• Environmental pleural plaques in residents of a Quebec chrysotile mining town — A. Churg, L. DePaoli (1988)
• Biological effects of tremolite — J.C. Wagner, M. Chamberlain, et al. (1982)
• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• Radiological survey of past and present vermiculite miners exposed to tremolite — J.C. McDonald, P. Sebastien, et al. (1986)
• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)

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