Chrysotile asbestos is the main cause of pleural mesothelioma

Our plain-English summary of Chrysotile asbestos is the main cause of pleural mesothelioma, by A.H. Smith, C.C. Wright (American Journal of Industrial Medicine, 1996).

The study

• Authors: A.H. Smith, C.C. Wright
• Journal: American Journal of Industrial Medicine
• Published: 1996;30(3):252-66
• Type of article: Review
• Cited by: 95 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In contrast to amphibole forms of asbestos, chrysotile asbestos is often claimed to be only a minor cause of malignant pleural mesothelioma, a highly fatal cancer of the lining of the thoracic cavity. In this article we examine the evidence from animal and human studies that relates to this issue. Reported data do not support widely quoted views regarding the relative inertness of chrysotile fibers in mesothelioma causation. In fact, examination of all pertinent studies makes it clear that chrysotile asbestos is similar in potency to amphibole asbestos. Since asbestos is the major cause of mesothelioma, and chrysotile constitutes 95% of all asbestos use world wide, it can be concluded that chrysotile asbestos is the main cause of pleural mesothelioma in humans.

In plain English

This 1996 article was a review. The authors examined human and animal evidence on chrysotile and pleural mesothelioma. They ranked asbestos-worker cohorts by pleural mesotheliomas per 1,000 deaths. Chrysotile was the primary exposure in at least two of the top 10 cohorts, crocidolite in three and amosite in none. Reanalyzing an earlier Hughes and Weill comparison with longer follow-up, the authors found pleural mesotheliomas equalled 20% of excess lung cancers in chrysotile-exposed groups and 72% in crocidolite-exposed groups. They suggested chrysotile may be three or four, not 14, times less potent than crocidolite. Pooled rat inhalation studies showed pleural mesothelioma in 1.8% (13/733) of rats given chrysotile, 1.3% (5/377) given amosite and 1.2% (4/328) given crocidolite. They judged that at least 80% of pleural mesotheliomas in adult males are likely due to asbestos. They noted chrysotile made up about 95% of world production. The authors concluded chrysotile is by far the main contributor to pleural mesothelioma in the U.S. and other countries where it has been the predominant fiber type. The animal data, they noted, do not allow per-fiber potency estimates.

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: Review of published epidemiological (cohort and case series), animal, and lung and pleural fiber-content studies, with the authors' own ranking and reanalysis of data from those studies
• Who was studied: Published asbestos worker cohort studies (most recent follow-up available as of October 1994), of which the 25 with the highest ratio of pleural mesotheliomas per 1,000 deaths were tabulated; also long-term animal inhalation studies and human lung and pleural fiber-content studies
• Exposure: Chrysotile (a serpentine fiber, often contaminated with small amounts of tremolite) compared with the amphiboles crocidolite and amosite; also tremolite
• Main results:
  • Among the 10 cohorts with the highest ratio of pleural mesotheliomas per 1,000 deaths, chrysotile was the primary exposure in at least two and crocidolite in three. Amosite was part of a mixed exposure in three but was not the primary exposure in any.
  • The highest ratio among the cohorts reviewed, 88.1 pleural mesotheliomas per 1,000 deaths, was in a cohort of railroad machinists exposed to chrysotile (Mancuso 1988). The authors noted that other researchers had criticized that study for not attributing the cases to possible amphibole exposure.
  • In the three gas mask studies the authors summed, workers exposed only to crocidolite had 19 pleural mesothelioma cases (per Table III), with a lung cancer excess of about 13.5.
  • In the same three gas mask studies, the chrysotile-only workers had one pleural mesothelioma case and an excess of 1.2 lung cancers. The authors took the low lung cancer excess as a sign of relatively low chrysotile exposure. They said these studies give no evidence that crocidolite is more potent.
  • In the authors' reanalysis of the cohorts used by Hughes and Weill, with later follow-up and pleural cases only, pleural mesotheliomas were 20% of excess lung cancers in chrysotile-exposed populations. The figures were 72% for crocidolite, 13% for amosite and 27% for mixed exposures.
  • From this reanalysis the authors suggested that chrysotile may be three or four times less potent than crocidolite in causing mesothelioma, rather than 14 times.
  • In rat inhalation studies, which the authors added together, the overall incidence of pleural mesothelioma was 1.8% (13/733) for chrysotile, 1.3% (5/377) for amosite and 1.2% (4/328) for crocidolite.
  • The authors judged that studies of lung fiber content were inconsistent.
  • The authors judged that studies of fibers found in the pleura showed chrysotile fibers preferentially reach the pleura.
  • Based on studies with careful work histories, the authors estimated it likely that at least 80% of pleural mesothelioma cases in adult males are attributable to asbestos exposure.
  • Chrysotile represents approximately 95% of total world asbestos production.
• The authors' conclusion: The authors concluded that chrysotile asbestos is by far the main contributor to pleural mesothelioma in the U.S. and other countries where it has been the predominant fiber type. They wrote that crocidolite may be 2 to 4 times more potent, but that there is no valid evidence that amosite is more potent than chrysotile.
• Limitations the authors state:
  • The animal data do not allow potency per fiber to be estimated.
  • Data on tremolite are quite limited.
  • For one gas mask study (McDonald and McDonald 1978), the expected number of lung cancer deaths was not reported, so the authors assumed a value of one.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Pleural Mesothelioma, Amphibole Group, Mesothelioma, Causation, Amphibole Asbestos.

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