Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis

Our plain-English summary of Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis, by L.T. Stayner, D.A. Dankovic, R.A. Lemen (American Journal of Public Health, 1996).

The study

• Authors: L.T. Stayner, D.A. Dankovic, R.A. Lemen
• Journal: American Journal of Public Health
• Published: 1996;86(2):179-86
• Type of article: Review
• Cited by: 154 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: This article examines the credibility and policy implications of the "amphibole hypothesis," which postulates that (1) the mesotheliomas observed among workers exposed to chrysotile asbestos may be explained by confounding exposures to amphiboles, and (2) chrysotile may have lower carcinogenic potency than amphiboles.

Methods: A critical review was conducted of the lung burden, epidemiologic, toxicologic, and mechanistic studies that provide the basis for the amphibole hypothesis.

Results: Mechanistic and lung burden studies do not provide convincing evidence for the amphibole hypothesis. Toxicologic and epidemiologic studies provide strong evidence that chrysotile is associated with an increased risk of lung cancer and mesothelioma. Chrysotile may be less potent than some amphiboles for inducing mesotheliomas, but there is little evidence to indicate lower lung cancer risk.

Conclusions: Given the evidence of a significant lung cancer risk, the lack of conclusive evidence for the amphibole hypothesis, and the fact that workers are generally exposed to a mixture of fibers, we conclude that it is prudent to treat chrysotile with virtually the same level of concern as the amphibole forms of asbestos.

In plain English

This article was a review, not a new study. The authors examined the "amphibole hypothesis": that mesotheliomas in chrysotile-exposed workers might be explained by small amounts of tremolite contamination, and that chrysotile may be less potent than amphiboles in causing asbestosis and lung cancer. They reviewed lung burden, epidemiologic, animal and mechanistic studies. Across 12 cohort studies of workers mainly exposed to chrysotile, the text reported 928 observed versus 618.9 expected lung cancer deaths, a pooled standardized mortality ratio of 1.50 (95% confidence interval 1.40 to 1.60). However, the article's own table totals 922 observed deaths against those 618.9 expected, leaving out one study that had no expected count. Mesothelioma made up 0.3% of deaths in these cohorts, versus 0.02% of 1988 United States deaths from pleural malignancies. The authors judged that chrysotile lung burdens poorly reflect past exposure. They found evidence that chrysotile may be less potent than crocidolite for mesothelioma, but no evidence that it is less potent for lung cancer. They noted that differences in fiber dimensions limit potency comparisons. The authors concluded that it is prudent to treat chrysotile with virtually the same concern as amphibole asbestos.

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: Critical review of published lung burden, epidemiologic, toxicologic and mechanistic studies, with a pooled summary of chrysotile cohort studies
• Who was studied: Published studies, including 12 retrospective cohort mortality studies of workers predominantly exposed to chrysotile (gas masks, textiles, friction products, cement, mining and milling and other industries), plus lung burden studies and experiments in rats
• Exposure: Chrysotile asbestos compared with amphibole asbestos (crocidolite, amosite, tremolite, anthophyllite), with attention to tremolite contamination of commercial chrysotile, reported as less than 1%, and to whether chrysotile is less potent for asbestosis and lung cancer
• Main results:
  • The article's text reported 928 observed and 618.9 expected lung cancer deaths across the chrysotile-exposed worker cohorts, a pooled standardized mortality ratio of 1.50 (95% CI 1.40, 1.60). Its Table 1, however, totals 922 observed deaths against the same 618.9 expected, so the 928 in the text does not match the expected count.
  • Table 1 excluded the Shiqu et al. study (6 observed lung cancer deaths) from the lung cancer total because it reported no expected number
  • Lung cancer potencies were similar among textile-industry cohorts with pure chrysotile exposure and those with mixed fiber exposure
  • In the chrysotile-exposed cohorts combined, mesothelioma accounted for 0.3% of deaths, compared with 0.02% of deaths from pleural malignancies in the United States in 1988
  • Comparing Quebec chrysotile miners and millers with South Carolina textile workers did not support the idea that tremolite explains the mesothelioma excess
  • Epidemiologic evidence indicated chrysotile may be less potent than crocidolite for mesothelioma. Among South African miners exposed to crocidolite, 4.7% of deaths were from mesothelioma, more than among the chrysotile-exposed Quebec miners (0.4%) or South Carolina textile workers (0.2%).
  • In the animal studies reviewed, chrysotile appeared at least as potent as amphiboles for lung tumors on a per-milligram basis
  • The intratracheal rat experiments of Coffin et al. gave some support to chrysotile having lower mesothelioma-inducing potential than amphiboles
  • The authors judged that chrysotile lung burdens poorly reflect past integrated exposure, which weakens the lung burden evidence for the hypothesis
  • Mechanistic arguments based on fiber iron content were contradicted by the lack of correlation between iron content and carcinogenic potency in experiments
• The authors' conclusion: The authors concluded that it is prudent policy to treat chrysotile with virtually the same level of concern as amphibole asbestos, given the evidence of lung cancer risk, the lack of compelling evidence for the amphibole hypothesis, and workers' exposure to fiber mixtures
• Limitations the authors state:
  • Comparisons across epidemiologic studies could not control for differences in exposure concentrations, fiber size distributions and length of observation
  • Exposure-measurement methods based on fiber length most likely caused substantial misclassification of exposure in the epidemiologic studies
  • Low tumor rates and small numbers of animals made quantitative comparisons of mesothelioma in rat inhalation studies uncertain
  • Asbestosis risk was not considered in the review

Topics

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

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Chrysotile, amphibole hypothesis, Mesothelioma, Confounding, Amphibole Group, Carcinogenic, Lung Cancer.

Related studies

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• Sumas Mountain chrysotile induces greater lung fibrosis in Fischer344 rats than Libby amphibole, El Dorado tremolite, and Ontario ferroactinolite — J.M. Cyphert, A. Nyska, et al. (2012)
• Differential activation of the inflammasome in THP-1 cells exposed to chrysotile asbestos and Libby "six-mix" amphiboles and subsequent activation of BEAS-2B cells — M. Li, M.E. Gunter, et al. (2012)
• Asbestos fiber concentrations in the lungs of brake repair workers: commercial amphiboles levels are predictive of chrysotile levels — G.M. Marsh, A.O. Youk, et al. (2011)
• Minimising health hazards associated with derogated products and in-situ materials containing chrysotile contaminated with amphibole asbestos fibres — R.M. Howie (1999)
• Mechanisms of asbestos carcinogenesis and toxicity: the amphibole hypothesis revisited — B.T. Mossman (1993)

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