Developments in asbestos cancer risk assessment

Our plain-English summary of Developments in asbestos cancer risk assessment, by M.A. Silverstein, L.S. Welch, R. Lemen (American Journal of Industrial Medicine, 2009).

The study

• Authors: M.A. Silverstein, L.S. Welch, R. Lemen
• Journal: American Journal of Industrial Medicine
• Published: 2009;52(11):850-8
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: Efforts have been made for 25 years to develop asbestos risk assessments that provide valid information about workplace and community cancer risks. Mathematical models have been applied to a group of workplace epidemiology studies to describe the relationships between exposure and risk. EPA's most recent proposed method was presented at a public meeting in July 2008.

Methods: Risk assessments prepared by USEPA, OSHA, and NIOSH since 1972 were reviewed, along with related literature.

Results And Conclusions: None of the efforts to use statistical models to characterize relative cancer potencies for asbestos fiber types and sizes have been able to overcome limitations of the exposure data. Resulting uncertainties have been so great that these estimates should not be used to drive occupational and environmental health policy. The EPA has now rejected and discontinued work on its proposed methods for estimating potency factors. Future efforts will require new methods and more precise and reliable exposure assessments. However, while there may be genuine need for such work, a more pressing priority with regard to the six regulated forms of asbestos and other asbestiform fibers is to ban their production and use.

(c) 2009 Wiley-Liss, Inc.

In plain English

This article, labelled a "Historical Perspective," was a review, not a new study. The authors reviewed cancer risk assessments by the U.S. EPA, OSHA and NIOSH since 1972. They focused on statistical models estimating how strongly chrysotile versus amphibole fibers, and fibers of different sizes, cause lung cancer and mesothelioma. Drafts prepared for EPA put chrysotile's lung cancer potency at 20%, then 27%, of amphibole potency, differences that were not statistically significant. One reviewer found that omitting a single study could reverse which fiber type appeared more potent. In a 2003 model, lung cancer potency factors across 15 studies varied by a factor of 50. The authors described flaws in the exposure data, such as a microscope method blind to thin fibers and uncertain conversions of old dust counts. They reported that EPA dropped its 2008 proposed method after expert review. The authors concluded that these estimates should not drive policy, that Nicholson's 1983 equal-potency approach remains useful, and that the top priority is banning production and use of the six OSHA-regulated forms of asbestos and other asbestiform mineral fibers.

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 (historical perspective) of government asbestos cancer risk assessments and related literature
• Who was studied: No new study population. The material reviewed was asbestos cancer risk assessments by USEPA, OSHA and NIOSH since 1972, which drew on sets of occupational epidemiology studies of asbestos-exposed workers.
• Exposure: Asbestos fiber types (chrysotile and the amphiboles) and fiber sizes, as modelled in risk assessments of lung cancer and mesothelioma
• Main results:
  • In the reviewed Berman and Crump drafts prepared for EPA, the 2001 draft estimated chrysotile at 20% of amphibole potency for lung cancer. The difference was not statistically significant, so equal potency was left open.
  • The 2003 revision raised chrysotile's relative lung cancer potency to 27% of amphibole potency. The difference was again not statistically significant. Chrysotile's estimated mesothelioma potency was 0.13% of the amphiboles.
  • In sensitivity calculations by one expert reviewer of the 2003 work, the result flipped depending on which study was left out. Without the Quebec miners and millers, chrysotile had twice the potency of amphiboles. Without the South Carolina textile workers, amphiboles had ten times the potency of chrysotile.
  • Under the 2003 model, lung cancer potency factors for 15 epidemiology studies varied by a factor of 50, even after adjusting for fiber type and size.
  • Under the 2003 model, mesothelioma potency factors varied by a factor of 30 after taking fiber type and size into account.
  • The 2008 EPA proposal left out of its mesothelioma analysis the Selikoff insulation worker cohort and the de Klerk crocidolite miners' cohort, which together held more than 80% of all mesothelioma deaths in its database.
  • Phase contrast microscopy, the most common fiber-counting method since the late 1960s, cannot resolve the thinnest fibers. The authors described this as one source of exposure misclassification.
  • Comparisons used to convert old dust counts to fiber counts gave conversion factors ranging from 0.1 to 52, yet the 2008 proposal used a single value of 3.
  • After review by EPA's Science Advisory Board Asbestos Committee, the EPA Administrator announced that the 2008 proposed risk assessment would not be pursued further.
• The authors' conclusion: The authors concluded that fiber-type and size potency estimates are too uncertain to drive occupational and environmental health policy. They regarded Nicholson's 1983 approach, which assumed equal potency, as still useful. They considered banning the production and use of the six OSHA-regulated forms of asbestos and other asbestiform mineral fibers the most important priority.

Topics

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

Medical terms in this study, explained in our Lexicon: Epidemiology, OSHA, NIOSH, Asbestiform.

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