Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk

Our plain-English summary of Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk, by D.B. Richardson, A.P. Keil, S.R. Cole, et al (American Journal of Industrial Medicine, 2018).

The study

• Authors: D.B. Richardson, A.P. Keil, S.R. Cole, J. Dement
• Journal: American Journal of Industrial Medicine
• Published: 2018;61(5):383-390
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Current regulations require that asbestos fibers are collected and examined using a light microscope. This method fails to enumerate fibers that are too short or thin to reliably count using a light microscope under normal conditions.

Methods: A cohort of 3054 workers employed at an asbestos textile plant was followed to ascertain causes of death. Exposure was almost entirely chrysotile. Fiber counts were quantified using light microscopy and electron microscopy. The g-formula was used to estimate impacts on lung cancer of policies defined in terms of fiber counts quantified using light and electron microscopy.

Results: Given exposure at the current standard, the estimated lung cancer risk was 7.33%, comparable to the risk expected under a standard of 1 fiber/mL counted using electron microscopy (7.30%). The lifetime risk of lung cancer under a standard of 0.1 fiber/mL counted by electron microscopy was estimated to be 7.10%.

Conclusions: We identify policies defined in terms of electron microscopy-based asbestos exposure metrics that yield comparable, or lower, lung cancer mortality than that expected under the current standard.

© 2018 Wiley Periodicals, Inc.

In plain English

This study used records of 3054 men and women who worked in production at a South Carolina asbestos textile plant for at least one month between 1940 and 1965. They were followed through 2001. Their exposure was almost entirely chrysotile. Each worker's yearly exposure was estimated in two ways: fibers counted with a light microscope, the method current rules require, and fibers counted with an electron microscope, which also captures shorter and thinner fibers. Using a statistical simulation method (the g-formula), the authors estimated the risk of dying of lung cancer by age 90 if exposure at work had been set at different limits. The estimated risk was 7.33% under the current OSHA standard of 0.1 fiber/mL counted by light microscopy. It was 7.30% under a limit of 1 fiber/mL counted by electron microscopy. A limit of 0.1 fiber/mL by electron microscopy gave 7.10%, a reduction the authors called small and highly imprecise. The authors concluded that electron-microscopy-based limits can give comparable or lower lung cancer mortality. They cautioned that the data came from one plant.

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: Cohort mortality study of an occupational cohort, analyzed with the parametric g-formula (a Monte Carlo simulation) to estimate lung cancer mortality under hypothetical exposure limits
• Who was studied: 3054 men and women employed in production at an asbestos textile plant in South Carolina who worked at least 1 month between January 1, 1940 and December 31, 1965, followed for vital status through December 31, 2001. At study entry the median age was 24 years; 42% were female and 81% were white.
• Exposure: Almost entirely chrysotile asbestos in textile production. Yearly exposure was estimated two ways. Light (phase contrast) microscopy estimates were based on 5952 sampling measurements taken between 1930 and 1975. Transmission electron microscopy estimates were based on a stratified random sample of archived dust samples collected in 1964-1971.
• Main results:
  • Under the 1976 OSHA standard of 2 fiber/mL counted by light microscopy, the estimated cumulative lung cancer mortality at age 90 was 9.35%.
  • Under the current OSHA standard of 0.1 fiber/mL counted by light microscopy, the estimated cumulative lung cancer mortality at age 90 was 7.33%.
  • The risk difference between the 1976 standard and the current standard was 2.02% (95% CI 0.26% to 3.79%). The authors estimated one lung cancer death prevented for every 50 workers regulated under the current standard compared with the 1976 standard.
  • A hypothetical limit of 45 fibers/mL counted by electron microscopy gave an estimated 9.41% lung cancer mortality by age 90, comparable to the 1976 standard.
  • A hypothetical limit of 1 fiber/mL counted by electron microscopy gave an estimated 7.30% lung cancer mortality by age 90, comparable to the current OSHA standard.
  • A hypothetical limit of 0.1 fiber/mL counted by electron microscopy gave an estimated 7.10% lung cancer mortality by age 90. Compared with the current standard, the risk ratio was 0.97 (95% CI 0.88 to 1.07).
  • The risk difference between the current standard and a 0.1 fiber/mL electron microscopy limit was 0.23% (95% CI minus 0.52% to 0.97%). The authors described this as a small, highly imprecise reduction and estimated one lung cancer death prevented for every 444 workers.
  • In a sensitivity analysis counting only fibers at least 5 µm long by electron microscopy, a 0.1 fiber/mL limit gave an estimated 7.21% lung cancer mortality by age 90.
• The authors' conclusion: The authors concluded that they identified exposure limits defined by electron microscopy fiber counts that yield comparable, or lower, lung cancer mortality than expected under the current OSHA standard.
• Limitations the authors state:
  • Interpreting the results as effects of interventions requires assuming no unmeasured confounding, for example from cigarette smoking or other workplace lung carcinogens.
  • The method assumes the statistical models are correctly specified, and this was not verified.
  • The estimates come from one facility with a specific distribution of fiber lengths and widths.
  • Because the study was within one industry, the ratio of electron-microscopy to light-microscopy fiber counts varied only within a narrow range. This limited the ability to assess the impact of shorter fibers.
  • Costs limited how many dust samples could be analyzed by electron microscopy.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Lung Cancer, Asbestos Exposure.

Related studies

• An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers — L. Stayner, E. Kuempel, et al. (2008)
• Lung Cancer Risk Associated with Regulated and Unregulated Chrysotile Asbestos Fibers — G.B. Hamra, D.B. Richardson, et al. (2017)
• Analysis of occupational asbestos exposure and lung cancer mortality using the g formula — S.R. Cole, D.B. Richardson, et al. (2013)
• Exposure and mineralogical correlates of pulmonary fibrosis in chrysotile asbestos workers — F.H. Green, R. Harley, et al. (1997)
• Follow-up study of chrysotile asbestos textile workers: cohort mortality and case-control analyses — J.M. Dement, D.P. Brown, et al. (1994)
• Pulmonary mineral fibers after occupational and environmental exposure to asbestos in the Russian chrysotile industry — A. Tossavainen, E. Kovalevsky, et al. (2000)

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