Potency factors for risk assessment at Libby, Montana

Our plain-English summary of Potency factors for risk assessment at Libby, Montana, by S.H. Moolgavkar, J. Turim, D.D. Alexander, et al (Risk Analysis, 2010).

The study

• Authors: S.H. Moolgavkar, J. Turim, D.D. Alexander, E.C. Lau, C.A. Cushing
• Journal: Risk Analysis
• Published: 2010;30(8):1240-8
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

We reanalyzed the Libby vermiculite miners' cohort assembled by Sullivan to estimate potency factors for lung cancer, mesothelioma, nonmalignant respiratory disease (NMRD), and all-cause mortality associated with exposure to Libby fibers. Our principal statistical tool for analyses of lung cancer, NMRD, and total mortality in the cohort was the time-dependent proportional hazards model. For mesothelioma, we used an extension of the Peto formula. For a cumulative exposure to Libby fiber of 100 f/mL-yr, our estimates of relative risk (RR) are as follows: lung cancer, RR = 1.12, 95% confidence interval (CI) =[1.06, 1.17]; NMRD, RR = 1.14, 95% CI =[1.09, 1.18]; total mortality, RR = 1.06, 95% CI =[1.04, 1.08]. These estimates were virtually identical when analyses were restricted to the subcohort of workers who were employed for at least one year. For mesothelioma, our estimate of potency is K(M) = 0.5 x 10(-8), 95% CI =[0.3 x 10(-8), 0.8 x 10(-8)]. Finally, we estimated the mortality ratios standardized against the U.S. population for lung cancer, NMRD, and total mortality and obtained estimates that were in good agreement with those reported by Sullivan. The estimated potency factors form the basis for a quantitative risk assessment at Libby.

In plain English

The authors reanalyzed Sullivan's mortality study of Libby, Montana, vermiculite workers. They analyzed 1,662 white male workers, followed through December 2001, at mines whose vermiculite was contaminated with winchite, richterite and tremolite. Adjusting for year of birth, a rough stand-in for smoking, a cumulative exposure of 100 fiber/mL-years was associated with hazard ratios of 1.12 (95% CI 1.06 to 1.17) for lung cancer, 1.14 (1.09 to 1.18) for nonmalignant respiratory disease and 1.06 (1.04 to 1.08) for all deaths. Estimates were similar for workers employed at least one year. Against U.S. rates, full-cohort standardized mortality ratios were 1.65 (1.33 to 2.01), 2.29 (1.89 to 2.74) and 1.17 (1.09 to 1.25) for the same three outcomes. The number of mesothelioma deaths was uncertain, between 2 and 15. Assuming 15, mesothelioma potency was 0.5 (95% CI 0.3 to 0.8) times 10 to the minus 8, half the EPA's 1986 value. Counting only 2 gave 0.07 times 10 to the minus 8. The authors concluded these potency factors form the basis for a risk assessment at Libby. They had no smoking data and could not address uncertainty in exposure 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: Reanalysis of an existing retrospective occupational cohort mortality study (Sullivan's Libby cohort). It used standardized mortality ratios against U.S. rates, time-dependent Cox proportional hazards models, and an extended Peto formula for mesothelioma.
• Who was studied: 1,662 white male vermiculite miners, millers and processors at Libby, Montana, hired 1935 to 1981. The original cohort was 1,672; 10 workers with unknown vital status were excluded. Follow-up for vital status ran through December 2001. A subcohort of 812 workers employed at least one year was also analyzed.
• Exposure: Cumulative exposure to Libby fibers (fibers/mL-years), reconstructed from job histories and job exposure matrices. The vermiculite was contaminated with the asbestiform amphiboles winchite, richterite and tremolite. Mean cumulative exposure for all workers was 91.4 fibers/cc-yrs.
• Main results:
  • Full cohort, lung cancer (95 deaths): hazard ratio 1.12 (95% CI 1.06 to 1.17) for a cumulative exposure of 100 f/mL-yr
  • Full cohort, nonmalignant respiratory disease (117 deaths): hazard ratio 1.14 (95% CI 1.09 to 1.18) for 100 f/mL-yr
  • Full cohort, all deaths (767 deaths): hazard ratio 1.06 (95% CI 1.04 to 1.08) for 100 f/mL-yr
  • Subcohort employed at least one year (812 workers): estimates were virtually identical. Lung cancer 1.12 (1.06 to 1.18), nonmalignant respiratory disease 1.14 (1.08 to 1.19), all deaths 1.07 (1.05 to 1.10).
  • Full cohort, lung cancer standardized mortality ratio against U.S. rates: 1.65 (95% CI 1.33 to 2.01), with 95 observed deaths versus 57.71 expected
  • Full cohort, nonmalignant respiratory disease standardized mortality ratio: 2.29 (95% CI 1.89 to 2.74), with 117 observed versus 51.15 expected
  • Full cohort, all-cause standardized mortality ratio: 1.17 (95% CI 1.09 to 1.25), with 767 observed versus 656.75 expected
  • Mesothelioma deaths could not be counted with certainty. Two deaths were coded as mesothelioma and four as pleural cancer. Sullivan reported nine more with some mention of mesothelioma, giving an upper bound of 15.
  • Mesothelioma potency (KM) was 0.07 times 10 to the minus 8 if there were 2 mesothelioma deaths, and 0.5 times 10 to the minus 8 if there were 15
  • Assuming 15 mesothelioma deaths, the 95% confidence interval for KM was 0.3 to 0.8 times 10 to the minus 8
  • The authors stated that the estimated mesothelioma potency was half the potency factor assumed by EPA in its 1986 risk assessment for asbestos. Across the lower and upper bounds, it was between 7% and 50% of the EPA 1986 value.
• The authors' conclusion: The authors concluded that the estimated potency factors form the basis for a quantitative risk assessment at Libby. They also stated that using them does not imply that risks are actually increased from environmental exposure.
• Limitations the authors state:
  • No smoking information was available. Year of birth was used only as a rough surrogate for smoking habits.
  • The authors could not address uncertainties in the exposure estimates or how these might affect the risk estimates
  • Mesothelioma deaths could not be clearly identified before 1999, because a separate death-certificate code for mesothelioma existed only from ICD-10 onward
  • The mix of fiber dimensions in environmental exposures may differ from that in the mining environment
  • Using these potency factors assumes that results from an occupational cohort apply to much lower environmental exposures
  • Comparisons with U.S. population rates may understate risk because of the healthy worker effect. They may overstate risk for smoking-related diseases if the workers smoked more than the general population.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Libby Vermiculite, Lung Cancer, Mesothelioma, Cumulative Exposure, Relative Risk, confidence interval.

Related studies

• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Exposure to asbestos-containing vermiculite ore and respiratory symptoms among individuals who were children while the mine was active in Libby, Montana — L.C. Vinikoor, T.C. Larson, et al. (2010)
• A review of the federal government's health activities in response to asbestos-contaminated ore found in Libby, Montana — K. Horton, V. Kapil, et al. (2006)
• Exposure to airborne amphibole structures and health risks: Libby, Montana — B. Price (2008)
• Select mortality and cancer incidence among residents in various U.S. communities that received asbestos-contaminated vermiculite ore from Libby, Montana — D.K. Horton, F. Bove, et al. (2008)
• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)

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