Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study

Our plain-English summary of Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study, by P.A. Sullivan (Environmental Health Perspectives, 2007).

The study

• Author: P.A. Sullivan
• Journal: Environmental Health Perspectives
• Published: 2007;115(4):579-85
• Cited by: 109 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Vermiculite from the mine near Libby, Montana, is contaminated with tremolite asbestos and other amphibole fibers (winchite and richterite). Asbestos-contaminated Libby vermiculite was used in loose-fill attic insulation that remains in millions of homes in the United States, Canada, and other countries.

Objective: This report describes asbestos-related occupational respiratory disease mortality among workers who mined, milled, and processed the Libby vermiculite.

Methods: This historical cohort mortality study uses life table analysis methods to compare the age-adjusted mortality experience through 2001 of 1,672 Libby workers to that of white men in the U.S. population.

Results: Libby workers were significantly more likely to die from asbestosis [standardized mortality ratio (SMR) = 165.8; 95% confidence interval (CI), 103.9-251.1], lung cancer (SMR = 1.7; 95% CI, 1.4-2.1), cancer of the pleura (SMR = 23.3; 95% CI, 6.3-59.5), and mesothelioma. Mortality from asbestosis and lung cancer increased with increasing duration and cumulative exposure to airborne tremolite asbestos and other amphibole fibers.

Conclusions: The observed dose-related increases in asbestosis and lung cancer mortality highlight the need for better understanding and control of exposures that may occur when homeowners or construction workers (including plumbers, cable installers, electricians, telephone repair personnel, and insulators) disturb loose-fill attic insulation made with asbestos-contaminated vermiculite from Libby, Montana.

In plain English

This study followed 1,672 white men hired from 1935 through 1981 to mine, mill or process vermiculite at Libby, Montana, tracking deaths through 2001. The vermiculite was contaminated with amphibole fibers (tremolite asbestos, winchite and richterite). Their deaths were compared with those expected among U.S. white men of the same age and calendar period. With a 15-year exposure lag, standardized mortality ratios were 1.7 for lung cancer (95% confidence interval 1.4 to 2.1), 2.4 for nonmalignant respiratory disease (2.0 to 2.9), 23.3 for cancer of the pleura (6.3 to 59.5; 4 deaths) and 165.8 for asbestosis (103.9 to 251.1; 22 deaths). Fifteen mesothelioma deaths appeared on death certificates. Asbestosis and lung cancer deaths rose with cumulative fiber exposure and employment length. Nonmalignant respiratory disease deaths were in excess even among workers with under 4.5 fiber/cc-years and those employed under a year. The author noted exposure estimates likely contained error and asbestosis ratios may be inflated because few deaths were expected. The author concluded the findings highlight the need to understand and control exposures from disturbed Libby vermiculite attic insulation.

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: Historical (retrospective) cohort mortality study using life table analysis. It compared workers' deaths with U.S. white male death rates (standardized mortality ratios, SMRs) and compared exposure groups within the cohort (standardized rate ratios, SRRs).
• Who was studied: 1,672 white male vermiculite miners, millers and processors hired at Libby, Montana, from September 1935 through December 1981, with deaths followed through 31 December 2001. Vital status follow-up was completed for 97.8% of the full cohort.
• Exposure: Airborne Libby amphibole fibers (tremolite asbestos, winchite, richterite) contaminating vermiculite. Exposure was estimated with a job-exposure matrix based on phase contrast microscopy fiber counts (fibers per cc) and expressed as cumulative exposure (fibers/cc-years) and as duration of employment. Median cumulative exposure for all workers was 8.7 fibers/cc-years.
• Main results:
  • Whole cohort, 15-year lag: lung cancer SMR 1.7 (95% CI 1.4 to 2.1) compared with U.S. white men
  • Whole cohort: nonmalignant respiratory disease SMR 2.4 (95% CI 2.0 to 2.9)
  • Whole cohort, 15-year lag: asbestosis SMR 165.8 (95% CI 103.9 to 251.1)
  • 40 workers had asbestosis mentioned on their death certificates. The asbestosis SMR was based on the 22 workers with asbestosis as the underlying cause of death.
  • Whole cohort: cancer of the pleura, 4 observed deaths against 0.17 expected, SMR 23.3 (95% CI 6.3 to 59.5)
  • Of 15 mesothelioma deaths identified on death certificates (1979 to 2001), 1 was peritoneal and 14 were pleural (or unspecified)
  • Mesothelioma SMR, based on 2 deaths in 1999 to 2001: 15.1 (95% CI 1.8 to 54.4)
  • Asbestosis SMR rose from 37.3 (approximate 95% CI 7.5 to 122.3) among workers with less than 50 fibers/cc-years to 749.1 (95% CI 373.0 to 1367.8) among those with 250 or more fibers/cc-years
  • Lung cancer SMR rose from 1.5 (95% CI 0.9 to 2.3) among workers with less than 4.5 fibers/cc-years to 1.9 (95% CI 1.2 to 2.9) among workers with at least 100 fibers/cc-years
  • For lung cancer, nonmalignant respiratory disease and asbestosis, tests for a linear trend in rate ratios across increasing cumulative exposure were statistically significant (p less than 0.01)
  • Nonmalignant respiratory disease SMR was 1.8 (95% CI 1.1 to 2.8) even among workers with less than 4.5 fibers/cc-years cumulative exposure
  • Lung cancer SMR was 1.6 (95% CI 1.1 to 2.1) among those working less than 1 year, rising to 2.5 (95% CI 1.4 to 4.3) among those working 10 years or more
  • Nonmalignant respiratory disease SMR was 2.1 (95% CI 1.6 to 2.8) among those working less than 1 year, rising to 3.6 (95% CI 2.2 to 5.7) among those employed 10 years or more
  • Asbestosis SMR among workers employed 10 years or more: 628.6 (95% CI 301.1 to 1185.1)
• The authors' conclusion: The author concluded that the workers had significantly elevated mortality from asbestosis, lung cancer and cancer of the pleura, with exposure-response relationships for asbestosis and lung cancer. Because Libby vermiculite was used in loose-fill attic insulation, the author stated that the findings highlight the need for better understanding and control of exposures when homeowners or construction renovation workers disturb that insulation.
• Limitations the authors state:
  • Retrospective exposure estimates, built from inspection reports, company monitoring data and professional judgment, likely contained measurement error
  • There were not enough sampling data to estimate exposure to possible lung cancer confounders such as diesel particulate and crystalline silica
  • Work history data were missing for 9% of workers who left before 1954; these workers were assumed to have worked about 1 year
  • The asbestosis SMRs may be inflated because the very small number of expected deaths may give unstable estimates
  • The mesothelioma SMR rests on only 3 years of data because mesothelioma had no separate cause-of-death code before 1999
  • Pneumoconiosis caused by other minerals may have been misclassified as asbestosis on death certificates
  • The fiber counting method could not tell the different amphiboles apart, so exposure estimates include winchite and richterite as well as tremolite

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: Asbestosis, Lung cancer, Talc and vermiculite, Tremolite.

Medical terms in this study, explained in our Lexicon: vermiculite, Asbestos Exposure, Tremolite, Amphibole Group, winchite, richterite, Libby Vermiculite, standardized mortality ratio (SMR).

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)
• Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole — T.C. Larson, V.C. Antao, et al. (2010)
• 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)
• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• Libby vermiculite exposure and risk of developing asbestos-related lung and pleural diseases — V.C. Antao, T.C. Larson, et al. (2012)

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