Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole

Our plain-English summary of Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole, by T.C. Larson, V.C. Antao, F.J. Bove (Journal of Occupational and Environmental Medicine, 2010).

The study

• Authors: T.C. Larson, V.C. Antao, F.J. Bove
• Journal: Journal of Occupational and Environmental Medicine
• Published: 2010;52(5):555-60
• Cited by: 32 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objective: To examine the relationship between cumulative fiber exposure (CFE) and mortality in a retrospective cohort study of vermiculite workers exposed to Libby amphibole (n = 1862).

Methods: Extended Cox regression was used to estimate the hazards associated with CFE as a time-dependent covariate of multiple-cause mortality.

Results: The Cox models for mesothelioma, asbestosis, lung cancer, and non-malignant respiratory disease were significant with rate ratios that increased monotonically with CFE. The model for deaths due to cardiovascular disease was also significant (rate ratio for CFE > or =44.0 f/cc-y vs <1.4 f/cc-y was 1.5; 95% confidence interval = 1.1 to 2.0).

Conclusions: By using a within-cohort comparison, the results demonstrate a clear exposure-response relationship between CFE and mortality from asbestos-related causes. The finding of an association between CFE and cardiovascular mortality suggests persons exposed to Libby amphibole should be monitored for this outcome.

In plain English

This retrospective cohort study followed 1,862 workers from the vermiculite mining and processing operation in Libby, Montana. The ore was contaminated with amphibole fibers, called Libby amphibole. By 2006, 952 workers had died. Exposure was estimated from job histories and earlier NIOSH estimates. Compared with the U.S. population, deaths from mesothelioma, asbestosis and lung cancer were elevated. Within the cohort (20-year lag), workers with 44.0 fiber/cc-years or more were compared with those below 1.4. Their rate ratios were 17.1 (95% CI 3.7 to 78.1) for mesothelioma, 11.8 (4.9 to 28.7) for asbestosis, 3.2 (1.8 to 5.3) for lung cancer, 2.5 (1.7 to 3.6) for non-malignant respiratory disease excluding pneumoconiosis and 1.5 (1.1 to 2.0) for cardiovascular disease. Cardiovascular disease showed no steady trend with exposure. The digestive system cancer model was not significant. Smoking was unmeasured for much of the cohort. Modelling based on smoking data from 336 screened workers led the authors to judge its impact minimal. The authors concluded there was a clear exposure-response relationship for asbestos-related causes, and that exposed people should be monitored for cardiovascular disease.

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: Retrospective cohort mortality study. It compared the cohort with the U.S. population (standardized mortality ratios) and made within-cohort exposure-response comparisons using Cox regression with cumulative fiber exposure as a time-dependent factor.
• Who was studied: 1,862 workers of the Libby, Montana vermiculite mining and processing operation, a cohort reconstructed by ATSDR from company records. Mortality was followed through the end of 2006. 1,667 workers were located, and 952 were identified as deceased.
• Exposure: Libby amphibole: a mixture of amphibole fibers contaminating Libby vermiculite, described as tremolite plus actinolite, winchite and richterite. Exposure was measured as each worker's cumulative fiber exposure in fibers/cc-years, estimated from NIOSH job exposure estimates and work histories.
• Main results:
  • Compared with the U.S. population, mesothelioma deaths had an SMR of 94.8 (95% CI 57.0 to 148.0), based on 19 observed deaths versus 0.2 expected.
  • Compared with the U.S. population, asbestosis deaths had an SMR of 142.8 (95% CI 111.1 to 180.8), based on 69 observed versus 0.5 expected.
  • Compared with the U.S. population, lung cancer deaths had an SMR of 1.6 (95% CI 1.3 to 2.0).
  • Compared with the U.S. population, ischemic heart disease deaths were significantly lower (SMR 0.7, 95% CI 0.6 to 0.8).
  • Within the cohort (20-year lag), the rate ratio for mesothelioma in the highest exposure group (44.0 f/cc-y and above, 11 deaths) versus below 1.4 f/cc-y was 17.1 (95% CI 3.7 to 78.1).
  • Within the cohort, the rate ratio for asbestosis in the highest exposure group (32 deaths) versus the lowest was 11.8 (95% CI 4.9 to 28.7).
  • Within the cohort, the rate ratio for lung cancer in the highest exposure group (38 deaths) versus the lowest was 3.2 (95% CI 1.8 to 5.3).
  • In the within-cohort models, non-malignant respiratory disease (NMRD) was defined to exclude pneumoconiosis.
  • Within the cohort, the rate ratio for non-malignant respiratory disease excluding pneumoconiosis in the highest exposure group (58 deaths) versus the lowest was 2.5 (95% CI 1.7 to 3.6).
  • For cardiovascular disease, only the highest exposure group had a significant rate ratio (1.5, 95% CI 1.1 to 2.0), and there was no monotonic trend across exposure groups.
  • The mesothelioma model was not significant with 0-, 5- and 10-year lags, but was significant with lags of 15 years and longer.
  • The Cox model for digestive system cancer was not significant.
  • According to the authors, the risk for mesothelioma, asbestosis, lung cancer, non-malignant respiratory disease and cardiovascular disease each increased by about 0.4% at the cohort's median exposure of 4.3 f/cc-y.
  • The authors modelled the possible effect of smoking using assumptions based on smoking data from 336 workers screened by ATSDR in 2000/2001.
  • After adjustment in this smoking model, the lung cancer rate ratio fell from 2.4 to 2.0. The authors judged that smoking had minimal impact on the exposure-response relationships.
• The authors' conclusion: The authors concluded that the within-cohort comparison showed a clear exposure-response relationship between cumulative fiber exposure and death from asbestos-related causes. They also concluded that the association with cardiovascular deaths suggests people exposed to Libby amphibole should be monitored for this outcome.
• Limitations the authors state:
  • The authors list potential exposure misclassification, reliance on death certificate information, and lack of smoking data as the major limitations.
  • Exposure estimates may have underestimated actual levels, especially for highly exposed workers. The authors say this would bias hazard estimates toward the null.
  • Earlier deaths may have been misdiagnosed, so hazards may be underestimated.
  • 10% of the cohort could not be located, and the impact of leaving them out is unknown.
  • Cause of death could not be obtained for 114 deceased workers. These workers had lower median exposure, so hazards in lower exposure groups might have been greater.
  • Smoking was an unmeasured confounder for much of the cohort. Smoking data were available only for 336 workers screened in 2000/2001.

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: vermiculite, Occupational Exposure, Libby amphiboles, Cohort Study, Mesothelioma, Lung Cancer, rate ratio, 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)
• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)
• Association between cumulative fiber exposure and respiratory outcomes among Libby vermiculite workers — T.C. Larson, V.C. Antao, et al. (2012)
• Libby vermiculite exposure and risk of developing asbestos-related lung and pleural diseases — V.C. Antao, T.C. Larson, et al. (2012)
• Cancer incidence and mortality associated with non-occupational and low dose exposure to Libby vermiculite in Minnesota — T. Konen, J.E. Johnson, et al. (2019)
• 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)

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