Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana

Our plain-English summary of Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana, by J.C. McDonald, J. Harris, B. Armstrong (Occupational and Environmental Medicine, 2004).

The study

• Authors: J.C. McDonald, J. Harris, B. Armstrong
• Journal: Occupational and Environmental Medicine
• Published: 2004;61(4):363-6
• Cited by: 71 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Fibrous tremolite is a widespread amphibole asbestiform mineral, airborne fibres of which constitute an environmental hazard in Libby, Montana, northern California, and elsewhere.

Aims: To determine excess risk from lung cancer, mesothelioma, and all-cause mortality in a cohort of men exposed to tremolite, but no other form of asbestos.

Methods: Mortality by certified cause and various measures of exposure to tremolite and related amphibole fibres was assessed in a cohort of 406 vermiculite mineworkers in Libby, Montana, employed before 1963 and followed until 1999.

Results: Total deaths were: lung cancer 44 (SMR 2.40), non-malignant respiratory disease (NMRD) 51 (SMR 3.09), all causes 285 (SMR 1.27); included among the total were 12 deaths ascribed to mesothelioma (4.21% of all deaths). Adjusted linear increments in relative risks (per 100 f/ml.y), estimated by Poisson regression, were: lung cancer (0.36, 95% CI 0.03 to 1.20), NMRD (0.38, 95% CI 0.12 to 0.96), and all deaths (0.14, 95% CI 0.05 to 0.26).

Conclusions: The all-cause linear model would imply a 14% increase in mortality for mine workers exposed occupationally to 100 f/ml.y or about 3.2% for a general population exposed for 50 years to an ambient concentration of 0.1 f/ml. Amphibole fibres, tremolite in particular, are likely to be disproportionately responsible for cancer mortality in persons exposed to commercial chrysotile, but to what extent cannot be readily assessed.

In plain English

This was an updated mortality study of 406 men employed at least one year, starting before 1963, at the vermiculite mine and mills in Libby, Montana. The vermiculite was contaminated with amphibole fibres the authors call fibrous tremolite. On average the men were exposed for nine years to 18 f/ml. Survivors were traced to the end of 1998. Compared with US white males, there were 44 deaths from respiratory cancer (SMR 2.40, 95% CI 1.74 to 3.22), 51 from non-malignant respiratory disease (SMR 3.09, 2.30 to 4.06) and 285 from all causes (SMR 1.27, 1.13 to 1.43). Twelve deaths (4.2% of all deaths) were from mesothelioma, all over 20 years after first employment. Respiratory cancer, non-malignant respiratory disease and all-cause risks rose with cumulative exposure. No mesothelioma exposure trend was more than chance could explain. The authors estimated a 14% rise in all-cause mortality per 100 f/ml years (95% CI 5% to 26%) and urged considerable caution in extrapolating this to residents. They concluded tremolite is likely responsible for some cancer mortality in people exposed to commercial chrysotile.

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 (an updated follow-up of an earlier study), with standardised mortality ratios (SMRs) calculated against US white males and Poisson regression of risk by exposure
• Who was studied: All 406 men first employed before 1 January 1963, with at least one year of net service, at the vermiculite mine and mills in Libby, Montana
• Exposure: Fibrous tremolite (the authors' simplified term for amphibole fibres contaminating the vermiculite; they note richterite and winchite are also present). The average exposure was nine years at 18 f/ml, with exposure estimated for each worker up to the end of 1982.
• Main results:
  • Follow-up: 165 men had died before 1 July 1983. All 241 survivors were traced to the end of 1998, and a further 120 deaths with certified cause were identified.
  • Respiratory cancers in the whole cohort: 44 deaths, SMR 2.40 (95% CI 1.74 to 3.22)
  • Non-malignant respiratory disease (NMRD) in the whole cohort: 51 deaths, SMR 3.09 (95% CI 2.30 to 4.06)
  • All causes in the whole cohort: 285 deaths, SMR 1.27 (95% CI 1.13 to 1.43)
  • Mesothelioma: 12 deaths, a proportional mortality of 4.2% of all deaths
  • Of the four mesothelioma cases before 1983, three were pleural and one peritoneal. Of the eight certified since 1983, four were coded to the pleura and one as abdominal.
  • All 12 mesothelioma deaths occurred more than 20 years after first employment. Time from first exposure to death ranged from 22 to 47 years, with a median of 35.5 years.
  • Linear model for respiratory cancer by cumulative exposure: relative risk increment of 0.36 per 100 f/ml.y (95% CI 0.03 to 1.20), p = 0.02
  • Respiratory cancer in the highest cumulative exposure category (113.8 f/ml.y and above): adjusted relative risk 3.20 (95% CI 1.16 to 8.84) compared with the lowest category
  • Linear model for NMRD by cumulative exposure: relative risk increment of 0.38 per 100 f/ml.y (95% CI 0.12 to 0.96), p = 0.0001
  • Linear model for all deaths by cumulative exposure: relative risk increment of 0.14 per 100 f/ml.y (95% CI 0.05 to 0.26), p = 0.0003. This is a 14% increase per 100 f/ml years, with a 95% CI of 5% to 26%.
  • Mesothelioma: for none of the three exposure measures was there a linear trend greater than chance could explain
  • The authors described the all-cause linear model as estimating a 14% increase in mortality after 100 f/ml years of exposure. By extrapolation to residential exposure, they estimated a 3.2% excess in all-cause mortality for 50 years at 0.1 f/ml.
• The authors' conclusion: The authors concluded that tremolite is likely to be responsible for some, possibly a large proportion, of the cancer mortality in persons exposed to commercial chrysotile. They said that assessing the size of this proportion was beyond the scope of the paper.
• Limitations the authors state:
  • The authors said the risk estimates for residents should be regarded with considerable caution. They rely on extrapolating from the exposure levels and working conditions of the mine workers.
  • Air measurements before 1975 were scanty, so the exposure estimates were based on discussion with long-term employees and involved many assumptions.
  • Work histories were not available after 1982.
  • There were only 12 mesothelioma cases. Exposure estimation and classification were not reliable enough to take account of biodurability.
  • In this small cohort, the three exposure measures were too strongly correlated to tell apart.
  • Mesothelioma deaths were coded inconsistently on death certificates.

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: Lung cancer, Mesothelioma, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: vermiculite, Amphibole Group, Tremolite, Asbestiform, Lung Cancer, Mesothelioma, Relative Risk, Chrysotile.

Related studies

• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)
• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• 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)
• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)
• Libby Amphibole Disease: Pulmonary Function and CT Abnormalities in Vermiculite Miners — A. Miller, J. Szeinuk, et al. (2018)
• 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)

Browse more research in Occupational and Environmental Medicine, or return to medical research.