ORIGINAL ARTICLE

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

J C McDonald¹, J Harris¹, B Armstrong²

¹ Imperial College School of Medicine, London, UK
² London School of Hygiene and Tropical Medicine, London, UK

Correspondence to:
Correspondence to:
Prof. J C McDonald
National Heart & Lung Institute, Imperial College School of Medicine, Department of Occupational and Environmental Medicine, Dovehouse Street, London SW3 6LY, UK; c.mcdonald{at}ic.ac.uk

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.

Keywords: amphibole; tremolite; vermiculite; mortality

Abbreviations: NMRD, non-malignant respiratory disease; SMR, standardised mortality ratio

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