Cohort study of mortality of vermiculite miners exposed to tremolite

Our plain-English summary of Cohort study of mortality of vermiculite miners exposed to tremolite, by J.C. McDonald, A.D. McDonald, B. Armstrong, et al (British Journal of Industrial Medicine, 1986).

The study

• Authors: J.C. McDonald, A.D. McDonald, B. Armstrong, P. Sebastien
• Journal: British Journal of Industrial Medicine
• Published: 1986;43(7):436-44
• Cited by: 118 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000419).

Abstract

A cohort of 406 men employed before 1963 for at least one year in a vermiculite mine in Montana was followed up until July 1983. The vermiculite ore as fed to the mill contained 4-6% of amphibole fibre in the tremolite series. Vital status was established in all but one of the 406 and death certificates were obtained and coded for 163 of the 165 men who died. Compared with white men in the United States, the cohort experienced excess mortality from all causes (SMR 1.17), respiratory cancer (SMR 2.45), non-malignant respiratory disease (SMR 2.55), and accidents (SMR 2.14). Four deaths were from malignant mesothelioma (proportional mortality 2.4%). Compared with Montana death rates, the SMR for respiratory cancer was somewhat higher (3.03). Man-year analyses of respiratory cancer and estimated cumulative exposure gave a relation that did not depart significantly from linearity. The results of this and case-referent analyses indicate an increased risk of mortality from respiratory cancer in this cohort of about 1% for each fibre year of exposure. In relation to estimated exposure the mortality experienced by the cohort from both lung cancer and mesothelial tumours was higher than in chrysotile mining.

In plain English

This was a cohort mortality study of 406 men who had worked for at least one year, starting before 1963, at a vermiculite mine in Libby, Montana. They were followed until July 1983. The ore was contaminated with fibrous amphibole in the tremolite series. By then 165 men had died. Compared with US white men, the cohort had more deaths than expected from all causes (SMR 1.17), respiratory cancer (SMR 2.45), non-malignant respiratory disease (SMR 2.55, with eight deaths certified as pneumoconiosis) and accidents (SMR 2.14). Four men died of malignant mesothelioma: three pleural and one peritoneal. Using Montana death rates, the respiratory cancer SMR was 3.03. In a comparison of cases and controls within the cohort, respiratory cancer deaths 20 or more years after first employment showed an estimated rise in relative risk of 1.0% per fibre-year of cumulative exposure (95% confidence interval 0.1 to 9.7). The authors concluded that the workers faced a serious hazard from lung cancer, pneumoconiosis and mesothelioma, presumably from the amphibole contaminant. They noted that the cohort was not large and that their analyses did not by themselves establish a linear relation.

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, with case-control (case-referent) analyses of exposure-response chosen from within the cohort
• Who was studied: 406 men employed for at least one year, starting before 1 January 1963, at the WR Grace & Co vermiculite mine in Libby, Montana; vital status followed up to 1 July 1983
• Exposure: Vermiculite ore contaminated with fibrous amphibole in the tremolite series (4-6% of the ore fed to the mill). The authors say airborne fibres were probably a mixture of amphibole types, including sodium-rich fibres. Cumulative fibre exposure (fibre-years) was estimated from job histories and past air measurements, with the highest levels in the dry mill before 1975.
• Main results:
  • On 1 July 1983, 226 men were alive and 165 were reported dead.
  • Compared with US white men, all-cause mortality in the whole cohort exceeded expected by 24 deaths (SMR 1.17).
  • Compared with US white men, the cohort had an excess of deaths from respiratory cancer (SMR 2.45).
  • Compared with US white men, the cohort had excess deaths from non-malignant respiratory disease (SMR 2.55) and accidents (SMR 2.14).
  • Eight of the deaths from non-malignant respiratory disease were certified as pneumoconiosis.
  • There was no evidence of excess deaths from cancers at non-respiratory sites.
  • Four deaths were from malignant mesothelioma (proportional mortality 2.4%).
  • Of the four mesothelioma cases, three were pleural and one peritoneal.
  • Using Montana death rates as the reference, the respiratory cancer SMR for the cohort was 3.03.
  • In the man-year analysis, the line constrained to pass through SMR 1 at zero exposure implied an increase in the respiratory cancer SMR of about 1.3% per fibre-year of cumulative exposure.
  • In the case-control analysis of respiratory cancer deaths occurring 20 or more years from first employment, relative risk rose an estimated 1.0% per fibre-year (95% confidence interval 0.1 to 9.7).
  • Positive relations with cumulative exposure were also seen for pneumoconiosis and mesothelioma, but conventional levels of statistical significance were not reached.
• The authors' conclusion: The authors concluded that the workers in this mine experienced a serious hazard from lung cancer, pneumoconiosis and mesothelioma. They attributed this presumably to the fibrous amphibole contaminant of the vermiculite ore, mainly under dust conditions before 1974 and particularly in the dry mill.
• Limitations the authors state:
  • The cohort was not large.
  • The analyses of respiratory cancer and cumulative exposure do not in themselves establish a linear relation.
  • For pneumoconiosis and mesothelioma, numbers were small and confidence intervals wide.
  • For most work locations, fibre measurements existed only for recent periods. Past levels were estimated with arbitrary correction factors, decided after discussion with company representatives.
  • 14 men could be traced only to 31 December 1981, and their later vital status was not established.

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, Tremolite.

Medical terms in this study, explained in our Lexicon: Cohort Study, vermiculite, Tremolite, Amphibole Group, Death Certificate, Mesothelioma, Proportional mortality, Cumulative Exposure.

Related studies

• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates — H.E. Amandus, R. Wheeler, et al. (1987)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part III. Radiographic findings — H.E. Amandus, R. Althouse, et al. (1987)
• Radiological survey of past and present vermiculite miners exposed to tremolite — J.C. McDonald, P. Sebastien, et al. (1986)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)

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