Mesothelioma in Quebec chrysotile miners and millers: epidemiology and aetiology

Our plain-English summary of Mesothelioma in Quebec chrysotile miners and millers: epidemiology and aetiology, by A.D. McDonald, B.W. Case, A. Churg, et al (The Annals of Occupational Hygiene, 1997).

The study

• Authors: A.D. McDonald, B.W. Case, A. Churg, A. Dufresne, G.W. Gibbs, P. Sébastien, J.C. McDonald
• Journal: The Annals of Occupational Hygiene
• Published: 1997;41(6):707-19
• Cited by: 85 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In a cohort of some 11,000 men born 1891-1920 and employed in the Quebec chrysotile production industry, including a small asbestos products factory, of 9780 men who survived into 1936, 8009 are known to have died before 1993, 38 probably from mesothelioma--33 in miners and millers and five in factory workers. Among the 5041 miners and millers at Thetford Mines, there had been 4125 deaths from all causes, including 25 (0.61%) from mesothelioma, a rate of 33.7 per 100,000 subject-years; the corresponding figures for the 4031 men at Asbestos were eight out of 3331 (0.24%, or 13.2 per 100,000 subject-years). At the factory in Asbestos, where all 708 employees were potentially exposed to crocidolite and/or amosite, there were 553 deaths, of which five (0.90%) were due to mesothelioma; the rate of 46.2 per 100,000 subject-years was 3.5 times higher than among the local miners and millers. Six of the 33 cases in miners and millers were in men employed from 2 to 5 years and who might have been exposed to asbestos elsewhere; otherwise, the 22 cases at Thetford were in men employed 20 years or more and the five at Asbestos for at least 30 years. The cases at Thetford were more common in miners than in millers, whereas those at. Asbestos were all in millers. Within Thetford Mines, case-referent analyses showed a substantially increased risk associated with years of employment in a circumscribed group of five mines (Area A), but not in a peripherally distributed group of ten mines (Area B); nor was the risk related to years employed at Asbestos, either at the mine and mill or at the factory. There was no indication that risks were affected by the level of dust exposure. A similar pattern in the prevalence of pleural calcification had been observed at Thetford Mines in the 1970s. These geographical differences, both within the Thetford region and between it and Asbestos, suggest that the explanation is mineralogical. Lung tissue analyses showed that the concentration of tremolite fibres was much higher in Area A than in Area B, a finding compatible with geological knowledge of the region. These findings, probably related to the far greater biopersistence of amphibole fibres than chrysotile, have important implications in the control of asbestos related disease and for wider aspects of fibre toxicology.

In plain English

This cohort study registered about 11,000 men born 1891 to 1920 who worked in Quebec chrysotile mining and milling or an asbestos products factory; after excluding men lost to view, 9780 remained. By 1992, 8009 had died, 38 probably of pleural mesothelioma. Mesothelioma caused 25 of 4125 deaths (0.61%) among Thetford miners and millers, 8 of 3331 (0.24%) at the Asbestos mine and mill, and 5 of 553 (0.90%) at the factory, where crocidolite or amosite exposure was possible. Case-referent analyses at the main Thetford complex showed risk rising with years of service (odds ratio 2.31 for 20 years, 90% confidence limits 1.35 and 3.93) but hardly with accumulated dust exposure. For men with any years in the central mines, the odds ratio for 20 adjusted years of service was 2.50 (90% confidence limits 1.49 to 4.20); for men only in peripheral mines the estimate was unstable. Tremolite fibres predominated in lung tissue from Thetford cases. The authors concluded that fibrous tremolite was the only candidate that could explain this pattern. They noted that diagnosis was partly subjective and lung analyses lacked controls.

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: Mortality follow-up of a historical cohort, with nested case-referent (case-control) analyses and electron microscopic lung tissue fibre analyses of the cases
• Who was studied: About 11,000 men born 1891 to 1920 who worked one month or more in chrysotile mining and milling in the Eastern Townships of Quebec (Thetford Mines region and the town of Asbestos) or in a small asbestos products factory in Asbestos. After exclusion of men lost to view, 9780 men remained, followed for deaths to 1992. The case-referent analyses used 35 cases and 326 referents.
• Exposure: Chrysotile mining and milling, with contamination by tremolite studied as a possible cause. Factory workers were potentially exposed to crocidolite and/or amosite. Dust exposure was estimated in million particles per cubic foot (mpcf) and as accumulated mpcf x years, along with years of service.
• Main results:
  • By the end of 1992, 38 of 8009 deaths in the cohort (0.47%) were probably from mesothelioma.
  • All 38 mesotheliomas were pleural; in one case the peritoneum was also invaded.
  • Thetford Mines miners and millers: 5041 men and 4125 deaths, of which 25 (0.61%) were from mesothelioma.
  • Asbestos mine and mill: 4031 men and 3331 deaths, of which 8 (0.24%) were from mesothelioma.
  • Asbestos factory: 708 men and 553 deaths, of which 5 (0.90%) were from mesothelioma.
  • The mesothelioma death rate for miners and millers was over 2.5 times higher at Thetford Mines (excluding the smallest mines) than at Asbestos (P value printed as 0.015).
  • The factory rate was 3.5 times the rate for the Asbestos mine and mill (P value printed as 0.02).
  • For the 38 cases, the time from first employment to death ranged from 21 to 60 years (mean 47 years).
  • Of the 33 miner and miller cases, 27 were in men employed 20 to 48 years. The authors considered that the six cases employed 2 to 5 years might have come from asbestos exposure elsewhere.
  • All but three of the 22 long-employed Thetford cases had worked at some time as miners; none of the five long-employed Asbestos cases had.
  • At the main Thetford complex, the odds ratio for mesothelioma in relation to 20 years of service was 2.31 (90% confidence limits 1.35 and 3.93).
  • Accumulated dust exposure and years in different dust categories showed no meaningful relation to risk at any workplace. Years of service also showed none at the Asbestos mine and mill or the factory. All but one P value lay between 0.58 and 0.99.
  • At the main Thetford complex, risk did not differ between non- and ex-smokers and cigarette smokers (likelihood ratio statistic 0.13).
  • Among 21 cases and 187 referents at the main Thetford complex, the odds ratio for 20 adjusted years of net service was 2.50 (90% confidence limits 1.49 to 4.20) for men with some or all years in the central mines. For men with all years in peripheral mines it was 0.80 (0.27 to 2.38), an estimate the authors called unstable.
  • In a further analysis of service in the two areas, the odds ratio for 20 years of work was 2.55 in the central area and 1.11 (90% confidence interval 0.47 to 2.62) in the peripheral area, a result of negligible significance for the peripheral area.
  • Lung tissue from 14 autopsied Thetford cases showed tremolite fibres predominating (geometric mean 104.1 versus 12.8 for chrysotile), with no crocidolite or amosite found. Crocidolite and amosite were found in lungs of cases from the factory and the Asbestos mill.
• The authors' conclusion: The authors concluded that the higher mesothelioma risk in the central Thetford mines was best explained by fibrous tremolite present in or near the ore body, not by chrysotile itself. They wrote that, if this hypothesis is correct, uncontaminated chrysotile carries very little mesothelioma risk.
• Limitations the authors state:
  • The authors described their assessment of each diagnosis as subjective: high certainty in 19 cases, moderate in 14, and low (though more likely than not) in 5.
  • Because of possible selection, small numbers, uncertain diagnoses and the absence of controls or denominators, the lung tissue analyses could not address risk or causation.
  • The estimates for men who worked only in peripheral mines were unreliable because that group contained only three cases.
  • The rising trend in mesothelioma deaths over time may partly reflect increased diagnostic awareness.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Jeffrey mine, Asbestos, Quebec

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma, Tremolite.

Medical terms in this study, explained in our Lexicon: Mesothelioma, Quebec Chrysotile, Epidemiology, Thetford Mines, Crocidolite, Amosite, Pleural Calcification, Tremolite.

Related studies

• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 — J.C. McDonald, F.D. Liddell, et al. (1993)
• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: development from 1904 and mortality to 1992 — F.D. Liddell, A.D. McDonald, et al. (1997)
• Mineralogical and exposure determinants of pulmonary fibrosis among Québec chrysotile miners and millers — A. Nayebzadeh, B.W. Case, et al. (2006)
• Fibers in lung tissues of mesothelioma cases among miners and millers of the township of Asbestos, Quebec — A. Dufresne, M. Harrigan, et al. (1995)
• Follow-up respiratory measurements in Quebec chrysotile asbestos miners and millers — M.R. Becklake, D. Thomas, et al. (1982)
• Fiber burden and patterns of asbestos-related disease in chrysotile miners and millers — A. Churg, J.L. Wright, et al. (1993)

Browse more research in The Annals of Occupational Hygiene, or return to medical research.