Dust exposure and mortality in chrysotile mining, 1910-75

Our plain-English summary of Dust exposure and mortality in chrysotile mining, 1910-75, by J.C. McDonald, F.D. Liddell, G.W. Gibbs, et al (British Journal of Industrial Medicine, 1980).

The study

• Authors: J.C. McDonald, F.D. Liddell, G.W. Gibbs, G.E. Eyssen, A.D. McDonald
• Journal: British Journal of Industrial Medicine
• Published: 1980;37(1):11-24
• Cited by: 259 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. 000837).

Abstract

We report a further follow-up of a birth cohort of 11 379 workers exposed to chrysotile. The cohort consisted of 10 939 men and 440 women, born 1891-1920, who had worked for at least a month in the mines and mills of Asbestos and Thetford Mines in Quebec. For all subjects, length of service and estimates of accumulated dust exposure were obtained, with a smoking history for the vast majority. Three methods of analysis, two based on the "man-years" methods, the other a "case-and-multiple-controls" approach, gave results consistent with one another and with previous analyses. By the end of 1975, 4463 men and 84 women had died. Among men, the overall excess mortality, 1926-75 was 2% at Asbestos and 10% at Thetford Mines, much the dustier region. The women, mostly employed at Asbestos, had a standardised mortality ratio (SMR) all causes, 1936-75) of 0.90. Analysis of deaths 20 years or more after first employment showed that in men with short service (less than five years) there was no discernible correlation with dust exposure. Among men employed at least 20 years, there were clear excesses in those exposed to the heaviest dust concentrations. Reanalysis in terms of exposure to age 45 showed definite and consistent trends for SMRs for total mortality, for lung cancer, and for pneumoconiosis to be higher the heavier the exposure. The response to increasing dose was effectively linear for lung cancer and for pneumoconiosis. Lung cancer deaths occurred in non-smokers, and showed a greater increase of incidence with increasing exposure than did lung cancer in smokers, but there was insufficient evidence to distinguish between multiplicative and additive risk models. There were no excess deaths from laryngeal cancer, but a clear association with smoking. Ten men and one woman died from pleural mesothelioma. If the only subjects studied had been the 1904 men with at least 20 years' employment in the lower dust concentrations, averaging 6.6 million particles per cubic foot (or about 20 fibres/cc), excess mortality would not have been considered statistically significant, except for pneumoconiosis. The inability of such a large epidemiological survey to detect increased risk at what, today, are considered unacceptable dust concentrations, and the consequent importance of exposure-response models are therefore emphasised.

In plain English

This study followed 11 379 people (10 939 men and 440 women), born 1891-1920, who had worked at least a month in the chrysotile mines and mills of Asbestos and Thetford Mines, Quebec. The authors estimated each person's accumulated dust exposure and collected smoking histories. By the end of 1975, 4463 men and 84 women had died. Among men, overall excess mortality for 1926-75 was 2% at Asbestos and 10% at Thetford Mines, the dustier area. The women's standardised mortality ratio for all causes, 1936-75, was 0.90. The risk of dying from lung cancer and from pneumoconiosis, compared with Quebec death rates, rose roughly linearly with heavier estimated exposure. Lung cancer deaths occurred in non-smokers too. Laryngeal cancer showed no excess but was linked with smoking. Ten men and one woman died of pleural mesothelioma. Among 1904 long-serving men in the lower dust groups (averaging 6.6 million particles per cubic foot, or about 20 fibres/cc), excess mortality was not statistically significant except for pneumoconiosis. The authors emphasised that even so large a survey could miss increased risk at levels now considered unacceptable, hence the importance of exposure-response models.

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 birth-cohort mortality study, analysed with man-years (standardised mortality ratio) methods against Quebec death rates and with a case-and-multiple-controls (Miettinen) approach
• Who was studied: 10 939 men and 440 women, born 1891-1920, who worked at least one month in the chrysotile mines and mills of Asbestos and Thetford Mines, Quebec, followed from first employment (earliest 1904) to the end of 1975
• Exposure: Chrysotile mine and mill dust. Exposure was estimated for each job and year in millions of particles per cubic foot (mpcf) and accumulated as mpcf-years. Smoking habit was also recorded.
• Main results:
  • By the end of 1975, 4463 men and 84 women in the cohort had died
  • Among men, overall excess mortality for 1926-75 was 2% at Asbestos and 10% at Thetford Mines, the dustier region
  • The women, mostly employed at Asbestos, had an all-causes standardised mortality ratio (1936-75) of 0.90
  • In men with less than five years' service, deaths 20 or more years after first employment showed no discernible correlation with dust exposure
  • Among men employed at least 20 years, there were clear excesses in those exposed to the heaviest dust concentrations
  • Of 42 pneumoconiosis deaths (20 or more years after first employment, 1951-75), 36 were in men with at least 20 years' service
  • With exposure measured to age 45, standardised mortality ratios rose with heavier exposure. The trends were clearest for pneumoconiosis and lung cancer and appeared in both mining areas. The response to increasing dose was effectively linear for both.
  • A fitted linear dose-response line for lung cancer (without regard for smoking) was: relative risk = 1 + 0.0014 per mpcf-year
  • Lung cancer deaths occurred in non-smokers and rose more with increasing exposure than lung cancer in smokers, but the evidence could not distinguish multiplicative from additive risk models
  • There was no excess of laryngeal cancer deaths, but there was a clear association with smoking (heavy smokers: 5 deaths, SMR 4.85)
  • There were 11 mesothelioma deaths (10 men, 1 woman). All were pleural and appeared to follow a clear exposure trend.
  • Among 1904 men with at least 20 years' employment in the low and medium dust groups (averaging 6.6 mpcf, or about 20 fibres/cc), there were 620 deaths, with an all-causes SMR of 0.94. Their excesses were not statistically significant except for pneumoconiosis.
• The authors' conclusion: The authors concluded that essentially linear relations were shown between exposure and lung cancer, pneumoconiosis and total deaths. They emphasised that even such a large survey was unable to detect increased risk at dust concentrations now considered unacceptable, and stressed the consequent importance of exposure-response models.
• Limitations the authors state:
  • Few dust measurements existed before 1949, so exposure for earlier years was estimated
  • Recorded causes of death may be biased towards over-diagnosis of dust-related disease in heavily exposed men
  • Healthy (or unhealthy) worker selection effects cannot be corrected for, especially in comparisons with an external reference population
  • The findings fit a linear dose-response but do not prove it
  • Smoking histories were not sought for 816 earlier deaths
  • One-tenth of the men were lost to view

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Thetford Mines, Lung Cancer, Pneumoconiosis, Laryngeal Cancer, Pleural Mesothelioma, Million Particles per Cubic Foot (mppcf).

Related studies

• Dust exposure and mortality in chrysotile mining, 1910-75. 1980 — J.C. McDonald, F.D. Liddell, et al. (1993)
• Exposure to chrysotile mining dust and digestive cancer mortality in a Chinese miner/miller cohort — S. Lin, X. Wang, et al. (2014)
• Respiratory effects of exposure to dust in taconite mining and processing — T.C. Clark, V.A. Harrington, et al. (1980)
• Mortality in the chrysotile asbestos mines and mills of Quebec — J.C. McDonald, A.D. McDonald, et al. (1971)
• Dust exposure and mortality in chrysotile mining, 1910-76 — M. Greenberg (1994)
• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 — J.C. McDonald, F.D. Liddell, et al. (1993)

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