The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88

Our plain-English summary of The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88, by J.C. McDonald, F.D. Liddell, A. Dufresne, et al (British Journal of Industrial Medicine, 1993).

The study

• Authors: J.C. McDonald, F.D. Liddell, A. Dufresne, A.D. McDonald
• Journal: British Journal of Industrial Medicine
• Published: 1993;50(12):1073-81
• Cited by: 92 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

A cohort of some 11,000 men born 1891-1920 and employed for at least one month in the chrysotile mines and mills of Quebec, was established in 1966 and has been followed ever since. Of the 5351 men surviving into 1976, only 16 could not be traced; 2508 were still alive in 1989, and 2827 had died; by the end of 1992 a further 698 were known to have died, giving an overall mortality of almost 80%. This paper presents the results of analysis of mortality for the period 1976 to 1988 inclusive, obtained by the subject-years method, with Quebec mortality for reference. In many respects the standardised mortality ratios (SMRs) 20 years or more after first employment were similar to those for the period 1951-75--namely, all causes 1.07 (1951-75, 1.09); heart disease 1.02 (1.04); cerebrovascular disease 1.06 (1.07); external causes 1.17 (1.17). The SMR for lung cancer, however, rose from 1.25 to 1.39 and deaths from mesothelioma increased from eight (10 before review) to 25; deaths from respiratory tuberculosis fell from 57 to five. Among men whose exposure by age 55 was at least 300 million particles per cubic foot x years (mpcf.y), the SMR (all causes) was elevated in the two main mining regions, Asbestos and Thetford Mines, and for the small factory in Asbestos; so were the SMRs for lung cancer, ischaemic heart disease, cerebrovascular disease, and respiratory disease other than pneumoconiosis. Except for lung cancer, however, there was little convincing evidence of gradients over four classes of exposure, divided at 30, 100, and 300 mpcf.y. Over seven narrower categories of exposure up to 300 mpcf.y the SMR for lung cancer fluctuated around 1.27 with no indication of trend, but increased steeply above that level. Mortality form pneumoconiosis was strongly related to exposure, and the trend for mesothelioma was not dissimilar. Mortality generally was related systematically to cigarette smoking habit, recorded in life from 99% of survivors into 1976; smokers of 20 or more cigarettes a day had the highest SMRs not only for lung cancer but also for all causes, cancer of the stomach, pancreas, and larynx, and ischaemic heart disease. For lung cancer SMRs increased fivefold with smoking, but the increase with dust exposure was comparatively slight for non-smokers, lower again for ex-smokers, and negligible for smokers of at least 20 cigarettes a day; thus the asbestos-smoking interaction was less than multiplicative. Of the 33 deaths from mesothelioma in the cohort to date, 28 were in miners and millers and five were in employees of a small asbestos products factory where commercial amphiboles had also been used. Preliminary analysis also suggest that the risk of mesothelioma was higher in the mines and mills at Thetford Mines than in those at Asbestos. More detailed studies of these differences and of exposure-response relations for lung cancer are under way.

In plain English

This study followed men born 1891-1920 who worked at least a month in Quebec's chrysotile industry, in mines and mills or a small asbestos products factory. Among the 5351 men alive in 1976, all but 16 traced, 2827 died in 1976-88; deaths were compared with Quebec male death rates. All-cause deaths were slightly above expected (standardised mortality ratio, SMR, 1.07); lung cancer was higher (321 deaths, SMR 1.40). Twenty-five men died of mesothelioma and 48 of pneumoconiosis. Lung cancer showed no trend with dust exposure to age 55 below 300 million particles per cubic foot-years. Above that it rose steeply, to 22 deaths against 7.2 expected at 1000 or more (SMR 3.04, 95% confidence interval 1.90 to 4.60). Pneumoconiosis rose sharply with exposure; mesothelioma similarly, on few deaths. Lung cancer SMR was 0.47 in cigarette non-smokers and 2.41 in men smoking 20 or more a day. Of 33 mesothelioma deaths in the cohort to date, 28 were miners and millers and five factory workers; the factory also used commercial amphiboles. The authors concluded that the asbestos-smoking interaction was less than multiplicative.

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 follow-up (subject-years method), comparing observed deaths with those expected from Quebec population death rates
• Who was studied: Men from a birth cohort of about 11,000 men born 1891-1920 who had worked at least one month in the Quebec chrysotile production industry. The 5351 who survived into 1976 were followed to the end of 1988. All but 16 were traced (5335): 2758 from mines and mills in the Thetford Mines region, 2158 from the mine and mill in Asbestos, and 419 from a small asbestos products factory in Asbestos.
• Exposure: Chrysotile mining and milling dust, estimated from job histories and dust counts and expressed as million particles per cubic foot x years (mpcf.y) accumulated to age 55. The small factory had also used commercial amphiboles.
• Main results:
  • Among the traced survivors into 1976, there were 2827 deaths in 1976-88. The all-cause SMR was 1.07. For lung cancer there were 321 deaths, SMR 1.40.
  • There were 25 deaths from mesothelioma in 1976-88 (best diagnosis after record review): 15 confirmed at necropsy and 10 by biopsy. There were 48 deaths from pneumoconiosis.
  • Compared with 1951-75, the lung cancer SMR rose from 1.25 to 1.39. Mesothelioma deaths increased from eight to 25, and respiratory tuberculosis deaths fell from 57 to five.
  • For deaths from all causes, there was little or no excess below 100 mpcf.y. Excesses were considerable above 300 mpcf.y, where the SMR was 1.27 at the Asbestos mine and mill, 1.35 at Thetford Mines and 2.76 at the Asbestos factory (18 deaths observed vs 6.5 expected).
  • Lung cancer was analysed among 5207 men followed from 20 years after first employment, with 310 deaths and an SMR of 1.38 (95% CI 1.23 to 1.55). There was no gradient over seven exposure classes up to 300 mpcf.y, where the SMR fluctuated around 1.27. Above 300 mpcf.y the risk increased greatly.
  • Among men followed from 20 years after first employment, at 1000 mpcf.y or more there were 22 lung cancer deaths against 7.2 expected, an SMR of 3.04 (95% CI 1.90 to 4.60).
  • Lung cancer SMR by smoking: 0.47 in non-smokers of cigarettes and 2.41 in smokers of 20 or more a day. The 'asbestos effect' compares the SMR at 60 mpcf.y or more with the SMR below 60 mpcf.y. It was 1.13 overall, 1.65 for non-smokers of cigarettes and 0.90 for smokers of 20 or more a day.
  • Deaths overall were strongly and systematically related to cigarette smoking. The all-cause SMR was 0.81 in non-smokers and 1.33 in smokers of 20 or more cigarettes a day.
  • Pneumoconiosis death rates rose sharply with exposure at each location. Mesothelioma showed similar trends, but on few deaths.
  • There have been 33 mesothelioma deaths in the cohort to date. Of these, 28 were in miners and millers and five in workers at the small factory where commercial amphiboles had also been used. They were primarily of the pleura.
  • A preliminary analysis suggested that mesothelioma risk was higher in the mines and mills at Thetford Mines than in those at Asbestos.
  • The authors judged that the lung cancer excess below 300 mpcf.y (SMR around 1.27) was largely explained by cigarette smoking.
• The authors' conclusion: The authors concluded that the interaction between smoking and asbestos exposure for lung cancer was less than multiplicative and far from simple. They gave a relative asbestos effect of 1.6, which they considered similar to the average of 1.8 found in an earlier review by others.
• Limitations the authors state:
  • The authors stated that the subject-years method cannot adequately examine lung cancer risk in relation to asbestos exposure and smoking. More detailed analyses were under way.
  • The authors stated that linear relations between SMRs and cumulative exposure are an oversimplification.
  • Exposure after November 1966 could not be estimated in the same way as earlier exposure, because of industry changes and a lack of information and comparable measurements. Only year-by-year trends were applied.
  • The mesothelioma exposure trend was based on few deaths.
  • The authors stated that the 1951-75 and 1976-88 periods cannot be compared exactly.

Topics

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

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

Related studies

• 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)
• Exposure to chrysotile mining dust and digestive cancer mortality in a Chinese miner/miller cohort — S. Lin, X. Wang, et al. (2014)
• The combination of effects on lung cancer of cigarette smoking and exposure in quebec chrysotile miners and millers — F.D. Liddell, B.G. Armstrong (2002)
• Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec — W.J. Nicholson, I.J. Selikoff, et al. (1979)
• Mortality in a Chinese chrysotile miner cohort — X. Wang, S. Lin, et al. (2012)
• Cancer mortality in chrysotile miners and millers, Russian Federation: main results (Asbest Chrysotile Cohort-Study) — J. Schüz, E. Kovalevskiy, et al. (2024)

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