The 1891-1920 birth cohort of Quebec chrysotile miners and millers: development from 1904 and mortality to 1992

Our plain-English summary of The 1891-1920 birth cohort of Quebec chrysotile miners and millers: development from 1904 and mortality to 1992, by F.D. Liddell, A.D. McDonald, J.C. McDonald (The Annals of Occupational Hygiene, 1997).

The study

• Authors: F.D. Liddell, A.D. McDonald, J.C. McDonald
• Journal: The Annals of Occupational Hygiene
• Published: 1997;41(1):13-36
• Cited by: 96 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This paper draws together the mortality experience for a cohort of some 11000 male Quebec Chrysotile miners and millers, reported at intervals since 1971 and now again updated. Of the 10918 men in the complete cohort, 1138 were lost to view, almost all never traced after employment of only a month or two before 1935; the other 9780 men were traced into 1992. Of these, 8009 (82%) are known to have died: 657 from lung cancer, 38 from mesotheliona, 1205 from other malignant disease, 108 from pneumoconiosis and 561 from other non-malignant respiratory diseases (excluding tuberculosis). After early fluctuations. SMRs (all causes) against Quebec rates have been reasonably steady since about 1945. For men first employed in Asbestos, mine or factory, they were very much what might have been expected for a blue collar population without any hazardous exposure. SMRs in the Thetford Mines area were almost 8% higher, but in line with anecdotal evidence concerning socio-economic status. At exposures below 300 (million particles per cubic foot) x years, (mpcf.y), equivalent to roughly 1000 (fibres/ml) x years-or, say, 10 years in the 1940s at 80 (fibres/ml)-findings were as follows. There were no discernible associations of degree of exposure and SMRs, whether for all causes of death or for all the specific cancer sites examined. The average SMRs were 1.07 (all causes), and 1.16, 0.93, 1.03 and 1.21, respectively, for gastric, other abdominal, laryngeal and lung cancer. Men whose exposures were less then 300 mpcf.y suffered almost one-half of the 146 deaths from pneumoconiosis or mesothelioma; the elimination of these two causes would have reduced these men's SMR (all causes) from 1.07 to approximately 1.06. Thus it is concluded from the viewpoint of mortality that exposure in this industry to less than 300 mpcf.y has been essentially innocuous, although there was a small risk or pneumoconiosis or mesothelioma. Higher exposures have, however, led to excesses, increasing with degree of exposure, of mortality from all causes, and from lung cancer and stomach cancer, but such exposures, of at least 300 mpcf.y, are several orders of magnitude more severe than any that have been seen for many years. The effects of cigarette smoking were much more deleterious than those of dust exposure, not only for lung cancer (the SMR for smokers of 20+ cigarettes a day being 4.6 times higher than that for non-smokers), but also for stomach cancer (2.0 times higher), laryngeal cancer (2.9 times higher), and-most importantly-for all causes (1.6 times higher).

In plain English

The authors updated mortality to 1992 for 10,918 male Quebec chrysotile workers born 1891 to 1920. Of 9,780 traced, 8,009 died, including 657 from lung cancer, 38 from mesothelioma and 108 from pneumoconiosis. Exposure to age 55 (million particles per cubic foot times years, mpcf.y) was estimated for all but 241 men. Below 300 mpcf.y, SMRs (observed versus expected deaths) for all causes and the cancers examined showed no discernible exposure link, averaging 1.07 overall and 1.21 for lung cancer. At higher exposures, all-cause and lung cancer SMRs rose with exposure. Stomach cancer SMR was 3.21 at 1000 mpcf.y or more (under 12 excess deaths), but its trend below that was negative. Pneumoconiosis death rates were associated with exposure at the two main workplaces, perhaps distorted by exclusions; mesothelioma showed little such sign. Among 5,274 men with acceptable smoking histories (deaths 1975 to May 1992), lung cancer SMR for those smoking 20 or more daily was 4.6 times non-smokers'. The authors concluded that exposure below 300 mpcf.y was essentially innocuous for mortality, apart from a small pneumoconiosis or mesothelioma risk.

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: Birth cohort mortality study: the final update of a cohort followed from first employment, which the authors present as a comprehensive review of their own investigation
• Who was studied: 10,918 men born 1891 to 1920 who worked at least one month in the Quebec chrysotile-producing industry (Asbestos and Thetford Mines areas), first employed from 1904 onward. 1,138 were lost to view and 9,780 were traced into 1992. Female workers were excluded.
• Exposure: Chrysotile mine, mill and factory dust, expressed as accumulated dust exposure to age 55 in mpcf.y. It was estimated from job histories, midget impinger dust surveys and interviews. This measure was not calculated for 241 men: 236 who had not started recorded employment by age 45 and five with inconsistent work histories. The authors equated 300 mpcf.y to roughly 1,000 fibres/ml times years.
• Main results:
  • Of 9,780 traced men, 8,009 (82%) had died: 657 from lung cancer, 38 from mesothelioma, 1,205 from other malignant disease and 108 from pneumoconiosis.
  • Among men with exposures below 300 mpcf.y, no association was discernible between exposure level and SMRs, for all causes or for any of the cancer sites examined.
  • For men below 300 mpcf.y, average SMRs were 1.07 for all causes, 1.16 for gastric cancer, 0.93 for other abdominal cancer, 1.03 for laryngeal cancer and 1.21 for lung cancer.
  • Men below 300 mpcf.y had almost half of the 146 deaths from pneumoconiosis or mesothelioma. Removing these two causes would have reduced their all-cause SMR from 1.07 to about 1.06.
  • At exposures of at least 300 mpcf.y, excess deaths from all causes and from lung cancer increased with degree of exposure.
  • For stomach cancer, the trend with exposure was negative even up to 1000 mpcf.y. The authors judged the average 17% excess probably due to socio-economic factors.
  • In the group exposed to at least 1000 mpcf.y, the stomach cancer SMR was 3.21, reflecting an excess of fewer than 12 deaths.
  • At exposures over 300 mpcf.y, the excess of lung cancer was 80.4 deaths. The authors judged that around one-fifth of this was due to smoking.
  • Pneumoconiosis death rates per 100,000 subject-years were associated with exposure at the two main places of employment. The authors warned that exclusions from this analysis may have distorted these associations.
  • There was little sign of an association between exposure and mesothelioma death rates.
  • The smoking analysis covered deaths from 1975 to May 1992 among 5,274 men with acceptable smoking histories. In that group, the lung cancer SMR for smokers of 20 or more cigarettes a day was 4.6 times that of non-smokers. The corresponding figures were 2.0 times for stomach cancer, 2.9 times for laryngeal cancer and 1.6 times for all causes.
  • SMRs in the Thetford Mines area were almost 8% higher. The authors described this as in line with anecdotal evidence on socioeconomic status.
• The authors' conclusion: The authors concluded that, in terms of mortality, exposure in this industry below 300 mpcf.y had been essentially innocuous, although there was a small risk of pneumoconiosis or mesothelioma. They described the higher exposures that led to excesses as several orders of magnitude more severe than any seen for many years.
• Limitations the authors state:
  • Analyses by exposure could only start at each man's 55th birthday, so deaths before age 55 were excluded from them.
  • Exclusions from the pneumoconiosis and mesothelioma analysis may have distorted the associations with exposure.
  • Reference rates for specific causes of death were available only from 1950.
  • The analysis by smoking was confined to the years after 1974, when the proportion of unclassifiable questionnaires became low enough.
  • The small manufacturing plant in Asbestos, whose workers were included, used some crocidolite and amosite.
  • Some mesothelioma cases in the cohort were later found to have resulted from crocidolite exposure in making filters for military gas masks.
  • Smoking histories were not obtained for men who died before 1950.

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, Lung Cancer, Pneumoconiosis, Thetford Mines, Million Particles per Cubic Foot (mppcf), Mesothelioma, Stomach Cancer, Laryngeal Cancer.

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 combination of effects on lung cancer of cigarette smoking and exposure in quebec chrysotile miners and millers — F.D. Liddell, B.G. Armstrong (2002)
• Exposure to chrysotile mining dust and digestive cancer mortality in a Chinese miner/miller cohort — S. Lin, X. Wang, et al. (2014)
• Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec — W.J. Nicholson, I.J. Selikoff, et al. (1979)
• Mortality of Talc Miners and Millers From Val Chisone, Northern Italy: An Updated Cohort Study — E. Pira, M. Coggiola, et al. (2017)
• Mortality in a Chinese chrysotile miner cohort — X. Wang, S. Lin, et al. (2012)

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