Mortality in Canadian miners and millers exposed to chrysotile

Our plain-English summary of Mortality in Canadian miners and millers exposed to chrysotile, by J.C. McDonald, F.D. Liddell (Annals of the New York Academy of Sciences, 1979).

The study

• Authors: J.C. McDonald, F.D. Liddell
• Journal: Annals of the New York Academy of Sciences
• Published: 1979;330:1-9
• Cited by: 21 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 001037).

Summary of the article

This paper reviewed how mortality findings developed over four stages of follow-up in a cohort study of the chrysotile asbestos mines and mills at Asbestos and Thetford Mines, Quebec. The study began in 1966. From a register of 28,000 employees, the cohort comprised persons born 1891-1920 and employed at least one month. After revision it numbered 11,379 (10,939 men, 440 women). By the end of 1975, 90.1% had been traced and 4547 had died. Dust exposure was estimated for each subject in millions of particles per cubic foot (mpcf), and smoking was assessed by questionnaire. Analyses used man-years methods with Quebec death rates and a matched case-control approach. In 1926-75 there were 4350 male deaths against 4107 expected (SMR 1.06). Relative risks for respiratory cancer rose with accumulated dust exposure to 2.9-3.2 in the highest category and were similar at each stage. In men employed 20 years or more with very heavy exposure, the SMR for respiratory cancer was 265. Dust and smoking fitted a multiplicative model. Ten male mesothelioma deaths occurred in the cohort against about 2 expected. The authors concluded that respiratory cancer and pulmonary fibrosis were the major hazards, that the exposure-response relationship might be linear, and that mesothelioma risk appeared much lower than with crocidolite.

PubMed has no abstract for this study, and the article doesn't print one. We wrote this summary from the full article with the help of AI. It is not the authors' wording.

In plain English

This paper gave an overview of a long-running study of deaths among workers in the chrysotile asbestos mines and mills at Asbestos and Thetford Mines, Quebec. The group comprised 11,379 people born 1891-1920 who had worked at least one month (10,939 men and 440 women), followed to the end of 1975. Dust exposure was estimated in millions of particles per cubic foot. From 1926 to 1975 there were 4350 male deaths, against 4107 expected from Quebec rates (SMR 1.06). Respiratory cancer risk rose with accumulated dust exposure. Among men employed 20 years or more, those with "very heavy" exposure had an SMR of 265 for respiratory cancer. Dust and smoking appeared to combine multiplicatively. Ten male deaths in the cohort were thought to be linked to malignant mesothelioma, against about 2 expected by the authors' estimate. All 12 cases later known among mine and mill workers were pleural. The authors concluded that the major hazards of chrysotile were respiratory cancer and complications of pulmonary fibrosis. They also concluded that mesothelioma risk appeared likely to stay much lower than with crocidolite.

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: Longitudinal (retrospective) cohort mortality study, reported as an overview of results at four follow-up stages. It used man-years analyses against Quebec death rates and a matched-control analysis for smoking.
• Who was studied: 11,379 people born 1891-1920 and employed one month or more in the Quebec chrysotile production industry, with a small number from an asbestos manufacturing plant in Asbestos, Quebec. They were 440 women and 10,939 men: 5302 at Asbestos and 5,637 at Thetford Mines. They were followed to the end of 1975, by which time 4547 were known to have died.
• Exposure: Chrysotile mining and milling dust. Each worker's exposure was estimated for each job and year in millions of particles per cubic foot (mpcf), using midget impinger dust counts from 1949-1966 and estimates for earlier years. Accumulated exposure was expressed as mpcf multiplied by years (mpcf.y).
• Main results:
  • All causes, men, 1926-75: 4350 deaths observed versus 4107 expected from Quebec rates, an SMR of 1.06. The SMR was 1.02 at Asbestos and 1.10 at Thetford Mines.
  • Respiratory cancer deaths in men: 94 among 2413 male deaths from 1910 to 1966. A further 156 occurred among an additional 2050 deaths by the end of 1975.
  • Relative risks for respiratory cancer by accumulated dust exposure, estimated at four stages (1966, 1969, 1973, 1975), were described as in good agreement, though not strictly comparable.
  • Among men employed 20 years or more, the SMR for respiratory cancer was 265 in those with 'very heavy' exposure. The SMR for cancers of the stomach and esophagus was 289. Respiratory cancer ratios rose fairly steadily with service and dust concentration, while those for stomach and esophagus cancer were very irregular.
  • Laryngeal cancer: 17 deaths observed versus 16.12 expected; relative risks were not related to exposure.
  • In the matched-control analysis of respiratory cancer deaths, a multiplicative model fitted the combined effect of asbestos dust and smoking.
  • Men employed less than 1 year (about 4 months on average) had relatively high SMRs, especially in low-dust jobs. The authors attributed this to factors probably related to selection.
  • Ten male deaths in the cohort to 1975 were thought to be associated with malignant mesothelioma. Only three of these were so stated on the death certificate. The authors estimated about 2 would be expected.
  • Including cases from 1976-77, 12 mesothelioma cases were known in chrysotile mine and mill workers: 9 among 3105 men employed 20 years or more and 3 among 7834 men employed less than 20 years. All 12 were pleural.
  • For comparison, 9 mesothelioma cases occurred among 199 people who made crocidolite filter pads for wartime gas masks; six of the nine were peritoneal. The median time from first employment to death was nearly 20 years longer in the chrysotile mine and mill workers.
  • Proportional mortality from mesothelioma was 2.2 per 1000 overall and 8.2 per 1000 for men with 20 years or more exposure. The authors expected this to rise somewhat but to remain much lower than with crocidolite.
• The authors' conclusion: The authors concluded that the major occupational hazards of chrysotile exposure are respiratory cancer and the complications of pulmonary fibrosis. They also found that excess respiratory cancer mortality was directly related to accumulated dust exposure.
• Limitations the authors state:
  • Dust was measured as particle counts. The authors considered conversion to fiber counts not justifiable where precision and certainty are required.
  • Before 1949 there were few dust measurements, so exposures were estimated from interviews and comparisons.
  • About 10% of the cohort could not be traced, mostly men with short service many years earlier.
  • The appropriateness of Quebec rates, or any reference population, is open to question.
  • Selection factors appeared to run counter to the exposure effect.
  • The relative risk estimates from the four study stages are not strictly comparable.
  • Smoking histories were not sought for deaths before 1950.
  • There was no easy way to obtain an expected number of mesothelioma deaths.

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, Cohort Study, Thetford Mines, Million Particles per Cubic Foot (mppcf), Relative Risk, Mesothelioma, Pulmonary Fibrosis, Crocidolite.

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)
• Asbestos exposure, cigarette smoking, and airflow limitation in long-term Canadian chrysotile miners and millers — R. Bégin, R. Boileau, et al. (1987)
• Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec — W.J. Nicholson, I.J. Selikoff, et al. (1979)
• 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 morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)

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