Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec

Our plain-English summary of Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec, by W.J. Nicholson, I.J. Selikoff, H. Seidman, et al (Annals of the New York Academy of Sciences, 1979).

The study

• Authors: W.J. Nicholson, I.J. Selikoff, H. Seidman, R. Lilis, P. Formby
• Journal: Annals of the New York Academy of Sciences
• Published: 1979;330:11-21
• Cited by: 39 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. 001041).

Abstract

Among a cohort of 544 men with at least 20 years of employment in chrysotile mining and milling at Thetford Mines, Canada, 16% of the deaths were from lung cancer and 15% from asbestosis. The excess over expected deaths from these causes account for 43 of 178 deaths in the group. The risk of death of asbestosis, at equal times fron onset of exposure, is very similar in miners and millers, factory workmen and insulators. The ratio of observed to expected deaths from lung cancer is similar in the miners and millers and factory workers, but higher in insulators. The risk of death of mesothelioma in miners and millers is decidedly less than the other two groups. The exact causes of the reduced risk in this category are not yet completely clarified.

In plain English

This study followed 544 men with at least 20 years' seniority in 1961 at four chrysotile mining and milling companies in Thetford Mines, Quebec. Their deaths through August 1977 were compared with numbers expected from Canadian national rates. There were 178 deaths against 159.9 expected, including 28 lung cancer deaths against 11.1 expected. Non-infectious lung diseases caused 30 deaths against 6.7 expected, 26 of them from asbestosis. In air samples from 1973 to 1975, average levels exceeded the then-current US limit of 5 fibers longer than 5 micrometers per milliliter in virtually all job categories. The authors compared the miners and millers with separately studied insulation and asbestos factory workers. The miners and millers had approximately the same asbestosis death risk at equal times from first exposure, though these deaths tended to occur earlier among them. Mesothelioma was far less common in miners and millers: 0.13 deaths per 1,000 person-years, against 2.92 for factory workers and 2.20 for insulators. The authors said the reasons for this lower risk were not yet clarified. They noted that no fiber concentrations were measured in Quebec mines and mills before 1970.

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 study comparing observed with expected deaths, plus air sampling and a comparison with two other asbestos-exposed cohorts studied separately
• Who was studied: 544 men employed in 1961 with at least 20 years' seniority at four chrysotile mining and milling companies in Thetford Mines, Quebec, Canada. They were followed from the date of each company's 1961 seniority list through August 1977, for 7,408 person-years.
• Exposure: Chrysotile asbestos only, from mine, crusher, dryer, mill, bagging, maintenance and laboratory work. Air samples were taken in 1973 to 1975 and measured fibers longer than 5 micrometers per milliliter of air.
• Main results:
  • Whole cohort: 178 deaths observed against 159.9 expected (ratio of observed to expected 1.11).
  • Whole cohort: 28 lung cancer deaths observed against 11.1 expected (ratio 2.52).
  • Whole cohort: 30 deaths from non-infectious lung diseases against 6.7 expected (ratio 4.48), including 26 asbestosis deaths. No general-population rates were available for asbestosis.
  • Asbestosis and lung cancer together accounted for 54 deaths, or 31% of all deaths. Deaths from all other causes were 15% below expected.
  • Mesothelioma death rate: 0.13 per 1,000 person-years in the miners and millers (1 death), against 2.92 in factory workers (25 deaths) and 2.20 in insulators (170 deaths).
  • Overall asbestosis death rates were 3.51 per 1,000 person-years in miners and millers (26 deaths), 3.27 in factory workers (28 deaths) and 2.04 in insulators (160 deaths). The authors described the risk as approximately the same in all three groups at equal times from first exposure.
  • Deaths from pneumoconiosis tended to occur earlier among the miners and millers than in the other two groups.
  • Air samples taken in 1973 to 1975: in virtually all work categories, average concentrations considerably exceeded the US standard of 5 fibers longer than 5 micrometers per milliliter.
  • Mortality was increased between 30 and 50 years after first exposure, with little excess after 50 or more years.
• The authors' conclusion: The authors concluded that the risk of death from mesothelioma was decidedly lower in the chrysotile miners and millers than in the factory workers and insulators. They said the exact causes of this lower risk were not yet clarified, and suggested differences in fiber size and nonfibrous dust as possible factors.
• Limitations the authors state:
  • Expected deaths were based on Canadian national rates. These were likely higher than rates in the rural mining counties, which would understate the excess, especially for lung cancer. County rates could not be obtained.
  • No fiber concentrations were measured in Quebec mines and mills before 1970, and the authors considered estimates from earlier total dust measurements unreliable. Workers therefore could not be grouped by past exposure. Past levels would have been higher than those measured.
  • The air samples were mostly taken during normal operation and missed intermittent higher exposures.
  • No comparative studies were available of the dust burden in the lungs of miners and millers versus other asbestos-exposed groups.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Thetford Mines, Lung Cancer, Insulator, Mesothelioma.

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)
• Lung mineral fibers of former miners and millers from Thetford-Mines and asbestos regions: a comparative study of fiber concentration and dimension — A. Nayebzadeh, A. Dufresne, et al. (2001)
• 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)
• Exploratory sampling of asbestos in residences near Thetford Mines: the public health threat in Quebec — M. Marier, W. Charney, et al. (2007)
• Mortality in Canadian miners and millers exposed to chrysotile — J.C. McDonald, F.D. Liddell (1979)

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