Cancer in Crysotile Mines and Mills

Our plain-English summary of Cancer in Crysotile Mines and Mills, by J.C. McDonald (1972).

The study

• Author: J.C. McDonald
• Published: Biological Effects of Asbestos Lyon, October 2-5, 1972. Paper 29.
• Type of article: Conference paper
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This conference paper reviewed evidence on cancer among chrysotile asbestos miners and millers. It drew mainly on the author's group's Quebec study of some 12,000 persons born 1891-1920 and employed in the industry for more than one month. It also covered an Italian study and other mining countries. In the original Quebec analysis, any respiratory cancer excess was virtually confined to those with a dust index above 200 mppcf-years. A provisional re-analysis found 136 male respiratory cancer deaths against 95 expected, with the excess emerging somewhere between 100 and 400 mppcf-years. The paper counted 11 recorded mesothelioma cases linked directly or indirectly to chrysotile production (9 probable, 2 doubtful), almost all in Canada. There was no evidence of an association with peritoneal tumours. Gastro-intestinal cancer was suggested to be raised in the most heavily exposed men, by less than 2 to 1. The author concluded there appeared to be a modest increase in respiratory and probably gastro-intestinal cancer, most serious with heavy, long exposure. The author also stated that mesothelioma cases were too few for firm conclusions on dose-response.

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: Review (conference paper) of studies of cancer in chrysotile mining and milling, drawing largely on the author's group's Quebec cohort mortality study
• Who was studied: Chrysotile mine and mill workers. Mainly the Quebec cohort of some 12,000 persons born 1891-1920 and employed in any part of the industry for more than one month; results are mostly reported for men (Table 2: 11,107 men, 9,692 traced). Also a Balangero, northern Italy cohort of 1,098 men, plus data from Rhodesia, Cyprus, South Africa and the USSR.
• Exposure: Chrysotile dust in mining and milling, measured as a cumulative dust index in millions of particles per cubic foot multiplied by years (mppcf-years). The paper also considered household and neighbourhood exposure.
• Main results:
  • Quebec cohort, original analysis (Table 1): any respiratory cancer excess was virtually confined to persons with a dust index above 200 mppcf, and was most evident above 400 mppcf
  • Quebec cohort, provisional re-analysis by another method (made because the original age-correction method did not allow for interaction between length of exposure and length of survival): 136 male respiratory cancer deaths observed compared with 95 expected
  • In the provisional re-analysis, the respiratory cancer excess became evident somewhere between 100 and 400 mppcf-years and was clearly present above this level
  • Quebec cohort: 135 respiratory cancers in the series, 134 in men, including 5 pleural mesotheliomas
  • Quebec men in the highest exposure category who were employed 30 years or more had about 5 times the respiratory cancer mortality rate of those in the lowest exposure group employed less than one year. Numbers were very small.
  • Eleven recorded cases of malignant mesothelioma were associated directly or indirectly with chrysotile production in 6 countries (9 probable and 2 doubtful). Almost all were in Canada.
  • Three Canadian cases (one man and two women) came from families in which the father worked in the mills
  • The author estimated that the 8 definite mesothelioma cases in Quebec were probably at least 4 times the expected number
  • No evidence of an association of chrysotile with peritoneal tumours
  • Quebec cohort: a suggested increase in gastro-intestinal cancer for men with a dust index above 400 mppcf, by less than 2 to 1, with no consistent pattern by site
  • The Italian (Balangero) cohort showed even less evidence of respiratory cancer than the Quebec cohort
  • A retrospective inquiry matching lung cancer cases in chrysotile workers with cases from the general population found an excess of non-smokers and light smokers in the asbestos group. The author said this did not support synergism but did not wholly exclude it.
• The authors' conclusion: The author concluded that there appeared to be a modest increase in respiratory and probably gastro-intestinal cancer in the chrysotile industry, most serious in heavily exposed long-term workers. The author judged this hazard would probably not be detectable below about 200 mppcf-years, and judged the link with mesothelioma to be weak.
• Limitations the authors state:
  • Chrysotile-related mesothelioma cases were too few for any firm conclusion on dose-response
  • The age-correction method used in the Quebec study did not allow for interaction between length of exposure and length of survival; the re-analysis by another method was provisional
  • Mesothelioma cases may have been missed in Cyprus, South Africa and Rhodesia
  • Diagnosis was likely more probable for industry-related cases than in the general population
  • The full analysis of the Italian study was not yet available

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Million Particles per Cubic Foot (mppcf), Mesothelioma, Dose-response relationship.

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