Chrysotile, tremolite, and mesothelioma

Our plain-English summary of Chrysotile, tremolite, and mesothelioma, by J.C. McDonald, A.D. McDonald (Science, 1995).

The study

• Authors: J.C. McDonald, A.D. McDonald
• Journal: Science
• Published: 1995;267(5199):776-7
• Type of article: Comment, Letter
• Cited by: 20 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Summary of the article

In this letter, the authors returned to a question they had raised in 1990: whether mesothelioma deaths among Quebec chrysotile miners and millers might have been caused by contamination with fibrous tremolite, an amphibole. Their cohort comprised almost 11,000 men born between 1891 and 1920 and followed continuously since 1966. By the end of 1992, 37 mesothelioma deaths had been identified among 8000 deaths from all causes. Twenty-four were from Thetford Mines and 13 from Asbestos, including five from a small asbestos products factory that had used crocidolite. For the 24 Thetford cases, up to 10 surviving controls with the same year of birth were sought, and two cases with too few controls were excluded. Years worked in each of 15 Thetford-region mines were calculated, excluding employment within 20 years of death. Case employment was concentrated in five central mines (Area A). There, the control-to-case ratio of man-years was 5.3, compared with 21.6 in 10 peripheral mines (Area B). Electron microscopy data on lungs from 83 cohort members who died of other causes (58 Area A, 25 Area B) showed geometric mean concentrations of fibers 5 micrometers or longer per microgram of dried lung of 7 versus 13 for chrysotile (not significant) and 32 versus 7 for tremolite (P = 0.0002). The authors concluded that the relatively rare mesothelioma cases mainly or perhaps wholly resulted from amphibole contamination. They stressed that these findings reflected long, heavy exposures 20 to 70 years earlier.

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 letter to the editor reported new analyses from the authors' cohort of almost 11,000 men born between 1891 and 1920. The men were chrysotile miners and millers in Quebec, plus workers at a small asbestos products factory. By the end of 1992, 37 mesothelioma deaths had been identified among 8000 deaths from all causes: 24 from Thetford Mines and 13 from Asbestos, including five from the factory. For the Thetford cases, the authors compared work histories with those of surviving cohort members born in the same year. The ratio of controls' to cases' years of employment was 5.3 in five localized mines (Area A) and 21.6 in ten peripheral mines (Area B), a fourfold difference. Lung tissue came from 83 Thetford cohort members who died of other causes. Counting tremolite fibers at least 5 micrometers long, the geometric mean per microgram of dried lung was 32 in Area A and 7 in Area B. Chrysotile did not differ significantly. The authors wrote that this suggests the cases mainly or perhaps wholly resulted from amphibole fiber contamination. They emphasized that the findings reflect long, heavy exposures 20 to 70 years ago.

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: Letter to the editor reporting a case-control comparison within an existing cohort of Quebec chrysotile miners and millers. Each mesothelioma case was matched to surviving cohort members born in the same year. The letter also compared lung-tissue fiber concentrations between two mining areas.
• Who was studied: A cohort of almost 11,000 men born between 1891 and 1920, followed continuously since 1966: chrysotile miners and millers in Thetford Mines and Asbestos, Quebec, plus workers at a small asbestos products factory in Asbestos. The detailed study covered the 24 mesothelioma cases from Thetford Mines (22 after two exclusions) and their controls. Lung tissue came from 83 Thetford cohort members (58 from Area A, 25 from Area B) who died of causes other than mesothelioma.
• Exposure: Chrysotile mining and milling in Quebec, with contamination by fibrous tremolite (an amphibole). Tremolite contamination was several-fold higher in Thetford than in Asbestos, and crocidolite had been used in the factory. Exposure was measured as years worked in each of 15 Thetford-region mines, excluding work within 20 years of death. Lung fiber concentrations were measured by electron microscopy.
• Main results:
  • In the whole cohort, 37 mesothelioma deaths had been identified among 8000 deaths from all causes by the end of 1992.
  • Of the 37 mesothelioma deaths, 24 were from Thetford Mines and 13 from Asbestos, including five from the factory.
  • Two Thetford cases had too few controls (six and one), so these two cases and their controls were excluded.
  • Among the Thetford cases and controls, the ratio of controls' to cases' man-years of employment was 5.3 (1881/354) in the five mines of Area A. In the 10 peripheral mines of Area B it was 21.6 (1275/59). The fourfold difference is between these two ratios. (The scanned text prints the Area B figure as '1275159'.)
  • The two excluded cases and their available controls showed the same pattern.
  • The lung-tissue groups (58 from Area A, 25 from Area B) were similar in duration of employment (36 and 37 years) and in time from end of work to death (8 years in both). Estimated cumulative dust exposure was about 30% higher in the Area B group.
  • Geometric mean concentration of chrysotile fibers 5 micrometers or longer per microgram of dried lung: 7 in Area A and 13 in Area B. The difference was not statistically significant.
  • Geometric mean concentration of tremolite fibers 5 micrometers or longer per microgram of dried lung: 32 in Area A and 7 in Area B (P = 0.0002).
• The authors' conclusion: The authors wrote that the more than fourfold difference in tremolite concentration between the two areas suggests that the relatively rare mesothelioma cases mainly or perhaps wholly resulted from amphibole fiber contamination.
• Limitations the authors state:
  • The authors emphasized that the findings represent the result of long, heavy exposures 20 to 70 years ago.
  • The authors noted that the geological distribution of tremolite within the chrysotile ore body may vary in time and place.
  • The authors stated that, at present levels of environmental control, any mesothelioma risk in either area would be far below the limits of epidemiological detection.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile, Lung cancer, Mesothelioma, Tremolite.

Medical terms in this study, explained in our Lexicon: Chrysotile, Tremolite, Mesothelioma, Quebec Chrysotile, Amphibole Group, Thetford Mines, Crocidolite.

Related studies

• Respiratory cancer in chrysotile textile and mining industries: exposure inferences from lung analysis — P. Sebastien, J.C. McDonald, et al. (1989)
• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)
• Sumas Mountain chrysotile induces greater lung fibrosis in Fischer344 rats than Libby amphibole, El Dorado tremolite, and Ontario ferroactinolite — J.M. Cyphert, A. Nyska, et al. (2012)
• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 — J.C. McDonald, F.D. Liddell, et al. (1993)
• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• Comparison of Calidria chrysotile asbestos to pure tremolite: final results of the inhalation biopersistence and histopathology examination following short-term exposure — D.M. Bernstein, J. Chevalier, et al. (2005)

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