The Incidence Of Pleural Mesothelioma In Chrysotile Asbestos Textile Workers

Our plain-English summary of The Incidence Of Pleural Mesothelioma In Chrysotile Asbestos Textile Workers, by J. Peto (1980).

The study

• Author: J. Peto
• Published: Biological Effects Of Mineral Fibres (J.C. Wagner, Editor), Lyon: Iarc Scientific Publications, Volume 30, Pp. 703-711 (1980)
• Type of article: Conference paper
• Our copy: from Johns-Manville's own medical library, J-M no. 002183. The article is still under copyright, so we describe it here rather than post it.

In plain English

This study followed 567 men first employed in an asbestos textile factory before 1951 for pleural mesothelioma to the end of 1978. The title calls them chrysotile textile workers. Cohort 1 was 69 men with 20 or more years' exposure, over 10 before 1933. Cohort 2 was 74 men with under 10 exposure years before 1933 and a minimum total exposure illegible in the scan. Cohort 3 was 424 men with 10 or more years' exposure, first employed 1933 to 1950. Dust levels fell sharply from 1933. Incidence rose with time since first exposure but, allowing for that, not with age. Allowing for time since first exposure, cohorts 1, 2 and 3 had 2, 5 and 7 mesotheliomas against 3.14, 5.52 and 5.35 expected. Cohort 1 had 13 lung cancers against 1.57 expected. The author concluded that risk did not appear much higher in men initially very heavily exposed. One possible explanation offered was that chrysotile is eliminated, or stops being biologically active, faster than other asbestos types. With only 14 cases, no firm conclusions could be drawn, the author stressed.

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 analysis (follow-up of a factory workforce), with internal comparisons of mesothelioma incidence by age, time since first exposure, period and exposure cohort
• Who was studied: 567 men first employed in an asbestos textile factory before 1951, including 143 who worked before 1933, followed to the end of 1978. They were divided into three groups. Cohort 1 was 69 men with 20 or more years' exposure, more than 10 of them before 1933. Cohort 2 was 74 men with fewer than 10 exposure years before 1933 and a minimum total exposure whose figure is illegible in the scanned text. Cohort 3 was 424 men with 10 or more years' exposure, first employed 1933 to 1950.
• Exposure: Work in an asbestos textile factory described as processing chrysotile. Dust levels were extremely heavy before 1933 and dramatically reduced from 1933 onwards. The extent of crocidolite exposure was not accurately known, but it appeared much lower than in the crocidolite factory cohort of Newhouse & Berry (1976).
• Main results:
  • 14 cases of pleural mesothelioma occurred in the whole population
  • Incidence increased with time since first exposure, and this increase persisted after adjustment for age
  • After adjustment for time since first exposure, there was no evidence of an increasing trend in incidence with age in any cohort
  • The first mesothelioma death was in 1936 and the second not until 1968. The marked rise in cases over successive periods appeared to be due largely to the long interval between first exposure and disease. There was little evidence of improved diagnosis since 1931.
  • Lung cancer in cohort 1: 13 observed against 1.57 expected at national death rates
  • Counting from 20 or more years after first exposure: cohort 2 had 7 lung cancers against 3.54 expected, plus 5 mesotheliomas. Cohort 3 had 22 lung cancers against 13.85 expected, plus 7 mesotheliomas.
  • Adjusted for time since first exposure, the expected numbers of mesotheliomas were 3.14 in cohort 1 (2 observed), 5.52 in cohort 2 (5 observed) and 5.35 in cohort 3 (7 observed)
  • Asbestosis was mentioned on the death certificate in 47% (31/66) of deaths in cohort 1 and in 13% (26/201) of deaths in cohort 3
• The authors' conclusion: The author concluded that age itself appears largely irrelevant to incidence. Past mesothelioma incidence may not have been much underestimated, so the recent rise in recorded rates is largely real. Risk did not appear much higher among men initially very heavily exposed. One possible explanation is that chrysotile is eliminated, or ceases to be biologically active, more quickly than other types of asbestos.
• Limitations the authors state:
  • Only 14 cases occurred, so no firm conclusions can be drawn
  • With so few cases, random variation must play a substantial role
  • The dimensions of respirable fibres may have changed substantially over the last 60 years
  • The extent of crocidolite exposure in this population is not accurately known

Topics

Medical terms in this study, explained in our Lexicon: Pleural Mesothelioma, Chrysotile, Mesothelioma, Lung Cancer.

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