The hygiene standard for chrysotile asbestos

Our plain-English summary of The hygiene standard for chrysotile asbestos, by J. Peto (Lancet, 1978).

The study

• Author: J. Peto
• Journal: Lancet
• Published: 1978;1(8062):484-9
• Cited by: 54 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. 004994).

Abstract

Previous studies, including the analysis on which the current 2 fibres/cm3 hygiene standard is based, may have underestimated the risk of morbidity or mortality following exposure to low levels of asbestos dust. Accurate dose-response data at levels below 2 fibres/cm3 are unlikely to be available for the foreseeable future, and the biologically plausible assumption that excess cancer mortality is approximately proportional to dust level should be provisionally accepted. It may be reasonable, however, to postulate a safe threshold for mortality from asbestosis. If excess mortality from asbestos-related disease is proportional to dust level for each cause, approximately 10% of male asbestos workers might, under certain assumptions, eventually die of asbestos-induced disease after 50 years' exposure at 2 fibres/cm3. Peritoneal mesothelioma is usually due to crocidolite (blue asbestos) or other amphiboles, but exposure to chrysotile (white asbestos) alone may lead to a substantial risk of pleural mesothelioma. These predictions are based on rather small numbers in a single factory, and further studies in other working environments are required. Fibre counts based on optical microscopy are likely to be less relevant than total counts by electron microscopy, and excess mortality is virtually confined to men first exposed more than 20 years ago, when little or no accurate data on dust levels were collected.

In plain English

This article, based on a report to the Advisory Committee on Asbestos, examined the 2 fibres/cm3 hygiene standard for chrysotile asbestos. The author argued that earlier analyses, including the one behind the standard, may have underestimated low-dose risk. The author analysed deaths among 679 men with at least 10 years' work in an asbestos textile factory, mostly exposed to chrysotile with some crocidolite. More than 25 years after first exposure, 80 deaths occurred against 61.44 expected. These included 14 bronchial carcinoma deaths (6.57 expected), 17 non-malignant respiratory deaths (9.17 expected; 5 attributed to asbestosis) and 4 pleural mesotheliomas. Assuming excess deaths from each cause rise in proportion to dust level (an assumption the author called dubious for asbestosis), and ignoring sampling error, the author estimated about 5 to 10% of men exposed 50 years at 1 to 2 fibres/cm3 might die of asbestos-induced disease. The author warned the predictions might be grossly inaccurate. None of the 34 mesotheliomas among the factory's workers was peritoneal. The author's suggested explanation, which needed confirming studies, was that chrysotile causes only pleural mesothelioma.

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: Risk analysis and modelling of mortality in a previously described cohort of asbestos textile workers (deaths compared with expected numbers from national rates), with a critique of earlier dose-response studies; based on a report to the Advisory Committee on Asbestos
• Who was studied: 679 men in the asbestos textile factory where the British Occupational Hygiene Society study was done, who entered scheduled areas after 1 January 1933, had worked at least 10 years by 31 December 1972, and were followed up to the end of 1974
• Exposure: Mainly chrysotile asbestos dust in a textile factory, with a small proportion of crocidolite at various times after 1933. Estimated average dust levels were about 13 fibres/cm3 up to 1950, falling to about 5 fibres/cm3 by 1955.
• Main results:
  • Cohort, more than 25 years after first exposure: 80 deaths observed against 61.44 expected.
  • Cohort, beyond 25 years after first exposure: bronchial carcinoma (lung cancers other than pleural mesothelioma) deaths were about twice the national average, 14 observed against 6.57 expected.
  • Cohort, over 25 years after first exposure: 17 deaths from non-malignant respiratory disease against 9.17 expected, including 5 attributed to asbestosis.
  • Cohort, beyond 25 years after first exposure: excess deaths from respiratory disease (including bronchial carcinoma and mesothelioma) exceeded the expected deaths from all causes by 31% (19.26/61.44).
  • Only 5 cases of pleural mesothelioma occurred in the cohort.
  • From a model fitted to these 5 cases, the author estimated a lifelong attributable risk of pleural mesothelioma of about 2% for a man exposed to 1 fibre/cm3 from age 15 to 65.
  • Under any linear model, the author estimated that about 20 years' exposure at about 1 fibre/cm3 is likely to cause bronchial carcinoma in roughly 1 man in 100.
  • Assuming excess deaths from asbestos-related disease are proportional to dust level for each cause (an assumption the author called dubious for asbestosis), and ignoring sampling error, the author estimated that about 5 to 10% of men exposed for 50 years to chrysotile at 1 to 2 fibres/cm3 are likely to die of asbestos-induced disease.
  • Whole factory workforce, preliminary results: only 1 case of pleural mesothelioma was known among approximately 15 000 men exposed for less than 2 years since 1933.
  • None of the 34 mesotheliomas among workers at this factory (including women and pre-1933 employees) was peritoneal. The author offered, as an obvious explanation, that chrysotile causes only pleural mesothelioma, while crocidolite causes both pleural and peritoneal tumours.
• The authors' conclusion: The author concluded that previous studies, including the analysis on which the 2 fibres/cm3 standard is based, may have underestimated the risk of illness or death after exposure to low levels of asbestos dust.
• Limitations the authors state:
  • The author stated that the predictions might be grossly inaccurate.
  • The author considered the assumption that risk rises in proportion to dust level dubious for asbestosis deaths, though likely for bronchial carcinoma and pleural mesothelioma.
  • Earlier work in other dusty environments may have contributed to the excess deaths in this cohort.
  • The estimates of exposure to asbestos before the war were not reliable.
  • The mesothelioma estimate rests on only 5 cases. Two of these men worked where raw asbestos was handled and perhaps should be excluded, which would almost halve the predicted risk.
  • Individual exposure histories had not yet been compiled for the whole group.
  • The author described the interpretation of how chrysotile and amphiboles are distributed in the body as speculation that needs electron-microscope studies to confirm or refute.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis.

Medical terms in this study, explained in our Lexicon: Chrysotile, Dose-response relationship, Foreseeability, Peritoneal Mesothelioma, Crocidolite, Amphibole Group, Pleural Mesothelioma.

Related studies

• Hygiene standards for asbestos — S.A. Roach (1970)
• Industrial hygiene and occupational health studies in Australian (New South Wales) shipyards — A. Bell (1976)
• Industrial hygiene perspectives in control of asbestos exposure — W. Reitze (1979)
• Mortality in the chrysotile asbestos mines and mills of Quebec — J.C. McDonald, A.D. McDonald, et al. (1971)
• Technological feasibility of the 2 fibers/cc asbestos standard in asbestos textile facilities — R.A. Curtis, P.H. Bierbaum (1975)
• Implementation of the federal asbestos standard in Connecticut — J.B. Schoenberg, C.A. Mitchell (1974)

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