Risk of asbestosis in crocidolite and amosite mines in South Africa

Our plain-English summary of Risk of asbestosis in crocidolite and amosite mines in South Africa, by L.M. Irwig, R.S. du Toit, G.K. Sluis-Cremer, et al (Annals of the New York Academy of Sciences, 1979).

The study

• Authors: L.M. Irwig, R.S. du Toit, G.K. Sluis-Cremer, A. Solomon, R.G. Thomas, P.P. Hamel, I. Webster, T. Hastie
• Journal: Annals of the New York Academy of Sciences
• Published: 1979;330:35-52
• Cited by: 22 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

X-rays of all while and mixed-race men employed in crocidolite and amosite mines and mills were read independently by three experienced readers according to the ILO U/C classification. Abnormality was regarded as present if reported by two or more readers. Parenchymal abnormality, defined as the presence of small irregular opacities of profusion 1/0 or greater, was found in 7.3% of the workers. Pleural thickening was found in 4.5% of the workers, costophrenic angle obliteration in 3.2%, and pleural calcification in 1.7%. The prevalences of both pleural and parenchymal abnormality were strongly related to the duration of exposure to asbestos at work. The overall prevalence of abnormality increase from 4.0% in men with exposure for 1 year or less to 47.9% in men with more than 15 years of exposure. After taking into account the effects of age and duration of asbestos exposure, the prevalence of pleural abnormality was not predicted by fiber concentration. However, white men working with amosite tended to develop a higher prevalence of pleural abnormality than did those working with crocidolite. Compared to whites, men of mixed race, who only work with crocidolite, had a high prevalence of pleural abnormality in each exposure duration category. In contrast to pleural abnormality, the prevalence of parenchymal abnormality, after taking into account the effects of age and duration of exposure, was significantly predicted by fiber concentration but not by race or asbestos type. Our results suggest that parenchymal abnormality in workers in South African asbestos mines could be largely prevented by reducing exposure to fibers visible under the light microscope. However, this may not be the case for pleural abnormality.

In plain English

The authors studied chest X-rays of white and mixed-race men working in South African crocidolite and amosite mines and mills between 1970 and 1975. Of 1801 eligible men, 1692 were analyzed: 882 white and 262 mixed-race crocidolite workers and 548 white amosite workers. Three readers classified each X-ray independently, and a finding counted if two agreed. Small irregular opacities (parenchymal abnormality) appeared in 123 men (7.3%) and pleural abnormality in 129 (7.6%). Any abnormality rose from 4.0% in men exposed 1 year or less to 47.9% in men exposed more than 15 years, and abnormality was also associated with age. After allowing for age and duration, average fiber concentration predicted parenchymal but not pleural abnormality. Among crocidolite workers, mixed-race men had more pleural abnormality than white men. White amosite workers exposed over 7 years had more than white crocidolite workers, significant only at the 10% level. The authors concluded that parenchymal abnormality could be largely prevented by reducing exposure to fibers visible under the light microscope, but perhaps not pleural abnormality. They had no unexposed comparison group.

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: Cross-sectional study of chest X-rays of a mine and mill workforce. Three readers worked independently using the ILO U/C classification, and an abnormality counted if at least two readers reported it.
• Who was studied: All white and mixed-race men employed at South African crocidolite and amosite mines and mills on November 30, 1970, or who entered employment between then and November 30, 1975. Men who had worked with more than one asbestos type were excluded. 1801 men qualified and 1692 (94%) were analyzed: 882 white and 262 mixed-race crocidolite workers, and 548 white amosite workers.
• Exposure: Work with crocidolite or with amosite. Exposure was measured as duration of asbestos exposure and as the average long-fiber and short-fiber concentration each worker experienced, estimated from fiber measurements in the mines and mills over time.
• Main results:
  • Among the 1692 workers, pleural thickening was found in 76 (4.5%), costophrenic angle obliteration in 54 (3.2%) and pleural calcification in 29 (1.7%).
  • Pleural abnormality (at least one of the three pleural findings) was found in 129 workers (7.6%). It rose from 2.5% in men exposed 1 year or less to 33.6% in men exposed more than 15 years.
  • Small irregular opacities of profusion 1/0 or greater (parenchymal abnormality) occurred in 123 men (7.3%). The prevalence rose from 2.3% in men exposed 1 year or less to 26.7% in men exposed more than 15 years.
  • Overall prevalence of pleural or parenchymal abnormality rose from 4.0% in men exposed 1 year or less to 47.9% in men exposed more than 15 years.
  • Abnormality was in general also significantly associated with age within each duration-of-exposure category.
  • Average short-fiber concentration did not predict the prevalence of pleural abnormality. A similar result was obtained for average long-fiber concentration.
  • In crocidolite mines and mills, mixed-race workers had a higher prevalence of pleural abnormality than white workers in each exposure-duration category.
  • Among white men exposed for more than 7 years, amosite workers had a higher prevalence of pleural abnormality than crocidolite workers. The difference was significant only at the 10% level.
  • Average short-fiber concentration did predict the prevalence of parenchymal abnormality. Long-fiber concentration had a similar but less significant effect.
  • Race and asbestos type did not predict the prevalence of parenchymal abnormality.
• The authors' conclusion: The authors concluded that parenchymal abnormality in South African asbestos mine workers could be largely prevented by reducing exposure to fibers visible under the light microscope, but that this may not be the case for pleural abnormality.
• Limitations the authors state:
  • There was no control group of people not exposed to asbestos. Workers exposed for 1 year or less were used as the basis for age adjustment instead.
  • Submicroscopic fibers, which the authors suggest might explain pleural abnormality, were not measured.
  • 229 men whose fiber concentrations were unknown were left out of the fiber-concentration analyses.

Topics

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

Medical terms in this study, explained in our Lexicon: Crocidolite, Amosite, Pleural Thickening, Costophrenic angle, Pleural Calcification, Asbestos Exposure.

Related studies

• Risk of asbestosis in crocidolite and amosite mines in South Africa: an erratum — L.M. Irwig, R.S. du Toit, et al. (1984)
• Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa — J.L. Botha, L.M. Irwig, et al. (1986)
• Re: "Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa" — H.P. Hart (1987)
• Thickening of pulmonary interlobar fissures: exposure-response relationship in crocidolite and amosite miners — A. Solomon, L.M. Irwig, et al. (1979)
• Health effects of amosite mining and milling in South Africa — J. Murray, G. Nelson (2008)
• Asbestosis in South Africa--certain geographical and environmental considerations — G.K. Sluis-Cremer (1965)

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