The mortality of amphibole miners in South Africa, 1946-80

Our plain-English summary of The mortality of amphibole miners in South Africa, 1946-80, by G.K. Sluis-Cremer, F.D. Liddell, W.P. Logan, et al (British Journal of Industrial Medicine, 1992).

The study

• Authors: G.K. Sluis-Cremer, F.D. Liddell, W.P. Logan, B.N. Bezuidenhout
• Journal: British Journal of Industrial Medicine
• Published: 1992;49(8):566-75
• Cited by: 80 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 004933).

Abstract

A cohort was established in 1981 of all 7317 white male employees in the amosite and crocidolite mines in South Africa whose names had appeared in the personnel records (initiated between 1945 and 1955) of the major companies. Some of the men had been employed as early as 1925, but only 8% had had more than 10 years of service. Three subcohorts were defined: 3212 men whose only exposure to asbestos was to amosite; 3430 exposed to crocidolite; and 675 to both amphiboles. No deaths or losses to view occurred before 1946, and 5925 men (81%) were known to be alive at the end of 1980. Losses to view numbered 167 (2%), and there had been 1225 deaths (17%), an excess of 331 over the number of deaths expected on the basis of the mortality of all white South African males. The fibre related excesses were of mesothelioma, lung cancer, and other respiratory diseases, but there were other excesses perhaps mainly related to socioeconomic factors including lifestyle. When cause of death was determined according to "best evidence" (after study of clinical, radiological, biopsy, and necropsy reports in conjunction with the death certificate), there were 30 deaths due to mesothelioma (22 pleural, six peritoneal, two other) and 65 due to cancer of trachea, bronchus, and lung. Various analyses of these deaths showed that crocidolite had higher toxicity than amosite for lung cancer and this was most pronounced for mesothelioma; there can now be no question that crocidolite is far more dangerous than amosite at least in so far as mesothelioma is concerned. Nevertheless, crocidolite induced mesothelioma appeared only in men who had been exposed for long periods, at least 12 months, but on average about 15 years.

In plain English

This cohort study followed deaths from 1946 to 1980 among 7317 white men who had worked at South African amosite and crocidolite mines. Of these, 3212 were exposed only to amosite, 3430 to crocidolite, and the rest to mixtures. In all, 1225 had died, 331 more than expected from national rates for white South African men. Medical record review attributed 30 deaths to mesothelioma: four among amosite miners, 20 among crocidolite miners and six with mixed exposure. Using death certificates, the ratio of observed to expected lung cancer deaths was 1.38 (90% confidence interval 0.97 to 1.91) for amosite miners, a range consistent with no excess, and 2.03 (1.43 to 2.80) for crocidolite miners. No mesothelioma occurred in men with less than 12 months of crocidolite exposure. Other causes of death were also in excess, which the authors linked partly to lifestyle. Black and mixed-race miners were excluded because of poor records, and the authors considered early dust measurements suspect. The authors concluded that crocidolite is far more dangerous than amosite, at least for 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: Cohort mortality study using the subject-years method against national death rates, with case-referent analyses within the cohort for mesothelioma and lung cancer; follow-up from January 1946 to the end of December 1980
• Who was studied: 7317 white male employees named in the personnel records of South African amosite and crocidolite mines, some first exposed as early as 1925
• Exposure: Mining and milling of amosite (3212 men exposed only to amosite) and crocidolite (3430 men). Average cumulative exposure was 15.2 (fibres/ml) x years in the amosite subcohort and 9.6 (fibres/ml) x years in the crocidolite subcohort. 675 men had mixed asbestos exposures.
• Main results:
  • There were 1225 deaths (17%) in the whole cohort, 331 more than expected from the mortality of all white South African males
  • Death certificates recorded 16 mesotheliomas and five deaths from asbestosis. On best available evidence (medical record review) the numbers were 30 and seven.
  • Of the 30 mesotheliomas, four were in the amosite subcohort, 20 in the crocidolite subcohort and six in the mixed-exposure subcohort
  • The authors estimated that crocidolite had a toxicity for mesothelioma about an order of magnitude (roughly ten times) higher than amosite
  • Lung cancer deaths were counted by cause on the death certificate. The observed-to-expected ratio was 1.38 (90% confidence interval 0.97 to 1.91; 26 deaths) in the amosite subcohort and 2.03 (90% confidence interval 1.43 to 2.80; 27 deaths) in the crocidolite subcohort.
  • The authors judged the lung cancer SMR for crocidolite miners substantially higher than for amosite miners, even though crocidolite miners had lower average cumulative exposure. They gave a rough relative toxicity for lung cancer of 1.85 for crocidolite compared with amosite.
  • No mesothelioma occurred among men with less than 12 months of crocidolite exposure, although such short durations were not uncommon among the comparison men (referents)
  • No excesses were found for cancer of the oesophagus, stomach, colon or rectum
  • The authors found no evidence that additional exposure to silica and radon in other mines affected lung cancer mortality
  • Amosite miners had excesses of ischaemic heart disease and suicide, which the authors suggested reflected a more reckless lifestyle
• The authors' conclusion: The authors concluded that crocidolite is far more dangerous than amosite, at least as far as mesothelioma is concerned
• Limitations the authors state:
  • The study was restricted to white employees because records for black and mixed-race men were poor; some of the excluded men had very high exposures
  • Fibre concentration data, especially before 1970, were considered suspect
  • Most of the cohort was still young at the end of follow-up
  • National reference death rates were not available for mesothelioma or asbestosis
  • The subject-years analyses had a potential weakness: an unexplained excess of deaths from 'other' causes
  • The relative toxicity estimates are imprecise

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite, Crocidolite.

Medical terms in this study, explained in our Lexicon: Amphibole Group, Amosite, Crocidolite, Mesothelioma, Lung Cancer, Biopsy, Autopsy, Death Certificate.

Related studies

• Factors influencing the reading of small irregular opacities in a radiological survey of asbestos miners in South Africa — G.K. Sluis-Cremer, P.A. Hessel, et al. (1989)
• 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)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Health effects of amosite mining and milling in South Africa — J. Murray, G. Nelson (2008)
• Risk of asbestosis in crocidolite and amosite mines in South Africa: an erratum — L.M. Irwig, R.S. du Toit, et al. (1984)

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