Case-control study of mesothelioma in South Africa

Our plain-English summary of Case-control study of mesothelioma in South Africa, by D. Rees, J.E. Myers, K. Goodman, et al (American Journal of Industrial Medicine, 1999).

The study

• Authors: D. Rees, J.E. Myers, K. Goodman, E. Fourie, C. Blignaut, R. Chapman, M.O. Bachmann
• Journal: American Journal of Industrial Medicine
• Published: 1999;35(3):213-22
• Cited by: 28 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: South Africa has, uniquely, mined, transported, and used crocidolite, amosite, and chrysotile. A multicenter case-control study was done in South Africa to examine the details of asbestos exposure in cases and controls, and to calculate relative risks for level of certainty of asbestos exposure, nature of exposure (e.g., environmental, occupational) and fiber type.

Methods: Cases and controls (one cancer and one medical per case) were collected by six study centers from referral hospitals, and exposure information was collected by interviewing cases and controls in life.

Results: One hundred and twenty-three cases were accepted into the study. None had purely chrysotile exposure. Twenty-three cases had mined Cape crocidolite; three had mined amosite; and three Transvaal crocidolite plus amosite. A minimum of 22 of the cases had exclusively environmental exposure, 20 were from the NW Cape crocidolite mining area. The relative risks associated with environmental exposure in the NW Cape (crocidolite) were larger than for environmental exposure in the NE Transvaal (amosite and crocidolite): 21.9 vs. 7.1 and 50.9 vs. 12.0 for the cancer control and medical control datasets, respectively.

Conclusions: The results confirm the importance of environmental exposure in the Cape crocidolite mining area, the relative paucity of cases linked to amosite, the rarity of chrysotile cases and are consistent with a fiber gradient in mesotheliomagenic potential for South African asbestos with crocidolite > amosite > chrysotile.

In plain English

This case-control study, run at referral hospitals in six South African study centers from late 1988 through early 1990, compared 123 people with mesothelioma (115 pleural, 7 peritoneal, 1 pericardial) with 119 cancer controls and 103 medical controls. All were interviewed in life about asbestos exposure. No case had purely chrysotile exposure. Twenty-three cases had mined Cape crocidolite, three amosite, and three Transvaal crocidolite plus amosite. Twenty-two cases (17.9%) had only environmental exposure in a mining region, 20 of them in the NW Cape crocidolite area. Compared with unlikely exposure, environmental exposure in the NW Cape gave odds ratios of 21.9 (95% CI 4.7 to 102.4) against cancer controls and 50.9 (7.2 to 360) against medical controls. For the NE Transvaal (amosite and/or Transvaal crocidolite) the figures were 7.1 (0.2 to 171) and 12.0 (1.2 to 117). The authors concluded the results were consistent with a gradient in mesothelioma-causing potential for South African asbestos of crocidolite, then amosite, then chrysotile, and said findings may need cautious generalization elsewhere. They noted the cases may not represent all South African cases: fifty-five percent were white.

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: Multicenter hospital-based case-control study, with exposure collected by in-life interview using a standard questionnaire, plus sputum examination for coated fibers
• Who was studied: 123 histologically confirmed mesothelioma cases from referral hospitals in six South African study centers (Johannesburg, Pretoria, Kimberley, Bloemfontein, Cape Town, Port Elizabeth), late 1988 to early 1990. By site, 115 were pleural, 7 peritoneal and 1 pericardial (Johannesburg). The cases were 102 men and 21 women, with a mean age of 55.4. Controls were 119 cancer controls and 103 medical controls, matched on hospital, skin color, gender and approximate age.
• Exposure: Asbestos exposure by certainty class (definite, probable, possible, unlikely); by nature (occupational, environmental, domestic, incidental); and by fiber type or district: NW Cape (Cape crocidolite), NE Transvaal (amosite and/or Transvaal crocidolite), E Transvaal (chrysotile). Also specific jobs: Cape crocidolite mining, insulation work, asbestos cement work.
• Main results:
  • Definite asbestos exposure was found in 94 cases (75%), compared with 24 cancer controls (20%) and 20 medical controls (19%).
  • No case had purely chrysotile exposure. No case had a history of chrysotile mining, and no case had exclusively environmental exposure to chrysotile.
  • Among the cases, 23 had mined Cape crocidolite, 3 had mined amosite, and 3 had mined Transvaal crocidolite plus amosite.
  • Twenty-two cases (17.9%) had exclusively environmental exposure in one of the three main mining regions, 20 of them in the NW Cape.
  • Environmental exposure in the NW Cape, compared with unlikely exposure: odds ratio 32.7 (95% CI 8.1 to 131) with both control groups combined, 21.9 (4.7 to 102.4) against cancer controls, and 50.9 (7.2 to 360) against medical controls.
  • Environmental exposure in the NE Transvaal, compared with unlikely exposure: odds ratio 12.7 (95% CI 1.9 to 84.7) with combined controls, 7.1 (0.2 to 171) against cancer controls, and 12.0 (1.2 to 117) against medical controls.
  • Definite exposure, compared with unlikely exposure: odds ratio 58.7 (95% CI 14.0 to 246) with combined controls, 40.8 (9.2 to 109) against cancer controls, and 104.5 (10.6 to 1026) against medical controls.
  • Work with asbestos cement, compared with unlikely exposure: odds ratio 27.7 (95% CI 4.9 to 154) with combined controls, 13.8 (2.3 to 83) against cancer controls, and 43.6 (3.9 to 486) against medical controls.
  • Working in a 'Risk' occupation without recall of exposure was significantly associated with mesothelioma only in the combined-control dataset.
  • Of the 20 subjects with coated fibers in sputum, 19 were classed as definitely or probably exposed. No subject in the unlikely exposure class had coated fibers.
• The authors' conclusion: The authors concluded that the study confirms the importance of environmental exposure in the NW Cape crocidolite mining area, the relative paucity of cases linked to amosite and the rarity of chrysotile cases. They concluded the results are consistent with a fiber gradient in mesothelioma-causing potential for South African asbestos: crocidolite greater than amosite greater than chrysotile. They cautioned that South African findings may need to be generalized to other settings with caution.
• Limitations the authors state:
  • Cases may not represent all South African cases. Fifty-five percent were white, although this group makes up only about 20% of the population.
  • The case series may underrepresent cases with environmental exposure to Cape crocidolite and cases with harbor-related exposure; no study team was successfully established in Durban.
  • Some suspected cases were diagnosed clinically without biopsy confirmation and were therefore not included.
  • Similar odds ratios for the possible and probable classes suggest these two exposure classes were not well distinguished.
  • The fiber type in insulation work and asbestos cement work could not be determined with confidence from the questionnaire.

Topics

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

Medical terms in this study, explained in our Lexicon: Case-Control Study, Mesothelioma, Crocidolite, Amosite, Chrysotile, Asbestos Exposure, Relative Risk, Transvaal.

Related studies

• Lung cancer, tobacco, and environmental factors in the African population of the Northern Province, South Africa — O. Mzileni, F. Sitas, et al. (1999)
• Pleural mesothelioma and occupational and non-occupational asbestos exposure: a case-control study with quantitative risk assessment — D. Ferrante, D. Mirabelli, et al. (2016)
• Asbestos and mesothelioma in South Africa — R.P. Abratt, D.A. Vorobiof, et al. (2004)
• Compensation for environmental asbestos-related diseases in South Africa: a neglected issue — N. Ndlovu, J.T. Naude, et al. (2013)
• Health effects of amosite mining and milling in South Africa — J. Murray, G. Nelson (2008)
• South African experience with asbestos related environmental mesothelioma: is asbestos fiber type important? — N. White, G. Nelson, et al. (2008)

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