Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa

Our plain-English summary of Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa, by J.L. Botha, L.M. Irwig, P.M. Strebel (American Journal of Epidemiology, 1986).

The study

• Authors: J.L. Botha, L.M. Irwig, P.M. Strebel
• Journal: American Journal of Epidemiology
• Published: 1986;123(1):30-40
• Cited by: 50 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. 000378).

Abstract

To determine whether exposure to crocidolite is associated with excess mortality, the authors calculated standardized mortality ratios based on deaths in South African crocidolite mining districts from 1968 to 1980 for selected causes of death. Contiguous districts were used as controls. To take account of background geographic variability, they divided the control districts into groups of population size similar to those of the crocidolite mining districts. Standardized mortality ratios in crocidolite mining districts were elevated for asbestosis and/or mesothelioma, and cancer of the lung and stomach. These findings could not be explained by background geographic variability in mortality and are likely to be due to exposure to South African crocidolite during mining and milling or to environmental contamination. The increased standardized mortality ratios for stomach cancer are of particular interest, since excess deaths have not previously been shown in individuals exposed to crocidolite alone.

In plain English

This geographic study compared 1968-1980 deaths among white and colored residents of South African crocidolite mining districts and surrounding control districts. Three districts (Kuruman, Hay and Prieska), where the average annual share of white or colored men aged 20-64 employed in crocidolite mines in 1921-1960 exceeded 2 per cent, formed the high crocidolite group. In these districts, standardized mortality ratios for asbestosis and/or mesothelioma were 7.86 to 10.30. Lung cancer ratios were significantly raised for white males (1.75) and colored females (2.47). Stomach cancer ratios were significantly raised for white males (1.67), colored males (2.18) and colored females (2.20). Women in these districts had raised asbestosis and/or mesothelioma risk although not employed in mines until 1950. That, together with deaths at early ages, led the authors to suggest much of the impact may have been environmental. The authors concluded that crocidolite exposure through mining, milling and/or environmental contamination results in increased mortality from asbestosis and/or mesothelioma and lung and stomach cancer. No smoking or district migration data were available, though census data indicated net out-migration, especially of whites.

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: Geographic (district-level) mortality study using death records and indirectly age-standardized mortality ratios. Crocidolite mining districts were compared with contiguous control districts, and the controls were grouped to measure background geographic variability.
• Who was studied: White and colored (mixed racial origin) residents of the northwestern Cape Province, South Africa, with deaths recorded from 1968 to 1980, mostly at ages 35-74. Blacks were excluded for lack of data. The high crocidolite group was three districts (Kuruman, Hay, Prieska), with 1,986 white males, 1,921 white females, 2,284 colored males and 2,398 colored females. The study also included two low crocidolite districts (Postmasburg, Vryburg) and 29 control districts.
• Exposure: Residence in districts where crocidolite was mined. Districts were classed by the average annual percentage of the male population aged 20-64 employed in crocidolite mines during 1921-1960. The rate was more than 2 per cent of adult white or colored males in the high crocidolite districts and 0.1 to 0.7 per cent in the low crocidolite districts. Exposure could be occupational (mining and milling) or environmental. Individual exposure was not measured.
• Main results:
  • Standardized mortality ratios for asbestosis and/or mesothelioma in the high crocidolite districts were 7.86 (white males), 10.30 (white females), 8.43 (colored males) and 8.72 (colored females), based on 14, 13, 17 and 17 observed deaths.
  • High crocidolite districts contained less than 1 per cent of the South African population. They accounted for 7 per cent (14/212) of South Africa's asbestosis and/or mesothelioma deaths in white males, 35 per cent (17/48) in colored males, 20 per cent (13/65) in white females and 49 per cent (17/35) in colored females.
  • For asbestosis and/or mesothelioma, the relative risks of the high crocidolite districts compared with the low crocidolite and control districts combined were 16.7 for white males, 29.6 for white females, 21.5 for colored males and 26.4 for colored females.
  • At ages 15-34, seven asbestosis and/or mesothelioma deaths occurred in the high crocidolite districts (an annual rate of 100 per million). There was one death in the low crocidolite districts (15 per million) and two in the control districts (2 per million).
  • From 1978 to 1980, when the two diseases were coded separately, there were 31 mesothelioma deaths and one asbestosis death in all districts combined. The authors took this to suggest that most of these deaths were due to mesothelioma.
  • Lung cancer standardized mortality ratios were elevated for all race-sex groups in the high crocidolite districts. The elevation was significant for white males (1.75) and colored females (2.47).
  • Stomach cancer standardized mortality ratios were elevated for all race-sex groups in the high crocidolite districts. The elevation was significant for white males (1.67), colored males (2.18) and colored females (2.20). The ratio for white females was 1.55.
  • There was no evidence of excess deaths from cancer of the esophagus, colon, rectum or other digestive organs or peritoneum in the crocidolite districts.
  • Standardized mortality ratios for all neoplasms in the high crocidolite districts were significantly elevated for white males (1.38), colored males (1.35) and colored females (1.41).
  • Tuberculosis standardized mortality ratios were significantly elevated for colored males and colored females, but these districts ranked only second and fourth among the groups of districts. The authors suggested the tuberculosis effect may be associated with mining in general rather than asbestos specifically.
  • Cancer of the lip, oral cavity and pharynx was significantly elevated only in colored males (SMR 2.72). The authors considered this association tenuous.
  • Standardized mortality ratios for all deaths in the high crocidolite districts were not markedly elevated: white males 1.02, white females 1.02, colored males 1.08 (p less than 0.05), colored females 0.98.
  • Females had raised asbestosis and/or mesothelioma risk although they were not employed in mines until 1950. Together with deaths at early ages, this led the authors to suggest that much of the impact may have come from environmental rather than occupational exposure.
• The authors' conclusion: The authors concluded that exposure to crocidolite through mining and milling and/or environmental contamination results in increased mortality from asbestosis and/or mesothelioma and from cancer of the lung and stomach. They added that stomach cancer mortality, already high in South African coloreds, was more than doubled.
• Limitations the authors state:
  • The authors had no migration data for the districts, although census data gave evidence of net out-migration from the crocidolite districts, especially of whites.
  • Only a small proportion of residents worked in the mines. Occupationally and environmentally exposed people could not be analyzed separately, so occupational effects would be diluted.
  • Because of migration and dilution, the authors stated that a lack of significant associations cannot be read as absence of any effect.
  • No smoking data were available. The authors argued the lung cancer excess was unlikely to be purely due to smoking, because other smoking-related respiratory diseases were not increased.
  • Asbestosis and mesothelioma were not recorded as separate causes of death until 1978.
  • Because the reference population included the high crocidolite districts, standardized mortality ratios may underestimate the relative risk, especially for asbestosis and/or mesothelioma.
  • Death certification may be less reliable for coloreds than for whites: 1.3 per cent of white deaths and 3.9 per cent of colored deaths were classified as ill-defined conditions.
  • Blacks could not be studied because the necessary information was unavailable.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Lung cancer, Mesothelioma, Stomach cancer.

Medical terms in this study, explained in our Lexicon: Stomach Cancer, Lung Cancer, Mesothelioma, Crocidolite, standardized mortality ratio (SMR).

Related studies

• Re: "Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa" — H.P. Hart (1987)
• Risk of asbestosis in crocidolite and amosite mines in South Africa — L.M. Irwig, R.S. du Toit, et al. (1979)
• Health effects of amosite mining and milling in South Africa — J. Murray, G. Nelson (2008)
• Risk of mesothelioma from exposure to crocidolite asbestos: a 1995 update of a South African mortality study — D. Kielkowski, G. Nelson, et al. (2000)
• Risk of asbestosis in crocidolite and amosite mines in South Africa: an erratum — L.M. Irwig, R.S. du Toit, et al. (1984)
• Asbestos-related disease in South Africa: the social production of an invisible epidemic — L. Braun, S. Kisting (2006)

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