Risk of mesothelioma from exposure to crocidolite asbestos: a 1995 update of a South African mortality study

Our plain-English summary of Risk of mesothelioma from exposure to crocidolite asbestos: a 1995 update of a South African mortality study, by D. Kielkowski, G. Nelson, D. Rees (Occupational and Environmental Medicine, 2000).

The study

• Authors: D. Kielkowski, G. Nelson, D. Rees
• Journal: Occupational and Environmental Medicine
• Published: 2000;57(8):563-7
• Cited by: 25 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objective: To find the risk of developing mesothelioma in a cohort born in 1916-36 in Prieska, Northern Cape Province, South Africa.

Methods: A birth cohort mortality study was carried out in a small town in the Northern Cape Province, South Africa, with a history of crocidolite asbestos mining and milling. The cohort comprised all white births registered in the magisterial district of Prieska from 1916 to 1936, inclusive (2390). Causes of death due to mesothelioma and other cancers as recorded on medical certificates of cause of death were investigated. Person-years analysis was used to calculate mortalities due to mesothelioma, other respiratory cancers, and other non-respiratory cancers. Proportional cancer mortalities were also calculated for mesothelioma and other specific neoplasms.

Results: The follow up rate for the cohort was 74.3% in 1995, and 683 traced members (38.6%) had died. Cause of death was unknown for 6.4% of deaths. There were 118 cases of cancer, 28 of them from mesothelioma, giving a cause specific mortality for mesothelioma of 277 (170-384) per 10(6) person-years. The rates for men and women were 366 and 172 per 10(6) person-years, respectively. The mortality for lung cancer (29 deaths) was 287 (135-436) per 10(6) person-years, and that for other non-respiratory cancers (60 deaths) was 593 (442-745). Two cases of laryngeal and four of colon cancer were observed. All cancer mortality, mesothelioma, and lung cancer proportional cancer mortality ratios were increased.

Conclusion: The mortality for mesothelioma in men was twice that in women, probably because men were more likely to have had both occupational and environmental exposure to asbestos. Nevertheless, the mortality in women was still high and is probably indicative of the environmental exposure as white women were rarely employed in the asbestos industry in the Prieska area. Due to the long latency from first exposure to diagnosis of the neoplasm, the cause specific mortality in this cohort could be expected to increase rapidly over the next 10 years.

In plain English

This birth cohort mortality study followed all 2390 white people whose births were registered in 1916–36 in the Prieska district of South Africa, a town near crocidolite asbestos mines with a crushing mill in town. By the end of 1995, 1776 (74.3%) had been traced and 683 had died; causes came from death certificates. There were 118 cancer deaths, including 28 from mesothelioma (one peritoneal, the rest pleural) and 29 from lung cancer. Among members over 30, the mesothelioma death rate was 277 per million person-years (95% CI 170 to 384): 366 in men and 172 in women. Compared with the white South African population, mesothelioma made up a far larger share of cancer deaths than expected (proportional cancer mortality ratio 31.1, 95% CI 20.7 to 43.6). Lung cancer ratios were raised but not statistically significant. The authors concluded that men's higher rate probably reflected work exposure in some, and that women's still-high rate was probably due to environmental exposure. They noted only white people could be studied, 614 had unknown vital status, and death certificates may misclassify causes.

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: Birth cohort mortality study using person-years analysis and proportional cancer mortality ratios (PCMRs), with causes of death taken from death certificates
• Who was studied: 2390 white people whose births were registered in the Prieska magisterial district, Northern Cape Province, South Africa, in 1916–36, followed to the end of 1995. Of these, 1776 (74.3%) were traced and 683 of the traced members had died. People who were not white had to be excluded because of poor official records and low tracing.
• Exposure: Crocidolite asbestos from nearby mining. A crushing mill in Prieska town operated from 1930, close to homes and businesses, until at least 1964. Other sources were dumps, spillage during transport, and tailings used for roads and bricks. Individual exposure was not measured. Some men may also have had occupational exposure.
• Main results:
  • Among the traced white cohort members there were 118 cancer deaths. These included 28 mesothelioma deaths (one peritoneal, the rest pleural) and 29 lung cancer deaths.
  • For cohort members over 30 (1975–95), the mesothelioma mortality was 277 per million person-years (95% CI 170 to 384). The rate was 366 per million in men and 172 per million in women.
  • Lung cancer mortality for cohort members over 30 was 287 per million person-years overall (95% CI 135 to 436). It was 403 in men (22 cases) and 151 in women (7 cases).
  • The comparison was with the white South African population. Proportional cancer mortality ratios for mesothelioma were significantly increased: 31.1 overall (95% CI 20.7 to 43.6), 18.2 in men (95% CI 11.1 to 27.7) and 26.7 in women (95% CI 11.5 to 52.5).
  • Proportional cancer mortality ratios for lung cancer were increased (1.2 overall, 95% CI 0.8 to 1.8), but the increases were not statistically significant.
  • There were fewer digestive system cancers than expected, and this deficit was significant (PCMR 0.5, 95% CI 0.3 to 0.8). The deficit for colon cancer (PCMR 0.5, 95% CI 0.1 to 1.4) was not significant. The authors suggested this could be a see-saw effect of the raised mesothelioma ratio.
  • For cohort members over 30, the all-cancer mortality was 1557 per million person-years in men and 689 per million in women.
• The authors' conclusion: The authors concluded that the higher mesothelioma rates in men were probably accounted for by occupational exposure experienced by some of them. They concluded that the still-high rate in women was probably a consequence of environmental exposure, since white women were rarely employed in the asbestos industry in Prieska.
• Limitations the authors state:
  • The cohort was restricted to white subjects. How completely asbestos-related disease was identified is uncertain, in both the study subjects and the reference population. The authors say there could be under-ascertainment (missed diagnoses) or over-ascertainment (doctors influenced by known crocidolite exposure).
  • Vital status was unknown for 614 of the 2390 cohort members.
  • Some causes of death were probably recorded wrongly on death certificates. For example, some gastrointestinal cancers may have been peritoneal mesotheliomas, or some pleural mesotheliomas may have been lung adenocarcinomas. The size of this bias cannot be estimated.
  • It was not yet known what proportion of men in the cohort died from work with asbestos, because a survey of members and relatives was still under way.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, Crocidolite, Prieska, Northern Cape Province, Neoplasm, Lung Cancer, Colorectal Cancer, Environmental Exposure.

Related studies

• 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)
• South African experience with asbestos related environmental mesothelioma: is asbestos fiber type important? — N. White, G. Nelson, et al. (2008)
• Are children more vulnerable to mesothelioma than adults? A comparison of mesothelioma risk among children and adults exposed non-occupationally to blue asbestos at Wittenoom — A. Reid, P. Franklin, et al. (2018)
• Effects of commodity on the risk of emphysema in South African miners — S.L. Mabila, K.S. Almberg, et al. (2020)
• Evidence for an amphibole asbestos threshold exposure for asbestosis assessed by autopsy in South African asbestos miners — G.K. Sluis-Cremer, E. Hnizdo, et al. (1990)
• Lung cancer, tobacco, and environmental factors in the African population of the Northern Province, South Africa — O. Mzileni, F. Sitas, et al. (1999)

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