Module 4: Asbestos And Disease - Note (Continued)

DISEASE IMPACT IN SOUTH AFRICA:

Asbestosis and pleural disease:

There have been a limited number of cross-sectional radiological surveys of workers in the asbestos production industry. For survey purposes, asbestosis is usually defined as a chest x-ray showing 1/0 or more on the ILO Classification (which would include borderline cases). A study of amphibole miners in 1979 found prevalences varying from about 4 percent in those exposed under one year to 23 - 29 percent in those employed over 15 years. Recent surveys of retrenched workers from a chrysotile mine in Mpumalanga (the only modern survey of a chrysotile workers in South Africa) found prevalences of as high as 38 percent. Figures of 10 to 20 percent have been quoted in manufacturing industry, while a study of stevedores handling asbestos cargoes at the harbour found a prevalence of 30 percent. The prevalence of pleural disease is typically higher than that of asbestosis in these studies.

In a survey of environmentally exposed residents in the Limpopo province, a prevalence of asbestosis of 3 percent and of pleural disease of 34 percent was found.

Mesothelioma:

Zwi et al. in a 1988 study found about 1 400 mesotheliomas for the period 1976 - 1984, with an apparent upward trend. Incidence differed by gender and racial group, with incidence highest among white males and Coloured men and women. By 1992, the National Cancer Registry was recording high incidence rates among white men and women, but much lower rates among other groups. This disparity existed despite the fact that African workers made up the vast majority of those employed in the mining industry. This suggests widespread underdiagnosis of mesothelioma. It was estimated that the total number of biopsy proven mesothelioma cases in South Africa in the early 1990s was of the order of 400 per year (age standardised incidence rate: 17 per million). This is a minimum estimate, and the true numbers are certainly much greater.

However, the best measure of risk is from long-term cohort studies. In a study of white amphibole workers by Sluis-Cremer et al., 4.7 percent of the cohort developed mesothelioma. A population study of death certificates by Kielkowski et al. from Prieska in the Northern Cape that found that 6.6 percent of all deaths were from mesothelioma. These are extraordinarily high risks for a rare tumour.

Given that crocidolite production peaked in 1977 and allowing a mean 30 year latency in the appearance of mesothelioma, it is likely that the peak of the mesothelioma epidemic is still to come.

Lung cancer:

The burden of asbestos related lung cancers is difficult to measure as there is a significant background rate of lung cancer in the general population, attributable mainly to tobacco. It has been suggested that the ratio of asbestos related lung cancers to that of mesothelioma may of the order of 1.5 to 3.5, but this has never been confirmed in South Africa.

In the cohort study of white amphibole miners cited above, 6 percent of the deaths were due to respiratory tract cancer, of which about half could be regarded as "excess", that is, potentially attributable to asbestos exposure. However, extrapolation to other groups of workers would have to take into account different background risks of lung cancer due to different smoking rates and possibly exposure to air pollution exposures. Lung cancers due to asbestos thus remain largely hidden from view.