Silicosis prevalence and exposure-response relations in South African goldminers

Aims: To measure the prevalence of silicosis among black migrant contract workers on a South African goldmine and to investigate exposure-response relations with silica dust. Methods: In a cross sectional study, 520 black goldminers (aged >37 years) were interviewed and had chest radiographs taken. Silicosis was defined as International Labour Organisation Classification radiological profusion of 1/1 or greater. Results: Mean length of service was 21.8 years ( range 6.3 - 34.5). The mean intensity of respirable dust exposure was 0.37 mg/m(3) ( range 0 - 0.70) and of quartz 0.053 mg/m(3) ( range 0 - 0.095). The prevalence of silicosis was 18.3 - 19.9% depending on reader. Significant trends were found between the prevalence of silicosis and length of service, mean intensity of exposure, and cumulative exposure. Conclusion: Results confirm a large burden of silicosis among older black workers in the South African goldmining industry, which is likely to worsen as such miners spend longer periods in continuous employment in dusty jobs. An urgent need for improved dust control in the industry is indicated. If the assumption of stability of average dust concentrations on this mine over the working life of this group of workers is correct, these workers developed silicosis while exposed to a quartz concentration below the recommended occupational exposure limit (OEL) of 0.1 mg/m(3). This accords with a mounting body of evidence that an OEL of 0.1 mg/m(3) is not protective against silicosis.