To compare risk of silicosis among cohorts of silica dust exposed Chinese tin miners, Chinese tungsten miners, Chinese pottery workers, and South African gold miners. The study was undertaken as part of an investigation to determine if the physical properties of silica dust, including the surface contamination of the silica particles, affect its toxicity, after allowing for intensity of exposure. The Chinese cohorts included 4,471 tin miners, 15,876 tungsten miners, and 4,798 pottery workers. The three cohorts started employment from 1950 and were employed for at least one year during 1960 to 1974. The radiological follow-up for a silicosis onset was from 1950 through December 31, 1994. Silicosis was assessed according to the 1986 Chinese pneumoconiosis roentgen diagnostic criteria that classified silicosis as suspect, or at Stage I, II, or III. Silicosis of Stage I or higher was related to cumulative respirable silica dust exposure calculated from a workplace, job title, and calendar year exposure matrix. The cohort of South African gold miners started employment from 1940 and was followed for silicosis onset up to 1990. The exposure-response for silicosis and cumulative respirable silica dust was estimated by the Life Table method. For the same level of exposure to cumulative respirable silica dust, the pottery workers had the lowest cumulative risk of silicosis, followed by tungsten, tin and South African gold miners who had the highest risk. Although the pottery workers were exposed to highest levels of cumulative respirable dust, their cumulative risk of silicosis was lowest, reaching about 54% at the highest levels, whereas 97% of the tin and tungsten miners developed silicosis at the highest dust levels. In the tin, tungsten and gold miners the intensity of silica dust exposure was the most important predictor for the development of silicosis, whereas in the pottery workers duration of silica dust exposure rather then intensity of exposure was most important factor. The results suggest that some physical properties of the silica dust, over and above exposure intensity, affect its toxicity. We plan to examine these relationships further with respect to the properties of the dust, in particular, indices of surface contamination.
La Medicina del Lavoro. 3rd International Symposium on Silica, Silicosis, Cancer and Other Diseases, S. Margherita Ligure, 21-25 October 2002