All white and mixed race men who were employed in South African asbestos mines and mills between 30 November 1970 and 30 November 1975 were studied. The men who had two radiographs available, the first taken some time between the above two dates and the latest available radiograph which had to be at least two years after the first one numbered 1454: 793 continued exposure after the first radiograph and 661 did not. The films were read by a panel of three readers. Data available included age, years of exposure to asbestos and other mining, intensity of exposure to asbestos and other dust, and smoking habit. Progression was expressed as the difference between the average readings of radiograph 2--radiograph 1 in minor categories per year of irregular opacities. Changes in pleural abnormality were also measured. No differences of progression in the profusion or change in size of the irregular opacities were found between the two groups or in the number of zones affected. "New attacks" appeared equally frequently between the two groups. No difference in the change in extent of any type of pleural change was seen. It appears that once a dose of asbestos sufficient to initiate the disease has been retained it is inexorably progressive.
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