Exposure to silica and silicosis among tin miners in China: exposure-response analyses and risk assessment

Objectives-To investigate the risk of silicosis among tin miners and to inv estigate the relation between silicosis and cumulative exposure to dust (Ch inese total dust and respirable crystalline silica dust). Methods-A cohort study of 3010 miners exposed to silica dust and employed f or at least 1 year during 1960-5 in any of four Chinese tin mines was condu cted. Historical total dust data from China were used to create a job expos ure matrix for facility, job title, and calendar year. The total dust expos ure data from China were converted to estimates of exposure to respirable c rystalline silica for comparison with findings from other epidemiological s tudies of silicosis. Each worker's work history was abstracted from the com plete employment records in mine files. Diagnoses of silicosis were based o n 1986 Chinese pneumoconiosis Roentgen diagnostic criteria, which classifie d silicosis as stages I-III-similar to an International Labour Organisation (ILO) classification of 1/1 or greater. Results-There were 1015 (33.7%) miners identified with silicosis, who had a mean age of 48.3 years, with a mean of 21.3 years after first exposure (eq uivalent to 11.0 net years in a dusty job). Among those who had silicosis, 684 miners (67.4%) developed silicosis after exposure ended (a mean of 3.7 years after). The risk of silicosis was strongly related to cumulative expo sure to silica dust and was well fitted by the Weibull distribution, with t he risk of silicosis less than 0.1% when the Chinese measure of cumulative exposure to total dust (CTD) was under 10 mg/m(3)-years (or 0.36 mg/mg(3)-y ears of respirable crystalline silica), increasing to 68.7% when CTD exposu re was 150 mg/m(3)-years (or 5.4 mg/m(3)-years of respirable crystalline si lica). Latency period was not correlated to the risk of silicosis or cumula tive dose of exposure. This study predicts about a 36% cumulative risk of s ilicosis for a 45 year lifetime exposure to these tin mine dusts at the CTD exposure standard of 2 mg/m(3), and a 55% risk at 45 years exposure to the current United States Occupational Safety and Health Administration and Mi ne Safety and Health Administration standards of 0.1 mg/m(3) 100% respirabl e crystalline silica dust. Conclusions-A clear exposure-response relation was detected for silicosis i n Chinese tin miners. The study results were similar to most, but not all, findings from other large scale exposure-response studies.