Crystalline silica exposure, radiological silicosis, and lung cancer mortality in diatomaceous earth industry workers

Background-The role of silicosis as either a necessary or incidental condit ion in silica associated lung cancer remains unresolved. To address this is sue a cohort analysis of dose-response relations for crystalline silica and lung cancer mortality was conducted among diatomaceous earth workers class ified according to the presence or absence of radiological silicosis. Methods-Radiological silicosis was determined by median 1980 International Labour Organisation system readings of a panel of three "B" readers for 180 9 of 2342 white male workers in a diatomaceous earth facility in California . Standardised mortality ratios (SMR) for lung cancer, based on United Stat es rates for 1942-94, were calculated separately for workers with and witho ut radiological silicosis according to cumulative exposures to respirable c rystalline silica (milligrams per cubic meter x years; mg/m(3)-years) ragge d 15 years. Results-Eighty one cases of silicosis were identified, including 77 with sm all opacities of greater than or equal to 1/0 and four with large opacities . A slightly larger excess of lung cancer was found among the subjects with silicosis (SMR 1.57, 95% confidence interval (CI) 0.43 to 4.03) than in wo rkers without silicosis (SMR 1.19, 95% CI 0.87 to 1.57). An association bet ween silica exposure and lung cancer risk was detected among those without silicosis; a statistically significant (p = 0.02) increasing trend of lung cancer risk was seen with cumulative exposure, with SMR reaching 2.40 (95% CI 1.24 to 4.20) at the highest exposure level (greater than or equal to 5. 0 mg/m(3)-years). A similar statistically significant (p = 0.02) dose-respo nse gradient was observed among non-silicotic subjects when follow up was t runcated at 15 years after the final negative radiograph (SMR 2.96, 95% CI 1.19 to 6.08 at greater than or equal to 5.0 mg/m(3)-years), indicating tha t the association among non-silicotic subjects was unlikely to be accounted for by undetected radiological silicosis. Conclusions-The dose-response relation observed between cumulative exposure to respirable crystalline silica and lung cancer mortality among workers w ithout radiological silicosis suggests that silicosis is not a necessary co -condition for silica related lung carcinogenesis. However, the relatively small number of silicosis cases in the cohort and the absence of radiograph ic data after employment limit interpretations.