Dose-Response Associations of Silica with Nonmalignant Respiratory Disease and Lung Cancer Mortality in the Diatomaceous Earth Industry

The potential carcinogenicity of crystalline silica to humans remains a controversial issue. The authors conducted an historical cohort mortality study of 2, 342 male workers exposed to crystalline silica, predominantly cristobalite, in a diatomaceous earth mining and processing facility in California. During the years 1942–1994, mortality excesses were detected for nonmalignant respiratory diseases (NMRD) (standardized mortality ratio = 2.01, 95% confidence interval (CO 1.56–2.55) and lung cancer (standardized mortality ratio = 1.29, 95% CI 1.01 –1.61). NMRD mortality rose sharply with cumulative exposure to respirable crystalline silica; allowing for a 15-year latency, the rate ratio for the highest exposure stratum (≥5.0 mg/m3-years) was 5.35 (95% CI 2.23–12.8). The rate ratio for lung cancer reached 2.15 (95% CI 1.08–4.28) in the highest exposure category. These associations were unlikely to have been confounded by smoking or asbestos exposure. The findings indicate a strong dose-response relation for crystalline silica and NMRD mortality. The lung cancer results, although less convincing, add further support to an etiologic role for crystalline silica. Am J Epidemiol 1997:145:680–8.