Crystalline silica exposure and lung cancer mortality in diatomaceous earth industry workers: a quantitative risk assessment

Objective-To use various exposure-response models to estimate the risk of m ortality from lung cancer due to occupational exposure to respirable crysta lline silica dust. Methods-Data from a cohort mortality study of 2342 white male California di atomaceous earth mining and processing workers exposed to crystalline silic a dust (mainly cristobalite) were reanalyzed with Poisson regression and Co x's proportional hazards models. Internal and external adjustments were use d to control for potential confounding from the effects of time since first observation, calendar time, age, and Hispanic ethnicity. Cubic smoothing s pline models were used to assess the fit of the models. Exposures were lagg ed by 10 years. Evaluations of the fit of the models were performed by comp aring their deviances. Lifetime risks of lung cancer were estimated up to a ge 85 with an actuarial approach that accounted for competing causes of dea th. Results-Exposure to respirable crystalline silica dust was a significant pr edictor (p<0.05) in nearly all of the models evaluated and the linear relat ive rate model with a 10 year exposure lag seemed to give the best fit in t he Poisson regression analysis. For those who died of lung cancer the linea r relative rate model predicted rate ratios for mortality from lung cancer of about 1.6 for the mean cumulative exposure to respirable silica compared with no exposure. The excess lifetime risk (to age 85) of mortality from l ung cancer for white men exposed for 45 years and with a 10 year lag period at the current Occupational Safety and Health Administration (OSHA) standa rd of about 0.05 mg/m(3) for respirable cristobalite dust is 19/1000 (95% c onfidence interval (95% CI) 5/1000 to 46/1000). Conclusions-There was a significant risk of mortality from lung cancer that increased with cumulative exposure to respirable crystalline silica dust. The predicted number of deaths from lung cancer suggests that current occup ational health standards may not be adequately protecting workers from the risk of lung cancer.