REANALYSIS OF MORTALITY FROM LUNG-CANCER AMONG DIATOMACEOUS-EARTH INDUSTRY WORKERS, WITH CONSIDERATION OF POTENTIAL CONFOUNDING BY ASBESTOSEXPOSURE

Objective-To evaluate the potential for confounding from asbestos expo sure, primarily chrysotile, on the relation between crystalline silica and mortality from lung cancer among diatomaceous earth (diatomite) w orkers. Methods-A reanalysis of a cohort mortality study of diatomite workers was performed to take into account quantitative estimates of a sbestos exposure. The reanalysis was limited to a subset of the origin al cohort, comprising 2266 white men for whom asbestos exposure could be reconstructed with greatest confidence. Comparisons between mortali ty from lung cancer (standardised mortality ratios (SMR)) were made be tween rates for 1942-87 for United States white men, and workers cross classified according to cumulative exposures to crystalline silica an d asbestos. Comparisons of internal rates, involving Poisson regressio n modeling, were conducted for exposure to crystalline silica, with an d without adjustment for asbestos exposure. Exposures were lagged by 1 5 years to take into account disease latency. Results-There was an ove rall excess of lung cancer (SMR 1.41; 52 observed). The SMRs for four categories of increasing crystalline silica among the workers not expo sed to asbestos were 1.13, 0.87, 2.14, 2.00. An SMR of 8.31 (three obs erved) was found for workers with the highest cumulative exposure to b oth dusts. Internal analysis, after adjustment for asbestos exposure, yielded rate ratios for categories of exposure to crystalline silica: 1.00 (reference), 1.37, 1.80, and 1.79. Conclusions-Asbestos exposure was not an important confounder of the association between crystalline silica and mortality from lung cancer in this cohort. Although based on a small number of deaths from lung cancer, the data suggest possibl e synergy between these exposures. An extended follow up of this cohor t is in progress and should enable better assessments of independent a nd combined effects on risk of lung cancer.