Lung-cancer mortality among Vermont granite workers: factors influencing exposure-response evaluation

Objective To understand why 2 studies relating crystalline silica exposure to lung-cancer mortality in Vermont granite workers yielded conflicting results. Methods Data used in the 2 studies were linked to identify discrepancies. Mortality data and employment histories from the earlier study were revised based on data obtained in the later study. SMR were computed and Poisson regressions corresponding to those in the earlier study were performed using the original and revised data. Analyses were repeated with the addition of workers omitted from the earlier study. Results After correction of incomplete mortality and employment information in the original data, the overall SMR for the cohort in the earlier study increased from 1.17 (95% CI 1.03 to 1.36) to 1.39 (95% CI 1.22 to 1.59), and was similar to the SMR of 1.37 observed in the later study (95% CI 1.23 to 1.52). The exposure-response relationship was attenuated, particularly when person-years in all exposure categories were included in the analysis. Inclusion of additional workers had a smaller impact on the SMRs but further attenuated the exposure-response relationship. Conclusions Differing results from the 2 studies are partly attributable to incomplete vital status and work history information used in the earlier study, as well as differences in cohort inclusion criteria. However, differences in length of follow-up and other factors likely play a larger role.