Cancer incidence among pesticide applicators exposed to chlorpyrifos in the agricultural health study

Background: Chlorpyrifos is one of the most widely used insecticides in the United States. We evaluated the incidence of cancer among pesticide applicators exposed to chlorpyrifos in the Agricultural Health Study, a prospective cohort study of licensed pesticide applicators in Iowa and North Carolina. Methods: A total of 54383 pesticide applicators were included in this analysis. Detailed information on pesticide exposure and lifestyle factors was obtained from self-administered questionnaires completed at the time of enrollment (December 1993-December 1997). Poisson regression analysis was used to evaluate the association between chlorpyrifos exposure and cancer incidence after adjustment for potential confounders. All statistical tests were two-sided. Results: A total of 2070 incident malignant neoplasms were diagnosed through 2001. The rate ratio for all cancers combined among chlorpyrifos-exposed applicators compared with nonexposed applicators was 0.97 (95% confidence interval = 0.87 to 1.08). For most cancers analyzed, there was no evidence of an exposure-response relationship. However, the incidence of lung cancer was statistically significantly associated with both chlorpyrifos lifetime exposure-days (P_trend = .002) and chlorpyrifos intensity-weighted exposure-days (P_trend = .036). After adjustment for other pesticide exposures and demographic factors, individuals in the highest quartile of chlorpyrifos lifetime exposure-days (> 56 days) had a relative risk of lung cancer 2.18 (95% confidence interval = 1.31 to 3.64) times that of those with no chlorpyrifos exposure. Conclusion: Our findings suggest an association between chlorpyrifos use and incidence of lung cancer that deserves further evaluation.