Occupational Exposure to Metribuzin and the Incidence of Cancer in the Agricultural Health Study

PURPOSE: Little is known about the potential carcinogenicity of the triazinone herbicide metribuzin. We evaluated the association between metribuzin use and cancer risk in the Agricultural Health Study, a prospective cohort study of licensed pesticide applicators in Iowa and North Carolina. METHODS: Applicators (N = 23,072) provided information on metribuzin use on a self-administered questionnaire at enrollment (1993-1997). Among metribuzin users (n = 8,504), there were 554 incident cancer cases. We used multivariable Poisson regression to evaluate potential associations between metribuzin use and cancer incidence by using two quantitative exposure metrics, lifetime days and intensity-weighted lifetime days. RESULTS: Using intensity-weighted lifetime days, the rate ratio (RR) and 95% confidence interval (CI) for the highest exposed tertile for lymphohematopoietic malignancies were 2.09 (95% CI: 0.99-4.29), P trend = 0.02 and 2.42 (95% Cl: 0.82-7.19), p trend = 0.08 for leukemia. For non-Hodgkin lymphoma, the RR was 2.64 (95% Cl: 0.76-9.11), p trend = 0.13 for lifetime days and 2.52 (95% Cl: 0.66-9.59), p trend = 0.13 for intensity-weighted lifetime days. Patterns of association were similar for both exposure metrics, but associations were generally weaker than for intensity-weighted days. CONCLUSIONS: The results from this Study suggest a potential association between metribuzin use and certain lymphohematopoietic malignancies; however, having not been observed previously, caution should be used in interpretation. Ann Epidemiol 2009;19:388-395. Published by Elsevier Inc.