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BackgroundParaquat dichloride is currently among the most widely used commercial herbicides in the United States. Exposure has been broadly linked to Parkinson’s disease (PD) through experimental and epidemiologic research. In the current study, we provide further epidemiologic assessment of ambient paraquat exposure and Parkinson’s risk in a large population-based study of PD in agricultural regions of Central California.MethodsBased on 829 PD patients and 824 community controls, we assessed associations between ambient paraquat dichloride exposure and PD. We estimated residential and workplace proximity to commercial agricultural applications in three California counties since 1974 using the CA pesticide use reporting (PUR) data and land use maps. We evaluated any, duration, and average intensity (pounds per acre per year) of exposure to paraquat in four time-windows prior to PD diagnosis or interview for controls.ResultsAmbient paraquat exposure assessed at both residence and workplace was associated with PD based on all three exposure measures, indicating that PD patients lived and worked near agricultural facilities that applied greater amounts of the herbicide than community controls. For workplace proximity to commercial applications since 1974, any exposure (yes/no, OR=1.25, 95% CI=1.00, 1.57), duration of exposure (per SD, OR=1.26, 95% CI=1.10, 1.44), and long-term average intensity of exposure (per SD, OR=1.30, 95% CI=1.10, 1.53) increased the odds of PD. Similar associations were observed for residential proximity (duration of exposure: OR=1.23 per SD, 95% CI=1.07,1.40; long-term average exposure: OR=1.22 per SD, 95% CI=1.03, 1.46). Risk estimates were comparable for men and women and the strongest odds were observed for those diagnosed ≤60 years of age.ConclusionThis study provides further evidence that paraquat dichloride exposure increases the risk of Parkinson’s disease.