Well Water and Parkinson’s Disease in Medicare Beneficiaries: A Nationwide Case-Control Study

BACKGROUND: Well water frequently is considered a risk factor for Parkinson's disease (PD), but few studies were designed appropriately to test whether geographic factors affect PD risk. OBJECTIVE: To determine the risk of PD in relation to residential use of private well water. METHODS: In a nationwide, population-based case-control study, we identified all incident PD cases (N = 89,790) and all comparable controls (N = 21,549,400) age 66-90 who solely relied on Medicare coverage in the U.S. in 2009. We estimated the probability of use of private well water using zip code of residence at diagnosis/reference and U.S. Census data on household water source. We modeled this exposure linearly in logistic regression to calculate the odds ratio (OR) and 95% confidence interval (CI) of PD risk in relation to well water use. We adjusted for age, sex and race/ethnicity, and verified that smoking and use of medical care did not confound results. We repeated analyses with a 2-year exposure lag and separately within each U.S. state. RESULTS: Use of well water was inversely associated with PD risk (OR = 0.87, 95% CI 0.85-0.89). We confirmed this association in a Cox survival analysis in which we followed controls for 5 years, death or PD diagnosis. There was little evidence that well water use increased risk of PD in any individual state. CONCLUSIONS: Although it remains possible that exposures in well water in more narrow geographic regions increase PD risk, in general these results suggest that exposures more common in urban/suburban areas might also be relevant.