Affiliations: [a] Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA
| [b] School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa
Correspondence to: Susan Searles Nielsen, PhD, Washington University School of Medicine, Department of
Neurology, Campus Box 8111, 660 South Euclid Avenue, St. Louis, MO 63110, USA. Tel.: +1 314 362 5291; Fax: +1 314
747 3274; E-mail: [email protected].
Abstract: 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.