Abstract
BACKGROUND AND OBJECTIVES: Numerous studies suggest that environmental exposures play a critical role in Parkinson disease (PD) pathogenesis, and large, population-based studies have the potential to advance substantially the identification of novel PD risk factors. We sought to study the nationwide geographic relationship between PD and air pollution, specifically PM(2.5) (particulate matter with a diameter <2.5 micrometers), using population-based US Medicare data. METHODS: We conducted a population-based geographic study of Medicare beneficiaries aged 66-90 years geocoded to US counties and zip+4. We used integrated nested Laplace approximation to create age, sex, race, smoking, and health care utilization-adjusted relative risk (RR) at the county level for geographic analyses with PM(2.5) as the primary exposure of interest. We also performed an individual-level analysis using logistic regression with cases and controls with zip+4 centroid PM(2.5). We adjusted a priori for the same covariates and verified no confounding by indicators of socioeconomic status or neurologist density. RESULTS: Among 21,639,190 Medicare beneficiaries, 89,390 had incident PD in 2009. There was a nationwide association between average annual PM(2.5) and PD risk whereby the RR of PD was 56% (95% CI 47%-66%) greater for those exposed to the median level of PM(2.5) compared with those with the lowest level of PM(2.5). This association was linear up to 13 μg/m(3) corresponding to a 4.2% (95% CI 3.7%-4.8%) greater risk of PD for each additional μg/m(3) of PM(2.5) (p (trend) < 0.0001). We identified a region with high PD risk in the Mississippi-Ohio River Valley, where the risk of PD was 19% greater compared with the rest of the nation. The strongest association between PM(2.5) and PD was found in a region with low PD risk in the Rocky Mountains. PM(2.5) was also associated with PD in the Mississippi-Ohio River Valley where the association was relatively weaker, due to a possible ceiling effect at average annual PM(2.5) levels of ∼13 μg/m(3). DISCUSSION: State-of-the-art geographic analytic techniques revealed an association between PM(2.5) and PD that varied in strength by region. A deeper investigation into the specific subfractions of PM(2.5) may provide additional insight into regional variability in the PM(2.5)-PD association.