Fine Particulate Matter and Parkinson Disease Risk Among Medicare Beneficiaries

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.

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.

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/m³ corresponding to a 4.2% (95% CI 3.7%-4.8%) greater risk of PD for each additional μg/m³ 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/m³.

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.