Effects of ambient air pollution on incident Parkinson’s disease in Ontario, 2001 to 2013: A population-based cohort study

Background: Despite recent studies linking air pollution to neurodegenerative illness, evidence relating air pollution and Parkinson’s disease (PD) remains scarce. We conducted a population-based cohort study in Ontario, Canada, to determine the association between air pollution and incident PD. Methods: Using health administrative databases, we identified all adults aged 55–85 years, free of PD, and who lived in Ontario on 1 April 2001 (2.2 million). Individuals were followed up until 31 March 2013. We derived long-term average exposures to fine particulate matter (particles 2.5 mm in diameter, or PM_2.5), nitrogen dioxide (NO₂) and ozone from satellite-based estimates, land-use regression models and optimal interpolation methods, respectively. Using 2-year lags in exposures, we linked these estimates to individuals’ annual postal codes from 1994 (7 years before cohort inception). We applied spatial random-effects Cox proportional hazards models, adjusting for individual- and area-level characteristics. We also performed sensitivity analyses, such as considering longer lags in exposures and stratifying by selected characteristics. Results: During the study period, we identified 38 745 newly diagnosed cases of PD. Each interquartile increment (3.8 mg/m³) of PM_2.5 was associated with a 4% increase in incident PD (95% confidence interval, 1.01–1.08) after adjusting for various covariates. We also found positive associations for NO₂ and ozone [hazard ratios (HRs) ranged from 1.03 to 1.04]. The associations for all exposures were unaltered with various sensitivity analyses except for considering longer lags, which somewhat attenuated the estimates, particularly for NO₂ and ozone. Conclusions: Exposure to air pollution, especially PM_2.5, was found to be related to incident PD.