Long-term exposure to low-level particulate air pollution and Parkinson's disease diagnosis - A Finnish register-based study.

Background: There is mixed evidence for an association between particulate matter air pollution and Parkinson's disease despite biological plausibility.

Objectives: We studied the association between particulate air pollution, its components and Parkinson's disease (PD) risk.

Methods: We conducted a nested case-control study within the population of Finland using national registers. A total of 22,189 incident PD cases diagnosed between 1996 and 2015 were matched by age, sex and region with up to seven controls (n = 148,009) per case. Time weighted average air pollution exposure to particulate matter and its components was modelled at the residential addresses, accounting for move history, for the 16 years preceding diagnosis. Conditional logistic regression analysis was used to evaluate the association between air pollution and PD. Different exposure periods (6-16 years, 11-16 years, 5-10 years, 0-5 years) before the index date (date of PD diagnosis) were applied.

Results: Time-weighted average exposures were relatively low at 12.1 ± 6.5 μg/m (mean ± SD) for PM and 7.7 ± 3.2 μg/m for PM. No associations were found between PM or PM exposure 6-16 years before index date and PD (OR: 0.99; 95% CI: 0.96, 1.02; per IQR of 3.9 μg/m and OR: 0.99; 95% CI: 0.96, 1.01; per IQR of 7.8 μg/m, respectively). However, inverse associations were observed for the same exposure period with black carbon (OR: 0.96; 95% CI: 0.93, 0.99; per IQR of 0.6 μg/m), sulphate (OR: 0.79; 95% CI: 0.68, 0.92; per IQR of 1.2 μg/m), secondary organic aerosols (OR: 0.86; 95% CI: 0.80, 0.93; per IQR of 0.1 μg/m) and sea salt (OR: 0.92; 95% CI: 0.87, 0.98; per IQR of 0.1 μg/m).

Discussion: Low-level particulate matter air pollution was not associated with increased risk of incident PD in this Finnish nationwide population. The observed weak inverse associations with specific particle components should be investigated further.