Ambient Air Pollution and Parkinson’s Disease and Alzheimer’s Disease: An Updated Meta-Analysis

Background: Previous epidemiological evidence regarding the associations between ambient air pollution and two major neurodegenerative diseases, Alzheimer’s disease (AD) and Parkinson’s disease (PD), remains inconclusive. Objective: This study aimed to evaluate the associations between long-term and short-term exposure to PM_2.5 and PM₁₀ (i.e., particulate matter with an aerodynamic diameter of, or smaller than, 2.5 μm or 10 μm), nitrogen dioxide (NO₂), ozone, sulfur dioxide, and carbon monoxide and the risks of AD and PD. Methods: A random-effects model was used to summarize individual effect estimates in the meta-analysis. A subgroup meta-analysis was further conducted to explore the potential sources of heterogeneity. Results: In total, 42 eligible studies were included. For each 5 μg/m³ increase in long-term PM_2.5 exposure, the odds ratios (ORs) were 1.16 (95% CI: 1.04, 1.30; I² = 95%) and 1.10 (95% CI: 1.03, 1.17; I² = 95%) for AD and PD, respectively. For each 5 μg/m³ increase in short-term PM_2.5 exposure, the OR was 1.01 (95% CI: 1.002, 1.01; I² = 77%) for PD. For each 1 ppb increase in long-term NO₂ exposure, the OR was 1.01 (95% CI: 1.0002, 1.02; I² = 79%) for PD. Conclusion: Ambient air pollution, particularly PM_2.5, may contribute to the increased risks of neurodegenerative diseases including AD and PD.