Abstract
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 PM(10) (i.e., particulate matter with an aerodynamic diameter of, or smaller than, 2.5 μm or 10 μm), nitrogen dioxide (NO(2)), 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/m(3) increase in long-term PM(2.5) exposure, the odds ratios (ORs) were 1.16 (95% CI: 1.04, 1.30; I(2) = 95%) and 1.10 (95% CI: 1.03, 1.17; I(2) = 95%) for AD and PD, respectively. For each 5 μg/m(3) increase in short-term PM(2.5) exposure, the OR was 1.01 (95% CI: 1.002, 1.01; I(2) = 77%) for PD. For each 1 ppb increase in long-term NO(2) exposure, the OR was 1.01 (95% CI: 1.0002, 1.02; I(2) = 79%) for PD. CONCLUSION: Ambient air pollution, particularly PM(2.5), may contribute to the increased risks of neurodegenerative diseases including AD and PD.