A systematic review with a Burden of Proof meta-analysis of health effects of long-term ambient fine particulate matter (PM2.5) exposure on dementia.

Previous studies have indicated increased dementia risk associated with fine particulate matter (PM_2.5) exposure; however, the findings are inconsistent. In this systematic review, we assessed the association between long-term PM_2.5 exposure and dementia outcomes using the Burden of Proof meta-analytic framework, which relaxes log-linear assumptions to better characterize relative risk functions and quantify unexplained between-study heterogeneity (PROSPERO, ID CRD42023421869). Here we report a meta-analysis of 28 longitudinal cohort studies published up to June 2023 that investigated long-term PM_2.5 exposure and dementia outcomes. We derived risk-outcome scores (ROSs), highly conservative measures of effect size and evidence strength, mapped onto a 1-5-star rating from 'weak and/or inconsistent evidence' to 'very strong and/or consistent evidence'. We identified a significant nonlinear relationship between PM_2.5 exposure and dementia, with a minimum 14% increased risk averaged across PM_2.5 levels between 4.5 and 26.9 µg m^-3 (the 15th to 85th percentile exposure range across included studies), relative to a reference of 2.0 µg m^-3 (n = 49, ROS = 0.13, two stars). We found a significant association of PM_2.5 with Alzheimer's disease (n = 12, ROS = 0.32, three stars) but not with vascular dementia. Our findings highlight the potential impact of air pollution on brain aging.