Objectives: This study aimed to analyse the existing evidence on the association between particulate matter 2.5 (PM2.5) and dementia, including two of its subtypes, namely, Alzheimer's disease (AD) and vascular dementia (VaD). Study design: This was a systematic review and meta-analysis. Methods: The PubMed, EMBASE, Cochrane and Web of Science databases were comprehensively searched for articles published between January 1900 and June 2022. All cohort studies that reported the influence of long-term exposure to PM2.5 on dementia, together with its subtypes, in adults aged >= 40 years, without any regional restriction were included. A random effects model was used to pool the hazard ratios (HRs) of PM2.5 for dementia, AD and VaD. Funnel plots, sensitivity analyses and subgroup analyses were performed to test publication bias and result stability. In addition, an explanation for the heterogeneity of the results was suggested. Results: In total, 20 articles were selected for review; 18 included results on the long-term effects of PM2.5 on dementia, 13 on AD, and eight on VaD. Three group meta-analyses were performed to obtain the HRs and 95% confidence intervals (CIs). The pooled HRs were 1.40 (95% CI 1.23, 1.60) for dementia, 1.47 (95% CI 1.22, 1.78) for AD and 2.00 (95% CI 1.30, 3.08) for VaD per 10.0 mg/m3 PM2.5 increase. Conclusions: Long-term exposure to PM2.5 may increase the risk of dementia, including AD and VaD. These results highlight the need for further study on the detrimental impact of PM2.5 and the importance of strategies to mitigate increasing air pollution. (c) 2022 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved.