Long-term PM2.5 exposure and various health outcomes: An umbrella review of systematic reviews and meta-analyses of observational studies

Adverse effects from exposure to particulate matter <2.5 μm in diameter (PM₂.₅) on health-related outcomes have been found in a range of experimental and epidemiological studies. This study aimed to assess the significance, validity, and reliability of the relationship between long-term PM₂.₅ exposure and various health outcomes. The Embase, PubMed, Web of Science, CNKI, WANFANG, VIP, and SinoMed databases and reference lists of the retrieved review articles were searched to obtain meta-analysis and systematic reviews focusing on PM₂.₅-related outcomes as of August 31, 2021. Random-/fixed-effects models were applied to estimate summary effect size and 95% confidence intervals (CIs). The quality of included meta-analyses was evaluated based on the AMSTAR 2 tool. Small-study effect and excess significance bias studies were conducted to further assess the associations. Registered PROSPERO number: CRD42020200606. This included 24 articles involving 71 associations between PM₂.₅ exposure and the health outcomes. The evidence for the positive association of 10 μg/m³ increments of long-term exposure to PM₂.₅ and stroke incidence in Europe was convincing (effect size = 1.07, 95% CI: 1.05–1.10). There was evidence that was highly suggestive of a positive association between 10 μg/m³ increments of long-term exposure to PM₂.₅ and the following health-related outcomes: mortality of lung cancer (effect size = 1.11, 95% CI: 1.08–1.13) and Alzheimer's disease (effect size = 4.79, 95% CI: 2.79–8.21). There was highly suggestive evidence that chronic obstructive pulmonary disease risk is positively associated with higher long-term PM₂.₅ exposure versus lower long-term PM₂.₅ exposure (effect size = 2.32, 95% CI: 1.88–2.86). In conclusion, the positive association of long-term exposure to PM₂.₅ and stroke incidence in Europe was convincing. Given the validity of numerous associations of long-term exposure to PM₂.₅ and health-related outcomes is subject to biases, more robust evidence is urgently needed.