Association between air pollution exposure and risk of hemorrhagic stroke: a systematic review and meta-analysis

Introduction Air pollution has been associated with multi-system health adversities. The cardiovascular effects of air pollution are especially prominent due to the pro-inflammatory nature of many pollutants. Common adversities that have been extensively studied include myocardial infarction, ischemic stroke, and systemic atherosclerosis. However, the association between air pollution and hemorrhagic stroke has not been sufficiently meta-analyzed in recent years. This was apparent when a literature search was conducted on PubMed, Google Scholar, and the International Prospective Register of Systematic Reviews (PROSPERO), and no comprehensive recent meta-analyses were found looking at this association. Purpose Considering that hemorrhagic stroke has a high incidence rate (28.8% of incident strokes) and accounts for almost 7.2 million deaths per year globally, it is important to identify risk factors and use it to guide preventative measures. Our goal is to conduct a comprehensive, and up-to-date meta-analysis assessing the risk of hemorrhagic stroke with air pollution exposure. Methods The study protocol was published in PROSPERO (CRD420251031712) on April 13, 2025. A systematic search was conducted on four databases (Embase, OVID Medline, Web of Science, PubMed) on March 10, 2025 for articles studying the association between air pollutants and hemorrhagic stroke. Studies selected for review included time series, case-crossover, and cohort studies in English, from all time periods, which investigated the risk of hemorrhagic stroke after exposure to air pollution (PM2.5, PM10, NO2, SO2, O3, or CO). The quality of evidence was assessed using Cochrane’s Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach. Data for different risk outcomes (risk ratio (RR), odds ratio (OR)) were meta-analyzed using an inverse variance statistical analysis and random effects model. The pooled effect sizes and 95% confidence intervals (CIs) were reported in forest plots. Results Among the 2,435 studies identified, 31 were included and deemed eligible for review and meta-analysis. The results illustrated a significantly increased risk of hemorrhagic stroke per 10 μg/m3 increase in NO2 exposure (RR = 1.0142; 95% CI: 1.0000–1.0286; P = 0.05). As for the other pollutants, they demonstrated a statistically insignificant (P>0.05) increased risk as follows: PM2.5 (OR = 1.0172; RR = 1.0002), PM10 (OR = 1.0076; RR = 1.0008), SO2 (RR = 1.0060), O3 (OR = 1.0024; RR = 1.0017), CO (OR = 1.0008; RR = 1.0001). Conclusion Among the six air pollutants that were studied, NO2 exposure was associated with a statistically significant increase in the risk of hemorrhagic stroke. This can help guide effective preventative protocols and public health measures in an effort to reduce the burden of cardiovascular and cerebrovascular disease.For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.