Global association between air pollution and risk of influenza-related outcomes: a systematic review and meta-analysis

Despite extensive research, air pollution-influenza associations remain inconsistent. This evidence synthesis aggregated evidence from 28 studies to quantify air pollution-influenza associations, employing the Office of Health Assessment and Translation (OHAT) and Navigation Guide tools for quality assessment. We derived combined relative risk (<i>RR</i>) per 10 µg/m³ increase in air pollution, 95% confidence intervals (<i>CI</i>s) and 95% prediction intervals (<i>PI</i>s) to quantify the link. PM_2.5 (<i>RR</i> = 1.037; 95% <i>CI</i>: 1.019–1.055), SO₂ (<i>RR</i> = 1.351; 95% <i>CI</i>: 1.107–1.649) and CO (<i>RR</i> = 1.003, 95% <i>CI</i>: 1.001–1.004) showed significant links to higher influenza risk. PM₁₀ (<i>RR</i> = 1.064, 95% <i>CI</i>: 0.964–1.174), NO₂ (<i>RR</i> = 1.207, 95% <i>CI</i>: 0.988–1.475), and O₃ (<i>RR</i> = 1.027, 95% <i>CI</i>: 0.962–1.097) demonstrated a nonsignificant tendency to increase influenza risk. Stratified analyses indicated heightened vulnerability to SO₂ in Asian populations relative to Australians, and to PM_2.5 in Europe compared to Asia. Males experienced greater risks for PM_2.5, PM₁₀, NO₂ and SO₂. However, these associations were characterized by substantial between-study heterogeneity even after stratification, which was reflected in 95% <i>PI</i> that suggested an inconsistent direction of effect in future studies. Our analysis confirmed air pollutants increase influenza risk, but confounding, exposure variability, and mechanistic pathways warrant further study, particularly in underrepresented regions.