Short‐Term Effect of Air Pollution on Out of Hospital Cardiac Arrest (OHCA) in Lombardy—A Case‐Crossover Spatiotemporal Study

Evidence linking short-term exposure to air pollution, particularly PM_2.5, PM₁₀, NO₂, O₃, SO₂, and CO, with out-of-hospital cardiac arrest (OHCA) risk remains inconsistent. Large-scale studies using advanced air pollution exposure assessment techniques and spatiotemporal modeling are limited. This study investigates the association between short-term air pollution concentration, derived from satellite data, and OHCA events (2016 - 2019) in Lombardy, Italy, using a time-stratified case-crossover design. A two-stage distributed lag non-linear model, adjusted for meteorological factors, long-term trends, seasonality, intraweek patterns and holidays, is applied across 96 districts to estimate the exposure-lag-response relationship, followed by a regional meta-analysis. Subgroup analyses by age and sex of OHCA patients, with models stratified by urban-rural areas and seasons, and interaction with co-pollutants, are conducted. Among 37,613 OHCA cases, significant associations between OHCA and air pollutants are observed: NO₂ shows a delayed effect at lag 4 per 10 µg m^- ³ increase (RR: 1.071; 95%CI: 1.029-1.114), while PM_2.5 (RR: 1.029; 95%CI: 1.006-1.053), PM₁₀ (RR: 1.025; 95%CI: 1.005-1.046), and O₃ (RR: 1.009; 95%CI: 1.001-1.016) have immediate effects at lag 0 or lag 1. CO exhibits a modest association per 100 µg m^- ³ increase (RR: 1.007; 95%CI: 1.000-1.013, lag 4). Risks are elevated in summer months, and varied by urban-rural setting. No significant effect modification is detected across age or sex subgroups. Bi-pollutant models reveal notable interaction and confounding patterns.