Joint Effects of Heat and Fine Particulate Matter on Out‐of‐Hospital Cardiac Arrest in Japan: A Nationwide Time‐Stratified Case‐Crossover Study

Background Out‐of‐hospital cardiac arrest (OHCA) is a leading cause of cardiovascular death. Although ambient temperature and particulate matter with an aerodynamic diameter ≤2.5 μm (PM 2.5 ) are established risk factors, evidence on their joint effects remains limited. We assessed the joint effects of heat and PM 2.5 on OHCA risk using nationwide data from Japan. Methods We collected daily data on cardiac‐origin OHCAs from the All‐Japan Utstein Registry and daily temperature and PM 2.5 from monitoring networks across 47 prefectures during 2012 through 2019. Using a 2‐stage time‐stratified case‐crossover design, we estimated prefecture‐specific associations with conditional quasi‐Poisson models and pooled them using meta‐regression. Interactions between temperature and PM 2.5 were assessed on multiplicative and additive scales using the relative excess risk due to interaction. Attributable cases were also estimated. Results A total of 610 613 cardiac‐origin OHCAs were included. Heat (relative risk, 1.04 [95% CI, 1.02–1.06]; 99th versus 80th percentile); and PM 2.5 (relative risk, 1.02 per 10 μg/m 3 [95% CI, 1.01–1.03]) were independently associated with increased OHCA risk. Heat effects were amplified at higher PM 2.5 concentrations, with each 10 μg/m 3 increase in PM 2.5 associated with an additional 1.8% increase in heat‐related risk. Conversely, PM 2.5 effects increased by 1.2% (95% CI, 1.1%–1.2%) per 25th percentile increase in temperature. Interactions were observed on both scales (relative excess risk due to interaction, 0.01 [95% CI, 0.00–0.02]; P =0.003). Overall, 2638 OHCA cases were attributable to coexposure, exceeding the combined burden attributable to heat (1215) and PM 2.5 (1276) separately. Conclusions Heat and PM 2.5 acted synergistically to increase OHCA risk, highlighting the need for integrated public health strategies addressing both extreme heat and air pollution under climate change.