Interactions between Ambient Ozone and Heat on Out-of-Hospital Cardiac Arrest: A Nationwide Study in Japan

High Resolution Image Download MS PowerPoint Slide Out-of-hospital cardiac arrest (OHCA) is a major cause of cardiovascular mortality. Both ground-level ozone (O 3 ) and high ambient temperature have been associated with cardiovascular risk, yet their combined effects on OHCA remain unclear. We examined the joint effects of daily O 3 concentrations and temperature on OHCA risk in Japan from 2012 to 2019. OHCA data were obtained from the All-Japan Utstein Registry and temperature data from the Japan Meteorological Agency. Daily maximum 8 h O 3 concentrations were estimated using the Community Multiscale Air Quality (CMAQ) model and population weighted. Prefecture-specific associations were estimated using time-stratified case-crossover analyses with conditional quasi-Poisson regression and pooled via multivariate random-effects meta-analysis. Interactions between O 3 and temperature were assessed on additive and multiplicative scales. A total of 610,613 cardiac-origin OHCA cases were analyzed. A nonlinear association between O 3 and OHCA was observed, with a threshold near 60 ppb. At extreme heat (99th percentile temperature), a 10 ppb increase in O 3 above this threshold was associated with a relative risk of 1.04 (95% CI: 1.03–1.05). Both additive (RERI = 0.01; 95% CI: 0.00–0.03) and multiplicative ( p < 0.01) interactions indicated synergistic effects. Elevated temperature amplified the adverse cardiovascular effects of O 3, and vice versa, highlighting the importance of integrated heat and air quality management strategies under a warming climate.