Ambient air temperature and out-of-hospital cardiac arrest: A national case-crossover study

Introduction: Out-of-hospital cardiac arrest (OHCA) is a major health concern. Roughly 300,000 cases occur annually in Europe; overall survival is 11%. Given the disease burden and increasingly extreme temperatures due to climate change, we investigated the association between air temperature and risk of OHCA in Sweden. Methods: This study employed a time-stratified case-crossover design to examine the relationship between daily mean air temperature and OHCA cases from the Swedish Registry for Cardiopulmonary Resuscitation between 2010 and 2019. Using a machine learning model, mean temperatures were estimated on a 1 × 1 km2 grid and converted into location-specific percentiles for Sweden's 290 municipalities. The relationship between temperature and OHCA was estimated using conditional logistic regression. Results: 29,604 OHCA cases (32% women, median age 74 years) were included. The 1st percentile of temperature ranged from −32.7 to −5.98°C; the 99th percentile ranged from 16.6 to 22.8°C. The reference point, where OHCA risk was lowest, was the 86th percentile. The odds ratio (OR) of OHCA comparing 1st percentile of temperature with the 86th was 1.197 (95% confidence interval [CI] 1.066–1.344), and the OR of OHCA comparing the 99th versus 86th percentile was 1.156 (95% CI 1.058–1.263). Conclusions: Location-specific extreme ambient air temperatures were associated with greater risk of OHCA in Sweden. This further adds to evidence suggesting that exposure to locally relative temperature extremes may increase risk of OHCA, even when temperatures do not represent extreme temperature events by absolute metrics. These results may be limited by potential discordance between ambient temperature and individual temperature exposure. © 2026 The Author(s). Journal of Internal Medicine published by John Wiley & Sons Ltd on behalf of Association for Publication of The Journal of Internal Medicine.