Short-term departures from an optimum ambient temperature are associated with increased risk of out-of-hospital cardiac arrest

•Ambient temperature and out-of-hospital cardiac arrest has been little studied.•We explored the exposure-response relationship and confounding by air pollution.•We observed a V-shaped relationship departing from a temperature optimum.•Higher risks were observed for 24- and 1-h temperatures below an optimum.•Associations for temperatures above the optimum were confounded by ozone. Associations have been reported between daily ambient temperature and all-cause and cardiovascular mortality. However, the potential harmful effect of temperature on out-of-hospital cardiac arrest (OHCA) is insufficiently studied. The objective of this study was to investigate the short-term association between ambient temperature and the occurrence of OHCA. In 5961 cases of OHCAs treated by Emergency Medical Service occurring in Stockholm County we investigated the association between the preceding 24-h and 1h mean ambient temperature, obtained from a fixed monitoring station, and OHCA using a time-stratified case-crossover design. We observed a V-shaped relationship between preceding mean 24-h and 1-h ambient temperature and the occurrence of OHCAs. For mean 24-h temperature we observed an odds ratio (OR) of 1.05 (1.00–1.11) for each 5°C below the optimum temperature and 1.05 (0.96–1.18) for each 5°C above the optimum. We observed similar results for 1-h mean temperature exposure. Results for temperatures above the optimum temperature showed evidence of confounding by ozone. Ambient temperature below an optimum temperature was associated with increased risk of OHCA in Stockholm. Temperature above an optimum temperature was not significantly associated with OHCA.