Mortality during heatwaves in Spain, 2015-2025: heat metrics, patterns and future risks

The summer of 2025 was exceptionally warm in Spain, ranking among the most extreme heat seasons on record and highlighting the need to better understand its health impacts. We analyzed the relationship between heat and mortality across the 2015-2024 period, using Spain's high-resolution mortality surveillance and meteorological data. We evaluated multiple heat metrics at different spatial scales to identify those most strongly associated with mortality during heatwaves. We found that event-level heat metrics can explain a large proportion of the variability in heatwave mortality, with extreme temperature and cumulative exposure providing the best performance. In particular, heat-attributable mortality increased by 15 % (95 % CI: 6.7-24.3 %) nationwide and by 24 % (95 % CI: 15.3-33.4 %) at the provincial level for each 1 °C increase in average daily maximum temperature during a heatwave. Over the study period, heat-attributable deaths declined annually by 3.9-5.8 % after adjusting for temperature, yet forecasts substantially overestimated the exceptional heatwaves of 2025. When combined with climate projections, results indicate increased risks in inland regions where warming and vulnerability converge. Spain's experience offers valuable lessons for other regions facing increasingly frequent and intense heatwaves.