How does geographical scale affect the effectiveness of public health heat-wave prevention plans?: the case of Spain

The summer of 2025 has been the hottest in Spain since records began, 2022registering temperatures one tenth of a degree Celsius higher than the hottest on record to date, i.e., the summer of 2022. However, mortality attributable to heat wave temperatures in 2025 was 908 deaths fewer than in 2022. Of the many factors that may have influenced the reduction in attributable mortality in 2025 with respect to 2022, this study sought to analyse the possible impact of an improvement made to heat-wave prevention plans in the form of enhancing the geographical resolution of health alerts. Using the annual heat wave intensity registered and the annual mortality attributable to these heat waves, a linear adjustment between these two variables was performed for all Spanish provinces in the above two periods. The results indicated that whereas the increase in annual mortality for every one-degree rise in annual heat wave intensity (mean slope of the lines of fit for Spain as a whole) from 2015 to 2023 was 1.74 deaths/⁰C, in the period 2024-2025 it was 1.66 deaths/⁰C. This result translates as follows: if annual attributable mortality in Spain were calculated for 2025 using the values of the slopes for the first period, this would yield a total of 4,082 attributable deaths, but if this same attributable mortality were calculated using the values of the slope corresponding to the 2024-2025 period, this would yield a total of 3,894 deaths. In conclusion, although this analysis has not taken into account many factors that could relate heat wave intensity to temperature-attributable mortality, the activation of heat wave thresholds in prevention plans at a smaller geographical scale could lead to a reduction in attributable mortality.