Temporal changes in the high-temperature–mortality association in the Netherlands and the potential impact of the implementation of the national heat plan

Background: To mitigate the health impact of high temperatures, heat plans (HPs) have become widespread around the world. Our aim was to evaluate the temperature–mortality associations and estimate the temperature-related deaths in the Netherlands in the years before (2000–2009) and after (2010–2019) the first activation of the national HP. Methods: We obtained data about daily all-cause mortality (2000–2019) for the entire Dutch population, and by age, sex, neighborhood socioeconomic status, and urbanization. We linked the daily maximum temperature based on 23 monitoring stations across the Netherlands. Time-series Poisson regression models with a distributed lag nonlinear model, adjusted for long-term and seasonal trends and day of the week, were used to assess relative risks (RRs, 95% confidence intervals [CIs]) in the warm months (May–September). Temperature-attributable mortality fractions for high-temperature exposures and potential HP days were calculated. Results: We observed positive associations between daily maximum temperature and mortality in 2000–2009 and in 2010–2019. Associations of high temperatures (28.9 °C—95th percentile) were weaker in 2010–2019 (RR: 1.07, 95% CI: 1.05, 1.09) than in 2000–2009 (RR: 1.17, 95% CI: 1.15, 1.20). The attenuation in temperature–mortality risk was strongest for the elderly, women, and individuals living in low-socioeconomic status neighborhoods. The estimated mortality attributable fractions of high temperatures (≥28.9 °C) were lower in 2010–2019 (0.72, 95% CI: 0.60, 0.84) than in 2000–2009 (1.21%, 95% CI: 1.07, 1.33). Conclusion: The impact of high temperatures on mortality attenuated in the Netherlands. This might be due to the implementation of the national HP, but other factors may have played a role as well.