Effect of different heat wave timing on non-accidental mortality in 20 French cities

Abstract

BACKGROUND AND AIM: Previous epidemiological evidence suggests that the impact of heat waves on mortality may change over time (within and between summer seasons). The consideration of heat wave timing could help to optimize the alert systems, especially under a warming climate. The aim of this study was to explore the effect of heat wave timing on mortality risk during the summer season in France. METHODS: Summertime daily mortality data for 20 French cities from 2000 to 2015 were obtained from the French National Institute of Health and Medical Research. Heat waves were defined according to the official definition of Météo France. The order of heat wave over time, from June to September, was assessed. To quantify mortality risk for the first and second heat waves, quasi-Poisson with distributed-lag models were performed for each city. RESULTS: Overall, we found that, compared with non-heat wave days, the first heat wave of the summer season was associated with a higher risk ratio (RR) for cardiovascular mortality (RR: 1.32, 95% Confidence Interval [CI]:1.18-1.47) as compared to subsequent heat waves within the same summer (RR, 95% CI: 1.28, 1.14-1.44). We found the opposite pattern for respiratory mortality with RRs of 1.59 (95%CI:1.35-1.88) and 1.66 (95%CI: 1.32-2.07) for the first and second heat waves respectively. We identified important heterogeneity in such patterns across all cities. For cities such as Paris or Nancy, the first heatwave of the season resulted in a greater mortality risk. In contrast, for cities such as Lille, Marseille, or Rouen, the second heat wave was more impactful regarding mortality risk. CONCLUSIONS: The timing of heat waves events modulates heat-related risks in France. Such information could be used to update local heat action plans to optimize health benefits across French cities.