French health officials are warning that a phenomenon known as the “effet retard” can cause severe medical symptoms and deaths days after a heatwave ends.
This delayed impact is critical because the public often lowers their guard once temperatures drop, while the most vulnerable populations remain at high risk of organ failure.
Medical experts describe the effect as a lag between exposure to extreme heat and the manifestation of critical health issues. Dr. Gérald Kierzek said there is a shift of five to 10 days [2] between the initial heat exposure and the appearance of severe symptoms.
This physiological stress can lead to dehydration, cardiovascular problems, and organ failure, particularly in the elderly and pregnant women. Dr. Jean-Christophe Calmes said the danger does not disappear with the drop in temperatures, and the delayed effect can lead to deaths in the following days [1].
The current heatwave, which peaked around June 24, has placed significant strain on the French healthcare system. Météo-France placed 72 departments under red vigilance [1], while 60 departments were under orange vigilance as of June 20 [4]. The impact has been felt across the country, with specific alerts issued for regions including Var, Bouches-du-Rhône, Vaucluse, and Gard [1].
Stéphanie Rist, the French Minister of Health, urged the public to maintain caution. Rist said the need for vigilance continues for several days after a heatwave because the delayed effect primarily impacts the most fragile individuals [2].
Beyond the immediate health crisis, the heatwave has created broader systemic instability. Reports indicate that the economic impact of such events is often measured two weeks after the peak, with some estimates placing the effect at 1.5% [3].
“The danger does not disappear with the drop in temperatures”
The 'effet retard' highlights a critical gap in public health communication, where the end of a weather event is mistaken for the end of the medical emergency. By shifting the focus from peak temperatures to a 10-day recovery window, French health authorities are attempting to prevent a secondary spike in mortality that occurs when patients and caregivers prematurely cease heat-mitigation protocols.
