Heat stroke, cardiovascular problems … The deleterious effects of hot weather on the body are well known. But how much do they accumulate when a heat wave lasts a long time, as it will probably be the case in August? The answer remains uncertain.
“Far from being a simple discomfort, the rise in temperatures can constitute an increasing risk for human health by causing death and suffering, and overloading health systems around the world,” recalled in June the European antenna of the World Health Organization (WHO).
Since then, two waves of heat have crossed the continent, the last currently reaching its peak in certain countries such as France. They were not only striking by their intensity, with temperatures exceeding 40 degrees, but also their duration. In France, high temperatures should thus last in certain regions beyond the weekend.
This observation fuels questions about the health effects of a heat wave extending beyond only a few days. Europe notably retains the memory of the emblematic heat wave of 2003 which had lasted more than two weeks and killed more than 70,000 people.
The answer is not obvious, because the effects of heat on health are sometimes slow to translate: at the moment, high temperatures can cause heat strokes and sometimes fatal dehydration, but they can also worsen existing pathologies, especially cardiovascular and respiratory, in which case death or hospitalization can take place after several days.
“The effect of heat on organisms is not necessarily felt at the moment T: we must be attentive in the days that follow,” the French Minister of Health, Catherine Vautrin, said on Monday.
And, after several days of heat wave, a crucial question arises: on organisms already tired by heat, does exposure to high temperatures have increasingly marked effects?
Sleep threatened
Scientific literature remains measured on the subject and few studies have specifically looked at the consequences directly linked to the duration of the heat wave. Already old, certain works, however, bring some elements of response.
A study, published in 2011 in the Epidemiology journal, from data on a hundred waves of heat in the United States, concluded that a “small effect” aggravating for more than four days.
But more often than not, the risk comes down “to the isolated effect of temperatures of each day”: in other words, the tenth day of a heat wave is not necessarily more dangerous or murderous than the third.
More broadly, studies do not all go in the same direction. “Some works conclude that a significant accumulation effect on mortality, but others record different conclusions from one city to another”, sometimes coming to the conclusion that the duration of heat exposure has only “minimal or negligible” effect, according to a synthetic work published in 2018 in the journal Science of the Total Environment.
The fact remains that in recent years, research has progressed on certain health aspects of hot weather, which could potentially change the situation.
This is particularly the case with the harmful effects of heat waves, or even simply heat, on sleep.
This impact was notably highlighted in 2024 by Lancet Countdown, a report published each year by the reference medical journal to take stock of the health effects of global warming. “Exposure to heat also affects more and more (…) the quality of sleep, which then has consequences on physical and mental health,” summarized the authors.
However, the negative effect of poor sleep tends to accumulate over the difficult nights. By disturbing the organization’s recovery capacities.
A summary of several scientific studies, published in mid-2014 in the journal Sleep Medicine, underlined that “the increase in temperatures induced by climate change and urbanization constitutes a planetary threat to sleep”.
