Health. Duration of heat waves and danger: a link that is difficult to establish

Heatstroke, cardiovascular problems... The harmful effects of extreme heat on the body are well known. But to what extent do they accumulate when a heatwave lasts for a long time, as will likely be the case this August? The answer remains uncertain.

"Far from being a mere nuisance, rising temperatures can pose a growing risk to human health by causing death and suffering, and by overloading health systems worldwide," the European branch of the World Health Organization (WHO) recalled in June.

Since then, two heatwaves have swept across the continent, the latest of which is currently peaking in some countries, including France. They were not only striking in their intensity, with temperatures exceeding 40 degrees, but also in their duration. In France, the high temperatures are expected to last in some regions beyond the weekend.

“Be attentive in the days that follow”

This observation fuels questions about the health effects of a heatwave lasting beyond just a few days. Europe particularly remembers the iconic heatwave of 2003, which 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 manifest themselves: at the time, high temperatures can cause heat stroke and sometimes fatal dehydration, but they can also aggravate existing pathologies, particularly cardiovascular and respiratory, in which case death or hospitalization can occur after several days.

"The effect of heat on organisms is not necessarily felt at a given moment: we must be attentive in the days that follow," explained French Health Minister Catherine Vautrin on Monday. And, after several days of heatwave, a crucial question arises: on organisms already exhausted by the heat, does exposure to high temperatures have increasingly marked effects?

Few scientific studies

The scientific literature on the subject remains measured, and few studies have specifically examined the consequences directly linked to the duration of the heatwave. Although some studies are already old, they nevertheless provide some answers. A study published in 2011 in the journal Epidemiology, based on data on around a hundred heat waves in the United States, concluded that there was a "small effect" of aggravation for episodes lasting more than four days. But more often than not, the risk boils down to "the isolated effect of each day's temperatures": in other words, the tenth day of a heatwave is not necessarily more dangerous or deadly than the third.

More broadly, studies do not all point in the same direction. "Some studies conclude that there is a significant accumulation effect on mortality, but others record different conclusions from one city to another," sometimes arriving at the conclusion that the duration of exposure to heat has only a "minimal or negligible" effect, according to a review published in 2018 in the journal Science Of The Total Environment.

Sleep threatened

However, in recent years, research has made progress on certain health aspects of extreme heat, 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 the Lancet Countdown, a report published annually by the leading medical journal to take stock of the health effects of global warming. "Exposure to heat also increasingly affects (...) the quality of sleep, which then has consequences for physical and mental health," the authors summarized. However, the negative effect of poor sleep tends to accumulate over the course of difficult nights, disrupting the body's recovery abilities.

A synthesis of several scientific studies, published in mid-2024 in the journal Sleep Medicine, thus highlighted that "the rise in temperatures induced by climate change and urbanization constitutes a global threat to sleep."