Heatwaves are becoming increasingly dangerous for health

After almost a week of sweltering temperatures, Western Europe is bracing for another heatwave. Unlike the first heatwave of the summer, which was relatively short, the current episode promises to be much more persistent. This raises a crucial question: does the body become more vulnerable as the heatwave days follow one another? The 2003 heatwave, which lasted more than two weeks, caused more than 70,000 deaths in Europe. Experts expect such episodes to become more frequent and longer due to global warming. A study published in 2025 in the journal Nature Geoscience shows that the duration of heatwaves is increasing at a faster rate than previous estimates.

• The effects of heat don't always appear immediately

The consequences of extreme temperatures are not limited to heatstroke and dehydration. Heat can worsen cardiovascular and respiratory diseases, and the effects can manifest themselves several days after the onset of a heat wave. French Health Minister Stephanie Rist warned that excess mortality associated with heat waves usually occurs five to 10 days after the onset of high temperatures. "We are now entering the period in which this impact is starting to manifest itself and we will be very vigilant in the coming days,” the French official said.

Is there a cumulative effect? Researchers do not have a clear answer. The scientific literature does not yet provide a definitive answer. A 2011 study published in the journal Epidemiology, based on an analysis of about 100 heat waves in the United States, found a modest aggravating effect for episodes lasting more than four days. However, the researchers concluded that the risk is mainly determined by the temperature of each individual day. Thus, the tenth day of a heat wave is not necessarily more dangerous than the third.

A 2018 meta-analysis published in Science of the Total Environment found that the results differed from city to city. Some studies show a cumulative effect on mortality, while others believe that the duration of exposure has a small or even negligible impact.

• Sleep, the hidden victim of heat waves

One of the most important recent findings concerns the effects of high temperatures on sleep. Tropical nights prevent the body from recovering, and lack of sleep affects both physical and mental health. The effects accumulate as insufficiently restful nights are repeated. A 2024 analysis published in the journal Sleep Medicine warned that global warming and urbanization pose a major threat to sleep quality worldwide.

• New measures in Europe: schools closed and trains canceled

Numerous European countries are preparing for unusually high temperatures, both day and night. France has closed schools and Belgium has cancelled some train services. Spain, Portugal, Belgium, the Netherlands, southern England, Wales, Switzerland, Austria and the Balkans are also under threat from extreme temperatures.

• A new climate reality

Doctors are well aware of the immediate effects of heatwaves, but the impact of weeks-long heatwaves is still poorly understood. The human body was not designed to function for days on end in temperatures approaching 40 degrees, with no respite even at night. As climate change turns exceptions into the norm, the question is no longer whether Europe will face more prolonged heatwaves, but how prepared societies are to cope with them.