“A silent killer”: what is heat stress that kills half a million people each year ?

“A silent killer”: what is heat stress that kills half a million people each year ?

En 2023 en France, 5 000 personnes sont décédées à cause de la chaleur. ILLUSTRATION MAXPPP – Alexis Sciard

La chaleur, techniquement le "stress thermique", tue plus que les ouragans, les inondations ou tout autre phénomène météo extrême.

Heat, technically “heat stress,” kills more than hurricanes, floods, or any other extreme weather event. The World Meteorological Organization (WMO) estimates that 500,000 people are victims each year, but specifies that the true toll could be 30 times higher than estimated.

With global warming, the toll continues to rise. In 2023 in France, more than 5,000 people died due to heat, according to data from Public Health France. That same year, Europe experienced a record number of days of "extreme heat stress".

More recently in India at the beginning of June during the legislative elections, the heat killed at least 33 polling agents in the state of Uttar Pradesh alone, in the north of the country, while more than 900 Muslim faithful died during the hadj in Saudi Arabia, a great annual pilgrimage to Mecca.

Internal heat increase

Heat stress occurs when the body is no longer able to naturally cool the body, causing symptoms ranging from dizziness and headaches to organ failure and death.

It is caused by prolonged exposure to heat and other environmental factors which, combined, prevent the body from regulating its temperature.

“Heat is a silent killer, because the symptoms are not so obvious. And when there are underlying health problems, the consequences can be very serious, even catastrophic,” says Alejandro Saez Reale of the World Meteorological Organization (WMO).

Infants, the elderly, people with health problems, and outdoor workers are particularly vulnerable.

Several parameters to assess the real impact of heat

To assess the real impact of heat on organisms, scientists take into account, in addition to temperature, a series of factors such as humidity, wind speed, clothing, direct sunlight, and even the presence of concrete or greenery in the environment. Several methods exist to measure heat stress and attempt to summarize all these factors in a single number or graph.

One of the oldest is the so-called wet bulb temperature. It allows us to realize the danger of an air temperature that may seem moderate but which, combined with humidity, can become unbearable, even fatal.

Six hours of exposure to 35 °C with a humidity level of 100 % is enough to kill a healthy person, scientists estimated in 2023. Beyond that, sweat can no longer evaporate, the body overheats, until death.

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