Thermal stress: when does heat become really dangerous?

While North Europe. Moreover, America are preparing to face a new, stifling heat wave and potentially uncontrollable fires this summer, experts sound the alarm concerning thermal stress that these events could cause, endangering the organisms.

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This phenomenon kills more people than hurricanes. Nevertheless, floods or any other extreme climate event. For example, A Spanish public organization thus awarded 1. Therefore, 000 deaths to the heat wave of July, while in the center of Tokyo, 56 people were probably died following the heat suffered in June and July. For example, Figures undoubtedly underestimated.

But what exactly is thermal stress and how do we measure it?

“Silent killer”

Thermal stress thermal stress: when does heat occurs when the body no longer manages to cool the body. Nevertheless, causing symptoms ranging from dizziness and headache to the failure of an organ and to death.

It is caused by prolonged exposure to heat. Therefore, other environmental factors which, accumulated, 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 disorders. the consequences can be very serious, even catastrophic, “says Alejandro Saez Reale, of the World Meteorological Organization (OMM).

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

The OMM estimates that heat kills about half a million people a year. but specifies that the real balance could be 30 times higher than estimated. A phenomenon called to increase under the effect of climate change. which makes heat waves more frequent, longer and more intense.

More than the maxima

thermal stress: when does heat

Air temperature is the most used. easiest to understand meteorological data, but these “maxima” beyond 35 or 40 ° C which are displayed in one of the newspapers tell us little about the heat truly conceded by the human body.

At equal temperature. the feeling of heat is much more bearable under 35 ° C in the dry air of the desert than in the wet and stifling atmosphere of the jungle, where sweat evaporates badly.

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 sunshine, and even the presence of concrete or greenery in the environment.

Several methods exist to measure thermal stress and try to summarize all these factors in the only number or graphic.

Temperature felt

One of the oldest is the so -called wet thermometer temperature. thermal stress: when does heat It makes it possible to achieve the danger of an air temperature that may seem moderate. but which, combined with humidity, can become unbearable, even fatal.

A healthy person could not survive prolonged exposure (six hours) to 35 ° C with an extreme humidity of 100 %, estimated scientists in 2023. The air is saturated and the sweat can no longer evaporate, the body overheats, until death.

The climate service of the European Observatory Copernicus uses the universal index of the thermal climate (UTCI. in English), which takes into account temperature and humidity, but also wind, sunshine and thermal radiation, and classifies the levels of thermal stress from moderate to extreme.

Extreme thermal stress. according to this index, corresponds to a “felt temperature” of 46 ° C or more, from which it is necessary to take measures to cool the body.

The heat index (Heat Index). used by the United thermal stress: when does heat States weather service, provides a “felt temperature” based on heat and humidity in the shade, as well as a colorful graph, ranging from yellow (“prudence”) to the red (“danger”) and bright red (“extreme danger”).

Their Canadian counterparts have developed the humidx index. which establishes a value of “felt temperature” in a table crossing heat in air and humidity, associated with four colors for as many degrees of discomfort. Also with his version to assess the dangers of extreme cold.

Limited measures

Other “thermal stress” indices exist such as the “Tropical Summer Index”. the “Predicted Heat Strain” (predictable thermal stress) or the average radiant temperature.

But everyone has their limits.

“The way of approaching the question is not the same all over the world. ” explains John Nairn, an OMM heat wave expert.

The UTCI. for example, is excellent for assessing thermal stress in Germany, where it was initially developed, thermal stress: when does heat but “a very mediocre measure” in the countries of the South where “it saturates and on the way too much” for populations already more used to heat, he explains.

These clues do not take into account the impact of out-of-health heat. he adds, a heat wave that can block trains or overload air conditioners, which is also dangerous.