The "wet thermometer", marker of the bearable limit for the human body

Even more than in absolute heat, the records of which fall regularly, the resistance capacity is evaluated according to the concept of "wet temperature" or "wet bulb".

Even a young and perfectly healthy person is at risk of dying after six hours at 35 degrees in the "wet bulb globe temperature" ("Wet Bulb Globe Temperature", TW), an index that takes into account both heat and humidity, according to research.

At this stage, the moisture in the hot air prevents the evaporation of perspiration, the body's main tool for lowering its temperature, which can lead to heat stroke, organ failure or even death.

A temperature of 35 degrees on a "wet bulb" has been reached a dozen times around the world so far, mainly in South Asia and the Persian Gulf, NASA researcher Colin Raymond told AFP.

These episodes having so far never exceeded two hours, no "massive mortality event" has been linked to them, notes this expert, main author of a study published in 2020.

But as temperatures continue to rise - July 2023 was the hottest month ever recorded on Earth - episodes of "wet bulbs" will multiply, warn scientists.

The frequency of damp heat peaks has more than doubled around the world since 1979 and temperatures "will regularly exceed 35°TW" in different parts of the globe if global warming reaches +2,5°C, according to the work of Colin Raymond.

South and Southeast Asia, the Persian Gulf, the Gulf of Mexico and parts of the African continent are the most exposed regions.

Now primarily calculated via heat and humidity data, the "wet bulb" effect was initially measured by placing a damp cloth over a thermometer and exposing it to air.

This made it possible to measure the rate at which water evaporated from the fabric, like perspiration from the skin. The theoretical limit of human survival of 35° at the "wet bulb" represents 35°C with 100% humidity - or 46°C with 50% humidity.

To test this limit, researchers at the University of Pennsylvania in the United States assessed the temperatures of healthy youngsters in a thermal chamber.

The participants reached their "critical environmental limit" - when their body was unable to prevent their internal temperature from continuing to climb - at 30,6° in the "wet bulb".

"Really dangerous"

It would take between five and seven hours before such conditions reached "really, really dangerous temperatures", Daniel Vecellio, who worked on the study, told AFP.

Joy Monteiro, an India-based researcher who recently published a study in Nature on the "wet bulb" in South Asia, points out that most of the killer heat waves in the region so far have been well below the 35-degree threshold. TW. But endurance limits vary greatly from person to person, he told AFP.

Young children are less able to regulate their body temperature and therefore more at risk.

The most vulnerable, however, remain the elderly, with fewer sweat glands and already more victims of heat waves.

People who have to work outdoors are also at greater risk.

The possibility or not of occasionally cooling one's body, for example in air-conditioned spaces, also plays a role. Not to mention access to toilets, because people who are deprived of them often drink less water and become more dehydrated.

Colin Raymond's research also shows that the El Niño weather phenomenon has increased the "wet bulb" effect in the past. Returning recently, this serious, cyclical meteorological episode will make its full effects felt towards the end of this year and will continue the following year.

The "wet bulb" peaks are also closely linked to surface ocean temperatures, says the researcher. However, the oceans broke a new world temperature record last week, higher than the previous one in 2016, according to the European Union's climate observatory, Copernicus.