It has not escaped your notice that absolute heat records have been making headlines since the beginning of the summer. But how does the human body deal with it? The ability to withstand the amount of heat and humidity that men and women can withstand is evaluated according to the concept of “wet bulb” or “wet bulb”.
And, according to experts, climate change promises to increase the episodes of “wet bulb”, potentially fatal. But what is this marker? How does it work ? 20 minutes tells you everything.
What is the “wet bulb”?
There is a limit to the amount of heat and humidity that the human body can withstand and this resistance capacity is evaluated with the index of the “wet bulb temperature” or “wet bulb Globe Temperature” or TW). This TW takes into account both heat and humidity, according to research.
Clearly, a young and perfectly healthy person risks dying after six hours at 35 degrees in “wet bulb” temperature. At this stage, the humidity 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.
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 human survival limit of 35° “wet bulb” represents 35°C with 100% humidity – or 46°C with 50% humidity.
Are we often confronted with a potentially fatal temperature in “wet bulb”?
A temperature of 35 degrees in the “wet bulb” has been reached a dozen times in the world so far, mainly in South Asia and the Persian Gulf, assures Colin Raymond, NASA researcher. 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 on the subject published in 2020.
Joy Monteiro, a researcher based in India, recently published a study in Nature on the “wet bulb” and this, mainly in South Asia. He points out that most of the killer heat waves in the region have so far been well below the 35 degree TW threshold.
Can these episodes occur more often?
As temperatures continue to rise – July 2023 was the hottest month on record on Earth – “wet bulb” episodes will increase, scientists warn. 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, specifies Colin Raymond. South and Southeast Asia, the Persian Gulf, the Gulf of Mexico and parts of the African continent will logically be the most exposed regions.
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. “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.
How was this TW calculated?
To test this index, researchers at the University of Pennsylvania, in the United States, evaluated the temperatures of young people in good health 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”. It would take between five and seven hours before such conditions reached “really, really dangerous temperatures,” said Daniel Vecellio, who worked on the study.
Who are the people potentially at risk?
Endurance limits vary greatly from person to person, according to Joy Monteiro. Young children, however, 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, adds the expert.
People who have to work outdoors are also at greater risk. Finally, 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.