Wet-bulb temperature

The "wet-bulb temperature", or WBT, measures the cocktail of high heat and high humidity that is fatal to humans. At 35° WBT, humans die within a few hours, as the humidity can no longer evaporate.

During the 2003 heatwave, 15,000 people died in France.

"When this happens, the evapotranspiration system – which regulates our internal temperature through the evaporation of sweat – can no longer function," points out anthropologist and biologist Alain Froment, former head of the skeleton and mummy collections at the French Musée de l'Homme (Museum of Man).

“Evapotranspiration and vasodilation put a huge strain on the heart, which has to send more blood to the skin; the kidneys also work at full capacity," says Ollie Jay, Director of the Heat and Health Laboratory at the University of Sydney's Faculty of Medicine. In extreme heat, vulnerable people are at risk. During the 2003 heatwave, 15,000 people died in France, despite the wet bulb temperature not exceeding 28° (82°F) WBT.

Work also slows down as a result of heat waves. "Heat stress costs our country 6.9 billion Australian dollars [about $4.6 billion] a year in lost productivity alone," says Ollie Jay.

Cultural adaption techniques

This drop in performance can be dramatic for individuals who are paid by the piece: if they don't want to see their pay drop, they have to stay on the job for longer, in trying conditions, until their bodies adapt. Normal people take around ten days to optimize their cardiovascular and thermoregulatory mechanisms.

So, last year, Ollie Jay launched a research project into the working environment of textile workers in Bangladesh. Sensors recorded air quality in one of their factories. Meanwhile, Jay reproduced the same conditions in a climate chamber in Sydney, testing easy-to-implement solutions: hydration, ventilation, white or vegetated roofs…

"It's what we call cultural adaptation to heat: dressing differently, shifting working hours, insulating buildings, creating cool oases in the city," says Guillaume Chevance, a health and environment researcher at ISGlobal, a global health institute in Barcelona, Spain. He studies, among other things, the effects of heat on sleep and mental health among vegetable growers, construction workers and, from next year, bicycle delivery drivers.

Genetic factor

Is there one last avenue of adaptation: will genetics select the human beings with the best-adapted genes? "It has already done so in some of the world's hottest regions in Africa," recalls Alain Froment. “The Shilluk and Dinka are among the tallest peoples: long legs, narrow bodies and short trunks favor evapotranspiration and thus suit the hot climate of Southern Sudan."

Will the same be true in Europe? A French study, carried out by the National Institute of Health and Medical Research (Inserm), and a German-Dutch study have shown that since the 1970s, Europeans seem to have adapted to higher average temperatures.

Epigenetics studies the additional layer of information that decides whether to activate certain genes.

"But we don't know the origins of this change: it could be cultural — for example, greater use of air conditioning — or physiological," warns Hein Daanen, Professor of Physiology at the Vrije University of Amsterdam, one of the authors of the German-Dutch study. "In any case, modern life and its interchange of populations do not favor genetic selection: it is more effective within isolated populations," warns Jonathon Stillman, Professor of Biology at the University of San Francisco.

There is one hope, however: epigenetics. While genetics studies genes, epigenetics is concerned with an additional layer of information that decides whether or not to activate certain genes. The DNA itself remains unchanged, but the epigenetic mechanisms that modify gene expression are nonetheless transmissible from generation to generation.

The role of epigenetics in heat adaptation has been demonstrated in plants and certain animal models," says Abderrezak Bouchama. In the future, we may be able to intervene in the human body to modify these mechanisms from the outside.

In the meantime, in the event of extreme temperatures, you should drink water, wet your body, avoid physical exertion, and close your curtains and shutters...