The waves of heat wave multiply, in France, and all over the world. In question, the global warming, which accentuates the number of extreme weather events. This week again, temperatures will rise above 35 ° C in several regions of France, especially in the Southwest. The opportunity to ask: what temperature can the human body withstand? Being homeotherms, our body temperature must remain constant, and be around 37°C. But, in practice, we feel the effects of heat long before theair reaches this temperature. Beyond a certain limit, our body no longer evacuates the heat it produces as easily: this is where the sensation of heat occurs. A 2010 study placed this limit at around 35°C for 100% humidity, but a new project from Penn State University, PSU HEAT Project, the results of which have just been published in the journal International Journal of Biometeorology, calls this value into question.
A problem with several factors
Rather than a given temperature above which the body gives out, it is a response to several factors. There is the temperature of course, but also the humidity which accompanies it, the duration exposure of a person and his health. An 80 year old, even in good health, will not withstand 40°C as well as a 25 year old in good condition. physical. The study released in 2010 had placed the threshold limit of tolerability at 35°C for a humidity rate of 100%, i.e. a “wet bulb temperature” of 35°C, and 46°C for a 50% humidity level. Remember that this rate represents the ratio between the amount of water vapor in the air, and the maximum amount of water vapor that the air can contain. Thus, at 100%, it is no longer possible to add water vapor to the air, so perspiration does not evaporate: impossible to regulate its temperature in this way.
But the new project led by Penn State University is, in a way, even more alarming. The team assessed from which temperature we are no longer able to perform daily tasks, such as cooking or short walks. The researchers have for this brought healthy young men and women to the Noll laboratory of the Penn State University so that they experience heat stress in a controlled environment,” describes their communicated. Each person then swallowed a video capsule, which makes it possible to monitor the internal temperature of the body, then simulated a day like any other in a room where the temperature and the humidity level were regulated. By gradually increasing the temperature and theair humidity and by looking at when the body’s internal temperature begins to rise, researchers have managed to assess a new limit of the human body.
31°C at 100% humidity, or 38°C at 60% humidity
And their results are weaker than those of the last study. This goes from 35°C to 100% humidity, at 31°C. And for 60% humidity, it is no longer 45°C, but 38°C which is the limit of the human body. Beyond these temperature-humidity associations, the temperature regulation mechanisms no longer function perfectly, and exposure should not last more than a few hours. Everything is dictated, at the base, by our hypothalamus, located at the heart of the human brain. It serves as regulator for many functions, including body temperature. When this ” thermostat natural” detects an internal heating of the body, it triggers regulatory actions. Sweating in particular, but which requires drinking a lot of water to compensate for the loss. But also the increase in debit cutaneous blood, which makes it possible to evacuate heat by radiation, through the skin. The human heart must therefore pump harder to allow this increase in flow.
In practice, the temperatures indicated in the study have not yet been reached in France: when temperatures rise, the humidity level does not exceed 30%, or even 40%. But in other countries where the climate is much wetter, like India and Pakistan, which experienced a particularly deadly wave of heat waves in May 2022, the risk is real. Because if the regulation no longer works, the person literally risks to die of heat. And not everyone is equal in this regard. Thus, in the future, the team intends to conduct a similar study, but this time on people who are more vulnerable to heat: the elderly.
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