Our bodies function optimally at 37°C. The consequences of prolonged hyperthermia range from headaches to unconsciousness and, above 41°C, death. Unfortunately, our main thermoregulatory system, sweating, loses its effectiveness in the excessively hot and humid environment promised by climate change. Simulations point to a limit temperature of 42°C at 51% relative humidity, or 26 to 31°C when the air is saturated with moisture, confirms a new study published in the journal PNAS.
In a room in the laboratory at the University of Ottawa in Canada, 12 volunteers (75% men, 25 to 32 years old) are resting on breathable fabric seats. In the room, it is 42°C with 28% of relative humidity, which expresses the saturation of the air with water – knowing that the warmer the air, the more water vapor it can contain. We then slowly increased the humidity in the climate chamber, until it reached around 51%,” explains the first author of this work, thermal physiology expert Robert Meade. 42°C at 51% relative humidity are the conditions beyond which the temperature in the volunteers' esophagus begins to increase, a sign that the subject's thermoregulation is under pressure. For 50 years it has been considered that beyond these conditions it is not possible for the person to thermoregulate, so that the core temperature increases relentlessly." explains Robert Meade.
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Heat and humidity increase our internal temperature
After nine hours spent in conditions slightly above (42°C at 57% relative humidity) and below (42°C at 45% relative humidity) this inflection point, the researchers confirm that the hours following this inflection result in an increase in internal temperature. This is one of the longest records ever made of human physiological responses to such hot conditions." points out Robert Meade.
This work formally confirms previous estimates made in 2023, setting the maximum temperature that can be tolerated by humans at 26-31°C when the air is saturated with humidity (equivalent to 42°C with a relative humidity between 28 and 51%). A striking result when we know that the limit previously accepted since 2010 was 35°C at saturated air with humidity, also called "wet bulb temperature" or wet thermometer temperature. No data have yet been available to determine whether the inflection of core temperature with gradual increases in air temperature or humidity actually corresponds to the upper limits of thermoregulation." explains Robert Meade. It's now done. " The method used to estimate the 26-31°C range was the same, the only difference being that the study participants were performing light physical activity and ours were resting.“
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Sweating becomes ineffective
If air humidity is so essential to our thermal regulation, it is because the evaporation of sweat, our main means of cooling, is greatly reduced. When the wet bulb temperature is high, it is much more difficult for the body to dissipate heat and regulate body temperature" adds Robert Meade. As global warming is already beginning to push back the limits of human tolerance to heat in certain regions of the world, urgent action is needed, warns the researcher. At present, there are few, if any, regions on the planet where critical wet-bulb limits are exceeded for prolonged periods." adds Robert Meade. This should, however, become more and more common," particularly in regions such as South and East Asia, North Africa and the Middle East" he lists.
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Global warming threatens our ability to thermoregulate
These measures still need to be refined according to the populations concerned. In fact, most, if not all, of the research on wet-bulb limits has been conducted in northern countries. To improve the accuracy of our warnings in the future, we need to determine whether these limits apply to people living in places where heat and humidity are already high." concludes Robert Meade. 30% of the world's population is already exposed for at least 20 days a year to this excessive humid heat, according to a 2017 study. By 2100, almost half of humanity would face this danger.
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