Robert Klatt
Humans are less able to withstand hot, muggy weather than previously thought. As little as 31 degrees Celsius in high humidity can be potentially deadly for a healthy young person.
Pennsylvania (USA). the person It usually manages to keep its core temperature at 37 degrees Celsius even on very hot days. This is made possible by the evaporative cooling of the body when sweating. If the humidity is too high, however, the sweat can no longer evaporate and there is no cooling. In extreme cases, this can lead to death from overheating.
the medicine has previously assumed that a healthy person can withstand a cool limit temperature of 35 degrees Celsius for around six hours. The cooling limit temperature is a combination of temperature and humidity. It corresponds to 46 degrees Celsius with a humidity of 50 percent and 35 degrees Celsius with a humidity of 100 percent. “Although this theoretical limit is based on physiological principles, it has not yet been verified by empirical data,” explains Daniel Vecellio from the Pennsylvania State University.
Cooling limit temperature checked experimentally
According to their publication in Journal of Applied Physiology the scientists have therefore now tested the cooling limit temperature experimentally. To do this, they exposed 24 healthy volunteers between the ages of 18 and 34 to various combinations of humidity and heat in climate chambers. The subjects wore summer clothes and moved slowly on a bicycle or treadmill. The researchers documented the skin temperature and sweat production on different parts of the body. A swallowed sensor capsule constantly measured the core temperature.
Core temperature limits too high
According to the measurements, the previous limit values for the core temperature are clearly too high. The subjects reached critical values well below 35 degrees Celsius. “None of the subjects in the six different experiment variants reached the theoretical limit of 35 degrees, their values became critical even below that,” explains Vecellio. At 100 percent humidity, the limit was already 31 degrees Celsius instead of the previously assumed 35 degrees Celsius.
The data also shows that a slight drop in humidity does not increase heat tolerance. Instead, the critical cooling limit temperature was already reached at 25 degrees Celsius to 28 degrees Celsius. “The critical cooling limit temperatures in hot, drier environments were thus almost ten degrees lower than the literature values,” the researchers report. This is primarily due to the fact that people no longer increase their sweat production above a certain temperature, despite lower humidity.
dangers of climate change
As the researchers explain, this could have major implications for decades to come. “The climate is changing and it will be more frequent and intense heat waves give. Our results indicate that in humid regions of the world we should already be concerned if the cool limit temperature rises above 31 degrees,” explains Larry Kenney. However, the problem also affects young people who, according to the results, could be physically overwhelmed.
According to studies, 1.2 billion people would live in regions with life-threatening heat waves in the coming years, even with the significantly higher limit values set previously. As a result, part of the earth could even become completely uninhabitable in summer. However, because the critical cooling limit temperature is actually below the limit values, even more regions and people are affected by the problem.
“The critical tolerance limits we determined document that parts of our planet are already regularly experiencing such non-compensable cooling limit temperatures,” the authors explain.
Universal critical cooling limit temperature
However, the study also shows that the critical cooling limit temperature depends more on the climatic conditions than was previously assumed. “Therefore, there cannot be a universal critical cooling limit temperature for human temperature tolerance across the globe,” say the researchers. The critical cooling limit temperature and that climate risk must therefore be examined regionally and not globally.
Journal of Applied Physiology, doi: 10.1152/japplphysiol.00738.2021