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Humans may not be able to take as much heat as we thought


More and more people die from extreme heat stroke. Night comes and we are still hot. Brutal heat waves are fast becoming the hallmark of the summer of 2022. Now, recent research suggests that people’s tolerance for heat stress may be lower than previously thought.

“Bodies are able to acclimate over a period of time” to changes in temperature, says Vivek Shandas , a researcher in environmental planning and climate adaptation at Portland State University in Oregon. Throughout geological time, there have been many climatic changes that humans have weathered, says Shandas. “[But] we are at a time when these changes are happening much faster .”

The current global rate of warming on Earth is unprecedented. And scientists have long predicted that human-caused climate change will increase the occurrence of heat waves.


Heat is already increasingly affecting human health

The human body has various ways of getting rid of excess heat and keeping the core of the body at an optimum temperature of around 37°C; the heart, for example, pumps faster, accelerating the flow of blood that carries heat to the skin. Air passing over the skin can absorb some of that heat. Sweating helps too. But there is a limit to how much heat humans can take.

When temperatures are so high that water evaporates from your skin or another surface , it steals energy as heat , briefly cooling that surface. That means that in drier regions, the wet-bulb temperature, where that short-lived cooling effect easily occurs, will be lower than the actual air temperature.

So when thinking about heat stress in the body, scientists use wet bulb temperatures because they are a measure of how much cooling through evaporation is possible in a given climate.

we are not efficient

Our bodies do not function at 100% efficiency , and regardless of body size, ability to sweat, age, or regional climate, for the past decade, the theoretical wet bulb number of 35°C has been regarded as the point beyond which humans can no longer regulate their body temperature. But recent laboratory research suggests that a general, real-world threshold for human heat stress is much lower, even for young, healthy adults.

The researchers tracked heat stress in two dozen subjects, ages 18 to 34, under a variety of controlled climates. Under hot, humid conditions, study subjects were unable to tolerate heat stress at wet-bulb temperatures approaching 30° or 31°C, the team estimates. In hot, dry conditions, that temperature was even lower, with a range of 25° to 28° C. These results suggest that much more work needs to be done to understand what humans can endure in the world’s hot and humid conditions. real, but that the threshold may be much lower than previously thought, the authors say in their study published in the Journal of Applied Physiology in February.

Be that as it may, regions that are not used to extreme heat may experience higher mortality, even at lower temperatures, simply because people are not used to the heat. Heat wave classifications could also help cities tailor their interventions to the severity of the event. Currently, six cities are testing the effectiveness of the system: four in the United States and in Athens, Greece and Seville, in Spain.

Referencia: D.J. Vecellio et al. Utility of the heat index in defining the upper limits of thermal balance during light physical activity. International Journal of Biometeorology. Published online July 1, 2022. doi:10.1007/s00484-022-02316-z.

D.J. Vecellio et al. Evaluating the 35° C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). Journal of Applied Physiology. Vol. 132, February 2022, p. 340. doi:10.1152/japplphysiol.00738.2021.

C. Tuholske et al. Global urban population exposure to extreme heat. Proceedings of the National Academies of Science. Published online October 4, 2021. doi:10.7927/fq7g-ny13.

A.M. Vicedo-Cabrera et al. The burden of heat-related mortality attributable to recent human-induced climate change. Nature Climate Change. Published online May 31, 2021. doi:10.1038/s41558-021-01058-x.

C. Raymond, T. Matthews and R.M. Horton. The emergence of heat and humidity too severe for human tolerance. Science Advances. Published online May 8, 2020. doi:10.1126/sciadv.aaw1838.

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