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How hot is "too hot" for humans?

July 11, 2023

On July 4, Earth recorded its hottest day ever. So what is the temperature limit that humans can safely tolerate? On the 6th, a research team from the University of Roehampton in the UK reported that there is an upper critical temperature (UCT) for humans, which may be between 40 ° C and 50 ° C. Understanding the temperature values that lead to an increase in metabolic rate, and how the temperature varies in different individuals, may have important guiding significance for work, sports, travel, etc. The study is a continuation of their previous work, published in the journal Physiological Reports.

The studies have found that when people are exposed to an environment between 40 °C and 50 °C,  the resting metabolic rate may be higher. Resting metabolic rate is a measure of how much energy the body expends to keep the body functioning properly, in short, the minimum amount of energy it expends at rest.

Previous experiments carried out by the research team have shown that when people are exposed to temperatures of at least 40 °C, their resting metabolic rate increases. At 40 ° C and 25 percent humidity, participants' metabolic rates increased by an average of 35 percent compared to baseline (so-called normal levels). At 50 ° C and 50 percent humidity, people's core temperature rose by an average of 1 ° C, their metabolic rate increased by 48 percent compared to baseline, and their heart rate increased by 64 percent.

According to the researchers, this means that the hotter the environment, the harder the body has to work to maintain normal function. In the latest experiment, published on the 6th, the researchers used echocardiography, or ultrasound of the heart, to examine the heart activity of 24 participants at 50 ° C and 25% humidity. They found that women's heart rates increased more, on average, than men's. This may indicate that women's bodies are less efficient at removing excess heat.