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

The study found that when people are exposed to an environment between 40°C and 50°C, their 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, or simply put, the minimum amount of energy expended at rest.

Previous experiments 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% humidity, participants' metabolic rates increased by an average of 35% compared to baseline (so-called normal levels). At 50°C and 50% humidity, people experienced an average 1°C increase in core temperature, a 48% increase in metabolic rate and a 64% increase in heart rate compared to baseline.

This means, the researchers say, that the hotter the environment, the harder the body has to work to keep functioning properly.

In the latest experiment, published today, the researchers used echocardiography, or ultrasound of the heart, to examine the heart activity of 24 participants at 50°C and 25% humidity. It was found that, on average, women's heart rates increased more than men's. This may indicate that women's bodies are not as efficient at shedding excess heat.

(Originally titled "How hot is "too hot" for humans?")