As heatwaves continue to scorch North America, you may be asking: How hot is too hot for the human body?
Research from the University of Roehampton in the U.K. found that at temperatures of 122 degrees Fahrenheit with moderate to high humidity, we can no longer control our core body temperature.
According to the U.S. Centers for Disease Control and Prevention, there are 702 heat-related deaths on average in the U.S. every year. With the mercury hitting a blistering 125 degrees Fahrenheit in some states last week, it is becoming more important than ever to understand how these temperatures might affect our bodies.
Lewis Halsey, a professor of environmental physiology at Roehampton, has been studying the effects of heat stress on the body for some time. Specifically, he has looked at how our basal metabolic rate—the amount of energy we use just to keep our bodies alive—changes at higher temperatures.
University of Roehampton
“Quite a lot of work has been done on the range of temperatures that different animal species prefer to live at in terms of their metabolic rates being minimal and thus their energy expenditure being low, but, weirdly, information is much less available for humans when considering the upper limits of our thermal neutral zone,” Halsey told Newsweek in a statement.
In a 2021 study, Halsey and his team subjected participants to four environments for one hour: 104 degrees Fahrenheit with 25 percent humidity; 104 degrees Fahrenheit with 50 percent humidity; 122 degrees Fahrenheit with 25 percent humidity; and 122 degrees Fahrenheit with 50 percent humidity.
Even at 104 degrees Fahrenheit and 25 percent humidity, the resting metabolic rate of the participants went up by 35 percent. At 122 degrees Fahrenheit with 50 percent humidity, this increased by a further 13 percent and the participants’ core body temperatures had increased by roughly 1.8 degrees Fahrenheit. This indicated that the participants were no longer able to dissipate enough heat from their bodies to maintain their core body temperature.
The exact mechanisms behind the increase are not entirely clear, and Halsey has put forward several theories. Firstly, it may be a result of changes in heart rate and contraction force—when we are hot, our blood vessels dilate, reducing our blood pressure and forcing our heart to work harder. A second theory is that at higher temperatures, the molecules involved in metabolic reactions inside our bodies have more energy and therefore react faster.
In a second set of experiments—the results of which Halsey presented on July 6 at the Society for Experimental Biology Centenary Conference 2023—the researchers measured the heart activity of 24 participants at 122 degrees Fahrenheit with 25 percent humidity. Interestingly, they found a clear difference between the men and women.
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On average, the team found that women’s heart rates increased more than men’s, suggesting that women’s bodies are less efficient at getting rid of excess heat. However, the study was done on a fairly small sample of people and more research is needed to draw concrete conclusions.
“There’s no way I can say to you: ‘Aha! Therefore women or men are going to struggle more in the heat because of this,'” Halsey said.
In our increasingly warming world, more research is needed to understand the impacts of heat on the human body and to realize which members of society may be more at risk.
“We are steadily building a picture about how the body responds to heat stress, how adaptable it can be, the limits to those adaptations, and, crucially, how varied responses are between individuals,” Halsey said. “In a warming world, this knowledge becomes ever more valuable.”
