“The upper limits of our thermal neutral zone.”

Understanding the temperatures at which human metabolic rates start to rise and how this varies among individuals has implications for working conditions, sports, medicine, and international travel.

As global temperatures continue to rise due to climate change, extreme heat poses an increasing health threat. The human body is resilient, but it has its limits. So, what is the highest temperature that people can endure?

Ongoing research led by Prof. Lewis Halsey and his team at the University of Roehampton in the UK has identified the existence of an upper critical temperature (UCT) for humans, estimated to fall between 40°C and 50°C. Further investigations are underway to understand the underlying factors behind the rise in metabolic energy costs at high temperatures.

Prof. Halsey and his team have discovered that the resting metabolic rate, which measures the amount of energy the human body consumes to maintain basic functions, can be higher when individuals are exposed to hot and humid conditions.

“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,” remarks Prof. Halsey.

Prof. Halsey and his team are also investigating the effects of temperatures above the UCT on heart function and how these effects differ among individuals based on factors such as age and physical fitness.

“We found some considerable changes in heart function responses to the heat between categories of people, the most novel being between the sexes,” says Prof. Halsey. “That is, on the average, men and women display some key differences in their cardiovascular responses to the heat”.

The team employed a state-of-the-art echocardiograph to measure detailed heart function, a challenging task in high-temperature environments.

“It was not easy to operate this kit in the heat!

“The sort of equipment you’d see in hospitals but rarely in research laboratories.”

Prof. Halsey will discuss the team’s latest findings during his talk at the SEB conference, highlighting the progress made in understanding the body’s response to heat stress.

“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.

“In a warming world, this knowledge becomes ever more valuable.”

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