Prof. Lewis Halsey and his team at the University of Roehampton, UK have conducted research to determine the upper critical temperature (UCT) for human survival. The researchers found that the resting metabolic rate, which measures the energy the body consumes just to keep functioning, can be higher when people are in hot and humid conditions.

Prof. Halsey explains,

“While a lot of work has been done on the optimal temperature range for different animal species, where metabolic rates are minimal and energy expenditure is low, there is less information available about humans and their upper thermal neutral zone.”

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

Professor Halsey and his team are currently investigating how higher temperatures affect heart function, and how this varies among people with different ages and physical fitness levels. They have discovered significant differences in heart function responses to heat between categories of people, with the most notable differences being between the sexes.

The team used a state-of-the-art echocardiograph to measure detailed heart function (which posed a challenge to operate in the heat). The team is steadily building a picture of how the body responds to heat stress, how adaptable it can be, the limits to those adaptations, and the varying responses between individuals. This knowledge is becoming increasingly valuable in a warming world.

So how hot is too hot for human survival? Prof. Halsey believes that is likely to be between 104°F and 122°F.

The team is now conducting further research to explain the high metabolic energy costs at high temperatures.