Scientists Identified How Hot is Too Hot for Humans

Is it possible to be too hot? Scientists say that the body does indeed have a threshold.

Evidence has been conflicting on what are the upper limits of the human thermal neutral zone (TNZ), which is the range of ambient temperatures where the body can maintain its core temperature with minimal metabolic regulation.

To fill the knowledge gap, the researchers at the University of Roehampton conducted an experiment that measured the participants' resting metabolic rate (RMR) at temperatures ranging from a normal room to 122F (50°C) and humidity of 25% and 50%.

RMR refers to the total number of calories burned when the body is completely at rest and comprises about 70% of total energy expenditure. While physical activity and diet choices can increase RMR, the researchers found that it can also be higher when people are exposed to hot and humid conditions.

The study indicates that an upper critical temperature (UCT) exists for humans and is likely to be between 40°C and 50°C (104°F and 122°F). Understanding at what temperatures human metabolic rates start to rise and how this varies between people can have implications for working conditions, sports, medicine, and international travel.

Using an echocardiograph, the research team also investigated how heart function is affected by temperatures above the UCT. They found some considerable changes in heart function responses to the heat between categories of people, especially between men and women. For example, women appeared to experience a greater increase in heart rate when exposed to high temperatures.

"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," says Professor Lewis Halsey, the lead research author.

The year 2022 was the world's 6th-warmest year on record since 1880, and the temperatures are expected to continue rising. As warming climate has already increased dehydration, tropical infections, allergies, and cardiovascular morbidity, some scientists call it the greatest threat to global public health.

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