The Human Heat Limit: Metabolic Rates Spike at 104-122°F

Summary: Researchers have discovered an upper critical temperature (UCT) for humans, likely between 104-122°F (40°C -50°C). Their research indicates that the human body’s resting metabolic rate, an indicator of energy consumption, rises in hot and humid conditions.

The team is currently investigating how heart function varies across different individuals in temperatures above the UCT. This study aims to improve our understanding of how we adapt to suboptimal environments, a key concern in a warming world.

Key Facts:

  1. An upper critical temperature (UCT) for humans exists, likely between 104°F and 122°F, causing a rise in resting metabolic rate in hot and humid conditions.
  2. This research reveals significant differences in cardiovascular responses to heat between sexes, alongside variations across age and physical fitness levels.
  3. The findings contribute valuable knowledge about the body’s adaptability to heat stress in a warming world and could have implications in diverse fields, including medicine, sports, work conditions, and international travel.

Source: Society for Experimental Biology

Ongoing research by Prof. Lewis Halsey and his team at the University of Roehampton, UK has identified that an upper critical temperature (UCT) exists for humans and is likely to be between 104°F and 122°F (40°C and 50°C).

Further research is now underway to explain this rise in metabolic energy costs at high temperatures.

Prof. Halsey and his team have found that resting metabolic rate, a measure of how much energy the human body consumes to keep ticking over, can be higher when people are exposed to hot and humid conditions.

This shows a man surrounded by flames.
Understanding the temperatures at which human metabolic rates start to rise, and how this varies between people, can have implications for working conditions, sport, medicine and international travel. Credit: Neuroscience News

“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,” says Prof. Halsey.

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

Credit: Neuroscience News

“This research provides fundamental knowledge about how we react to suboptimal environments, and how ‘optimal’ differs between people with different characteristics,” adds Prof. Halsey.

Prof. Halsey and his team are also investigating how heart function is affected by temperatures above the UCT, and how effects on the heart vary between people with different characteristics 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,” adds Prof. Halsey. “That is, on the average, men and women display some key differences in their cardiovascular responses to the heat”.

Prof. Halsey’s team measured detailed heart function using a state-of-the-art echocardiograph. “It was not easy to operate this kit in the heat!”, he adds. “The sort of equipment you’d see in hospitals but rarely in research laboratories”.

These experiments are ongoing and Prof. Halsey’s talk at the SEB conference will highlight their most recent findings.

“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,” he adds.

About this climate, heat, and metabolism research news

Author: Alex Evans
Source: Society for Experimental Biology
Contact: Alex Evans – Society for Experimental Biology
Image: The image is credited to Neuroscience News

Original Research: The findings will be presented at the SEB Centenary Conference 2023

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  1. It was 115 deg F in Texas and 117 in Arizona the other day. DO humans actually believe they will survive abrupt exponential climate change? It is impossible. We are already at the limits of 104 deg and 122 deg. It was 122 deg F, 500 miles from the Arctic, in Canada 2 years ago. Wet bulb conditions will be the main modality of death as humans go extinct. Starvation and dehydration will be 2nd. Severe weather is 3rd. Heat is what will kill us all. On a 115 deg F day, everyone runs inside and turns on the AC, this in turn over stresses the power grid, which will collapse. Because it is so hot outside, work crews cannot safely fix the power outages. Time is now very short for homo sapiens on Earth. We will be lucky if a single human being remains on Earth by 2028. Yes…it is happening that quickly now. This is a time to get your affairs in order.

  2. Curious if there is a control for those born and raised in a hot climate vs. cold.

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