While Earth Hits Record Temps, Scientists Find the Upper Heat Limit for the Body

Throw in some humidity and temperatures in the triple digits.

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A boy plays among jets of water of a fountain amid hot weather in Beijing on July 5, 2023. (Photo by...

If the world felt like it was on fire earlier this week, it kind of was. According to data from the US National Centers for Environmental Prediction, the average global temperature reached 62.62 degrees Fahrenheit (17.01 degrees Celsius) on Monday and spiked to nearly 63 degrees (around 17.2 degrees Celsius) on Wednesday, making July 3 the hottest day our planet has experienced since at least 1979.

“[National Centers for Environmental Prediction] has placed Earth's average temperature yesterday as the hottest single day thus far measured by humans,” Robert Rohde, lead scientist at Berkeley Earth, tweeted on Tuesday. “This is driven by the combination of [natural climate phenomenon] El Niño on top of global warming, and we may well see a few even warmer days over the next [six] weeks.

As we’ve seen for the past few years, dangerous heat waves are becoming a regular occurrence with no intention of going away until we can curb global greenhouse gas emissions. Heat stress can affect the body in a myriad of deleterious ways but what’s harder to pin down are these temperature shifts impact on health and well-being on a large scale.

To that end, researchers at the University of Roehampton in the UK determined that the human body isn’t able to regulate its core body temperature when the weather gets to 104 degrees Fahrenheit or more, especially if there’s moderate to high humidity. According to findings presented Thursday at The Society for Experimental Biology’s Annual Conference in Edinburgh, Scotland, this physiological malfunction appears to be linked to the body's metabolic rate.

“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,” Lewis Halsey, the lead researcher and professor of life and health sciences at the University of Roehampton, said in a press release. “[B]ut, weirdly, information is much less available for humans when considering the upper limits of our thermal neutral zone.”

Finding the UCT

Scientists have been studying the range of temperatures in which our body's normal processes, like our metabolic rate, can maintain our core temperature without using extra energy. This range is called the thermoneutral zone (or TNZ), and research on it dates back to the 1930s.

While the temperature at which we need to generate heat — called the lower critical temperature — is well understood, the upper critical temperature (or UCT) where our body's energy expenditure at rest increases isn’t. It is uncertain whether humans have a UCT because mechanisms we use to cool down our core temperature like sweating and widening blood vessels in the skin aren’t considered energy intensive.

To find this elusive UCT, Halsey and his colleagues conducted a round of experiments in 2021 where 13 participants under age 60 had to lie semi-nude in a chamber for an hour under four different temperature and humidity conditions: 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. Resting metabolic rates, core temperatures, blood pressure, heart rate, and breathing rates were measured. Results were compared against baseline measurements taken at 82 degrees Fahrenheit with 50 percent humidity where the human body can comfortably regulate core temperature.

University of Roehampton’s Lewis Halsey as a participant in an experiment led by PhD student Chris Wolfe.

Lewis Halsey / University of Roehampton

The experiment found that under 104 degrees Fahrenheit and 25 percent humidity, the resting metabolic rate went up to 35 percent compared to baseline, although core temperatures didn’t go up. But at 122 degrees Fahrenheit and 50 percent humidity, core temperatures spiked by 1.8 degrees Fahrenheit along with metabolic rates at 56 percent and heart rates at 64 percent compared to baseline.

In a second set of experiments, the results of which were presented at the conference, 24 participants, some from the original cohort, had their hearts examined with echocardiograms (or heart ultrasounds) while exposed to 122 degrees Fahrenheit and 25 percent humidity.

Halsey and his team found there were differences between how women and men fared under the unpleasant conditions — women's heart rates were increased more than men's, suggesting their bodies may be less efficient at getting rid of excess heat.

It’s important to note, though, these studies are based on a small sample of people in the UK. While lending important insight into how metabolic rate may play a role in heat stress, these findings may not be generalizable to all populations, for instance, folks living and acclimated to higher temperatures. It’s also unclear how the increase in resting metabolic rate causes the heat-related changes seen in the studies.

However, the researchers have some theories — it could be that when blood vessels dilate, it reduces blood pressure and forces the heart to work more. Another idea is that with hotter temperatures, molecules and chemicals involved in metabolic processes get all fired up and react faster. (Higher humidity does prevent you from cooling as sweat on the skin can’t evaporate.)

Said Halsey: “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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