As a deadly heat wave continues to ravage the US, new evidence suggests that the human body may stop functioning optimally when the outside temperature rises from 104 to 122 degrees Fahrenheit.
Research presented Thursday at the Society for Experimental Biology annual conference in Edinburgh, Scotland, suggests that temperatures in that range increase a person’s resting metabolic rate, the amount of energy needed to function at rest.
When that happens, people may start to breathe harder and their heart rate may increase. Once a person’s body can no longer get rid of excess heat, a condition known as “heat stress,” the core temperature rises. This can cause confusion, nausea, dizziness, headache, or fainting.
“People are usually quite good at acclimating to heat, up to a point,” said Lewis Halsey, professor of life and health sciences at the University of Roehampton in England, who conducted the research.
halsey who published the results From his first set of experiments in 2021, he shared the findings Thursday from a second set that focused on heat and heart activity.
For the first experiments, Halsey and her research team asked 13 adults under the age of 60 to lie down in a chamber and exposed them to different temperatures and humidity levels for one hour each. The researchers measured the people’s resting metabolic rates, core temperature, blood pressure, heart rate and respiratory rate.
The results indicated that people’s resting metabolic rates increased when they encountered temperatures of at least 40 degrees Celsius (104 degrees Fahrenheit).
As a benchmark for comparison, the researchers used 28 degrees Celsius (about 82 degrees Fahrenheit) with 50% humidity, because humans can comfortably maintain their core body temperature in those conditions.
At 40 degrees Celsius and 25% humidity, participants’ metabolic rates increased by an average of 35% compared to baseline, but core body temperature did not increase.
However, at 50 degrees Celsius (122 degrees Fahrenheit) and 50% humidity, people’s core temperatures rose by an average of 1 degree Celsius. The people’s metabolic rates also increased by 56% and their heart rates by 64%.
A threshold where sweating no longer helps
Various parts of the US have had temperatures exceeding 104 degrees in recent weeks. Death Valley, California hit 122 degrees Monday, and temperatures in several Texas cities topped 110 degrees last week.
Globally, the data from the Institute for Climate Change at the University of Maine suggests that this July 4th may have been the hottest day ever recorded on Earth.
Halsey, who was personally involved in their experiments, said the 50 degree Celsius environment is “pretty bleak” because even sweat can’t cool you down.
“Being able to sweat is kind of a superpower, unless you’re really humid, and then the sweating mechanism doesn’t work, because the sweat isn’t going anywhere. It just accumulates in the skin,” he said.
Halsey estimated that if the participants had stayed in the chamber at 50 degrees Celsius and 50% humidity for a long time, they might not have survived, although the researchers aren’t sure how much exposure would be fatal.
“They would have died eventually, because their core temperature would go up and up,” Halsey said. “The body would be fighting to discharge the heat.”
On Thursday, Halsey presented the results of her most recent experiments, which involved 24 participants, some of whom were part of the original study. This time, the researchers used echocardiograms, or heart ultrasounds, to examine people’s heart activity at 50 degrees Celsius and 25% humidity.
They found that, on average, the women’s heart rates increased more than the men’s. That could suggest that female bodies aren’t as efficient at getting rid of excess heat, Halsey said.
But he added: “There’s no way I can tell them, ‘Aha! So women or men are going to struggle more in the heat because of this.'”
‘Don’t be a hero’
Halsey’s research helps explain why extreme heat can be deadly, said Kim Knowlton, an assistant clinical professor of environmental health sciences at Columbia University.
“There are many people around the world, millions if not more, who are exposed to these conditions” in Halsey’s study, Knowlton said.
People with pre-existing heart and lung problems are most vulnerable to extreme heat, along with older adults, pregnant people and newborns, he said.
united states averages 702 heat-related deaths each yearaccording to the Centers for Disease Control and Prevention.
But Michael Sawka, a physiologist at Georgia Tech’s School of Biological Sciences, questioned whether metabolic rate plays a role in heat stress. He pointed to his own research showing that, at 40 degrees Celsius, people exhibit lower metabolic rates during exercise after acclimating to the heat.
“Saying that when it’s hot out people increase their resting metabolic rate doesn’t make a lot of sense to me,” Sawka said.
He surmised that the participants’ metabolic rates might have increased in the study because they were restless due to discomfort. He also questioned whether the results would apply to people who are used to higher temperatures, because the UK has a temperate climate.
However, Knowlton said, everyone should take precautions on extremely hot days.
“If you feel like you’re passing out or dizzy or feel sick, take it very seriously,” he said. “Don’t be a hero and keep forging through the heat.”