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By Sachi Kitajima Mulkey, Grist
In the summer of 2023, a dozen people willingly walked into a steel chamber at the University of Ottawa designed to test the limits of human survival. Outfitted with heart rate monitors and temperature probes, they waited in temperatures of 42 degrees Celsius, or 107 degrees Fahrenheit, while the humidity steadily climbed, coating their bodies in sweat and condensation. After several hours, their internal body temperatures began ratcheting upward, as the heat cooked them from the outside in.
“Few people on the planet have actually experienced temperatures like this,” said Robert Meade, a postdoctoral researcher in epidemiology at the Harvard School of Public Health who led the study. “Imagine moisture condensing on the skin like a glass of water on a hot day. That’s how hot it was, compared to skin temperature.”
Their experiment tested the body’s ability to cope with extreme heat by exposing participants to temperatures at which they could no longer cool themselves. Their study, published last week in the science journal PNAS, confirmed that this dangerous threshold is much lower than scientists had previously thought: a so-called wet bulb temperature, which accounts for heat and humidity, of 26 to 31 degrees C.
Scientists call this limit the point of “uncompensable” heat stress, “because the body cannot compensate for the heat load placed upon it,” Meade said. “With climate change driving heat waves, there’s been a lot of interest in defining these upper limits.”
When studying the health risks of heat, scientists often refer to wet bulb temperatures because moisture in the air can make heat waves much deadlier by blocking the body’s ability to sweat out heat effectively.
For over a decade, it was widely believed that the maximum wet bulb temperature that bodies could handle was 35 C — unlikely to become a common occurrence until global warming had reached a staggering 7 C over preindustrial temperatures.
It wasn’t until 2022 that a group of researchers tested this limit with human subjects, and found that things could get dangerous much sooner, at wet bulb temperatures as low as 26 C. This threshold means that vast areas of the planet could become risky to live in with 2 C of global warming — which could be reached as early as 2045 if greenhouse gas emissions aren’t reduced.
“With a warming climate, we expect that those thresholds will start to be exceeded more often.” said Tony Wolf, an assistant professor at the University of Georgia who studies heat stress and co-authored the 2022 study. “The heat waves are larger in magnitude, and they last longer.”
Other studies, like Wolf’s, have tested this lower heat tolerance over a couple of hours. But Wolf said the latest study is the first to do so over nine hours, closer to what a person might actually experience during a heat wave. Only a few participants were unable to complete the full nine hours while exposed to the temperatures at the “uncompensable” heat limit, although the researchers estimated heatstroke would occur after 10 hours. At slightly lower temperatures, participants were on track to experience heat stroke within 35 hours.
“It’s very rare that you would have such high wet bulb temperatures for more than a day,” Meade said. “But if you think about what it would be like for a person actually exposed to these temperatures, that limit still indicates the point at which core temperature is on this crazy train, streaming up and up.”
Different factors can make heat stress more likely at lower temperatures, too. Working outdoors, having preexisting health conditions and lacking access to air-conditioning can make even moderate heat waves deadly. And while Meade’s study tested young, healthy adults, Wolf’s research has found that older adults experience heat stress at lower temperatures.
“Any elderly person’s circulatory system isn’t going to be as good at dispersing heat,” said Radley Horton, a professor at the Columbia Climate School. “When the temperatures start to get really extreme, the body has to start making some difficult choices,” he said.
In February, Horton published a study in Nature that found 2 C of warming could make more than a third of Earth’s land too hot for those over 60 years old — an estimated danger zone five times larger than it would be for younger adults. The study found that regions with especially hot and humid climates, like the Middle East, West Africa and Southeast Asia, would be hit the hardest. Cities such as Karachi, Pakistan, could bake under temperatures too hot for older adults 20 percent of the year.
Research from Penn State University predicts that keeping global warming under 2 degrees C nearly eliminates the risk of widespread uncompensable heat. But in the past year, global temperatures have surged beyond scientists’ predictions, marking 2024 as the first year to breach 1.5 C of warming.
The rising heat has already taken a serious toll. Between 1999 and 2023, heat deaths in the United States more than doubled, rising from roughly 1,000 fatalities a year to more than 2,000. Over the same amount of time, nearly a quarter-million people have died from heat worldwide. In 2023 alone, more than 47,000 Europeans died from heat, with countries in the Mediterranean — which is warming 20 percent faster than the rest of the planet — hit the hardest.
“People already die from heat waves now,” Wolf said. “So regardless of what happens to the climate of the future, it’s important to understand, right now, what are these thresholds above which we start to see greater risk of heat related illness and death?”
This article originally appeared in Grist at https://grist.org/health/science-extreme-heat-humidity-research/. Grist is a nonprofit, independent media organization dedicated to telling stories of climate solutions and a just future. Learn more at Grist.org. Banner photo: A man shields himself from the sun (iStock image).
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