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As the climate keeps warming, with blistering heat waves on the rise , scientists are pondering the pressing question: Exactly how much ambient heat can the human body tolerate? The conventional belief among researchers has been that humans can withstand temperatures up to 35 degrees Centigrade (or 95 degrees Fahrenheit) without suffering major consequences like heat strokes or heart attacks. 

But in a new study published in the Proceedings of the National Academy of Sciences, Penn State University researchers challenge that limit. They say it doesn’t take into account factors that amplify heat’s effects. For example, many such estimates rely on the dry heat-tolerance level. Dry heat—that is, heat with little to no moisture in the air—is easier for humans to withstand. That’s because humidity—the level of water vapor in the air—affects how human bodies cool off, says graduate student Qinqin Kong, one of the study co-authors. Our bodies regulate temperature in a few different ways. Our skin naturally releases some heat into the air, for one. And wind or a light breeze on a hot summer day can whisk even more heat away. But sweating is best, Kong says.

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Subjects could no longer cool off and their bodies’ temperature started to rise uncontrollably.

That’s why, when speaking about heat’s effect on humans, scientists often rely on a measure called “wet-bulb temperature”—it is calculated to include other factors like humidity. “Our bodies are kind of like big wet bulbs,” says Daniel Vecellio, a bioclimatologist and a postdoctoral researcher at Penn State who led the study. To understand the idea of a big wet bulb, imagine a thermometer’s bulb wrapped in a wet cloth. As water evaporates from the cloth, it cools the thermometer. Likewise, “the sweat on top of our skin has to be evaporated away to cool the skin, which allows more heat to move from our core up toward our skin.” When the air around us is too humid, it can’t take up more moisture so we can no longer evaporate water through sweat, losing our most efficient heat-releasing mechanism. That leads the body to rapidly and perilously overheat. “Humid heat is more dangerous,” Vecellio says. 

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In an earlier study from 2022, Vecellio and his team used heat chambers to investigate people’s tolerance thresholds to high temperature and humidity. They recruited healthy volunteers to pedal a no-resistance bike inside the chambers. Having them move this way heated up their bodies no more than daily activities like “brushing your teeth, doing the dishes, doing some light housework,” says Vecellio. That mimicked what people typically do when they are hot. “During heat waves they’re not out running a 26-mile marathon. They’re sitting around their homes doing as little as they can to try to stay cool.” 

As volunteers pedaled, the scientists would slowly increase the heat and humidity in the chamber to see at which point the subjects could no longer cool off and their bodies’ temperature started to rise uncontrollably. “We found that in warm and humid conditions,” the uncontrolled rise in body heat “started to happen at a wet-bulb temperature of 31 degrees Celsius,” says Vecellio.

He acknowledges that the study participants were from central Pennsylvania, so they weren’t accustomed to temperatures and humidity spiking off the charts. Some people living in typically hot and humid climates may be better able to tolerate those conditions. However, he adds, volunteers were also young and healthy, which presumably would have made them relatively more resilient: Age as well as heart conditions, diabetes, and other diseases inevitably bring the threshold down for many. For some of them, 31 Centigrade (88 Farenheit) at 90 percent humidity might be deadly.

These findings are particularly important for densely populated geographies that feature high humidity and intolerable temperatures multiple days a year, says Vecellio. These include Pakistan’s and India’s Indus River Valley, home to 2.2 billion residents; eastern China, with 1 billion residents; and sub-Saharan Africa, where 800 million people live. If global temperatures rise by 2 degrees Centigrade above pre-industrial levels, about 4 billion people—half of humanity—will be suffering from intolerable humidity-intensified heat multiple days a year, which will essentially render such places unlivable. 

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“Around the world, official strategies for adapting to the weather focus on temperature only,” Kong says. “But our research shows that humid heat is going to be a much bigger threat than dry heat.”

Yes, air conditioning would help. But the real solution to killer heat is cooling our fossil fuel use. “We need to make a concerted effort to rapidly accelerate our greenhouse gas reductions,” Vecellio says—before 4 billion people literally feel the heat.

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