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How Hot is Too Hot?

The human body can survive at surprisingly high temperatures, so long as you’re prepared to sweat.

By Randall Munroe
 
“What is the hottest ‘room’ temperature at which a human body can, by sweating, keep itself cool enough to avoid health damage?”

— Matt B., Etterzhausen, Germany

It’s not the heat, it’s the humidity.

Your body avoids overheating by taking advantage of a bit of physics: When water evaporates from a surface, it leaves the surface cooler. When your body gets too hot, it pumps water onto your skin and lets it evaporate, carrying away heat. This effect can actually lower the temperature of your skin to below the air temperature. This allows humans to survive in places where the air temperature is as high as human body temperature — as long as we keep drinking water to produce more sweat.

Sweating works best in dry air.

If there’s a lot of moisture in the air, then evaporation slows to a crawl, because water condenses onto your skin almost as fast as the moisture evaporates off it. When you feel sticky from sweat pooling on your skin, it means your body is struggling to evaporate water fast enough to keep you cool.

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I asked Zachary Schlader, a researcher at Indiana University who studies how our bodies handle extreme heat, about the hottest temperature a normal human could tolerate under ideal conditions. He sent me a 2014 study by Ollie Jay, of the Thermal Ergonomics Laboratory at the University of Sydney, and colleagues. The study found that a person who is at rest, wearing minimal clothing, in a very dry room — 10 percent relative humidity — and drinking water constantly could probably avoid overheating in temperatures as high as 115 degrees Fahrenheit.

The limiting factor for our heat tolerance is sweat — how quickly we can produce it and how quickly it evaporates. If you kept your skin wet with a steady spray of water, and sat in front of a powerful fan, you could increase the evaporation rate and keep your skin cool in even higher temperatures.

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But even if you do everything you can to increase sweat as fast as possible, there is a limit to how cool you can make a surface through evaporation. This limit is called the wet-bulb temperature, and it depends on both temperature and humidity. Its precise value can be found using various calculators.

Models of human thermoregulation like the one in the 2014 paper don’t usually cover such extreme conditions, but I tried adjusting their formulas to approximate what would happen under extreme evaporation and high wind. The results suggested that, with the help of a pool of water and a powerful fan, a human could conceivably tolerate heat of up to 140 degrees Fahrenheit in air with 10 percent humidity.

That seemed awfully high, so I ran the number by Dr. Schlader. “Doing some rough calculations, I come up with a similar number,” he said. “Honestly, I was surprised.” But, he added, these models are likely not reliable at such extremes. “I would interpret such findings with caution.”

Personally, my advice would be to avoid any room where the thermostat has a setting labeled “potentially survivable, under some circumstances, according to theoretical calculations.”

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