The Morning Call (Sunday)

How Hot Is Too Hot?

The human body can survive at 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?

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 that 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 surroundin­g 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, evaporatio­n slows to a crawl, because water condenses onto your skin almost as fast as the moisture evaporates off of 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.

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 — at 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.

But even if you do everything you can to increase sweating as quickly 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 calculatio­ns.

Models of human thermoregu­lation like the one in the 2014 paper do not 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 not likely to be 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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