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MADELEINE BRAND, host:

It is hot. Here in Southern California, we're having a heat wave, temperatures in the 90s, the 100s in some places. I know I shouldn't complain because, well, the weather here is usually perfect, and it's summer after all. It's supposed to be hot. Still, a lot of us are wondering: How much heat can a person stand? Well, here's an answer from our science correspondent Robert Krulwich.

ROBERT KRULWICH: Our subject is: When it's even hotter than frying-eggs-on-the-sidewalk hot…

(Soundbite of frying egg)

Bernd, are you there?

Professor BERND HEINRICH (Biology, University of Vermont; Author, "Summer World"): Yeah, I'm there, Robert.

KRULWICH: Okay, Bernd Heinrich, esteemed biology professor from the University of Vermont, let's begin with a fact most people know. Water boils at 212 degrees Fahrenheit when you're down at sea level.

Prof. HEINRICH: Mm-hmm. Mm-hmm.

KRULWICH: So you've got to wonder: What would happen if you, say, stepped into a room heated to 211 degrees, just a single degree shy of boiling, and then ever so slowly, you made that room just a little bit hotter? Would your blood start to boil? In your book "Summer World," you describe how once somebody actually tried this.

Prof. HEINRICH: Yup, yup.

(Soundbite of music)

KRULWICH: More than 230 years ago, around 1775 at the Royal Society in London, three very prominent gentlemen - Sir Charles Blagden, doctor and chemist, Sir Joseph Banks, botanist and explorer, and Dr. Daniel Solander, naturalist, also an explorer - they decided to conduct a heat experiment inside a room of their own design.

Prof. HEINRICH: A small room, oh, 10, 15 feet or something like that square, and they put a metal stove in there, and they got heating it red-hot, and so they conducted experiments and observation in a heated room.

KRULWICH: And those experiments, says Professor Heinrich, began with the room at 210 degrees. Then they moved it up to 211. And one by one, the gentlemen entered, reported it unpleasantly hot but very bearable. So they made the room hotter, to 212, to 220, to 230.

Prof. HEINRICH: They got it up to 240 or even more, yeah, 30 or more points above the boiling point of water.

KRULWICH: And Sir Charles, was he…

Prof. HEINRICH: Blagden stayed apparently eight minutes at 240 degrees, and afterwards, he felt he could have actually stayed even longer. So this is amazing thing, but there are certain conditions that are involved here.

KRULWICH: First of all, the air in that room was dry, not wet heat. It was dry heat. Then, the gentlemen didn't move around hardly at all, and third, and this is the most important, Sir Charles reported almost immediately he began sweating.

Prof. HEINRICH: Sweating profusely.

KRULWICH: And why is that important?

Prof. HEINRICH: Well, the sweat is removing heat.

KRULWICH: Meaning, when you sweat, and then the sweat evaporates, that act of evaporation pulls heat away from your skin. When water on your skin turns to gas, the heat gets removed. So you sweat some more, then there's more evaporation, and as long as you keep up this sweating, evaporating, sweating, evaporating, the air right round your skin never gets too hot because evaporation protects you.

Prof. HEINRICH: So that tiny, thin layer of air right next to your skin, maybe it's only a millimeter or so, that little space is being cooled.

KRULWICH: And that's what keeps you from burning or boiling?

Prof. HEINRICH: Yeah, by your body temperature, by the evaporating water, we can cool the whole body through the sweating.

KRULWICH: But what happens if you can't sweat? Well, to find that out without doing anybody serious harm, Sir Charles brought a chicken egg - chicken eggs, after all, don't sweat - into the room with him.

And what happened to the egg?

Prof. HEINRICH: Well, it was cooked solid because the egg doesn't sweat. Of course, a beef steak can evaporate water for quite a while.

KRULWICH: Boy, I did forget. They also did bring a juicy beef steak into the room.

Prof. HEINRICH: But the beef steak, in 33 minutes, got dry, and then it got really hot, and then it cooked.

KRULWICH: So this is a hooray-for-sweat story is what this is.

Prof. HEINRICH: Exactly. It shows the power of sweating, which is an amazing power that humans have.

(Soundbite of dog growling)

KRULWICH: Well, let's be careful here because I understand there was a non-human involved.

Prof. HEINRICH: Well, they had a 32-pound bitch. By that, they mean a little…

KRULWICH: Being a family radio station, we have to ask you to explain that.

(Soundbite of laughter)

Prof. HEINRICH: Well, that's a female dog.

KRULWICH: Okay. So they bring a dog with them.

Prof. HEINRICH: Yeah, and the dog stays in there at 220 degrees Fahrenheit for half an hour, and actually during that time it's in there, the temperature rises to 236 and the dog comes out, it's panting, but it's perfectly all right.

KRULWICH: And one month later, says Sir Charles, the dog was still perfectly brisk, showing that if you cool yourself either on the inside through panting or on the outside through sweating, either way, it is apparently possible to survive temperatures, at least for a little while, that would normally boil water.

Robert Krulwich, NPR News.

BRAND: We've recreated that original experiment with the gentlemen, the eggs, the beef steak and, yes, the dog in an animation at our Web site, npr.org, and bring a towel.

(Soundbite of song, "Gonna Make You Sweat (Everybody Dance Now)")

Ms. MARTHA WASH (Singer): (Singing) Put me on line. Come on let's sweat, baby. Let the music take control. Let the rhythm move you. Sweat, sweat. Let the music take control. Let the rhythm move you. Everybody dance now.

ROBERT SIEGEL: host:

You are listening to ALL THINGS CONSIDERED from NPR News.

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