人类耐热的极限时多少?

STEVE INSKEEP, host: In many parts of the country, it's been too hot to go anywhere near a tennis court. But the heat wave hovering over a large section of the country raises this question: How much heat can a person stand?

Back in 2009, we got an answer from science correspondent Robert Krulwich, one that stands the test of time.

ROBERT KRULWICH: Our subject is: when it's even hotter than frying-eggs-on-the-sidewalk hot...
Bernd, are you there?

Dr. BERND HEINRICH (Biology, Emeritus, University of Vermont): Yeah, I'm there, Robert.

KRULWICH: OK. 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( 华氏温度)when you're down at sea level.

Dr. 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.

Dr. HEINRICH: Yup, yup. 

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( 植物学家) 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(火炉)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.

Dr. 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...

KRULWICH: Sir Charles, was he...

Dr. HEINRICH: Blagden stayed, apparently, eight minutes...

KRULWICH: Oh ... 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.

Dr. HEINRICH: Sweating profusely(汗流浃背).

KRULWICH: And why is that important?

Dr. 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.

Dr. 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?

Dr. 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?

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

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

Dr. 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.

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

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

Dr. HEINRICH: Well, they had a dog.

KRULWICH: OK. 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.

INSKEEP: And that's "Your Health" for this Monday morning. It's MORNING EDITION, from NPR News.