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LINDA WERTHEIMER, HOST:

A look at the weather forecast for the first few days of the London Olympics shows temperatures in the 70s and 60s. That's good news for the athletes. Bodies generate lots of heat running up and down a soccer field or racing around a track. And when it's cooler outside, the body doesn't have to work so hard to cool off.

RENEE MONTAGNE, HOST:

The Olympic athletes are lucky they're not competing in the U.S. this year - much of which is sizzling this summer.

As part of our series Summer Science, we've asked NPR science correspondent Joe Palca to explain what clothing is the best for outside activity when temperatures are high.

JOE PALCA, BYLINE: When you get hot, you sweat. But it's not enough to just sweat. To cool off, you need that sweat to evaporate. It's evaporation that drains the heat from your body.

To help the sweat evaporate, you want air to flow over your skin - as much of your skin as possible. So according to George Havenith, professor of Environmental Physiology and Ergonomics at Loughborough University in England, the best clothing for people to wear when exercising is none at all.

GEORGE HAVENITH: They would probably want to wear some underwear to, you know, just cover up and, you know, be comfortable in that way. But in terms of the heat loss, the naked person is best able to lose heat.

PALCA: That's only if the sun's not shining. If the sun's out, you need clothing to protect your skin from burning.

HAVENITH: So then you have to find a balance with a certain amount of clothing coverage without the clothing actually hampering the sweat evaporation.

PALCA: But that made me wonder, why wear clothes? Why not just wear sunscreen?

HAVENITH: That's an interesting point. In theory, yes, but unfortunately sunscreen sometimes affects sweating as well. So the sunscreen then might reduce the amount of sweat evaporation and sweat production. So that's where then things, again, could go wrong.

PALCA: So if you're exercising in the heat, wear as little as possible, especially after the sun goes down.

WERTHEIMER: OK. So that explains the skimpy outfits worn by Olympic track stars. But for those of us who don't move that fast, we asked Joe what's to best to wear when we're not exercising, but we're outside when it's hot.

PALCA: In that case, wear loose-fitting clothing.

HAVENITH: Even if you don't realize you're sweating, you will have some moisture evaporation already in the heat, and that means you have to take that moisture away, ideally straight away from the skin. And that's where the loose-fitting clothing comes in, allowing drier air to pass along your skin from outside your clothing and leave the clothing again.

PALCA: You don't want to wipe the sweat off, you want it to evaporate.

George Havenith says with tight-fitting clothes, the sweat doesn't evaporate as efficiently, so it doesn't cool you down as much. Then the question comes up, should you wear dark- or light-colored clothing? Havenith says there are studies that show that if you wear thicker clothing, like say what Bedouins wear in the desert, color doesn't matter. Yes, the outer layer of the Bedouins' black garments gets hotter than their white ones, but that heat doesn't get transmitted to the skin because of the thick fabric. If you wear clothes made from thin fabrics, it's a different story.

HAVENITH: And there we find clear differences where we find that the black clothing absorbs more heat that will actually end up inside the body.

PALCA: Havenith says light-colored lightweight fabrics are best. So going back to what we learned about the best way to cool off when exercising once the sun goes down, the implications are clear. People should wear loose-fitting, light-colored clothes in the summertime during the day, and everybody should get naked at night.

(LAUGHTER)

HAVENITH: Yeah. I'll leave that to you to be said. But in essence, that's probably quite, quite the basics. Yeah.

(SOUNDBITE OF MUSIC)

MONTAGNE: And that's NPR science correspondent Joe Palca with wardrobe choices for sultry summer days.

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