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What Are Those Foil Wrappers at the Marathon?How to stay warm when you can't stop sweating.

By Daniel Gross
Updated Monday, Nov. 6, 2006, at 4:05 PM ET

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Spectators and television viewers who watched yesterday's ING New York City Marathon may have noticed that, soon after finishing, most runners were cloaked in feather-light wrappers, emblazoned with the names of sponsors like Continental Airlines and Foot Locker. What are these wrappers for?

They warm you up. Physicians and fitness experts say the HeatSheets, as they are known, serve an important function for runners who compete in cool-weather marathons. Marathoners don these sheets for the same reason campers and hikers pack them as survival equipment, and for the same reason they are a staple of emergency medical-supply kits: They help prevent hypothermia by reflecting body heat.

Runners tend to shed layers of clothing as they progress in a race, with most finishing in shorts and a shirt. The best way to keep cool is to sweat, but marathoners typically can't perspire very well due to dehydration. Long-distance runners find it difficult to take in more liquids than they lose through sweating. As they become more dehydrated over the 26.2-mile course, it becomes more difficult for the body to cool. Consequently, the body's core temperature rises.

After runners cross the finish line, their internal sensors tell their bodies to keep shedding heat. In the meantime, when external temperatures are cool, runners will lose body heat rapidly because their heads, legs, and arms are exposed. (It was down in the 40s for yesterday's race.) Under these conditions, runners completing a marathon can run the risk of venting off too much heat and becoming hypothermic.

HeatSheets are designed to combat this problem. They are made by taking sheets of Mylar, a polyester film developed by DuPont in the early 1950s, and coating them with a thin veneer of aluminum. If the sheet is wrapped around a body, the aluminum coating allows the material to reflect heat and capture it in an envelope around the skin. Since they're lightweight, HeatSheets are easier to store and hand out to large groups of people than, say, sweatshirts. And since they're nonabsorbent, they won't retain water on rainy days. The wrappers are designed to enable runners to cool down gradually until they can find some dry clothes and fluids.

For the sake of experiment, the Explainer yesterday ran about 205 minutes through the streets of New York. Upon finishing, he donned a HeatSheet and stumbled toward Central Park West. He felt many things: intense pain, relief, burning hunger, thirst, exhilaration, and exhaustion. But no chills.

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Daniel Gross is the Moneybox columnist for Slate and the business columnist for Newsweek. You can e-mail him at . He is the author of Pop! Why Bubbles Are Great for the Economy.
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