What Are Those Foil Wrappers at the Marathon?

Answers to your questions about the news.
Nov. 6 2006 4:05 PM

What Are Those Foil Wrappers at the Marathon?

How to stay warm when you can't stop sweating.

Download the MP3 audio version of this story here, or sign up for The Explainer's free daily podcast on iTunes.

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.

Advertisement

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.

Got a question about today's news? Ask the Explainer.

TODAY IN SLATE

Medical Examiner

The Most Terrifying Thing About Ebola 

The disease threatens humanity by preying on humanity.

I Bought the Huge iPhone. I’m Already Thinking of Returning It.

Scotland Is Just the Beginning. Expect More Political Earthquakes in Europe.

Students Aren’t Going to College Football Games as Much Anymore

And schools are getting worried.

Two Damn Good, Very Different Movies About Soldiers Returning From War

The XX Factor

Lifetime Didn’t Think the Steubenville Rape Case Was Dramatic Enough

So they added a little self-immolation.

Politics

Blacks Don’t Have a Corporal Punishment Problem

Americans do. But when blacks exhibit the same behaviors as others, it becomes part of a greater black pathology. 

Why a Sketch of Chelsea Manning Is Stirring Up Controversy

How Worried Should Poland, the Baltic States, and Georgia Be About a Russian Invasion?

Trending News Channel
Sept. 19 2014 1:11 PM Watch Flashes of Lightning Created in a Lab  
  News & Politics
Weigel
Sept. 20 2014 11:13 AM -30-
  Business
Business Insider
Sept. 20 2014 6:30 AM The Man Making Bill Gates Richer
  Life
Quora
Sept. 20 2014 7:27 AM How Do Plants Grow Aboard the International Space Station?
  Double X
The XX Factor
Sept. 19 2014 11:33 AM Planned Parenthood Is About to Make It a Lot Easier to Get Birth Control
  Slate Plus
Slate Picks
Sept. 19 2014 12:00 PM What Happened at Slate This Week? The Slatest editor tells us to read well-informed skepticism, media criticism, and more.
  Arts
Brow Beat
Sept. 20 2014 3:21 PM “The More You Know (About Black People)” Uses Very Funny PSAs to Condemn Black Stereotypes
  Technology
Future Tense
Sept. 19 2014 5:03 PM White House Chief Information Officer Will Run U.S. Ebola Response
  Health & Science
Bad Astronomy
Sept. 20 2014 7:00 AM The Shaggy Sun
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.