Does the majority of the body’s heat escape through the head? (Image source: Copyright Getty Images)
Our guide told us that no matter what measures we take, capsizing must be avoided. Once the kayak overturns and you end up in the water, you can catch a cold within just a few seconds. We were in Torres del Paine National Park in Patagonia, surrounded by countless emerald-colored icebergs—some shaped like giant pianos, others like towering rockets. As we paddled along, we could occasionally hear the cracking sounds of ice breaking off the glaciers.
Before this trip, I had never seriously thought about how the human body loses heat. Now, every action I took had to account for the extreme cold. At night, the chill was bone-deep—even hiding in a tent didn’t shield us from it. I slept in a regular sleeping bag, wearing all the clothes I had—four pairs of pants, three jackets, two hats, and some waterproof sheets. But it still wasn’t warm enough. Our guide, Mariano, slept outside the tent, covered only by an extra-thick sleeping bag against the wind. He insisted he liked sleeping this way, even without a hat. I was worried about him: there’s a popular belief that the majority of the body’s heat escapes through the head. But is that really true?
Over the past few years, physiologists have tested various theories about how colds develop, using a wide range of volunteers—from robust students to soldiers. Since the experiments involve enduring periods of physical discomfort, volunteers are usually required to prepare in advance.
Thea Pretorius from the University of Manitoba in Canada used scuba gear to help assess how much heat is lost through the head. The volunteers in these studies weren’t just physically fit—they also had thick body hair, or at least thick hair on their heads. Baldness would introduce unnecessary complications to the experiments.
In one study, to prevent the volunteers from shivering—a reflexive response where the body tries to stop heat loss—they were given a medication. Then, they were lifted by a crane into cold water. Some volunteers were fully submerged, while others kept their heads above the surface. The second variable was clothing. Some wore only swimsuits, while others had insulating drysuits, two pairs of socks, and wool gloves. But everyone was immersed in 17°C (62°F) water for 45 minutes.
As expected, all volunteers lost heat most rapidly right after entering the cold water. But the data from the rest of the time contradicted popular beliefs. Those who lost heat almost entirely through their heads (fully submerged in drysuits) lost only half as much heat as those who lost heat mostly through their bodies (wearing swimsuits but with heads above water). In other words, most of our body heat isn’t lost through the head. The results showed that submerging the head in cold water only increased overall heat loss by 10%. Since the head makes up about 7%–9% of the body’s surface area, this figure doesn’t seem particularly high.
But before you toss all your hats in the trash, there’s another finding to consider: when your head is in the water while your body is insulated, your core body temperature drops much faster than most people would imagine.
One reason might be that the scalp contains many blood vessels close to the skin’s surface—the reason why head injuries often bleed heavily is a clear example. On a cold day, if you’re wearing warm clothes but no hat, blood flowing from your warm body up through the scalp will be cooled by the surrounding environment. When that blood flows back into the warmer parts of your body, it lowers your overall temperature. The second reason relates to shivering. Physiologists were puzzled to find that volunteers who only immersed their heads in the water didn’t shiver. Since shivering slows down the rate of body cooling, not shivering causes it to speed up.
In extreme environments like Patagonia, what matters more is how low your body temperature ultimately drops—not how quickly it declines. Mariano seemed to understand this well. What he truly needed was to keep his body warm in order to maintain his core temperature, not to cover his head. Years of experience had taught him that what he really needed was a warm sleeping bag. Next time, I’ll bring one too.