This article was originally published in the San Francisco Chronicle’s health section on December 11, 2012.
Myth: You lose the most body heat through your head.
Fact: As winter descends, scores of parents will send their kids off to school with stern warnings: Cover your head if you want to stay warm. In truth, wearing a hat will keep you warm, but the percentage of heat you lose through your head depends on how well you bundle up the rest of your body. And losing even a small amount of heat through your face, as well as your hands and feet, affects your body’s internal temperature.
The British Medical Journal suggests the idea that you lose most of your heat through your head originated with a 1950s military study in which researchers exposed subjects to Arctic temperatures. Their bodies were insulated by warm clothing, but their heads were exposed, and – not surprisingly – they lost more heat through their heads than through covered body surfaces.
Scientists reconsidered this question in 2006. Perhaps, they thought, more heat was lost through the head because it was the only body part directly exposed to the cold. They devised a series of experiments that put volunteers into 63-degree water. In one test, the volunteers wore only swimsuits, and in another they wore warm clothes covered by wetsuits. In each condition, they spent 30 minutes with their heads above water, and later, 30 minutes with their heads fully submerged in the water, breathing compressed air.
When the subjects wore wetsuits and their heads were above water, they lost an average of 298 kilojoules of heat, compared with the 440 kilojoules they lost when they wore wetsuits and submerged their heads. When they wore only swimsuits, they lost between two and three times more heat, an average of 914 kilojoules when their heads were above water and 988 kilojoules when their heads were submerged.
Though the head accounts for about 7 percent of the body’s surface area, clothing and exposure conditions on the rest of your body dictate how much heat you lose through your head.
The researchers learned something else. In both cases when subjects’ heads were submerged in cold water, it led to a disproportional drop in the body’s internal, or core, temperature. This is likely because certain parts of the head – the ears, nose and cheeks – have specialized blood vessels that play a direct role in cooling or warming the body’s internal organs. The same blood vessels are present in the palms and the soles of the feet. Even a relatively small amount of heat loss through these surfaces, the scientists found, impacts the entire body’s internal temperature.
Stanford scientists H. Craig Heller and Dennis Grahn discovered that people can use this principle to their benefit, especially when overheated by heavy exercise. Heller and Grahn created the CoreControl, a vacuum-sealed cooling device. Athletes insert their hands, and the machine lowers the internal temperatures of their entire bodies. Several college and pro football teams use CoreControl on the sidelines to help hot, tired players get back on the field.