The Dive Hood, Your Head and Heat Loss

After reading last month's article about a dive hood on the head as being the best way to prevent hypothermia, subscriber Harvey Cohen (New York, NY) wrote to suggest that we should get our heads examined for stating that. "If you leave significant body surface area exposed, that's where you'll lose heat. For equivalent area, heat loss is about the same, whether it's your head, your thighs, etc. Of course, divers are more likely to expose their heads than other areas, but a full wetsuit and bare head doesn't lose more heat than a shorty wetsuit and hood."

While our story's source, Steve Muscat, chief diving medical officer for the government of Malta, stands behind the fact that most of the hypothermia problems he sees in divers are usually due to heat loss from the uncovered head in cold water, Cohen has it right, too: The amount of heat released by any part of the body depends largely on its surface area, and in a cold environment, you would lose more heat through an exposed leg or arm than a bare head.

Dr. Daniel I. Sessler, an anesthesiologist at the Cleveland Clinic, said the popular myth that most heat escapes from the head stemmed from military experiments done 50 years ago. Researchers dressed subjects in Arctic survival suits and exposed them to frigid conditions. But the suits only covered the subjects from the neck down, so naturally most of their body heat escaped through their heads. But Sessler says that's not a fair comparison. "If you did the same experiment with someone wearing a swimsuit, only about 10 percent of the heat loss would come from the head."

The face, head, and upper chest are up to five times as sensitive to changes in temperature as other areas, says Sessler. "This creates the illusion that covering up those areas traps in more heat, but clothing another part of the body does just as much to reduce overall heat loss." While the body does not lose most of its heat through the head, you'll be a lot warmer if you pair a hood along with your dive gloves and wetsuit.