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It is vital to wear a hood on cold dives, as most body heat is lost through the head.

The origin of this particular myth seems to be poor reporting of a poorly conducted and only vaguely scientific experiment carried out by the US military in the 1950s. Volunteers were dressed in Arctic survival suits, without hoods, and exposed to bitterly cold conditions. Because their heads were the only part of their bodies left uncovered, most of their heat was lost this way. It might have been considerably more instructive, not to mention fun, to carry out experiments in similar conditions with other body parts uncovered; if the experiment had been performed with people wearing only pants, it is estimated they would have lost no more than 10% of their body heat through their heads.

A US army survival manual from 1970 prolonged this misinformation by strongly recommending covering the head when it is cold, since "40 to 45 percent of body heat" is lost from the head. And further whispers of the Chinese variety have ballooned this proportion up to 95 percent.

If this were true, you would expect to be fully thermally protected whilst naked in subzero temperatures by wearing a nice woolly hat. From personal experience, I know this to be false. Any exposed part of the body loses heat in proportion to its surface area. Depending on the size of head, the heat loss through it is usually no more than 20-30 percent.

The rate of head heat loss does change depending on how cold it is (the lower the temperature, the quicker heat is lost) and with exercise (heat loss through the head is reduced with increasing exertion). However the rate of heat loss is not dependent on the body part in question; it’s the same whether it’s the head, hand, foot or any other appendage. You do not lose heat any faster through the scalp. Interestingly, hair on your face makes absolutely no difference either. And as we all know, bald is beautiful.