Do you really lose 80% of body heat through your head?

We've all heard it, but do you really lose 80% of body heat through your head?

Your mother’s constant entreaties to wear a hat in cold weather seemed to make a certain sense because, as we all knew, some huge percentage of your body heat escaped through your head, a bit like the steam rising from a rapidly cooling cup of tea. Except that it’s not really true.

This old wives’ tale apparently has its roots in a 1970s US army survival guide, which took the information from military experiments in the 1950s. In those experiments, recruits were wrapped up in Arctic survival suits with only their heads left exposed.

No surprise then, when the scientists discovered that it was their heads that were leaking most of their body heat. There was simply nowhere else it could have come from.

In fact, our heads lose heat at about the same rate as other parts of the body. Researchers discovered this in the early 2000s when they convinced volunteers to plunge into cold water dressed either in a swimsuit or insulated clothing, and with their head either submerged or out.

"They calculated that there was only an 11% increase in heat loss when the head was exposed as well as the body," explains Dr Mark Waldron, physiology lecturer at Swansea University.

Do I still need to wear a hat?

So now we know that only around 10% of body heat is lost through our heads, does that mean we don’t need to bother about a hat in cold weather?

No, says thermal physiology specialist Professor George Havenith at Loughborough University.

"If you are fully dressed and your head is exposed, then you are losing most of the heat from your head, so it makes sense to insulate it."

So even though our head doesn’t lose any more heat than the rest of us, it is more sensitive to changes in temperature. So we actually feel colder with an uncovered head.

Prof Havenith even suspects that the British reluctance to wear a hat may in part explain why mortality rises when the mercury drops.

"If you go to zero degrees you see very little impact on mortality in Scandinavia, where, unlike in the UK, you will see young children in hats even at 10C. I do think there’s a link to mortality rates."

Why? Blood pressure rises when you’re cold (because blood vessels constrict), and in the frail that might be enough to trigger a heart attack or stroke, or so the theory goes.