Don't mind me nerding out, but here ya go: http://www.ucsdbglab.org/tools/RuleOfNines.htm
So call it 9% of adult body surface is head. But the assumption that's nicely proportionate to actual heat loss, which appeals to those who need to simplify things, including some guys of unclear scientific credentials at a "center for science policy" is just as wrong as the 50% figure. Actually, more wrong.
Subcutaneous fat and muscle distribution accounts for a lot of insulation. We have relatively little of either on our heads, which are richly vascularized. And head and face skin's full of sweat glands, which carry away heat through insensible perspiration.
So actual contribution to heat loss - or gain - by the head is considerably greater than 9% After you also adjust for the head being higher up and more exposed to convective cooling from airflow, probably 1/5 of all heat loss. And can exceed 1/4 in children, who have disproportionately larger heads.
By contrast, we have a bunch of fat and muscle on our trunks. In general, the upper back is the most important part of the body for retaining heat; Stegman showed that the back muscles fill with blood and create a sort of blanket over the rear of the heart and lungs, where core heat production collides with incoming cold air. So even though our arms and legs have a lot more surface area, relatively speaking, the reason vests keep us warm is that they further insulate the upper back.
And legs also have a thing called counter-current circulation, which evolved in vertebrates to reduce cold damage risk in lower extremities. Reason birds can perch on a leg in the north atlantic and not get frostbite. Our hands have some of that too. We also habituate to cold in our lower limbs better than to our heads; meaning we don't feel it as much. Same idea; we evolved to have bare legs and feet in variable climates. This is why, incidentally, we see people during January thaws walking around in shorts when it's 40. Or women walking around in heels and short skirts when it's 25. We just don't feel it the same way.
So bottom line: most of these science policy folks' fundamental premises are wrong. But it's a cute idea, along with then proceeding to query sugar and kids' behavior: Replace those myths with even more problematic reasoning.
The Army's old model isn't unreasonable in any case, it just assumes we aren't living naked in a cold box with thermocouples up our ass. Take an extreme case of the Inuit; prolly 3/4 of their heat loss is from the face. The rest is covered so well they needed to invent vented parkas to let out the warm humid air during work. So is it a myth to say that most their heat loss is from the head?
Whereas partly naked peeps at football games do fine because a) they're so drunk. Which keeps them from feeling pain, and also turns into excess calories rapidly and also vasodilates to boot, so their skin feels toasty even as they lose core heat, b) the crowd around them actually creates a microenvironment that's a lot warmer and with less moving air than even a few feet above their heads, and c) they can get away with it as long as the air is dry and cold. If it starts to snow, and their skin gets wet, it's bye bye or the ER.
You will be tested on this.
Edited by beyond - 1/10/14 at 9:31pm