View previous topic | View next topic

Heat loss

Page 1 of 1

61195.  Tue Mar 21, 2006 8:47 am Reply with quote

Flash suggested I repost this from my FT column:

MYTHCONCEPTIONS: Hot heads by Mat Coward

THE MYTH: You lose most of your heat through your head, so the most important piece of outdoor clothing is a hat.

THE "TRUTH": Everyone knows this. Look at an outdoorsy website and you will be told it again, just in case you’ve forgotten it. But apparently, it’s not true. Anything from 5% to 55% of a body’s heat loss can be from the bonce area, depending on various factors, including how much hair you’ve got. But there’s nothing special about the head: any exposed body bit will lose heat, and how much will depend to a great extent on how big it is (flashers be warned). Logically, if this myth were true, wearing a hat but no trousers would keep you warmer than wearing trousers but no hat: it won’t. (Flashers be doubly warned). Hypothermia expert Dr Daniel I. Sessler, of the University of Louisville medical school, blames the belief on military experiments, using Arctic survival suits, conducted half a century ago. The suits did not cover the subjects’ heads, so of course most heat loss was from the top. Someone wearing just a swimsuit in cold conditions, says Dr Sessler, would lose only about 10% of their heat via their heads.

SOURCES: miniurl,696

DISCLAIMER: If you have information proving that the experts are talking through their hats, it’d be healthier to send it to FT’s letters page rather than steaming in silence.

61218.  Tue Mar 21, 2006 10:17 am Reply with quote

But there’s nothing special about the head...

This doesn't quite add up. There must be something 'special' about the head because you can lose 55% of your body heat through it, even though it takes up far less than 55% of the body's surface area. This is because the head generates far more heat than any other part of the body.

Heat loss through skin only has one really significant variable, and it's surface area, so proportionally speaking 55% is definitely 'most' in terms of body heat lost.

Logically, if this myth were true, wearing a hat but no trousers would keep you warmer than wearing trousers but no hat: it won’t.

But legs have a far larger surface area than your head, so obviously the heat loss will still be large. I'd still bet a hat is more important than trousers, though, simply because more blood pumps through the head per second than through the legs. It's core temperature that matters in survival situations.

I'd like to see more than one source for this one (especially as that link is dead). All the mountain and antarctic survival experts I know (which is bizarrely quite a few, as my sister is one) would tend to disagree.

61221.  Tue Mar 21, 2006 10:24 am Reply with quote

This seems like another good explanation:
Since the blood vessels in your head do not constrict when exposed to cold, heat literally blasts out of your exposed head on a winter day. So covering your head allows body heat to be transferred to other areas keeping you warmer overall.

Feet cold? Put on a hat! Adequate clothing for your head and neck is essential to your outdoor wardrobe in order to keep radiant heat loss to a minimum.

61230.  Tue Mar 21, 2006 10:49 am Reply with quote

I just reports ‘em, Gray - I make no claim to understand them!

If you google “heat through your head” + “myth” you’ll get hundreds of hits - but I suspect they mostly, or entirely, quote the same professor. On the other hand, if he’s the only one who’s ever researched it, and everyone else is going by "common sense" ...

61232.  Tue Mar 21, 2006 10:58 am Reply with quote

The paper cited is:

Thermoregulatory vasoconstriction decreases cutaneous heat loss: Sessler DI, Moayeri A, Støen R, Glosten B, Hynson J, McGuire J.
Anesthesiology 1990;73:656-660

a similar result can apparently be found in

Regional heat loss in newborn infants: I. Heat loss in healthy newborns atvarious environmental temperatures. Simbruner G, Weninge RM, Popow C, Herholdt WJ:
S Afr Med J 1985; 68:940–4

the arctic survival suit experiment is:

Heat losses from the human head. Froese G, Burton AC:
J ApplPhysiol 1957; 10:235–41

I haven't managed to check any of these papers as they all seem to require subscription. Ho hum.

61247.  Tue Mar 21, 2006 11:58 am Reply with quote

I think we can understand them, though...

Vasoconstriction is the process whereby heat is preserved in the skin by constricting the veins, thereby keeping it away from the cold surface. This stops cold blood from returning to the heart and cooling the core of the body. You get frostbite in the extremities for exactly this reason - the toes and fingers lose most of their blood supply and get so cold that the cells rupture.

This doesn't happen in the head (although it does in the nose), which explains why so much heat can be lost from it - it's not 'auto-regulated' like the rest of the skin.

Newborn infants don't have any thermoregulation ability at all until they're about 24 hours old (which is why they all wear little hats on the way home from the hospital).

I'd be surprised if this hasn't been tested since the 50s, however. Although it might not have, I suppose, if one accepts all the medical wisdom that predicts that heat loss will be higher through the head than any other same-area part of the body. The army absolutely must have done these tests, and I'm sure I can get sis to send in some data from the Antarctic chaps.

61396.  Wed Mar 22, 2006 7:40 am Reply with quote

Sis has come back with some excerpts from a couple of training manuals that her antarctic boys took with them, which make a few more good points.

The rest of the body has a fat covering which is a very good insulator (see seals, for example). The head has none.

The deep veins are close to the skin's surface in 3 areas - armpits, the groin and the sides of the neck. These areas can carry heat or cold directly to the core which equals dangerous heat-loss.

The "core" includes the organs that are vital to survival, including the heart, lungs and liver. However, the brain too is important to survival. Though in deep hypothermia, blood flow to the extremities might slow to a trickle, the blood flow in the great vessels of the neck will continue. Unlike the arms and legs, there is no counter-current heat exchange* in the neck. Therefore when the body is trying to conserve heat, and the limbs are cold, the neck and head will continue to lose heat at a fast rate.

*Counter-current heat exchange occurs when heat from an artery (from the core) can be passed to blood in a vein returning from a cold part of the body, warming it up slightly and stopping it from extracting heat from the core when it gets there. You see this a lot in, e.g. birds' legs.


Page 1 of 1

All times are GMT - 5 Hours

Display posts from previous:   

Search Search Forums

Powered by phpBB © 2001, 2002 phpBB Group