Heat loss through the head

[B the Hiker]

I don't have the link for this article from The Guardian, as a friend clink-n-dragged it into an email, but it is mildly interesting. It's not going to stop me from wearing a hat or anything...

B

p.s. I'm actually more interested in the "five other myths" they claim to have dispelled!

# # #

Scientists debunk the myth that you lose most heat through your
head

Ian Sample, science correspondent
The Guardian, Thursday 18 December 2008

If as much as 45 percent of body heat were lost through your head,
going out without a hat would feel like going out without trousers.

When it comes to wrapping up on a cold winter's day, a cosy hat is
obligatory. After all, most of our body heat is lost through our
heads--or so we are led to believe.

Closer inspection of heat loss in the hatless, however, reveals the
claim to be nonsense, say scientists who have dispelled this and five
other modern myths.

They traced the origins of the hat-wearing advice back to a US army
survival manual from 1970 which strongly recommended covering the
head when it is cold, since "40 to 45 percent of body heat" is lost
from the head.

Rachel Vreeman and Aaron Carroll, at the centre for health policy at
Indiana University in Indianapolis, rubbish the claim in the British
Medical Journal this week. If this were true, they say, humans would
be just as cold if they went without a hat as if they went without
trousers. "Patently, this is just not the case," they write.

The myth is thought to have arisen through a flawed interpretation of
a vaguely scientific experiment by the US military in the 1950s. In
those studies, volunteers were dressed in Arctic survival suits and
exposed to bitterly cold conditions. Because it was the only part of
their bodies left uncovered, most of their heat was lost through
their heads.

The face, head and chest are more sensitive to changes in temperature
than the rest of the body, making it feel as if covering them up does
more to prevent heat loss. In fact, covering one part of the body has
as much effect as covering any other. If the experiment had been
performed with people wearing only swimming trunks, they would have
lost no more than 10% of their body heat through their heads, the
scientists add.

 

[Brambor]

I only have pants.

I've got to start wearing trousers.

 

[Little Rickie]

I would think people with large heads would loose more heat.

We know lots of people with large heads so it should be easy to test.

I for one wear a hat because my head gets cold and I'm more comfortable.

How much heat is lost when we don't wear gloves or mittens?

 

[SAR-EMT40]

In fact, covering one part of the body has as much effect as covering any other. If the experiment had been performed with people wearing only swimming trunks, they would have
lost no more than 10% of their body heat through their heads, the
scientists add.

I agree with almost all the information in this article but I disagree with the statement “covering one part of the body has as much effect as covering any other” and I’ll try to explain why. This statement is true as far as actual heat loss but when your arms and legs get cold and bring that cold blood to the core your body has a mechanism to vasoconstrict and also shunt your blood so that you lose less heat through your limbs. I am aware of no such mechanism in your head. So if you were to walk around without pants, shirt and hat eventually you would start loosing less heat through your legs and arms because your body would shunt and vasoconstrict your limbs but you will continuously lose the same amount of heat from your head. Of course the upshot of this is that if you keep your limbs covered and warm and only your head is exposed there is no body mechanism to reduce the heat loss from your head (10% according to the article), other than putting a hat on. So while actually uncovering your arms or legs your body can compensate to help keep your body core warm it has no method to do the same with the heat loss from your head. For obvious reasons the body doesn't want to shunt or vasoconstrict blood from the head area. At least for most of us.

This is just my take on it. I would be interested to hear other opinions on this.

Regards,
Keith

 

[Little Rickie]

Will the body increase blood flow to the Head (brain) if the head is cooling off too quick? I know my head will start to hurt real bad if I don't keep it covered in the cold, especially if my head is wet and the wind is blowing.

I do take off my gloves to help regulate body heat.

 

[nartreb]

Keith's got a point: reducing blood flow to the head does not improve one's odds of survival. Mitigating the importance of this are two factors:
1) vasoconstriction can and does occur within the skin, and especially the ears and nose;
2) Most people's heads are already partially covered with a decent insulator (hair)

The old skin-burn estimation guidelines hold that the head (with neck) is about 9% of your surface area, equivalent to an arm (or half a leg, or half a torso). That's roughly consistent with the "new" 10% estimate.

I made that "sure, if the rest of you is covered" remark to a friend many years ago; you mean I could have gotten published? Not often enough is the question asked: is our doctors learning?

Edit: point 1) is actually an argument for wearing a hat and mask. Subfreezing temperatures + vasoconstriction = frostbite.

 

[Peaks]

Well, I for one, wear a hat more often than I used to when there was hair up there.

 

[SAR-EMT40]

The old skin-burn estimation guidelines hold that the head (with neck) is about 9% of your surface area, equivalent to an arm (or half a leg, or half a torso). That's roughly consistent with the "new" 10% estimate.

Interesting point about the rule of 9's. Didn't think of that but it is an interesting thought.

Edit: point 1) is actually an argument for wearing a hat and mask. Subfreezing temperatures + vasoconstriction = frostbite.

I totally agree but frostbite is an entirely different conversation though. But bottom line is if your body needs to vasodilate and shunt some part long enough, frostbite is going to be the result, guaranteed.

Keith

 

[SAR-EMT40]

Will the body increase blood flow to the Head (brain) if the head is cooling off too quick? I know my head will start to hurt real bad if I don't keep it covered in the cold, especially if my head is wet and the wind is blowing.

I do take off my gloves to help regulate body heat.

I wonder if it it could be like an ice cream headache which I just checked is caused from the ganglion in the roof of the mouth being cooled by the ice cream. This causes the constriction and dilatation of the vessels in the head causing refered pain. I wonder, (out loud) if the constriction and dilatation of vessels in the head from the cold could be the cause.

I also use pit zips, clothing removal and hat removal to help regulate the amount of sweating I do. Actually to totally prevent the sweating I do if at all possible.

Keith

 

[Trainwreck]

I think the ice cream headache is a combo vagal response and the proximity of the hard palate to the brain.
One thing is for sure. When my feet are cold, I put a hat on, and they get warmer.

 

[J.Dub]

Well, I for one, wear a hat more often than I used to when there was hair up there.

As part of the High Forehead Club, ("I'm not just the president...I'm also a member") I have to agree...

 

[DougPaul]

Edit: point 1) is actually an argument for wearing a hat and mask.

If you are in a cold-weather survival situation and have a facemask--put it on. It will conserve a significant amount of heat even if frostbite is not an issue. (A significant number of cases of hypothermia occur above freezing.)

Subfreezing temperatures + vasoconstriction = frostbite.

It isn't quite that simple. There is a mechanism whereby the body will send a bit of warm blood down to a cold extremity periodically (every ~15min or so, IIRC) in an attempt to minimize damage to the extremity without losing too much core heat. This mechanism is evidently common in whites, but lacking in blacks.

This info is from a lecture by Dr. Murray Hamlet, formerly of the US Army's Research Institute of Environmental Medicine.

Doug

 

[DougPaul]

Will the body increase blood flow to the Head (brain) if the head is cooling off too quick?

Maybe not to the brain, but certainly to the skin. Notice how red people's faces get when they are out in the cold? That is the high blood flow to keep the skin (and ultimately the brain) warm.

Doug

 

[DougPaul]

The face, head and chest are more sensitive to changes in temperature than the rest of the body, making it feel as if covering them up does more to prevent heat loss. In fact, covering one part of the body has as much effect as covering any other. If the experiment had been performed with people wearing only swimming trunks, they would have lost no more than 10% of their body heat through their heads, the scientists add.

I agree with Keith (SAR-EMT40) here: arms and legs can vasoconstrict far more than the head. And the body routes warm blood to the face when it is chilled in spite of the heat loss.

Doug

 

[DougPaul]

Scientists debunk the myth that you lose most heat through your head
<snip>

IMO, this is one of those "let's change the conditions of the experiment and make a fuss" articles. (All too frequently a conclusion gets repeated and the conditions get forgotten which can lead to this kind of article.)

Many people will insulate their bodies in cold weather, but will resist hats (fashion, etc). Also, one's hands and/or feet may feel cold while one's head is comfortable--thus it is not intuitive that the solution may be to insulate one's head. In the context where a hat is an issue, a disproportionate amount of heat will be lost from a bare head.


"Cotton kills" is another one of these "truisms"... In the cold yes it can, but in the desert heat the heat loss through damp cotton can save your life.

"A pound on your feet is equivalent to 5 on your back". Supposedly from an Army experiment. I can believe it for a quick-march on smooth level terrain, but on a rugged hiking trail? Maybe or maybe not.

Doug

 

[Craig]

p.s. I'm actually more interested in the "five other myths" they claim to have dispelled!

Here ya go.

 

[SAR-EMT40]

I think the ice cream headache is a combo vagal response and the proximity of the hard palate to the brain.
One thing is for sure. When my feet are cold, I put a hat on, and they get warmer.

I actually wrote that it was a vagal response originally because I was pretty sure that was what it was. I tried to check that fact after I had written it and could not find any reference that corroborated that. I was pretty sure that was what it was from but I could not verify it at the time so I changed what I had written to match what I found in several references.

Keith

 

[ColdRiverRun]

The fact that they try and break heat loss down to only being about surface area exposed is nonsense. The amount of blood flow, proximity of vessels to the surface, thickness and natural insulation of those areas is highly critical. You lose a lot more heat through areas like the crotch, armpits and neck than other areas of equal surface area due to things like being closer to the core, relatively thinner skin, amount of blood vessels next to the skin and such. The neck alone gives off more heat through the carotids and jugulars than the skull region itself. I agree with Doug on the type of article.

IMO, this is one of those "let's change the conditions of the experiment and make a fuss" articles.

 

[DougPaul]

In searching for something else, I uncovered the following: "Frozen Mythbusters" http://www.box.net/public/static/lehlmxuu38.pdf. Some good info from three experts.

Doug

 

[Stan]

Ahhhh. Science trumps common sense. I wonder if there are more important myths to debunk. Meanwhile, I'll continue to wear my hat ... sometimes two.