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If you didn't grow up in a tropical climate, you probably heard something like this as a child...
Put on a hat before you go out because you lose the most heat through your head.
Some say that the reason you lose more heat through your head is because "heat rises" and most of your body heat naturally escapes though your head.
Is this true?
Does more body heat escape from our heads than from other parts of the body?
Apperently...
You can lose 40 to 45 percent of body heat from an unprotected head and even more from the unprotected neck, wrist, and ankles. These areas of the body are good radiators of heat and have very little insulating fat. The brain is very susceptible to cold and can stand the least amount of cooling. Because there is much blood circulation in the head, most of which is on the surface, you can lose heat quickly if you do not cover your head. - FM 21-76 US ARMY SURVIVAL MANUAL, BASIC PRINCIPLES OF COLD WEATHER SURVIVAL, PG 148.
Can this be right ? Does more body heat escape from our heads than from other parts of the body ?
Short Answer: No.
In general, heat loss is essential to provide thermoregulation and prevent excessive heat build-up in the body to avoid the consequences of heat stroke. However, heat loss in the head is the same as for any exposed body part. The exposed surface area of the body, along with relative humidity and temperature, determines heat loss primarily through evaporation and our head makes up only 10 percent of body surface area. So, at rest, heat loss through the head accounts for only 7-10 percent of total heat dissipation. - source
See below: General heat loss through the head.
As you begin exercise, cerebral blood flow increases due to increased cardiac output and the percentage of heat lost through the head accounts for about 50 percent of total body heat loss. As exercise continues, more oxygen is directed toward muscle and blood flow to this tissue increases. Core temperature has to be maintained and as body heat increases, the skin arterioles expand, or vasodilate, redirecting blood flow to the skin which cools the blood. Hence, total blood flow to the brain is decreased and the percentage of total body heat lost through the head is reduced to about 10 percent. The percent lost through the scalp returns to 7 percent after sweating begins. - source
Shivering is exercise...
But, this is important, if they are shivering, the percent of heat loss via the scalp can increase to upwards of 55%, so protecting the head well is a very important part of treating the hypothermia patient. And as you can imagine, the primary defense against the cold and hypothermia is vasoconstriction of the peripheral circulation, this shunts blood to the core, reduces circulation to the skin, and increases the percent of heat loss through the scalp. - source
General heat loss through the head
source: Thermal effects of whole head submersion in cold water on nonshivering humans
One hypothesis predicts a substantial heat loss through the head due to the great amount of surface blood flow in the scalp and because scalp blood vessels do not vasoconstrict in response to cold as do surface vessels in other body areas (8). An alternative hypothesis predicts minimal heat loss from the head because submersion of the head and neck would only involve 7–9% more of the body surface area (20). As well, mathematical modeling predicts minimal conductive heat loss directly through the scalp and skull (27).
The present results are consistent with previous data (9) in that the supposition of proportionately greater heat loss from the head was not supported.
Whole body cutaneous heat loss (n = 8) values for baseline (20 min) and immersion (30 min) periods. *Greater than body-insulated conditions, P < 0.001. †Greater than head-out in the same subconditions, P < 0.05.The measured heat loss from the head in both head-in conditions was only 100 kJ (compared with 17 kJ in the two head-out conditions). In contrast, total heat loss in the body-exposed configurations was 914 and 988 kJ, for head-out and head-in conditions, respectively. In the latter case, the head accounted for only 10% of the total body heat loss when both the head and body were submersed. The surface area of the submersed head is 7% of the total surface area of the body.
Energy production and loss during 30 min. of immersion in 17°C water (n = 8). Total loss includes whole body cutaneous and respiratory heat loss. Body loss includes trunk, legs, and arms. *Greater than body-insulated conditions, P < 0.001. †Greater than head out in the same subconditions, P < 0.005. ‡Greater than head-out conditions, P < 0.001.
These results thus indicate that heat loss from the head is not disproportionately increased over what would be expected from the head’s contribution to total body surface area.
However...
With the body exposed to cold water and shivering intact, additional dorsal head immersion increased core cooling from 3.8 to 9.4°C/h (250%) in 1–2°C water.
Oliver_C is correct: "No, mom, Science says I don't need to wear a hat !..."
"...Ok, mom, I'll bring one along just in case I get cold !"
Cited and worth a read...
(8) Froese G and Burton AC. Heat loses from the human head. J Appl
Physiol 10: 235–241, 1957.
(9) Giesbrecht G, Lockhart T, Bristow G, and Steinman A. Thermal
effects of dorsal head immersion in cold water on nonshivering humans.
J Appl Physiol 99: 1958–1964 2005.
(20) Layton R, Mints WJ, Annis J, Rack M, and Webb P. Calorimetry with
heat flux transducers: comparison with a suit calorimeter. J Appl Physiol
54: 1361–1367, 1983.
(27) Xu X, Tikuisis P, and Giesbrecht G. A mathematical model for human
brain cooling during cold-water near-drowning. J Appl Physiol 86: 265–
272, 1999.
Bear in mind that the rest of the body is usually clothed which provides the most important insulation of the body, as can be seen in a thermal scan image:
[Scale is heat loss from least (bottom, blue) to most (top, red/white); source: Britannica.com]
However, this image also shows that hair is actually a pretty good insulator as well. So while a clothed human indeed loses much of their heat through the head, donning a hat won’t help.
There is another aspect: the body will strive to keep the core body temperature constant. If necessary, it does this by down-regulating blood circulation (and consequently heat exchange) in the periphery (arms, legs); in contrast, the blood circulation through the head (in particular, the brain) stays almost constant:
In fact, next to the trunk (clothed!) the head emits the most amount of heat per area (skin temperature of 34.6°C) [source: Blatteis, Physiology and pathophysiology of temperature regulation, p. 17], and also the most total energy of any non-clothed area [ibd.].
So yes, when clothed (and not immersed in water; this diminishes the insulating effect of clothing significantly!) we do indeed lose most of our body heat through our head. However, except for bald people, hats may not help that much (to maintain the core body temperature).
Reports say that the claim is a misunderstanding of the user manual for an early survival suit. If your body is well wrapped, then the head becomes the main vector of heat loss.
But not overwise.
http://www.guardian.co.uk/science/2008/dec/17/medicalresearch-humanbehaviour
There are real life consequences to cold water immersion, such as in the sea. The head is the only part of the body exposed, so what happens is waves continually wet the head and then the wind evaporates the water thereby cooling the head far more effectively than if the head remained dry and free from wind. In this case the head does lose a lot more heat than other parts of the body which will typically have clothing on which acts in some way as an insulator. This is why some life jackets have built in wind proof hoods. Couple that with a thermal hat and the head is no longer a major heat loss.
