Feature Christmas 2008: Seasonal Fayre

Festive medical myths

BMJ 2008; 337 doi: http://dx.doi.org/10.1136/bmj.a2769 (Published 18 December 2008) Cite this as: BMJ 2008;337:a2769

Head cover in the cold

I agree that the simple statement that "40-50% of body heat" is lost
through
the head is inaccurate. The true statement is that at – 4 degC half the
heat
production of a resting (clad) man may be lost through the head and that
at –
15 degC this may rise to 70%. Also in normal circumstances people do not
venture into the cold wearing only swimsuits. The head is also different
to
other parts of the body.

1. The insulating layer of subcutaneous fat over the head is much
less than
over the rest of the body, and becomes even less in the elderly.

2. Vasoconstriction is much less effective over the head.

3. The head has a covering of hair over a large area which provides
some
additional protection but, mainly in males, this covering becomes less
with
age though the extent varies.

4. Covering the head reduces heat loss caused by temperature
difference
between the skin and the environment. However the effect of wind and wet
are much more important (factors often not measured in laboratory studies)

and using a hat provides protection from these factors as well.

5. In babies, where the surface area of the head is a much larger
proportion
of the total body surface than in adults, and the brain is a major heat-
producing organ, prevention of heat loss from the head can be critical.
In
fact the use of a close fitting gamgee-lined hat reduced the rate of fall
of
core temperature in a naked neonate, and also reduced the oxygen
consumption. The same gamgee insulation over the lower abdomen did not
have a measureable effect.

6. During personal experience of camping above the snow line with
an
inferior sleeping bag, I found that I felt too cold and uncomfortable to
sleep
till I put on a wooly hat. I then slept comfortably.

To summarise, covering the head in cold weather can produce a marked
effect in subjective comfort and reduction of heat loss. I would hate
to think that this myth-busting exercise, while discouraging them from
venturing out in swimwear, may lead to people going out in very
cold weather (cold, windy and wet) without bothering to protect their
heads.

References
All these points, except number 6, are cited in
Lloyd Evan L Hypothermia and Cold Stress. Croom Helm, Kent. 1986.

Competing interests:
None declared

Competing interests: No competing interests

21 December 2008
Evan L Lloyd
retired
72 Belgrave Road, Edinburgh EH12 6NQ
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