I have read figures that one can lose between 20-40% of your body heat through your head alone. Although 13-16% of the 
body’s blood volume is in the head at any given time it is a very exposed structure. The problem is that the head is only about 10% of the body surface area. Thus the head would have to lose about 40 times as much heat per square inch compared to the rest of the body.
Wilderness Medicine took volunteers, wired them to monitor their core temperatures, and discovered we lose heat through any exposed part of the body and the amount of heat we lose depends on the amount of exposed surface area. The rate of heat loss is relatively the same for any exposed part of the body not simply the head. You do not lose heat significantly faster through the scalp than any other portion of the body with the same surface area. The idea that we lose heat faster through out scalp because of the constant blood supply to the brain is simply a myth. This and other useful wilderness medicine information is available through their blog.
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I remember hearing the 90% thing and laughing my ass off, but I still imagined that you lost more heat though your head. This is useful info. Thanks for finding it.
The art of winter camping is so much fun. i laughed so hard as well, and i just love it. i will be a winter camper for life thanks to you and your web site. thank you for uniting the winter campers of the world!