There is widespread misconception regarding heat loss through one’s head. You have probably heard one or more of these myths:
- If your feet are cold, cover your head.
- You can lose up to 75% of your body heat through your head alone.
Although 13-16% of the 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 or centimeter 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.
Wilderness Medicine reports the cerebral blood flow is constant; blood flow to the brain does not change as the demand for oxygen is constant. As a result, when you look at total heat loss, the head accounts for about 7% of the heat lost. The cerebral blood flow does, however, vary based on cardiac output – the harder your heart beats, the greater the blood flow to the brain. When you begin to exercise you increase the blood flow to the brain and increase the percentage of heat loss through the head to about 50% of total body heat loss. But as the person continues to exercise, the muscles demand more oxygen which increases blood flow. To ensure thermoregulation and maintain normal core temperature (exercises increases body heat), the skin vasodilates which increases blood flow to the skin to cool the blood. The net result is a decrease in the total blood flow to the brain and a decrease in percentage of total body heat lost through the head to about 10%. Once sweating begins, the percent lost through the scalp returns to 7%. Research at the Army Research in Environmental Medicine labs showed that there was a temporary increase in heat loss through the scalp that returned to the baseline of 7% as the subjects continued to exercise.
CNN published a set of 10 health myths including heat loss through one’s head: “You Lose 75 Percent of Your Body Heat Through Your Head”. CNN reports the truth is: “This adage was probably based on an infant’s head size, which is a much greater percentage of the total body than an adult head”. That’s why it’s important to make sure an infant’s head remains covered in cold weather. (This also explains those ubiquitous newborn caps at the hospital.) But for an adult, the figure is more like 10 percent. And keep in mind that heat escapes from any exposed area (feet, arms, hands), so putting on a hat is no more important than slipping on gloves.
It is still a good idea to put on a hat if your feet are cold. But what is BUSTED is that there is nothing peculiar or unique about the head. The idea that we lose heat faster through out scalp, because of the constant blood supply to the brain, is simply a myth.
A collection of comments from previous discussions on this topic are listed below:
- firstname.lastname@example.org: I read the referenced article and still do not think that this addresses issues such as the proximity of blood vessels to the skin and the rate of heat loss.
- Ezra Burgess, Former Nordic Advisor to the Eastern Division National Ski Patrol.
I have been involved in winter camping for over 35 years. The problem here is that the original statistic is not being stated properly. The original statistic made many years ago was that 50% to 70% of body heat is lost thru the head, neck and shoulder areas of the body. The solution to prevent this was and is to put on a good hat, close up your coat and/or wear a scarf. The heat-loss doesn’t come from the head only; but from the overall collection of heated air trapped in the clothing layers rising from the “chimney effect” caused by an open neck,head and shoulder area allowing all this heated air to escape thus chilling the body. Don’t forget, however, that when engaging in strenuous activities the body will produce greater amounts of heat, requiring you to open these areas for proper ventilation and then re-closing them when activity ceases.
- Compass Points: Whether we lose 50 percent or 10 percent of our body heat through our heads …It doesn’t matter.Cover up the noggin anyway. Wear a liner while engaging in strenuous activity like hiking up a mountain, then add a thicker hat for camping, sleeping, etc. That is unless you don’t want to keep your ears.
- Josh Turner: That’s a pretty cool fact. I use an Under Armour face mask/liner that does a pretty good job.
- Trailsauce: So, we lose most heat from the head because it’s the most frequently uncovered? Makes perfect sense. Even if you wear a beanie your face is still exposed.
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February 19th, 2011 | Tags: heat loss | Category: Cold Weather Camping, Winter Camping