Have you heard somebody (probably your mom) tell you to put a hat on because you lose a significant percentage of body heat out your head alone? I’ve heard that number quoted from 50-90%. When I asked, “Why do you lose so much heat out your head?” I got an answer along the lines of “Your brain needs lots of oxygen and the blood takes it there and so there is lots of blood in your head, which lets all of your heat out.” Regardless, it didn’t make sense to me when I was a defiant youngster. “But I am not cold! I never get cold! My brain doesn’t get cold!” I’d explain to my mom when I wanted to play outside in a blizzard with no hat as a kid. And it still doesn’t make sense to me now, even as a sensible hat-wearing-when-it-is-cold adult.
40-45% of Body Heat is Lost Through Your Head
Most of my research points to the 1957 FM-21-76 US Army Survival Manual  as the source of this myth. On page 148, under the heading “Basic Principles of Cold Weather Survival” the manual states,
“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 area 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.”
The US Army likely got its information from a Canadian Defence Research Board study from 1957 which showed that 40-45% of your body heat is lost out your head… when wearing insulative protective cold weather gear from the neck down. The study was performed on three subjects who were sat in a chair in a room of varying temperatures, which got as cold as -20 deg C (-4 deg F). The subjects wore clothing so that the sensation of cold was only felt at their head. Their head was also encapsulated in a styrofoam box calorimeter which measured approximate heat loss. One of the conclusions from the study is that the body attempts to keep the brain temperature normal, which causes significant blood flow to the area to keep the brain warm . However, the study neglected to account for the heat loss from the skin across the rest of body, which was not the same due to the fact that the body was insulated and the head was not.
Well meaning moms everywhere since 1957 have perpetuated the myth that a large percentage of your heat loss is through the head.
9-10% of Body Heat is Lost Through Your Head
The original Canadian study was performed in 1957. Since then, two subsequent studies of brave subjects in cold water have shown that heat loss from the surface area of your head is comparable to heat loss from a similar surface area elsewhere on your body  . Heat loss out your head averages about 10-11% of your total body heat. Your head is about 7-10% of the total surface area of your body. The amount of heat loss out of your head is proportionate to the amount of surface area of your body. If you covered your head and everything else, but left 7-10% of your legs exposed, you would experience about the same amount of heat loss as if everything except your head was covered.
Though, there is a caveat. If you are exercising (shivering would fall into this category of “exercising”),your heart pumps faster and your blood flow increases. The increased blood flow to the brain increases the heat loss out of your head to about 50%, until your muscles demand more oxygen and blood, and then the heat loss returns to about 10%. If streaking during cold weather is your schtick, wearing a hat will help you retain some body heat initially, but it will become irrelevant as you continue running.
However, don’t throw away your collection of warm wool caps. Even though the heat loss is similar to the surface area exposed, the body core temperature drops significantly when the head is exposed and the body is insulated. The significant core temperature drop may be due to physiological factors controlled by the trigeminal nerve, which innervates your face . Basically this means if your face gets cold, then your body will react in a way that drops your core temperature significantly, even though the heat loss rate out your head is the same as the heat loss rate of other areas of your body.
Newborns Have Big Heads
The old adage of losing lots of body heat out your head is somewhat true for newborn babies, as newborns have a proportionately larger head than adult humans. Newborn head surface area measures about 20.8% of the total surface area of the body , whereas in adult humans the surface area of the head accounts for about 7 -10% of the surface area of the adult human body . A newborn wearing a hat retains 23% more body heat than a naked newborn, or a newborn wearing a cumberbund of equivalent surface area to the hat. The ubiquitous newborn hats drastically help in retaining body heat .
Thin, wicking hats (affiliate) are great for running and aerobic exercise during cold weather. When it is super cold, wear a warm aviator type hat (affiliate) which will help keep your trigeminal nerve endings in your face warm. This will prevent your core temperature from dropping.
Wearing a hat isn’t as crucial as Mom implies it to be. Although you don’t lose the majority of body heat out your head, it is still prudent to wear a hat when it is cold, just like you wear pants and long sleeves when it is cold. Wear a hat, it feels nice.
 U.S. Department of the Army. “Basic Principles of Cold Weather Survival.” FM 21-76 US Army Survival Manual. U.S. Department of the Army, 1957. 148. Print.
 Froese, Gerd, and Alan C. Burton. “Heat Losses from the Human Head.” Journal of Applied Physiology 10.2 (1957): 235-41. Print.
 Pretorius, Thea, Farrell Cahill, Sheila Kocay, and Gordon G. Giesbrecht. “Shivering Heat Production and Core Cooling during Head-in and Head-out Immersion in 17 Degree C Water.” Aviation Space and Environmental Medicine 79.5 (2008): 495-99. ResearchGate. Web. 7 Nov. 2016.
 Pretorius, Thea, Gerald K. Bristow, Alan M. Steinman, and Gordon G. Giesbrecht. “Thermal Effects of Whole Head Submersion in Cold Water on Nonshivering Humans.” Journal of Applied Physiology 101.2 (2006): 669-75. American Physiological Society. Web.
 Stothers, J. K. “Head Insulation and Heat Loss in the Newborn.” Archives of Disease in Childhood 56 (1981): 530-34. PubMed. U.S. National Library of Medicine. Web. 08 Nov. 2016.
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