In an adult who is evenly dressed, or evenly undressed, the head gives off heat in rough proportion to its size. That works out to something closer to a tenth of body heat than the half many people have heard. The old figure isn’t pure invention, though. It comes from cold-weather tests where the head was the only bare part, which is why whether a hat matters depends less on your head than on what the rest of you is wearing.
The Head Loses About Its Fair Share of Heat
Your body sheds heat mostly through its skin, so what counts is surface area: the total amount of skin exposed to the air or water around you. If everything is equally exposed, a body part loses heat roughly in proportion to how much skin it has. The adult head and neck make up only around 7 to 10% of total skin area. So for an adult who is naked or evenly dressed, the head gives off about that same share of heat.
Cold-water tests back this up. When the back of the head and upper chest were dipped in cold water, extra heat loss rose by about 10%, matching the roughly 10% of extra skin put in the water.1PubMed. Thermal effects of dorsal head immersion in cold water on nonshivering humans With the whole head under 17°C water, the head lost no more heat for its size than the rest of the body did.2PubMed. Thermal effects of whole head submersion in cold water on nonshivering humans
The head does feel colder than most places, and that helps the myth seem believable. Your face and scalp are packed with nerve endings that sense cold sharply. A chilly face or pair of ears can make you feel cold all over even when your total heat loss is modest. Feeling cold and losing a lot of heat are not the same thing.
A comparison with legs makes the point. Your two legs together hold far more skin than your head, so bare legs in winter lose more total heat than a bare head. A warm coat and trousers matter more than any single hat.
Still, a bare head in a cold wind counts for something. In winter the face is often the only part left uncovered outdoors, so in practice it can account for more than its share of heat loss. The useful question is “how much skin is uncovered?” and not “which body part is this?”
Blood flow adds to the confusion. The scalp and face have a rich blood supply near the surface, so a bare head can look flushed when you are hot. In the cold, the skin of your limbs and trunk narrows its blood vessels to hold heat in, and the face responds less dramatically, so it tends to stay a bit warmer than your hands. That extra warmth doesn’t mean the head loses more heat than its size predicts; the cold-water tests show it doesn’t. It does make the head glow on a thermal image, which can reinforce the belief without proving it.
Where the Half Your Heat Myth Came From
In the 1950s, US military researchers put volunteers in bitterly cold conditions wearing Arctic survival suits.3the Guardian. Scientists debunk myth that most heat is lost through head The volunteers’ heads were left bare, the only uncovered part of the body.4the Guardian. Scientists debunk myth that most heat is lost through head About 40 to 45% of their heat loss came from the head.5Knowledge Stew. The Myth of Losing Heat Through Your Head But that figure reflected the covered bodies, not anything special about the head.6Knowledge Stew. The Myth of Losing Heat Through Your Head
The suits sealed in almost all the skin that would normally lose heat, leaving the bare head to do most of the work. The experiment measured where heat escaped in that particular outfit, not in an ordinary person in ordinary clothes.
Then the number traveled. Later retellings kept the striking percentage and dropped the detail about survival suits. Army survival guidance repeated the idea of covering the head in cold weather, and over decades it turned into “you lose most of your heat through your head” as general advice.
It lasted because a single number like 40 to 45% is easy to remember, while “the rest was in a survival suit” is dull and gets dropped. The advice that followed, to wear a hat, was sensible, so nobody had a reason to question the number behind it.
When a Hat Really Does Make a Difference
In cold-weather activity, most of the body is usually covered while the head and face stay partly exposed.7DexShell official. Do We Really Lose More Heat Through Our Heads? That makes the head a meaningful route of heat loss because it lacks the same protection, not because it behaves differently.8DexShell official. Do We Really Lose More Heat Through Our Heads? There’s another effect too: cooling the face alone tightens skin blood vessels and raises blood pressure.9PubMed Central. Control of blood pressure in the cold: differentiation of skin and skeletal muscle vascular resistance A cold face changes how your body responds to cold, not just how you feel.
So cover your head, ears, and neck when the rest of you is warmly dressed for the cold. Put warm layers on your torso and legs first, since they hold most of your skin.
For most healthy adults that blood pressure response is harmless, but people with heart or blood pressure problems may want to think about it on very cold days. A scarf or balaclava pulled over the nose and cheeks in strong wind or bitter cold limits the effect.
A hat only helps if it insulates. Thick knit, wool, or fleece traps still air and holds warmth far better than thin fabric. Cover the ears too: they are thin and have little tissue to hold heat, so they chill, and can be injured by cold, sooner than thicker areas. On windy days, loose weaves let the wind through, so a windproof outer layer or a tightly woven hat is worth having. A wet hat loses much of its insulation, so carry a spare or swap it if rain or snow soaks it.
Your own hair is natural insulation. It forms a layer that slows heat moving to and from the scalp. Thick hair gives some protection, but less than a proper hat in real cold. With thin hair or none, the scalp is more exposed, so bald readers may notice a cold head sooner and benefit more from a hat.
Why Babies Lose Much More Heat From Their Heads
Newborns lose a large share of heat from the head because it has so much skin for their size.10PubMed. Head insulation and heat loss in naked and clothed newborns using a thermal mannequin A newborn’s busy brain also produces a lot of heat that flows out through the head.11PubMed. Reduction of neonatal heat loss by an insulated head cover That is why insulated hats cut heat loss from the baby’s head by about 63 to 73%.12PubMed. Reduction of neonatal heat loss by an insulated head cover In a 27°C room, babies in hats lost about 25% less total heat than when naked.13PubMed Central. Head insulation and heat loss in the newborn A thin tube-gauze hat, however, gave no measurable protection.14PubMed Central. Head insulation and heat loss in the newborn Choose a hat with real insulation, not a thin stretchy cap.
Newborns also can’t shiver effectively or adjust to the cold the way adults do, so they depend on caregivers to keep them warm. That’s why hospitals commonly put hats on babies soon after birth, when they are wet and most at risk of chilling. The head’s share of heat loss falls as children grow, until by adulthood it settles near one-tenth.
Hats can be overdone, though. When a newborn is already heavily dressed, a bonnet can leave only the face to release heat, which may lead to overheating.15PubMed. Head insulation and heat loss in naked and clothed newborns using a thermal mannequin Once a baby is warm and well dressed, it is better to adjust body layers than to keep a hat on indoors.
To check, feel the back of the neck or the chest rather than the hands or feet. Cool hands are normal and don’t mean the baby is cold. A sweaty neck, flushed cheeks, or damp hair suggest overheating. Follow your nurse or doctor’s advice about hats indoors and at sleep.
Older Adults, Cold Water, and Emergencies
Older people cannot narrow skin blood vessels to hold in heat as well as younger people.16Journal of Thermal Biology. Thermal physiology of the elderly and handicapped Physiological responses and thermal sensations of the elderly in cold and hot environments They also notice cold later than younger people do.17Journal of Thermal Biology. Thermal physiology of the elderly and handicapped Physiological responses and thermal sensations of the elderly in cold and hot environments So older adults should dress fully, head included, before they feel chilly. Dress for the forecast, not for how you feel, with a hat, scarf, and warm layers. It matters indoors too, where a cool house can slowly lower body temperature without the person noticing.
Hypothermia is a dangerous drop in body temperature. In emergency care, insulating from the ground, adding a vapor barrier, and covering the head early helps stop heat loss.18ISAR Journal of Medical and Pharmaceutical Sciences. Prevention of Accidental Hypothermia in Pre-Hospital Care: The Critical Role of Head Protection But in a simulated snow burial, insulating the head and face did not slow cooling.19PubMed. Effect of head and face insulation on cooling rate during snow burial In cold water, a head dip can cool the core more than its size suggests, because a chilled body circulates blood through less tissue.20PubMed. Thermal effects of whole head submersion in cold water on nonshivering humans So in cold water, keep your head out when you can.
If someone is getting too cold, get them out of the wind, water, or snow first. Put insulation beneath them, since the ground draws heat fast. Replace wet clothes and cover the head and body. Call emergency services if they become confused or stop shivering.
How the Head Helps You Cool Down in Heat and Exercise
The head is also a major cooling outlet. Head sweating rises as you work harder and your core warms.21PubMed. Local differences in sweat secretion from the head during rest and exercise in the heat Evaporative heat loss, which is cooling as sweat dries, matters especially from the head when protective clothing blocks the rest of the body.22PubMed. Quantification of head sweating during rest and exercise in the heat During moderate exercise, the head’s skin and breath together can shed about 200 watts of heat.23PubMed. Heat loss from the human head during exercise That is more heat than the brain receives and produces when you overheat.24PubMed. Heat loss from the human head during exercise Fanning the face can lower head temperatures and make you feel less overheated.25PubMed Central. Natural selective cooling of the human brain: evidence of its occurrence and magnitude
Hats and helmets change this. Any covering over the head reduces how well sweat evaporates.26PubMed Central. Human scalp hair as a thermoregulatory adaptation In strong sun, though, hair cuts the heat you absorb, with tightly curled hair best.27PubMed Central. Human scalp hair as a thermoregulatory adaptation Ventilated helmets keep the head cooler and drier and are more comfortable.28Measurement: Journal of the International Measurement Confederation. Experimental investigation on the thermal and moisture mapping of cricket helmets using microsensor technology In hot weather, choose a light, airy sun hat and a top-vented helmet when you need one.29PubMed. Helmet design to facilitate thermoneutrality during forest harvesting
For summer workouts, splash water on your face and head, since evaporating water takes heat from the skin. If you’re sweating heavily in the shade, take your hat off so sweat can dry. In direct sun, shield your head, then let it breathe when you can.
Winter exercise brings the reverse problem. Hard effort under a thick hat can overheat you. A thinner hat or headband lets heat escape while still protecting your ears. Take the hat off when you’re sweating and put it back on during rest, because damp hair chills fast once you stop moving.