What are the higher heat loss areas of the body and how can I reduce heat loss in those areas?
My original answer to this question sparked a surprisingly intense debate, so I'm rewriting it to clarify a few points and offer a more holistic answer.
Let me start by saying that every square inch of skin on the human body is capable of allowing heat to escape. That is to say, if you wear a jacket with no pants, your legs will lose more heat than your torso -- but if you wear pants with no jacket, your torso will lose more heat than your legs. Both areas are capable of losing heat -- and will do so more quickly when exposed. Therefore, any part of your body that's more exposed to the cold than another part has the potential to lose more heat than other than parts that are less exposed.
This is fairly obvious, but I mention it for completeness' sake. This generally means that you should try to cover yourself as much as possible, especially larger skin surfaces like the torso.
However, the point that I think the OP is trying to get at is that -- all else being equal -- every square inch of exposed human skin does NOT lose heat at the same rate. That is to say, some areas of skin lose heat faster than others. If you were to stand in the cold totally naked, some parts of you would lose heat more quickly than others. This is an indisputable, biological fact. Allow me to explain:
Human blood serves many functions -- one of which is thermoregulation. As an evolutionary adaption, humans have areas on their body that are especially suited for releasing heat, in order to cool us off when we get too hot. In these areas, there is a high volume of blood flow, and that blood passes very close to the skin. These areas are:
- The skull
- The groin
- The armpits
- Some might also say the extremities, like fingers and toes.
In these areas, there is a lot of blood flowing around, and the skin is fairly thin, allowing heat to escape from the blood more quickly than in other areas.
Therefore, if you are in a desperate survival situation where you risk hypothermia, these are the areas of the body you should focus on keeping the warmest.
Armpits are fairly easy to keep warm, just keep your elbows down and they'll be insulated by your torso and upper arm.
It's important to keep warm, dry underwear in order to keep your groin warm as well.
And yes, YOU SHOULD WEAR A HAT. While you might not lose the "majority" of body heat through your head, it is one of the areas of skin that loses heat most quickly and it should be one of the main things you focus on keeping warm.
The extremities are also important to keep warm. I have friends that go out jogging in the winter wearing shorts and a t-shirt. They don a warm beanie and gloves right before they go out, and those are all it takes to keep them warm in freezing temperatures.
In summary: If you have clothes or ways to protect every inch of skin on your body, you should do that. I thought this was basic common sense, but people were squabbling so I'll reiterate.
However, if you only have limited means of protection and you need to focus on warming only a small area of your body, you're best off focusing on the four areas listed above.
It's probably worth pointing out that a lot of people reading this question may be thinking along the (commonly quoted) line that about 80% of body heat is lost through the head - which is much more of a myth than people realise (See here for details.) From what I remember, it was an experiment done with people fully kitted out apart from the fact that they weren't wearing anything on their heads... and, somewhat unsurprisingly, most of the body heat lost in this scenario was from the head.
Of course, it is important to keep the head wrapped up, but it's equally important to keep other parts warm - especially the torso. Even though extremities may feel colder, this is because your body is reserving the heat for its vitals and cutting it off more to things such as your hands and feet. Warm up the core of your body and it'll start warming your extremities as a result!
In terms of how to reduce heat loss in those areas, it's generally as simple as wrapping up as warm as you feasibly can (and as much as is appropriate for the conditions. Obviously full arctic gear isn't required for a damp autumnal day!)
In short, wrap up as best you can all over, but it's especially important on the torso. In practice though, it shouldn't come down to deciding what to wrap up in particular - if it's cold, wrap up as best you can all over!
Heat loss can occur from anywhere on the body through a number of processes:
- Conduction e.g. when sitting on the ground.
- Convection e.g. due to wind chill.
- Radiation i.e. heat loss direct to the environment from exposed skin.
- Evaporation i.e. heat loss through perspiration.
- Metabolism generates heat; conversely, without enough sustenance the body won't generate heat.
- Respiration the body moistens and warms the air on inhalation, this heat is lost on exhalation which increases the colder and dryer the air is.
- Body size the smaller the body the greater the relative surface area to lose heat through.
Combat heat loss by:
- Minimising exposed skin, which will radiate heat.
- Wear wind-proof clothing to minimise convective heat loss.
- Use a sit mat or sit on your rucksack when taking a break to prevent conductive heat loss.
- Wear moisture-wicking undergarments to minimise heat loss by evaporation.
- Eat high-energy foods to keep your metabolism up.
- Breathe through a scarf in cold dry air.
- Monitor children and be aware that they will feel the effects of cold before you.
As for specific, high heat loss, areas of the body, it depends:
- If you are sitting on a rock or camping without ground insulation the high heat loss areas will be those in contact with the ground through conductive heat loss.
- If your back is sweaty under your rucksack, heat loss through evaporation could make your back the high heat loss area.
- Cold, dry air can make respiration a high heat loss area.
In general, the high heat loss areas will be those not adequately protected from the various mechanisms of heat loss prevalent to the situation you are in:
Here is a thermogram pictorial answer to try and figure this out! I believe this is a un-clothed male under a thermal camera (SFW)
There's not a simple answer to the question of where your body loses heat, because the body is a sophisticated thermoregulator, and the amount of heat your body loses, and where it loses heat, varies depending on how warm or cold it "thinks" it is.
The primary means of thermoregulation are by controlling blood flow near the surface of the skin, and by evaporative cooling (i.e., sweating). Let's assume you're concerned with heat loss in cold conditions and just talk about that. All parts of your body can lose heat, but it is obviously not equal, because heat loss will be greater where blood flows close to the skin in exposed parts of the body. The thermogram link posted by mjrider shows many of these areas.
When cold, your autonomic nervous system uses vasoconstriction to reduce blood flow to areas. In your extremities, blood flow through capillary beds which accelerates heat loss can be completely shut down, and blood flows directly from arteries to the veins through vascular shunts. Note there is some argument about the presence of precapillary sphincters involved in this process, but there is more recent evidence of the involvement of the vascular shunts. Not sure anyone has explained the role of the AV shunts in Raynaud's syndrome, but Raynaud's clearly illustrates just how effective these vascular shunts are when in effect. Similarly, rosacea and erythromelalgia reveal locations of capillary beds used for dissipating heat when hot.
However, as mentioned in other answers, blood flow to the brain is critical and isn't constricted, so areas like the neck and temple where blood flows next to your skin, when not covered, can represent a major loss of heat.
Also note that these mechanisms can also inadvertently contribute to hypothermia or frostbite, and in some cases it's better to maintain some blood flow. Laplanders and Inuit have an intrinsic ability to withstand cold conditions far better than the rest of us. The rest of us would have to practice ancient yogi techniques for maintaining warmth, recently popularized by Wim Hof (The Iceman).
The short answer is age-old knowledge: in addition to regular clothing, wear warm socks, gloves, a hat and a scarf. While the feet and hands won't lose a lot of heat when blood flow is constricted, you will become dysfunctional if you don't keep them warm enough.
Over most of your body your system can restrict blood flow to the skin, cooling the skin (and making you feel cold)
Locations where bloodvessels run close to skin big sources of heat loss. Parts of your body that have sub-cutaneous fat lose heat more slowly. Wehre there is little fat (sides of the chest, under the arms) heat loss is more rapid.
The head has no auto-regulation for heat, and receives about 25% of your blood supply. If you were standing naked, then initially some fairly large percentage (the head isn't 25% by area, so it will be under 25%) will come off your head.
As your body realizes, "hey, it's cold out here" surface circulation will be reduced. Later even main flows to limbs will be reduced. Muscles get stiff. Fingers get inept at doing up zippers and buttons. (Fumbles) You are now in trouble. Further chilling will result in cooler blood going to the brain. Large muscle coordination gets erratic (Stumbles) Judgement starts to go, and speaking is impaired (mumbles) Note that the 'umbles are also characteristic of low blood sugar. Eat.
But backing up, as you cover up, what heat losses there are come from the parts that are still uncovered. So yes, if you are wearing winter boots, good mitts, pants over fleece and a storm parka, then, yes your head is likely to be some very high percentage of your heat loss.
Prevention: Stay cool to stay dry. Open up when you even start to sweat. If your hands get cold put on your hat. If your still cold, put up your hood.
If you start to shiver, your body metabolism is falling behind. Reduce the heat loss. Get out of the wind, get more clothes on. Change wet or sweat soaked clothes for dry ones. You need to fuel that shivering. Eat. Drink. Hot sweet liquids are best. Dehydration interferes with metabolism.
Prevent dehydration. Eat snow if you must.. If you aren't already cold it won't hurt you. The energy of half a snickers will melt a kilogram of snow into water. (The ice crystals however will scrape your throat raw) One trick I use on winter trips is to bring a thermos of triple strength juice at as close to boiling as I can. Fill the cup 1/3 full and pack snow into it until it doesn't melt. One liter in the back becomes 2-3 liters in the field.
Lots of ways to reduce heat loss: Wear a hat scarf and gloves as these keep the most important parts of your body warm. Wear warm knickers (even if you're a guy). You can get fleecy knickers but not fleecy pants. Also, you can huddle, like penguins as it reduces your surface area and conserves bodyheat.
I don't think heat versus temperature is addressed in any answers.
Assuming all skin is the same temperature then cover all skin with equal insulation is optimal. Starting warm all skin is about the same temperature. Cover nose mouth and eyes is problematic.
What happens is a smaller mass will lose temperature faster. There is just not as much thermal capacity (reserve) behind the skin. If you go outside naked on a cold day your fingers, toes, ears, and nose will get cold first. The body could send more blood to those parts to basically share the cold but it does not (and for good reason). The body actually restricts blood flow to the extremities to preserver heat (temperature) for the vital organs. This also reduces heat loss as a cold hand, arm, foot, and leg loses heat at a lower rate. Those parts also recover better from cold with the exception of frostbite. The body considers the brain a vital organ. So from the perspective of what the body does when it is cold then torso and head are most important but most people don't want to rob from extremities and make cold feet and hands even colder.
While I personally disagree to other answers, I'd like to make a point. According to my understanding an uncovered head will loose a lot of heat. Put a hat on and no top and see what happens. I'm a hunter and when I sit, I find the cold area are the shoulders, kidney and wrist.
I had a Saab 900 with heat vents that would blow on my wrist when I put my hand on the dash. It was amazing to experience how quickly my body was warmed up when heat blew on my wrist. These in my experience are the key areas to keep warm when all else in you clothing is equal.
To answer the question directly, the three areas that lose heat the fastest are actually:
- The Head
- The Groin
- The Armpits
This is why those are the areas that naturally grow the most hair on human bodies. It's an evolutionary adaption to keep us warm. In those areas, there is a lot of blood flowing and it passes very close to the outer layer of skin, allowing it to cool of more rapidly and lower our body's temperature. It's all of the blood rushing around your skull, into your legs, or into your arms, respectively.
Covering your head is a no brainer. As others have mentioned and most people already know, the head it where you lose most of your body heat, so wearing a dry, windproof hat with a lot of loft is always a smart idea if you're cold.
The groin and the armpits are a bit more tricky. If you're dressed according to normal human social standards, you'll probably be fine. But if, for some reason, you're out in the cold with little or no clothing, those are the areas to focus on the most.
Keep your arms tight at your sides and cover your groin with your hands. Not only is that our natural reaction to the whole "being naked" thing, it also happens to be the best way to stay warm with no clothes on!