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The topic of human response to cold water presents many questions. Does body fat matter? Does surface area to mass ratio matter? Is cold adaptation limited to commercial divers of the Orient? Will pouring warm water in your suit help or hurt? Why are women not at greater risk of hypothermia than men? Why are men's hands and feet often warmer in winter than women's and what does that have to do with penguins?

My interest in cold was 'sparked' as a small child as I sat in the snow watching my grandfather, the oldest member of the Brooklyn Icebergs, walk barefoot across the ice on Coney Island beach to go swimming as he did every day. Later I became a long distance competition swimmer racing in cold lakes and rivers. Still later I did two Army tours overseas stationed in mountains so cold you could frostbite your thighs just trying to go to the bathroom. Today, as a researcher in thermoregulation and immersion, I have the luxury of studying cold water immersion 23 hours a day and still have time to call out for pizza. My occupational perspective follows in hopes of clearing up harmless, yet common misconceptions.

In the dive reflex, your heart rate and limb blood flow decrease. Careful science (that means we didn't accidently step on the thermisters) shows time and again that the dive reflex does not reduce your need for oxygen underwater as it does in marine mammals (Bove, 1979; Craig & Medd, 1968; Furedy, et al., 1983; Heistad & Wheeler, 1970; Rapper et al., 1967). Field studies (where you're allowed to step on the thermisters) indicate that the dive reflex does not extend breathholding time (Pierce, 1969). Occasional cases of human survival after very cold water near-drowning are not due to the dive reflex. Cold is the likely mechanism behind the reduced metabolism that permits survival (Hayward et al.,1984).
Decreased heart rate from the dive reflex does not reduce metabolism or the oxygen demands of your body's vital organs. The lowered heart rate and sometimes abnormal heart rhythms resulting from the dive reflex may contribute to blackout underwater, not save you from it (Bove et al.,1973; Landsberg, 1975; Thornton et al., 1964; Tuttle & Templin, 1942). The medical term for this is death, which ends these personal dive reflex experiences prematurely. What good is the dive reflex in human divers? The blood vessel constriction in your limbs reduces heat loss.

You lose heat all the time. You have to lose heat because your body generates heat in the process of being alive. If you didn't lose heat you'd literally cook to death. Losing heat doesn't mean you are in danger of cooling. It depends on how much you keep and how much you lose.

Your body loses heat in four specific ways: radiation, conduction, convection, and evaporation. When you lose heat to the environment though breathing, sweating, peeing, or through your skin, you do it combinations of those four ways. In respiration for instance, you lose heat though evaporation, conduction, and convention. There is no single respiratory heat loss pathway per se.

These pathway distinctions don't matter a jot to a freezing person, as Robert Falcon Scott and his entire Antarctic expedition party would readily attest to from the grave. However this differentiation is far from mere semantics. The key to knowing which gas mixture to breathe, which to inflate your dry suit with, what materials to make protective garments from, and which survival protocols to entrust to your troops, depend on knowing which heat pathway determines what effect.

Cold hands, feet, or skin does not mean you have hypothermia. Shivering and teeth chattering do not mean you have hypothermia. Feeling cold does not mean you have hypothermia. Someone who loses more heat than someone else is not necessarily more susceptible to hypothermia. Only core temperature below 95 degrees F (35 degrees C) determines hypothermia. People can become incapacitated by cold without ever going hypothermic (Bridgman, 1990). Hypothermia is not a common event in diving. Well before you become hypothermic, just getting cold, which what scientists call it because we were unable to be any cleverer about it, reduces the safety and fun of diving.

Hypothermia from water imersion is sometimes called immersion hypothermia. It is not a reflex and has nothing to do with the dive reflex. Like heat loss pathway names, distinctions among cold injuries are also not semantics. Sucessful treatment depends on proper identification.

The complex science of heat transfer can be simply summed up: heat energy flows naturally in only one direction, from areas of higher temperature to lower temperature. When the difference is large, more heat flows than when it is small. Temperature differences called gradients exist all over your body. Two are important to understanding heat loss in the cold. One is between your insides and your skin, called your core-to-skin gradient. The other is between your skin and the outside environment, called the skin-to-environment gradient. Here is where thermoregulation gets really interesting.

Your skin temperature is not 98.6 degrees F (37 degrees C). That familiar number is the average temperature of your insides called your core. Core temperature drops a degree or two in early morning and rises three to six degrees during exercise. Healthy core temperature maintains a narrow range. Skin is cooler and marvelously changeable. In the cold, skin temperature quickly drops to that of the surrounding air or water, which is a good thing.

A skin temperature close to the surrounding temperature decreases the gradient and with it, heat loss. Two concepts follow: the surrounding temperature need not be 98.6 degrees F to be thermoneutral, and cooler skin, common to women, is an advantage in the cold to lose less core heat. Men with higher skin temperature lose more heat in the cold.  An analogy is if you stand outside your house in cold weather, touch the outside wall and find it warm, you would notice the expensive waste of heat and know your home needed better insulation.

Polar animals have bodily insulation plus the equivalent of protective clothing. The fur of arctic seals and bears is an effective wet suit. It adds exterior insulation to their thick fat layer by trapping a two to ten millimeter water layer near their skin. The feather pelt of penguins, on the other hand, works like a dry suit, maintaining an insulative layer of air. Even the hairiest New York cabbie can't match it. Humans insulation consists of fat, muscle, and the thickness of your 'shell.'

It's useful, though oversimplified, to think of your body as a central  core surrounded by an adjustable temperature insulating shell. Shell thickness changes with the amount of blood flowing through it. Your body's first response to skin cooling is to decrease circulation to your shell. You do this by constricting blood vessels. Decreased blood flow from this vasoconstriction does two things. It transforms skin and subskin tissues to up to two inches of additional insulation. Lowered skin temperature decreases your skin-to-environment gradient further restricting heat loss.

Studies of cold water immersion report lower skin temperature in women than men. That does not mean women were at increased risk of hypothermia. The lower temperatures were of the surface skin, not the deep skin layers or, more important, the core. Deep skin temperatures remained higher compared to those of men under the same laboratory cold conditions (Malkinson, et al., 1981). Women's  better insulation and vasoconstriction at the surface reduced deep heat loss. Cool extremities are not, in this case, the result of 'poor circulation." It's a healthy vasoconstrictive adaptation.

There's a growing body of opinion (that means that two people have said it) that fat does not keep you warm, or does not help unless you are obese. However, research consistantly substantiates body fat as a major deterrent to heat loss.

Body insulation increases directly with the average thickness of the fat layer under the skin (Park et al., 1984) and with deep body fat (Carlson et al.,1958). People with thicker fat layers lose less core heat at rest and during exercise both in cold air and cold water (Dulac, 1987; Wolff, 1985). Thicker people tolerate a lower temperature before shivering, and their core temperature does not drop as fast during swimming in cold water compared to thinner people (Veicsteinas & Rennie 1982). Thin people raise their metabolic rate higher than fatter people in a none too successful attempt to keep as warm as the more calorically challenged (Keatinge,1960). There is no question that the advantage is to the young and the globular.

Muscles are wonderful. Nothing wrong with having more muscle than less. However the statement that muscle is somehow 'better' than fat at insulation doesn't lend itself to sweeping conclusion. Muscle and skin provide less thermal resistance than fat (Veicsteinas et al., 1982). During exercise, muscle rapidly convects away the heat it produces, making fat a better insulator during exercise in cold water like scuba diving (Rennie,1988).

Penguins stand around in ice and snow in their little bare feet with negligible heat loss. The arteries and veins in their feet closely intertwine. Warm arterial blood going to their feet warms the cool venous blood coming back. Cold venous blood returning from the feet cools the outward bound arterial blood. Heat is not lost to the feet, and cold blood from the feet does not cool the penguin's core. Penguin feet are nearly the temperature of the snow. The arterial-venous swap meet is called counter current heat exchange, and is not limited to penguins. This example demonstrates how cool feet can be a healthy adaptation to cold.

The term surface area to mass ratio is thrown around a lot when cold exposure is mentioned. Like the term 'thermal coefficient' which can mean just about any number, the phrase 'surface area to mass ratio' has high dazzle value. It sounds intellectual enough to invest the speaker with instant thermal authority. Equivalent oral footwork occurs when yet another military jet impacts unfavorably (crashes) and energetically disassembles (explodes) causing the pilot to have an unfavorable shift in health status of high magnitude (die), but they'll take the problem under consideration (just as soon as hell freezes over).

Now what does surface area to mass ratio mean? It's the amount of surface that can lose heat compared to the internal mass that holds and, in animals, generates heat. Car and home heat redistributors are built to have long thin shapes so their high surface area to mass ratio gives off, or radiates, lots of heat. Imaginatively, they are called radiators.

Spaghetti cools rapidly. Baked potatoes stay hot. Like spaghetti, your fingers and ears are relatively long and thin with much exposed surface. Fingers and ears chill faster than your torso. Your torso, very much like a potato, has relatively high internal mass compared to its outer surface of skin giving it a lower surface area to mass ratio. Even though fingers have less total surface than your body they have a higher ratio, and more capacity to radiate heat.
Do women have a larger surface area to mass ratio than men putting them at greater risk of hypothermia? The ratio is not the main determinant of chilling, it is much less important underwater than above, and it is only one star in a constellation of temperature regulating mechanisms.

The ratio is also not determined by gender. A tall thin man's ratio might exceed that of a shorter heavy woman. Short thin men have a high ratio compared to large men. A large man will also lose more total heat from his larger skin surface area than a smaller man or woman. If a given woman has a higher ratio than a man, like all warm blooded creatures women have more than one protective mechanism against cold. No single heat loss pathway creates a global risk of hypothermia.

The ratio becomes an interesting player in the heat where lower ratio people have an odd habit of overheating more than larger ratio people. That is for another article on heat. When is the ratio a serious player in the cold? In fingers and ears compared to bodies. In jockeys compared to linebackers. In children compared to adults. Otherwise the ratio matters, just not as much as you've heard.

The statement that women have more curves than men and therefore increased surface area to mass ratio to lose heat, and consequently higher susceptibility to chilling should be taken with a millimole of NaCl.

For one thing, curves don't always increase the ratio. As moderate math can show, both the surface area and the mass increase with curves. The ratio could stay the same or change in either direction. A sphere, for instance, can have a lower surface to mass ratio than a cylinder. Then too, men's bodies are full of curves, from muscles to other normal structures. Men's genitals don't fare well in the cold for similar reasons as fingers and ears. Here is where surface area to mass ratio comes home to the, ahem, heart. 'Frostbite Shorts,' under various names, is a documented medical malady.

Moreover, thermal influence about another male consideration, namely fertility, was known anecdotally for centuries. The, dare I say it, seminal work in this area was conducted on pearl diving men of the South Seas. Their fertility dived after cold water excursions for two cool reasons.

On the whole, pouring warm water in your dry suit would enjoy a spectacular lack of success. On the other hand, (arm, leg) with wet suits it helps. Adding warm water to your wet suit is the principle behind hot water suits used commercially. You will not overheat or begin to vasodilate or sweat. That would occur only past a certain heat load that could not be reached from a few cups of warm fluid. The small heat load gained is that much more in the BTU bank for later when you're back in the water spending heat or tyring to warm up between after diving.

Cold acclimatization or habituation is a process of gradually increasing your resistance to cold injury through regular cold exposure (McMurray & Hovath,1979). Following the recommendation of the International Union of Physiological Sciences, the term acclimatization is distinguished from acclimation. Acclimatization refers to adaptive change due to seasonal or geographical exposure; acclimation, if produced in a laboratory.

Anyone can acclimatize, if you'll excuse the expression, to a degree (Shephard,1985). The extent varies among individuals (Kundu et al.,1977). You'll miss out on this natural phenomenon if you spend each winter in a heated home, sleep nestled in comforters or heated water beds, and bundle up like a kid in a snowsuit thereby remaining in artificial tropics.

In some cases physical exertion in cold water intensifies cooling. But not all cases. Exercise in cold water can generate enough heat to match (Doubt & Smith,1990) or surpass the heat you lose, depending on water temperature in some work, (Toner et al., 1985) and in other work regardless of temperature (McArdle 1984). Arm and leg exercise prevents a fall in core temperature (Craig Dvorak, 1969) benefiting individuals with a low shivering response (Hayward & Keating). Exercise prevents core heat loss particularly in fatter subjects (Pugh, et al., 1960) again demonstrating the benefit of body fat in keeping warm. It is noteworthy that these studies are all of subjects exercising in cold water with no protective garments. Divers wearing exposure suits are more likely to benefit from the heat generation of exercise.

It is also possible to overheat, as swimmers doing laps in warm pools and divers sweating into their masks can tell you. A recent Navy study looked at overinsulated divers swimming in cold water and found they needed a bit of heat extraction to prevent overheating (Beckett MB, Hodgdon JA, Derion T, Moffatt RJ, Webb P, 1993).

Studies confirm women protect their core temperature equally or better at rest in the cold than men (Bagian & Kaufman 1990; Bolstad et al.,1991). Although still true during activity, (Bolstad et al.,1991; Mannino & Kaufman 1986; McArdle et al., 1992 ) the extra variables introduced, particularly during immersion, make understanding exercise in cold water an eyeglazing knee-bone's-connected-to-the-thigh bone affair, yielding answers as definitive as those of the psychic advisor eightball "situation unclear try again later," the details of which we needn't go into here.

In general, male and female swimmers in a long distance cold water swimming competition displayed similar metabolic and hormonal responses in a 1987 study (Dulac et al., 1987). They also differ in several aspects. Women have greater ability than men to limit heat loss through the skin due to greater constriction of skin blood vessels, and thicker subcutaneous fat layer. Men lose more heat through radiation and their poor vasoconstrictor response, but counter with increased heat production. Men display a greater blood pressure response than women to cooling the hand or the face (Graham 1988). Women seem to be more likely and willing to get out of the cold.

When a diver gets out of the water saying she's cold, what information do you have? One, that the buddy still in the water is not stating that he's cold, and two, that he's not getting out of the cold water. Without thermisters up some funny places, that's all you know. In military ops, (that's 'ops' as in operations, not 'oops' as in military decision making) the ranks suffer more hypothermia cases than the officers. What information do you have? Are officers more resistant? Turns out they're less exposed. What about studies and observations that African Americans have lower skin and core temperatures in the cold, and Black soldiers suffer more cases of cold injury than White soldiers? (Iampietro et al.,1959). I'm not touching that one. It's all yours.

A thermal stress workshop held at the Institute for Naval Medicine in England by the Diving Medical Advisory Committee discussed what they called the 'non-responder to cold.' They stated, "It is still not known what the differences are between the man who responds to and complains of the cold, and another man who cools and is unaware that he is cooling. Presumably this latter type of diver is a potential hypothermic casualty."

American statesman Daniel Webster said, "There is nothing so powerful as truth — and often nothing so strange." What good is it to clarify that the dive reflex does not conserve oxygen in humans, that women protect their core temperature in the cold as well or better than men, and fat is a major help to keep you warm? Does this information make us build a better automobile than the Japanese? Furthermore, scientists are not so ivory tower that they have never seen a woman leave the water saying she's cold.

Scientists don't just sit in small dark labs worrying if the term 'anal retentive' takes a hyphen. They use several approaches to try to understand what occurs in the real world. Only one is the experimental method of putting small numbers of cheerful volunteers in freezing vats of water. Another well defined method is careful description of large numbers of triathletes during races in cold open water, marathoners running in the heat, or hundreds of divers using computers on a liveboard. Actual observation of the real life absence of males among the Korean breath hold divers led to the conclusion that males may have an inherently poorer tolerance to cold than females (Hong, Rennie, Park, 1986).

In life many interactions confuse what is cause and effect, and what is correlation. Science tries to determine if what you see is due to what you think it is, to something else entirely, or to pure chance. Science works by separating things that go together or not, so that causality is not misattributed. If an isolated factor like surface area to mass ratio determines one avenue of heat loss, science clarifies how much is important and what is background noise. Best of all, it's nothing personal. In reporting science, it's just the facts, Ma'am.

In conclusion: if a woman leaves a cold environment before a man, it may be for reasons other than gender or even physiology, and if a fat person chills faster than a thinner person in one instance, it's not because their fat didn't help.

But do people listen? Noooooooooo. If people listened to scientists, the Spielberg movie would be called "Cretaceous Park", and it would be men who ride side saddle. There are still unanswered questions like "Does interpolation mean being buried in the polar ice cap?" and "Do two thermal people make one thermocouple?" At least we can tell you what will happen if you stick your tongue on the ice tray.

About The Author
Dr. Jolie Bookspan earned Master's and Doctoral degrees in exercise physiology and underwater physiology, a fellowship in cold immersion, and post docs in saturation decompression and altitude. Five years of her work involved unraveling the results of extension of oxygen tolerance research in humans. Far from the ivory tower, her father taught her to dive in the Hudson River in the late 1960's. After serving in the Army she went on to become research scientist for the Navy, demonstrating that even skilled scientists can make the same mistake twice. She taught anatomy at a college in México in the mountains where the entrance exam was getting up there without a nosebleed, and has conducted intensive work in cold chambers and other scientific thing-a-ma-bobs which funding organizations have gone out of their way to meticulously ignore.


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