: Hypothermia (hypo
, low; thermia
, temperature) is an effective, but infrequently used suicide method. It is a poor choice for a suicidal gesture, unless one is sure of timely intervention.
Fatality rate: More than 30%.
Permanent injury: Moderately likely.
What are the pros and cons of hypothermia for suicide?
What Is Hypothermia?
- Not severely painful;
- Often lethal in the absence of intervention;
- Requires no special equipment and minimal knowledge;
- Usually some time to change your mind, without ill effects.
- Rate of hypothermia highly variable: dependent on ones physical fitness and body fat content as well as on temperature and weather conditions;
- Sometimes severe injury in survivors;
- Requires temperatures that are seasonally and geographically limited;
- May take a long time;
- Victims may be revivable even several hours after clinical death;
- Insidious: people often dont notice signs of hypothermia in themselves.
The freezing point of water, under standard conditions, is 32°F (0°C) and its boiling point is 212°F (100°C). Normal human body temperature is around 98.6°F (37°C), and is tightly regulated by a variety of physiological mechanisms. Even a 3.6°F (2°C) change is significant; a 5.4°F (3°C) fever is a medical emergency.
Systemic hypothermia is generally defined as a body temperature below 95°F (35°C). Severity is rated by how low the core body temperature falls: 89.6-95°F (32-35°C) is mild hypothermia; 80.6-89.6°F (27-32°C) is moderate; below 80.6°F (27°C) is severe.
Hypothermia is also divided into acute and chronic categories. Acute hypothermia occurs quickly, roughly within two or three hours; chronic hypothermia takes longer. While the boundaries are fuzzy, the distinction is more than academic, since both treatment and prognosis differ between the two. This will be discussed in more detail in the Rate of cooling section.What Causes Hypothermia?
Hypothermia occurs when the quantity of body heat lost to the external environment substantially exceeds the heat generated from metabolism. This can be caused by a decrease in heat production, or an increase in heat loss, or both.
Low heat production is usually due to insufficient food, illness, injury, or some drugs.
An average resting body gives off about 50 kilocalories (kcal) or kitchen calories the calories you see on food labels of heat per square meter of skin per hour. Multiplied by the average persons 1.7 square meters of skin and 24 hours, you lose around 2,000 kcal per day to the outside world, which is about the amount that you generate at rest from the food that youve eaten. This is called basal metabolism.
If your muscles are working, metabolic heat production goes way up. For example, while youre shivering, you generate roughly five time as much heat as when sitting quietly; while exercising, ten times as much.1
However, exercise is a two-edged sword in hypothermia. While it produces heat, which is needed to keep body and mind functioning, it also increases blood flow to your active muscles, and thus heat loss from them. In addition, it uses up energy stores quickly. The decision whether to exercise in a potentially hypothermic situation depends on the circumstances: how long the conditions are expected to last, availability of food and shelter, and need for clear thinking.
To some degree there is no choice in the matter: shivering is a form of involuntary exercise. It increases heat production by 200 to 700%,2 but is accompanied by an increase in blood flow to/from the muscle, resulting in about a 25% increase in heat loss, too. Shivering is an effective means of generating heat, until muscles run low on energy.
The common warning against going to sleep in the cold is certainly appropriate if youre in the process of walking out of trouble; but that is not always the right thing to do. For example, someone with reasonable cold-weather clothing caught in a mountain blizzard might be better off building a snow-hole or shelter, staying as dry and warm as possible, and sleeping through the storm.
Excessive heat loss is due to cold surroundings or to a failure in the bodys temperature regulating mechanism.
There are four physical mechanisms of heat loss:
- Convection: heat loss by means of molecular transfer of energy via air or water currents.
- Conduction: heat loss by touching something thats colder than you are. This is, fundamentally, the same process as convection, but mostly applies to solids.
- Radiation: heat loss from invisible infrared-wavelength energy you give off.
- Evaporation: heat loss from the cooling effect of changing a liquid into a gas.
Looking at these in more detail:
Convection and conduction
Most heat loss in hypothermia is from contact with cold air or cold water: Youre always generating a micro-environment of warm air (or water, if thats where you are) around your body, but this is easily stripped away by air/water currents or your own movements. You can easily demonstrate this by putting your hands into a container of cold water, and holding them still for a few seconds. If you then move your hands, they will feel suddenly colder, as unwarmed water contacts them.
At a molecular level, what happens is that cold air/water molecules hit your skin, pick up some heat (energy) and bounce off, leaving cooler skin behind (hot air/water warms you by the same process, with the hot molecules transferring energy to your skin). This cooler skin, being in contact with the rest of the body, is in turn warmed by the body core, which is thereby cooled. The colder the air/water molecules, the faster this occurs.
The rate of convective heat loss also depends on the density of the moving substance (heat loss in water is much faster than in air of the same temperature) and the velocity of the moving substance (the faster the air/water current, the faster the heat exchange).
Another variable in heat loss is surface area. The more surface area, the more heat transfer. Curling up in the fetal position minimizes heat exchange by minimizing surface area, and thus evaporation, radiation, and convection. It may also be comforting for atavistic reasons.
With cold air, some of the variables that affect heat loss are: temperature, wind speed, insulation, and humidity. Well look at their effects in a bit more detail.
Wind speed is the cause of the often-misunderstood wind-chill factor. Wind-chill is just a practical demonstration of air convection, and is nothing more complicated than wind removing the warmed (by you) air molecules from near your body and replacing them with cold ones. This results in your body being hit by more cold air molecules per minute, and so being cooled more. For example, 0°F (-18°C) with no wind causes heat loss at the same rate as 30°F (-1°C) and 25 mile-per-hour (40 kilometer-per-hour) breeze.3 However, the latter case would not drop below 30°F (-1°C), in the absence of evaporative effects.
A second consideration, which is not technically part of wind-chill factor calculations but may be biologically important, is that more wind will cause faster evaporation of sweat. This will remove from your body around two a half kilocalories per teaspoon of sweat, if any is present, and thus additional cooling.
Evaporation cools you because energy (heat) is always required to change a liquid into a gas. If the liquid is on or near your skin (sweat, water, or wet clothing), the heat comes from you, leaving you cooler. This is handy in the desert, but not so good if youre trying to keep from freezing. Thus, a dry body will cool faster in the presence than absence of wind (wind-chill), but it wont get below ambient temperature. A wet body in the wind will cool both faster and further, and may drop far below ambient temperature.
Another way heat is lost is by radiating it away in the form of infrared (electromagnetic) energy.a This requires a temperature difference between you and the outside world. The greater the difference, the larger and faster the heat loss (or gain, if the environment is at a higher temperature).
Infrared wavelengths are longer than the unaided human eye can detect, but electronic enhancement permits these wavelengths to be translated into our visible range.b One sort of night-vision goggles is sensitive to infrared wavelengths, allowing you to see things that are at a different temperature than their background. (Another kind amplifies existing light.) Since about a fifth of the hearts output of blood goes to the head (15% to the brain)4, its not surprising that in cold weather an uncovered head may be responsible for more than half the bodys total heat loss.5 Under these circumstances a bare head shows up, on an infrared detector, like a beacon.
External insulation consists of clothing and shelter. In cold climates both function to keep the air nearest your body, which you have spent precious calories warming, from leaving quickly and being replaced by cold air.
Sweating increases heat loss in two ways: In addition to causing heat loss from evaporation, it replaces some trapped air in clothing and blankets. Since water conducts heat about 25 times faster than does dry air, this will increase heat loss under cold conditions.6
Some materials,c like wool and a variety of synthetics (Fiberfill, Holofil, Capilene, Thermax, etc.) maintain their loft (and thus insulating ability) better than others (cotton, down) that mat down when they get wet.d Nevertheless, any clothing that gets wet or even damp will allow faster heat loss than would the same item dry, both because water conducts heat better than air does, and from increased evaporation.
Internal insulation involves fat and bulk. It is not an accident that mammals in cold climates are, as a rule, larger than closely related warmer-environment species.e They also have a smaller surface-to-volume ratio; are rounder, and have smaller ears, nose, and tail, and thus less heat loss. You can see these differences if you compare, say, cold- and warm-region rabbits or bears.
Fat is very useful in cold climates. Because fat tissue has relatively low blood flow, it acts as a good barrier to heat loss from important core organs. Equally important, it also is a concentrated metabolic energy (heat) source, providing about 9 kilocalories per gram, compared to some 4 kcal per gram of protein or carbohydrate.
The low humidity level of frigid air can play a role in hypothermia. On the one hand, the higher the humidity (gaseous water molecules), the more particles will be available to snatch heat from the skin (though this is offset by the lower density of warm air). On the same side of the equation, humidity will decrease the insulating properties of clothing. However, the humidity is limited by the temperature. For example, the maximum possible humidity at 32°F (0°C) is only one tenth the maximum at 98°F (36.6°C). Thus, at temperatures below the freezing point of water there simply isnt enough gaseous water in the air for these processes to matter much.
On the other hand, low humidity increases the net evaporation of sweat, which is a major cooling mechanism (and which is why people are much less comfortable on a hot, humid day than a hot, dry one). Low atmospheric humidity can also contribute to dehydration for other reasons, since (1) the body sweats to keep skin from drying out (a humidity level of 70% nearest the skin is ideal); (2) exhaled air is close to 100% relative humidity. (The condensation you see from your breath on a cold day is a visual reminder of this.) I estimate the daily amount of water lost in breath at freezing temperatures to be
about 1.3 pints (0.6 liters) .
~0.4 gm water/min > ~0.6 L/day... x 600 kcal/L to heat & evaporate > ~350 kcal/24 hrs If the heat capacity of air is ~1/1,000 that of liquid water (check) 1L x 10 br/min x 60 x 24 x 1mol/20L = 720 mol air... x .018gm/mol x 40 (delta T) = ~ 0.5 kcal/24 hrs. Thus, very little heat is used to warm up the air that you breathe. Whew!Demographics: Who Dies From Hypothermia?
In a recent study of 234 cases of hypothermia in Switzerland, 43 (18%) were attempted suicides; 141 were due to accidents or mountaineering-related; the rest to a variety of causes. Three quarters occurred in cold air; the others in cold water. 68 (29%) of the 234 died. The coldest survivor had a core temperature of 63.5°F (17.5°C) and the longest heart stoppage (in a survivor) was 4.75 hours.7
Hypothermia is not uncommon among the elderly, who often are in frail health, frequently live alone, and may not be able to afford adequate heating or food. These people are at high risk of dying of cold. In some British studies, 3-4% of elderly admissions to hospitals were hypothermic.8 Similar circumstances apply to the homeless; in Chicago, 8 of 22 (36%) hypothermia deaths were among the homeless.9 In the U.S. there were an average of 780 annual deaths attributed to hypothermia between 1979-199010; however only a handful (less than ten a year) are officially suicides.
You might be surprised I certainly was at the geographical distribution of the top ten hypothermic-death-rate States in the U.S.11 While Alaska and Illinois are northern states with severe winters, the others are not: Alabama, South Carolina, North Carolina, Virginia, Arizona, New Mexico, Oklahoma and Tennessee.
Whats going on here? Cold weather turns out to be only one of several relevant factors. Another is how quickly and unpredictably temperatures change, due to unstable weather patterns, as in the Carolinas and Virginia. High elevation and clear skies cause large temperature drops from day to night time; this is the situation in New Mexico and Arizona (23 people died of hypothermia in New Mexico during the 1993-94 winter). It may also be that people in the South have little experience with severe cold and underestimate its danger. Two other inter-related factors are poverty, such that people cant afford adequate shelter and heat, and a high proportion of older residents; older folks are more susceptible to cold injury, more likely to be poor, and more likely to live alone and unnoticed. Half of hypothermia deaths in this country are among people 65 or older.12What are the Physiological Effects of Cold?
Central temperature regulation in mammals is located in the anterior hypothalamus section of the brain. It is sensitive to blood temperature changes of as little as 1°F (0.5°C) and also reacts to nerve impulses sent from nerve endings in the skin. Injury or tumors in the hypothalamus can lead to fatal loss of thermal control even at room temperatures.
In cold conditions, heat is first conserved by decreasing blood flow to the skin and skeletal muscles, and is controlled by the sympathetic nervous system.
Interestingly, some marine mammals, such as seals, have an additional full-time heat conservation method called countercurrent blood flow. Normally, warm-blooded creatures in cold water would be expected to lose a lot of heat from their flippers and fins, which have and need a large surface area for propulsion. However this heat loss is decreased because the arteries and the veins in the flippers/fins run right next to each other. This allows heat transfer between the two and means that some of the heat contained by the arterial blood warms the returning venous blood rather than being wasted in warming the seawater. Very crafty.
In humans, typical blood flow to the skin is 300-500 ml/min. Maximum physiological constriction can decrease this to around 30 ml/min. One of the consequences of this is that blood will pool in the core organs at higher-than-normal pressure. This, in turn, forces the kidneys to increase urine output (cold diuresis), which is only a minor heat loss, but may be a significant cause of dehydration, and increases the load on the heart because the blood is more viscous. Nothing has only one effect.
Additional heat is generated by shivering. This reflex is set off by lowered temperature either in the skin or deeper in the body. The fact that it is not under voluntary nervous control is shown by its existence in paraplegics and in people temporarily paralyzed by curare.f 13 Shivering continues down to a core temperature of around 90°F (32°C), but falters below there, and stops altogether at around 86°F (30°C).14 It also ceases, during slow hypothermia, if muscles use up their energy stores (glucose/glycogen); this occurs after a few hours. In either case, body temperature drops precipitously after the shivering reflex fails.
Goosebumps (piloerection), which lift hairs or feathers away from the body, increase the amount of trapped air insulation near the skin. Unfortunately, since humans are more-or-less hairlessg
over most of their bodies,
this process doesnt do us much good. But it was a fine idea when we were furry and is still an interesting sensation.h
Rate of heat loss
As a practical matter, most hypothermia is the result of contact with either cold air or cold water. Since air has a much lower ability to remove heat than does water, hypothermia in air normally takes many hours and is usually chronic or slow. Acute (fast) hypothermia, generally in or due to water, has some significantly different characteristics from the chronic variety.
In slow, as compared to fast, hypothermia, the body has more time to make some physiological responses:
blood flow to the limbs and skin decreases,
blood pools in the core organs, and shivering continues until metabolic energy supplies are depleted. People develop the physical and mental lethargy commonly associated with hypothermia.
One of the hazards specific to fast hypothermia is a phenomenon called afterdrop, in which the core temperature continues to fall even after a person has been taken to a warm place. This is because blood vessels in the arms and legs get larger (dilate) as they start to be rewarmed. As a consequence:
- Blood moves from the core into the limbs (which, despite warming, are still colder than the core), and gets chilled further. This results in more cold blood moving from the periphery into the core, further decreasing core temperature, which can be lethal;
- Redistribution of blood from the core to the periphery decreases blood pressure, and may lead to heart failure (shock);
- The stagnant blood from the limbs is very acidic (from lactic acid and carbon dioxide buildup) which may cause cardiac arrhythmias and death.
Afterdrop can best be avoided by not rewarming the periphery until after warming the core. This was observed by one of Napoleons army doctors during the retreat from Moscow, when he noticed that the hypothermic soldiers placed
closest to warming fires tended to die more often than those further away.
Fatal chilling may occur before physical and mental abilities are seriously retarded: in 1980, 16 Danish fishermen spent about one-and-a-half hours in the North Sea when their boat went down. When rescued, all could climb into the
cargo net, and walk across the deck of the rescue vessel. They went below to the galley for hot drinks (good idea) and to warm up quickly (bad idea). Every one of them died of afterdrop hypothermia.15
Susceptibility to cold
There are age, racial and gender differences in susceptibility to cold. Infants and the old are much more likely to be injured than are young adults. Fat people are less subject to hypothermia than are thin ones. Fashion models drop like flies. Men are more vulnerable to hypothermia than are women, even though they are larger, because women have more subcutaneous fat, which is better insulation (less blood flow) and energy store (more calories per gram) than muscle. Blacks seem more sensitive to cold than are Inuit or caucasians.16 Interestingly, Australian Aborigines apparently lower their body temperature (and, as a result, metabolism and food needs) to around 95°F (35°C) on cold nights.i 17
Effects of cold
Behavior and reflexes
In slow hypothermia, by the time body temperature drops to 90°F (32°C) both the central and peripheral nervous systems are impaired, primarily due to decreased blood flow to the brain (6-7% per °C18): People are physically and mentally clumsy, show decreased sensitivity to pain, have slowed reflexes, and may hallucinate. Thus, a medical school mnemonic for hypothermia symptoms: stumbles, mumbles, fumbles, and grumbles, which summarizes changes in motor coordination and consciousness.
Sleepiness (cold narcosis) occurs at around 86°F (30°C) core temperature. At around 81°F (27°C),
people stop responding to verbal commands,
and some reflexes (such as the reaction of eye pupils to light) stop working entirely. Knee jerk is the last reflex to go,j19 at 79°F (26°C). The bodys temperature-regulating mechanisms also fail and there is quick cooling until the body reaches ambient temperature. However, there is the usual individual variability, with recorded reflexes as low as 68°F (20°C).20
The hearts response to hypothermia is usually the actual cause of death. Initially, the heart merely slows down and shows some electrocardiograph changes. Dehydration blood really is thicker than water makes it harder for the heart to pump increasingly viscous blood. Ventricular fibrillation is often but not always seen below 89.6°F (32°C), and reaches a maximum between 82.4-86°F (28-30°C).21
Ventricular fibrillation occurs when different parts of the muscle surrounding the main pumping chambers beat in a chaotic, unsynchronized fashion; as a result the heart cant send blood through the arteries with fatal
consequences unless reversed. In hypothermic conditions fibrillation may easily be set off by even minor exertion. If the heart escapes fibrillation, it slows further as the temperature drops. In one case, a woman had a pulse of four beats per minute (room-temperature normal is around 75 per minute) with a body temperature of 52°F (11°C), and the heart stopped pumping entirely at 51°F (10.5°C). Her heart restarted when she was rewarmed.22
At normal body temperature, brain damage starts in about five minutes, in the absence of blood flow; at lower temperatures the length of time before brain (and other organ) injury is substantially longer. This is because cellular
metabolism and oxygen needs drop sharply with lowered temperature. For example, oxygen consumption is reduced by 50% at 82.4°F (28°C); 75% at 71.6°F (22°C) and 92% at 50°F (10°C).23
However, people whose bodies cool down faster than they run out of oxygen (e.g. fast hypothermia in air, drowning in icy water), can be revived longer. Those who run out of oxygen faster than their temperature drops (e.g. avalanche, drowning in warmer water) will not be revivable as long. Some hypothermic people have been given CPR for up to 3.5 hours and have recovered without neurological damage.
Cause of death
Death is generally from circulatory failure: the heart either goes into ventricular fibrillation or slows down and stops altogether. In people who have survived the first couple of days after rescue, organ failure, particularly of the pancreas, may lead to delayed death.24 A wide range of other organs also may show acute damage from cold, but these injuries, while sometimes severe, are not usually fatal.Signs and Symptoms of Hypothermia Associated with Specific Temperatures
Looking at signs and symptoms in relation to temperatures, rather than by organ systems, give a somewhat different perspective.
The following chart25
shows the body core temperature and corresponding signs and symptoms. Not all hypothermic people exhibit all of these symptoms, which will also change as the persons core temperature changes.
What are the Clinical Signs of Hypothermia?
||Signs and symptoms
|99 to 97°F
(37 to 36°C)
|Normal temperature range, shivering may begin.
|97 to 95°F
(36 to 35°C)
|Cold sensation, goosebumps, unable to perform complex tasks with hands, shivering mild to severe, skin numb.
|95 to 93°F
(35 to 34°C)
|Shivering intense, muscle incoordination becomes apparent, movements slow and labored, stumbling pace, mild confusion, may appear alert, unable to walk straight.
|93 to 90°F
(34 to 32°C)
|Violent shivering persists, difficulty speaking, sluggish thinking, amnesia starts to appear and may be retrograde, gross muscle movements sluggish, unable to use hands, stumbles frequently, difficulty speaking.
|90 to 86°F
(32 to 30°C)
|Shivering stops in chronic hypothermia, exposed skin blue or puffy, muscle coordination very poor with inability to walk, confusion, incoherent, irrational behavior, but may be able to maintain posture and the appearance of awareness.
|86 to 82°F
(30 to 27.7°C)
|Muscles severely rigid, semiconscious, stupor, loss of awareness of others, pulse and respiration slow, pupils can dilate.
|82 to 78°F
(27 to 25.5°C)
|Unconsciousness, heart beat and respiration erratic, pulse and heart beat may be unobtainable, muscle tendon reflexes cease.
|78 to 75°F
(25 to 24°C)
|Pulmonary edema, failure of cardiac and respiratory centers, probable death.
DEATH MAY OCCUR BEFORE THIS LEVEL.
In addition to the behavioral symptoms mentioned, there is often swelling, especially of the face and ears, as fluid leaks out of the circulation. This is another cause of viscous blood, which overworks the heart. Blood pressure is low
and sometimes unmeasurable, as are pulse and respiration. There may be no signs of life whatsoever below temperatures of 64-81°F (18-27°C).26 And with good reason: Most of these people are dead. However, there have been occasional cases of survival despite extreme temperatures. Perhaps the most astonishing was that of a man found with a body temperature of 32°F (0°C), who felt frozen and had no signs of life.27 Yet, when thawed, he revived.k Thus, in the felicitous phrase of Dr. R.T. Gregory, No one is dead until warm and dead.28Some Risk Factors for Hypothermia: Alcohol, Other Drugs and Exercise
The role of alcohol in hypothermia is controversial. On the one hand, it predisposes to hypothermia by several mechanisms:
- It produces an increase in blood flow (and thus heat loss) near the skin (cutaneous vasodilation). Since the skin contains many temperature receptors, drinking alcohol generates a sensation of warmth, but this comes at the expense of internal heat;
- Alcohol causes hypoglycemia (low blood sugar), which decreases the bodys ability to produce heat;
- As a central nervous system (CNS) depressant, alcohol slows metabolism and promotes sleepiness;
- And certainly alcohol impairs judgment, which may be critical under adverse conditions.
Many hypothermia victims have blood alcohol concentrations (BAC) ranging from 0.13 to 0.25 gm/100 ml blood.29 Legally impaired in most of the U.S. is 0.08-0.10 gm/100 ml BAC. Since exercise also increases blood flow to the skin, the combination of alcohol with strenuous exercise would seem to cause maximum heat loss.
On the other hand, alcohol appears to protect the heart against fibrillation (and has been used for that purpose during low-temperature medical operations, at levels of 0.40 gm/100 ml blood.)30 Perhaps this protective effect causes the observed higher survival rate in hypothermics who had been drinking, compared to those who were sober.31
In addition, alcohol protects limbs against frostbite (freezing) by increasing their blood flow, but again, this is at the expense of core temperature.
Some other drugs that have central nervous system depressant effects can also produce hypothermia, for example barbiturates, opiates, and the major tranquilizer chlorpromazine and related compounds; but also some non-sedatives like acetaminophen (Tylenol) and lithium ion (used to treat manic-depressive behavior). A more complete list is found in Appendix Table C-1. These drugs interfere with temperature regulation at the hypothalamic regulatory center in the brain, and may cause hypothermia even at room temperatures. Of 103 consecutive Intensive Care Unit drug overdose admissions, 27 were hypothermic.32
In the Swiss study of 234 cases of hypothermia,33 some of the factors that tended to be associated with the death of the victims were, in decreasing order of importance:
- asphyxia (e.g. in avalanche);
- invasive rewarming methods;
- slow rate of cooling.
Positive survival factors were:
What are the Medical Treatments for Hypothermia?
- fast cooling rate;
- presence [sic] of ventricular fibrillation in cardiac arrest cases;
- presence of alcohol and/or narcotics in the body.
Treatment depends on the severity of the hypothermia, it#146;s underlying cause, and the age and general condition of the patient. In the mildest cases, warm, non-alcoholic drinks, food, and a few blankets are enough.
If the hypothermia is moderate, or if the patient has low heat production due to exhaustion, illness, drugs, or malnutrition, more active warming is appropriate. This usually involves heating pads/blankets, hot water bottles, or
other external sources of heat applied to the trunk. In wilderness situations, the best emergency rewarming may consist of sandwiching the (dried) hypothermic person in between two warm people, all inside or under a sleeping bag.
In severe or acute hypothermia active core rewarming (ACR) is often used. There are many varieties of ACR. For example, heated liquids may be circulated around the stomach by a tube running through the nose, or heated IV fluids may be administered. In the most critical cases, those whose hearts have stopped, blood is often withdrawn from the femoral veinl (the largest vein in the leg, conveniently close to the surface near the groin), heated to 104°F (40°C) and oxygenated, and reinjected into the femoral artery.34How Long do People Survive under Cold Stress?
In one recent study, 8 of 11 people with deep hypothermia and cardiac arrest were resuscitated.35 Five of the eleven had no heartbeat and six had ventricular fibrillation. None were breathing and all were clinically dead with wide, non-reactive pupils, and were supported by external heart massage and ventilation (CPR). The average (mean) length of exposure to the cold in the survivors was 4.4 hours, and average core temperature was 72.5°F (22.5°C). All three of the patients who died had also been asphyxiated (1 avalanche, 2 drownings).Cold Water
Data from shipwrecks and accidents suggest that a well-nourished man can survive roughly two hours in water at 39°F (4°C).36 At 32°F (0°C) survival time is only about a half an hour.m37 Since swimming, like other exercise, increases heat loss (as well as heat production) due to increasing blood flow near the skin, this may account for some accidental drownings in cold water amongst good swimmers. On the other hand, at temperatures above around 68°F (20°C), fit swimmers can continue for many hours. Swimmers attempting the English Channel, which has summer water temperatures in the mid-to-upper 50s °F (13 to 15°C) coat themselves with grease (more for insulation and to decrease water absorption than for buoyancy), and take hot drinks and quick-energy carbohydrate foods provided by their support boat every half hour or so.
However, if one is not exercising and taking in lots of calories, the same water temperature can be deadly: In March, 1995, four U.S. Army Rangers died of hypothermia in Florida after spending several hours in water between
52-59°F (11-15°C) while training. Similarly, a man died of hypothermia after getting stuck in the mud of a 15-foot-diameter pond, nowhere more than 3 feet deep.38 Air temperature was 70°F (21°C) and water temp was 52°F (11.5°C). Even warm (e.g., 95°F, (35°C)) water will chill you after an hour or so unless you are actively exercising.
As always, there is substantial variability between people. For example, the lowest temperature water in which different young men and women at rest could maintain a steady body temperature ranged from below 53.6°F (12°C) to 89.6°F (32°C). This depended on both insulation (primarily thickness of the fat layer on the trunk) and metabolic heat production. Each of these factors was about equally important: Fast metabolizers could maintain body temperature in water 18°F (10°C) colder than slow metabolizers who had the same amount of body fat.39Cold Air
Immersion in cold water seems to cause loss of body heat about twenty-five to thirty times as fast as air at the same temperature.n As a result, survival time in cold air is much longer than in water of the same temperature. Consequently, secondary factors, such as wind speed, precipitation,
and amount and type of clothing worn, are more important in cold air than in cold water.Cold Comfort: How to Do It
Make arrangements so you wont be looked for: e.g., tell people youre going away for a few days. Go to a cold, secluded spot where you wont be seen. Drink alcohol and/or take sedatives. You can speed up the process by removing clothing and/or getting wet. Obviously, the amount of time needed will also be very dependent on temperature and wind conditions, and your size, weight, and fat content. These are too many variables to make even rough time estimates.
Use of a home freezer has been recommended40 but the air supply in such freezers is so limited that asphyxia will occur long before hypothermia. Commercial freezers are a possibility, but the risk of untimely discovery must be considered.
Since heat loss in water is much faster than in air of the same temperature, the amount of time that one is subject to being saved is correspondingly less. The main concern will be to avoid being seen, both to avoid unwanted rescue and to prevent risk to potential rescuers. Depending on the temperature of the water and your physical condition, fatal hypothermia can occur in as little as 30 minutes or so; less after alcohol and/or sedatives. Be aware that unless the water is very shallow (and even then if you end up face down), you are likely to actually die of drowning while insensible from cold, rather than from hypothermia.
Hypothermia is an under-appreciated means of suicide: It is relatively painless, though certainly uncomfortable, and generally fatal if not interrupted. The main disadvantage is that it normally takes several hours on land (but much less in water), during which time permanent injury is quite possible if rescue occurs. A second major disadvantage is that the availability of hypothermia is limited by climate and season.
As a suicidal gesture, hypothermia is a bad choice, because the length of time is so dependent on both weather and individual variables. And while, in principle, you can change your mind, in practice you lose too much capacity to think and/or act rationally when hypothermic for this to be a reliable survival net.