Underwater exposure can lead to body heat loss and the development of hypothermia. Thermal balance is best achieved by preventing direct contact between skin and water via various suits. Hypothermia can be treated by rewarming under medical supervision.

One of the important problems in underwater exposure is loss of body heat and development of hypothermia. This term indicates that body temperature is below normal. Skin temperature is usually about 85oF and any water temperature less than that will cause a loss of heat from the body. Water of 70 to 72oF is cool enough to produce significant thermal loss when a diver is submerged without protective cover. Because water conducts heat rapidly, the normal pattern of heat loss in comfortable air temperatures (65 to 75oF) is changed to a high rate of heat loss. The table included in this article shows survival times for water immersion for an unprotected diver.

Notice even in water that may be considered reasonable, a diver stranded without protective clothing would risk death from prolonged exposure. Survival time becomes short as the temperature falls. Immersion in water of 30 to 40oF without protective dress would be rapidly fatal. When a person is immersed in cold water, there is a reflex hyperventilation and cooling of the muscles. This combination often leads to inability to remain afloat and drowning quickly follows.

Humans require a constant temperature environment (usually about 98.6oF) and any exposure that threatens the stability of body temperature evokes a response to restore temperature to this normal value. Cold stress produces increased metabolism because of the need to generate heat internally to account for losses through the skin. With even small amounts of heat loss, the heart and circulation increase their activities to provide more blood flow to the muscles for shivering. In some divers, the added load to the heart may cause abnormal heart rhythms and severe fatigue when exercise is attempted in cold water.

If temperature falls below 96oF, body metabolism starts to fall and, as temperature declines further, metabolism goes even lower – ultimately, unconsciousness occurs. Hypothermia of this severity is rare in sport diving.

Blood flow to various tissues of the body varies with thermal stress. Cold exposure causes constriction of blood vessels in the skin, which makes it a better insulator. This constriction is controlled by the brain, which senses temperature and protects the body from heat loss when a fall in temperature is detected. The brain signals include blood vessel constriction in the skin, increase in heart activity and hair erection on the skin (goose bumps) to add insulation to the skin layers and also signals increased activity of muscles. There is a period when muscles generate heat without shivering but, eventually, shivering occurs and muscle metabolism increases to produce heat.

Cold induced constriction of blood vessels supplying the muscles in the forearm, combined with cooling of the nerves to the muscles, can temporarily paralyze the muscles. Ultimately, control of the fingers and hands becomes impossible in a cold environment. When planning a cold water dive, you should consider the contingency of extreme cold exposure to the arms and fingers. This might occur if a drysuit was torn or leaked during the dive. Finger and hand motions may be impossible to perform once significant cold exposure has occurred.

Diving medicine cold and heat stress in diving


Because ambient U/W temperatures, even in the best of conditions, will produce heat loss, it is always necessary to consider thermal loss a problem when diving. Even in tropical latitudes, where water temperatures may be 70 to 75oF, prolonged exposure (several hours) can cause significant heat loss. Other animal species that are exposed to frequent water immersion generally develop a thick layer of fat beneath the skin for insulation. Whales and seals, for example, are known for their thick layers of fat and their tolerance to cold water. In addition to the fatty insulation, there is a physiologic or dynamic insulation that comes from changes in blood flow to the skin.

The best way to achieve thermal balance (no heat loss or gain) is to eliminate direct contact between skin and water, which rapidly conducts heat from the body. This can be accomplished in several ways.

FABRIC BODY SUITS – There are a number of fabric suits that offer a small amount of thermal protection but are not made to provide cold water protection. These suits will hold a water layer near the skin but, since they are water permeable, there will be a flow of water away from the skin when motion occurs. These suits provide some thermal protection in relatively warm water (above 75oF) but do not offer the protection of a true wetsuit. Some fabric suits have a liner that holds water in the suit and offers more protection than a single layer of fabric. Fabric suits offer increased comfort in tropical water but could lead to significant hypothermia if used in water below 70oF.

WETSUITS – A wetsuit provides good thermal insulation and takes advantage of the insulating properties of water. Once the water layer formed between the skin and the wetsuit is raised to body temperature, it is possible to maintain a reasonable level of thermal protection. Water should not flow through the suit, since the warm water will be replaced by cold if this occurs. A loose fitting wetsuit that allows water to flow in at the head and out at the feet is poor thermal protection. A wetsuit will not provide as much thermal protection for deep diving (100 feet) because of compression and loss of the insulation characteristics of the suit itself.

A mechanism somewhat equivalent to a wetsuit can be achieved with the fabric suits mentioned above or with ordinary clothing as long as rapid motions in the water are avoided. Clothing of any type can be helpful if the water layer trapped between the clothing and skin is not disturbed by motion. A wetsuit allows tolerance of water temperatures down to 50oF for reasonable periods of time. At 50oF and below you will lose heat even with a wetsuit.

DRYSUITS – Since the development of reliable drysuits, cold water sport diving is now possible. Air is an excellent insulator and as long as an air space is maintained between skin and suit a drysuit provides excellent insulation. If a drysuit becomes flooded, however, the thermal properties of the suit are lost and the suit may become a severe handicap to safe diving. Drysuits can inflate rapidly and prevent the diver from maneuvering in the water but, with proper training, such problems can be avoided. Drysuits are needed in extremely cold waters and provide a significant advantage over wetsuits in water between 35 and 50oF.


Temperature       Survival Time

32 [degrees] F        1/4 hour
38 [degrees] F        1/2 hour
45 [degrees] F      2 1/2 hours
55 [degrees] F          6 hours
65 [degrees] F          8 hours

When deep commercial diving is done in cold water, the divers wear heated suits. Heating is usually accomplished by pumping warm water from a surface heater through the suit.

TREATMENT OF HYPOTHERMIA – The treatment of hypothermia is rewarming. In severe cases this must be done in a hospital under medical supervision. For mild hypothermia, rewarming by exposure to a warm environment is successful. Being wrapped in blankets, sitting in a tub of hot water or taking a hot shower all contribute to rewarming. Ingestion of alcohol is not recommended. Alcohol dilates blood vessels in the skin and causes further heat loss. Drinking warm fluids will add heat to the body. Rewarming should continue until body temperature is normal and the subject is not shivering.

There is a phenomenon called after-drop, which causes the body temperature to fall further when rewarming begins. The cause of afterdrop is thought to be a shift of cold blood from the skin into the central core of the body as blood vessels in the skin dilate from the warmth. Because of afterdrop, internal body temperature can fall even lower after leaving the cold and, rarely, the effects of severe hypothermia may occur even though a diver has been removed from the cold.

Hypothermia can be insidious. It can dull the mind to the point where inappropriate behavior may prolong exposure to the cold. As function in the hands and fingers diminishes because of cooling of nerves and muscles in the forearm and as mental state is dulled, a diver may find himself or herself in a dangerous underwater situation.


Diving is usually not a sport that causes problems with excess heat. Most of us would welcome some extra heat during and after diving. Usually, heat is lost from the body when diving because the water is colder than the skin and heat flows from skin to water.

If water temperature reaches 85oF, there is little or no difference in temperature between the skin and water and heat cannot be transferred. This situation might occur while diving in tropical waters and I know of commercial divers who dive in inactive nuclear reactors for inspections and encounter warm water.

In sport diving you can become hyperthermic in very warm water if you try to perform heavy exercise. During the summer, in tropical areas, water temperatures will reach the 80 to 85oF range and there is a potential for hyperthermia.

The more important problem with hyperthermia in sport diving is on the surface, when preparing for a dive or when the dive is finished. In areas where a full wetsuit is needed, exposure to the sun on a boat while waiting to start a dive can produce hyperthermia.

The risk of hyperthermia is increased by dehydration. Divers are prone to dehydration where the air temperature is high and exercise or work is involved. It is difficult to detect a state of slight dehydration. A little thirst is the signal to drink water but often we ignore it in the press of getting ready to dive. Dehydration increases your chances of becoming hyperthermic because of the reduced volume of blood available for circulation into the skin, where heat is lost to the environment. Dehydration can also increase your chances of getting decompression sickness because of inadequate flow of blood to muscles and slowed removal of excess nitrogen from the tissues. For all of these reasons you should make a strong effort to maintain adequate water intake when diving, whether in hot or cold environments. Note that alcohol is a diuretic – it causes water to be lost through the kidneys and can make the problem worse.

Effects of heat on the body can take several forms. Collectively, when they are associated with medical problems, we call them hyperthermia. The most common problem with heat is heat exhaustion. In this disorder, the body temperature rises above normal because of excess heat generation from work or exercise, inability to remove heat because of a hot environment, heat retaining dress or exercising in warm water. The victim becomes dizzy and disoriented, blood pressure falls, the pulse is rapid, hyperventilation occurs and often the victim loses consciousness because of the inability of the circulatory system to maintain blood pressure at a normal level. The victim is sweating and usually will recover rapidly if allowed to lie in a cool area and given fluids to replace the water lost from sweating. Often the fluids must be replaced intravenously and this is best done in a hospital. This disorder is very common in runners who race in hot climates when they are not heat acclimatized. Divers can develop the same problem in hot weather when they work or exercise in a full wet or drysuit on the surface.

Heat stroke is a more serious form of hyperthermia and can cause death. The stimulus for heat stroke is also too much heat. Usually this is owing to prolonged work or exercise in very hot environments. It is associated with unconsciousness and very high body temperatures (105 to 108oF) and results in heart, brain and kidney damage that may cause death. Treatment is rapid cooling and should be done in a hospital under medical supervision. If a diver develops any type of hyperthermia, he or she should be placed in a cool, shady location, given fluids if conscious and cooled with water poured over the skin until body temperature is normal. Immersion in cool water will help heat loss but the water should not be excessively cold. If a diver is fully dressed, remove the wet or drysuit first. This action by itself will make a large contribution to reducing body temperature.


Keeping warm is a persistent problem in diving but, occasionally, the opposite occurs. Because our diving dress is designed to retain heat, it can cause hyperthermia if the wrong combination of temperature, work or exercise and dress cause an increase in body temperature.

Hyperthermia can be avoided by maintaining adequate fluid intake, dressing appropriately for the diving environment and avoiding excess work or exercise when air temperature and humidity are high or when the water temperature is high.


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