Things to know about IMMERSION HYPOTHERMIA

The crucial factors in immersion hypothermia, the loss of body temperature due to protracted immersion in water, are water temperature and the duration of immersion. A nude man of average build will survive twenty to thirty minutes when immersed in water that is 5 C (41 F), but at 15 C (59 F) he would proba­bly live for 1 ½ to 2 hours. Ordinary, thick clothing will just about double these survival times. Fat is an important factor; fat persons are more likely to survive immersion. Survival is not threatened by hypothermia at water temperatures above 21 C (69.8 F), though nude subjects cannot maintain heat balance even in water at 24 C (75.2 F). It may be possible for thin individuals to develop hypothermia while swimming in waters considerably warmer than 21 C, however. In winter, the average temperature of ocean waters is 20 C or less along the entire coast of the United States except the eastern Gulf states, Florida and Georgia.

The specific heat and thermal conductivity of water are much greater than those of air. Because of this, the rate of heat loss from a body in water is approximately twenty-five times that in air at the same temperature.

In moderately cold water, subcutaneous fat acts as an inert layer of insulation. The amount of subcutaneous fat is one of the most important factors in the susceptibility to immersion hypothermia.

The rectal temperature of a fat man who is immobile in cold water will drop much more slowly than that of a thin man. If both are swimming, the difference is much greater and the fat man may actually raise his rectal temperature. During vigorous swimming, a 1.0 mm (about 1/25 inch) thickness of subcutaneous fat may be equivalent to an increase in water temperature of 1.5 C. Women generally have more subcutaneous fat than men. Fat individuals who remain motionless in the water may be able to maintain their body temperature even at water temperatures as low as 5 C (41 °F).

Another factor in heat loss in the water is body size. Children cool faster than adults because they are smaller. A small person has a larger surface area in proportion to his size than does a large person. It follows that a small person has a greater area from which to lose heat than does a large person, in proportion to weight.

A physiological phenomenon of vasodilation occurs early during immersion in cold water. This is dilation or widening of the blood vessels of the skin and is especially pronounced in the fingers. It begins about five minutes after immersion and continues periodi­cally for about twenty minutes. The greater the blood flow to the skin, the more heat is lost from the core of the body. If it were not for this vasodilation, very fat men could probably maintain their body temperature for prolonged periods while in very cold water. Alcohol ingested prior to immersion causes vasodilation and accelerates the fall in body temperature.

One of the most important factors in the development of immersion hypothermia is the movement of water relative to the body. Water is a very poor insulator and conducts heat very well. When there are currents around the body, therefore, much more heat is lost. This situation is analogous, in some respects, to the effect of wind discussed previously. Any movement of the water relative to the body, or vice versa, will result in a new layer of cold water against the body. The amount of heat lost from a person at rest in moving water is double that of a person at rest in still water. If the individual is swimming, the heat loss is 2% times greater than if he was at rest in still water. Swimming or struggling also increases blood flow to the muscles and superficial tissues, therefore increasing heat loss. Heat loss is increased by exercise, regardless of whether the individual is fat or thin, clothed or nude, and whether the water is still or moving. This is a major factor at water temperatures below 25 C (77 F).

Clothing may considerably reduce the amount of heat loss. Ordinary clothes may decrease heat loss by 50 to 75 percent.

This benefit is not completely lost when the individual swims or struggles. A life jacket (PFD or personal flotation device) also decreases heat loss, in part by allowing the individual to remain still. Swimming and treading water increases the cooling rate by about 35 percent and the drownproofing technique does so by over 80 percent. A PFD will also keep afloat a person who has become unconscious or helpless from hypothermia, or trauma. A PFD will invariably prolong the life of a man immersed in cold water, though the ultimate result may be death from hypothermia rather than from drowning. An individual not wearing a PFD will become helpless and drown before dying of hypothermia.