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3-32 NOAA Diving Manual However, if flooded, the suit loses its insulating value and can become a severe thermal hazard. Protective suits cre- ate an interesting complication. The body’s defense is reduced by the thermal barrier of the clothing. This com- plication, long known, is only just being recognized as an important contributor to designing protective systems. Body fat, ability to generate heat, ability to constrict blood vessels in the limbs to shunt and save heat for the core, physical conditioning, and regular cold exposure are important contributors to cold tolerance and protection. There is evidence that vasoconstriction, a heat-preser- vation response, may be highly efficient in women. During vasoconstriction, blood vessels in the shell narrow and restrict cutaneous blood flow, thereby decreasing convective heat transfer from core to skin and subsequent loss to the environment and shunt the warm blood to the vital organs. Vasoconstriction, however, is only one of many factors involved in thermal stress. Each diver will respond to the cold water environment based on his own specific physio- logical makeup, level of training and conditioning, and the environmental factors in that particular situation. Prevention of Hypothermia: • Check air and water temperature conditions before the dive. • Wear adequate thermal protection for the dive. • After a dive, get out of wet clothes. • Move to a warm, protected area. • Dry your hair. • Wear a hat. • Drink warm liquids in between dives. • When considering adequate thermal protection, fac- tor in the duration of decompression or safety stops. • Be adequately nourished, stay well hydrated, and avoid alcohol and caffeine. • Repetitive dives should not be made until diver is completely rewarmed. • For maximum cold water performance, divers should swim in cold water on a regular basis to improve cold tolerance. 3.4.4 Thermal Stress Irrespective of Ambient Temperature Hypothermia is not a problem only in frigid environ- ments and can occur irrespective of ambient temperature. Similarly, divers may also suffer extremes of hot and cold thermal stress simultaneously during the same dive. There have been documented cases of severe heat exhaustion in Arctic waters by commercial divers as a result of wearing thick, occlusive dry suits, aggravated by dehydration from breathing dry compressed gas and perspiring from pro- longed underwater swimming or heavy underwater work. Perspiration from excessive or from predive overheating can also cause the diver’s dry suit underwear to lose insula- tion, thus predisposing him to hypothermia. Warm Water Hypothermia. Divers also have to be wary of hypothermia in warm environments. A phenomenon called “warm water hypothermia” can occur even in the tropics, especially during long dives and repetitive dives made without adequate rewarming between dives. In warm water hypothermia, long slow cooling can take place in water temperatures as warm as 82º– 91ºF (27–33C). Although warm water hypothermia is not as easily recognized as its cold water counterpart, it definitely warrants attention. The physiological mechanisms of warm water hypothermia have been demonstrated in various medical studies, but they still are not clearly understood. The victim in this situation may not shiver, because the drop in core temperature may not be rapid enough to activate the body’s thermoregulator defense mechanism. There may be a discrepancy between the input of the receptors in the body’s shell and core, making the diver’s skin feel warm while his core is cooling.