Cold water immersion is the most rapidly lethal environmental exposure on planet earth. This is illustrated by work-related death statistics obtained by the U.S. Coast Guard on commericial fishing vessels where death rates approach 180 deaths per 100,000 people per year and where 75% of these deaths result from fishermen falling overboard. The risk of death of cold water immersion on commercial fishing boats is quoted to be 15 times more deadly than firefighting. While cold water immersions will not present to most Emergency Departments on a daily basis, a brief introduction to human survival in cold water will compliment any practicing Emergency Medicine provider.

Prediction of survival time is not exact as many variables come into play. However two factors have the greatest contribution to survival or death:
1. Temperature of the water in relation to area of the exposed body
2. Individual's response to the cold water immersion

Temperature of the Water:

• Sudden Disappearance Syndrome occurs when an immersion occurs in water temperatures below 60 F (15.5 C). This syndrome is caused by reflexes which trigger uncontrollable gasping, profound hyperventilation, and cardiac arrythmias. These reflexes also interfere with voluntary coordination of breathing while swimming. Finally, blood vessel constriction and increased viscosity of blood increase the rate of fatigue and impairs ability to swim. Death occurs within minutes.
• If the individual survives the initial exposure, body heat is lost at a rapid rate which is inversely related to the temperature of the water.
• Rate of cooling is also inversely related to body mass, subcutaneous fat thickness, or other insulation (clothes, wet suit, dry suit, etc.)

Survival Behavior in Cold Water:

• What a person does while being immersed in cold water is a critical independent survival factor. Knowledge that different areas of the body have varying degrees of heat loss can increases ones chances of survival.
• The head, neck, lateral chest walls, and groin are the areas of highest heat loss. This is due to large blood vessels (carotid, femoral, axillary) that travel through these regions that are not amenable to vasoconstriction and contribute to heat loss.
• To combat heat loss form these regions, people immersed in cold water should employ the Heat Escape Lessening Posture (HELP)

Heat Escape Lessening Posture (HELP) and the Decision to Exercise

• The head is kept out of the water, the arms are held next to the body, and the legs are drawn up and pressed together tightly.
• Remaining still is crucial to success of HELP. 
• Exercise or shivering cannot generate enough heat to offset heat loss.
• In fact, the convective heat losses from exercising in cold water (water stirring around the body) can be much greater than in still water and can be 100 times greater than in cold air at the same temperature.
• It is recommended that the best chance of survival is to keep as much of the body out of the water as possible and to not exercise. The only reason to swim in an cold water immersion would be if safety is less than 200 yards away.

References:

1. Piantadosi, C. "Survival in Cold Water." The Biology of Human Survival. Oxford University Press. 2003. Pg 121-125.