Immersion Hypothermia

The quickest route to hypothermia is cold-water immersion. If you wanted, for example, to cool a hot frying pan very quickly, would you A) leave it in 40-degree air, or B) immerse it in 60-degree water? Almost anyone who's ever fried bacon knows they'd put the pan in the water. The water cools the pan faster because water conducts away heat 25 to 30 times faster than air.

If you think of your 98.6-degree body as that frying pan, you can see that you don't want to be in the water when hypothermia is an issue. The conductive powers of water will cool you much faster than air. Air, after all, is a better insulator than conductor of heat.

How cold does water have to be?

How cold does water have to be to cause hypothermia?"...It just has to be colder than you are," says Captain Paul Ouellette, boating safety coordinator for the Florida Fish and Wildlife Conservation Commission. Obviously, the colder the water, the faster it's going to take away your heat, but spend enough time in even relatively warm waters, and you could be in trouble.

Let's say you're a Florida boater, and a fire, collision, or other catastrophe causes you to end up in the water. What's the deal with regard to hypothermia? That depends, of course, on the water temperature. According to the U.S. Coast Guard Boat Crew Seamanship manual (COMDTINST M16114.5B), if the water temp is 50 to 60 degrees (50-something degrees is not unusual in Florida winters), most adults have about 1 or 2 hours before they become exhausted or lose consciousness. Losing consciousness may lead to death by drowning. (Your life jacket--especially if it's a Type I--may keep your face out of the water even if you are unconscious, another reason to wear it. ) At 60 or 70 degrees, most adults have about 2 to 7 hours before exhaustion or unconsciousness; at 70 to 80 degrees, 2 to 12 hours; over 80 degrees, the time period is "indefinite."

What happens to you in cold water?

The shock of sudden entry into cold water causes you to gasp immediately for air, then begin to hyperventilate. If circumstances place you underwater at this time, this reflexive gasp may cause you to drown. Also, the shock of sudden entry itself may cause problems--a heart attack, a stroke, or sudden unconsciousness, especially in older people with a history of cardiovascular problems.

Assuming you survive the sudden immersion, the body seeks to maintain core temperature by shifting blood from the arms and legs to the core. You start to shiver--another of the body's attempts to stay warm--and your lips may turn blue. With further cooling, you soon lose the use of your arms and legs because their muscles just won't do what you want them to. Most victims drown at this point, well before they might have died from hypothermia.

How long this takes depends not only on water temperature, but also on variables relating to the individual involved. Factors such as age, weight, clothing worn, water convection, alcohol in the blood, and hydration level can all affect survivability. In this case, older persons and very young children would be at a disadvantage. Those with a few extra pounds of body fat are likely to fare better than the very thin. Clothing, even in the water, can act as insulation, similar to a diver's wet suit. Remaining motionless in the water provides an edge that active swimming does not. Active swimming causes two problems: 1) warmth from the body's core is brought by the blood to the surface where it's sapped away, and 2) movement causes increased convective action of cold water flowing over your skin. Active swimming, as opposed to remaining relatively motionless, reduces your survival time as much as 50 percent.

Nearer the final stage of hypothermia, mental impairment leads to disorientation and irrational behavior. Shivering soon stops as the body runs out of energy. Constricted blood vessels begin to dilate, allowing core body heat to escape rapidly through the periphery. Eventually, the heart stops.

Cold-water Survival

Rule number 1: Get as much of your body out of the water as possible. (Remember the frying pan example discussed above.) If possible, climb on your overturned hull or on any piece of flotsam that will support you. The air may feel colder than the water, but air won't conduct away your body heat as fast as water. Don't remove clothing, but fasten it as much as possible. You lose a lot heat through your head and neck. Keep these parts out of the water as much as possible. If in the water wearing a life jacket, assume the H.E.L.P. (Heat Escape Lessening Posture) position. Cross your legs so they touch as much as possible. Draw your knees toward your chest, and cross your arms tightly over your chest. The idea is to make as much of your body touch another part of your body as possible. Legs touching legs, for example, lose less heat than legs touching cold water. A group should huddle in a circle, arms on one another's shoulders, touching chests and bodies as much as possible. Place small children in the center of the huddle. Sleepiness is your enemy. Don't let anyone fall asleep. Swim for safety only if there is no hope of rescue, or if safety is very nearby. The choice to swim weighs the risk of increased heat lost by swimming to a known point against the reduced heat lost by remaining still for an uncertain period.

Be competent on the water

This information is not meant to scare you away from boating, but to help keep you safer. Knowledge is power. If you know what to expect, you can help prevent problems, and deal with them more effectively should they arise.

In fact, wilderness competency (and by some definitions, open water is wilderness) is the central theme of my book on wilderness survival in Florida. I wrote it with you, the Florida adventurer, in mind.



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