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Hypothermia
Cold water removes heat from the body 25 times faster than cold air. About 50% of that heat loss occurs through the head. Physical activity such as swimming, or other struggling in the water increases heat loss. Uncontrolled rapid breathing follows the initial gasping response and may cause loss of consciousness. The victim must attempt to recover control of his/her breathing rate. Sometimes people with hypothermia will feel hot, and start removing their protective clothing. (Don't let them do it.)
An adult dressed in average clothing may remain conscious for an hour at 40 degrees F (4.4 degrees C), and perhaps 2-3 hours at 50 degrees F (10 degrees C) of water temperature. Without thermal protection, swimming is not possible. The victim, though conscious, is soon helpless. Without a life jacket, drowning is unavoidable. Survival time can be reduced to minutes.
Strong swimmers have died before swimming 100 yards in cold water. In water under 40 degrees F (4.4 degrees C), victims have died before swimming 100 feet.
Most hypothermia cases develop in air temperatures between 30 and 50 degrees F (-1.1 and 10 degrees C). Most outdoor enthusiast simply can't believe such temperatures can be dangerous. They fatally underestimate the danger of being wet at such temperatures. Fifty degree water is unbearably cold.
Try to get back in or on your boat immediately. Do not leave the boat. If you are not wearing thermal protection and can not get out of the water, stay as still as possible. Fold arms, cross legs and float quietly on the buoyancy of your PFD until help arrives (Heat Escape Lessening Posture; H.E.L.P.). If 2 or more people are in the water, put your arms around one another. Stay still and close together (Huddle posture).
Cold shock: immeditae events in cold water
1, Without a life jacket, a victim may inhale while under water (involuntary gasping reflex) and drown without coming back to the surface. This can only be prevented by wearing a life jacket at all times on the water in the off-season. There is no second chance.
2, Exposure of the head and chest to cold water causes sudden increases in heart rate and blood pressure that may result in cardiac arrest.
3, Other responses to cold water immersion result in immediate loss of consciousness and drowning.
Breathing control
Untrained peopel may drown in the immediate abandonment due to the inability to control breathing in the first three minutes of immersion.
If you are in choppy water, there is an inability to coordinate and control breathing with wave splash.
There is an inverse relationship between water temperature and breath hold ability. Thus, for abandonment in 25 degrees C water, average breath holding is 38 seconds, whereas for 15, 10 and 5 degrees C water it is 28, 24, and 19 seconds respectively.
Research has also shown that hand grip strength was reduced by up to 60% (Reference 7, 8, 20, 29), manual dexterity was reduced by 30% (Reference 11, 37, 56) and speed of finger flexion was decreased by 15-25%.
Rescue
Victim should be kept lying down with torso, thighs, head and neck covered with dry clothes, coats or blankets to stop further heat loss.
If victim resist help or are semiconscious or unconscious, it is severe hypothermia when shivering may have stopped. Arms and legs must not be stimulated in any manner. Cold blood in extremities, that suddenly returns to the core, may induce cardiac arrest. Do not apply heat to arms and legs or give them a hot bath. This forces blood out through the cold extremities and back to the heart, lungs and brain, which will further drop the core temperature. This can cause "after drop" which can be fatal. Do not massage or rub the victim, rough handling may cause cardiac arrest. Hot drinks are also dangerous as they draw warm blood away from vital organs. Nearly 3 gallons would be needed to raise core body temp 1 degrees C.
If victim appears dead. Little or no breathing or pulse, body rigid. Assume victim can be revived. Look for faint pulse or breathing for 2 minutes. If any trace is found, do not give CPR. It can cause cardiac arrest. Medical help is imperative. If pulse and breathing are totally absent, CPR should be started by trained medical personnel.
Transport canada. Survival in Cold Waters. http://www.tc.gc.ca/marinesafety/TP/Tp13822/chapter-2.htm#Breath-Holding
American canoe association. OFF-SEASON BOATING, COLD SHOCK and HYPOTHERMIA. Available URL from: http://www.enter.net/~skimmer/coldwater.html#offseason
Melody Miller & Mark Zen. My Page for Hypothermia Fundamentals. Available URL from:http://www.dotzen.org/paddler/cpr/hypo.html
Hyperthermia
Heat stress from enclosed vehicles
There has been many child fatalities in vehicles due to hyperthermia after being left in hot cars, trucks, vans and SUV's. Some of these even occurred on days with relatively mild (i.e., ~ 70 degrees F or 21.1 degree C) temperatures. Many victim childens were forgotten by caregiver or playing in unattended vehicle.
The atmosphere and windows are relatively transparent to the sun’s shortwave radiation and are warmed little. The shortwave energy does however warm objects that it strikes. These objects (e.g., dashboard, steering wheel, childseat) heat the adjacent air by conduction and convection and also give off longwave radiation (red) which is very efficient at warming the air trapped inside a vehicle.
Regardless of the outside ambient temperature, the rate of temperature rise inside the vehicle was not significantly different. The average mean increase was 3.2°F per 5-minute interval, with 80% of the temperature rise occurring during the first 30 minutes. The final temperature of the vehicle depended on the starting ambient temperature, but even at the coolest ambient temperature, internal temperatures reached 117°F (47.2 degrees C). On average, there was an 40°F increase in internal temperature for ambient temperatures spanning 72 to 96°F (22.2 to 35.6). Cracking windows open did not decrease the rate of temperature rise in the vehicle (closed: 3.4°F per 5 minutes; opened: 3.1°F per 5 minutes or the final maximum internal temperature.
Even at relatively cool ambient temperatures, the temperature rise in vehicles is significant on clear, sunny days and puts infants at risk for hyperthermia. Vehicles heat up rapidly, with the majority of the temperature rise occurring within the first 15 to 30 minutes.
** McLaren C, Null J, Quinn J. Heat stress from enclosed vehicles: moderate ambient temperatures cause significant temperature rise in enclosed vehicles. Pediatrics. 2005 Jul;116(1):e109-12 . Available URL from:http://www.ggweather.com/heat/
High temperature and water
Given amples of water to drink, humans can tolerate extremely high temperatures while preventing a rise in body temperature. It was demonstrated by a British scientist remained for 45 minutes in a room heated to 260 degree F (126 degree C). Men would have cooked had the oven been hot and humid rather than hot and dry. Sweating rates may exceed 3 liters per hour and cannot be long tolerated unless the lost water is replaced by drinking. Without water, a human continue to sweat unabatedly until the water deficit excedds 10% of the body weight, when collapse occurs.
----Hickman CP, Roberts LS, Larson A. Homeostasis: Osmotic Regulation, Excretion, and Temperature Regulation. In: Biology of Animals, 7ed. McGraw-Hill, Boston 1998;170
-- Essentials of sea survival