Cold Water Shock
Inland water can be surprisingly cold, as low as 2 degrees in summer. On hot days people looking for a little fun may be encouraged to jump into the cold water.
This difference in temperature can result in a phenomenon known as "cold water shock". Not a great deal is known about this effect but research has suggested that when a warm body is quickly immersed in cold water, it reacts with sudden constriction of the capillaries under the fat just under your skin.
This is an involuntary reaction that cannot be prevented. The result is a sudden increase in blood pressure; the heart rate rises dramatically putting extra stress on the heart.
This cannot be consciously controlled, and can result in immediate loss of consciousness followed by drowning.
Staying in very cool water for long periods can lower your body temperature. Your body temperature drops far more quickly in water than it does on land.
And if you're swimming, you're using energy and losing body heat even faster than if you were keeping still. Monitor yourself when swimming in cold water and stay close to shore.
If you feel your body start to shiver or your muscles cramp up, get out of the water quickly as it doesn't take long for hypothermia to set in.
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Swimming Pool Safety