Cold water can kill you in less than a minute. It's actually so dangerous that it kills a lot of people within seconds. Thousands of people have drowned after falling into cold water and a lot of them died before they even had a chance to reach the surface.

That's a scientific and medical fact that most people have trouble understanding - because they have no personal experience actually being in cold water. When they hear or think about 50F (10C) water, it doesn't sound particularly cold - or dangerous - because they're mentally comparing it to 50F (10C) air. It's a big mistake that gets a lot of people killed each year. This is explained in much greater detail in the section Why Cold Water is Dangerous.

You should treat any water temperature below 70F with caution.

Water Temperature Safety Guide

Below 77F (25C)

Breathing begins to be affected.

This is why the official water temperature required for Olympic swimming competition is 77-82F (25-28C).

70-60F (21-15C) Dangerous

Controlling your breathing and holding your breath becomes progressively more difficult as water temperature falls as water temperature falls from 70°F to 60°F (21°C to 15°C).

True or False: You don't need thermal protection when the water temperature is above 60F (15C).

False. You should certainly be wearing a wetsuit or drysuit below 60F, however, 60F (15C) is not the temperature at which most people should start wearing thermal protection.

60-50F (15-10C) Very Dangerous/Immediately Life-threatening

Total loss of breathing control. Maximum intensity cold shock. Unable to control gasping and hyperventilation.

Fact: Cold shock is as extreme between 50-60F (10-15C) as it is at 35F (2C).

Most people who are unaccustomed to cold water will experience a maximum cold shock response somewhere between 50-60F (10-15C). For some individuals, this happens at 57F (14C), for others, the peak occurs at 52F (11C) and so on.

This means that an unprotected immersion in this temperature range will cause most people to completely lose control of their breathing – they will be gasping and hyperventilating as hard and fast as they can.

Since cold shock reaches its maximum intensity between 50-60F (10-15C), it can’t get any more intense at lower water temperatures. In other words, breathing control, once completely lost, cannot be lost to a greater degree.

Below 40F (5C) Very Dangerous/Immediately Life-threatening

Total loss of breathing control. Unable to control gasping and hyperventilation. Water feels painfully cold.

Below 40F (4.5C), water is so painfully cold that it often feels like it’s burning your skin. For many people, the notorious “ice cream headache” can be triggered simply by water touching your face. Even though cold shock is no more intense than it was between 50-60F (10-15C), the severe pain makes a desperate situation even worse because it greatly increases your psychological stress. Clear thinking becomes almost impossible.

See for Yourself

If you're in good physical shape and feeling adventurous, a very memorable way to find out about cold water is by conducting a personal experiment. First, make sure the tap water is as cold as it will get by running the faucet for a minute or two, then fill a glass and measure the temperature.

When you're feeling brave, get in the shower and turn it on full blast. No shower? No problem. Have a friend spray you with cold water from a garden hose while you're wearing a bathing suit.

Warning:Don't try this unless you're completely healthy because the shock of cold water hitting your skin will cause an immediate, and often dramatic, increase in your blood pressure and heart rate. If there's any doubt in your mind, check with your doctor

Interesting Temperatures

98.6F(37C) Normal body temperature measured with an oral thermometer.

Normal body temperature measured with an oral thermometer. 99.6F(37.5C) Deep body or core temperature measured with a rectal thermometer.

Deep body or core temperature measured with a rectal thermometer. 95F(35C) For medical purposes, this is the clinical point at whichh hypothermia begins.

For medical purposes, this is the clinical point at whichh hypothermia begins. 91F(32.7C) The temperature of your skin.

The temperature of your skin. 85F(29.4C) Water feels pleasantly cool rather than warm.

Water feels pleasantly cool rather than warm. 77-82F(25-28C) Swimming pool temperature range for Olympic competition.

Swimming pool temperature range for Olympic competition. 70F(21C) Water feels quite cold to most people. Treat any water temperature below 70F (21C) with caution.

Water feels quite cold to most people. Treat any water temperature below 70F (21C) with caution. 40F(4.4C) or lower Water is painfully cold.

Different Strokes

Most people unfamiliar with cold water find 70F (21C) to be quite cold. On the other hand, a competitive open-water swimmer who is used to swimming in 55F (13C) water will probably think that 70F (21C) doesn’t feel very cold at all. What’s important to your safety is how you personally respond to cold water.

Acclimation and body fat can make a significant difference in how someone responds to cold water.

Acclimation is a process by which your body gradually adapts itself to cold water through repeated exposure. Through acclimation, it’s possible to improve circulation to the hands during cold water immersion, and to greatly reduce or eliminate cold shock.

Body fat is an excellent insulator. Seals, whales, and other warm-blooded aquatic mammals have a lot of this insulating fat - called blubber - which enables them to keep warm while swimming in cold oceans.

Because fat provides insulation from the cold, it can delay incapacitation and hypothermia and also improve physical stamina in the water. Repeated exposure to even cool water increases the layer of fat directly under the skin surface (subcutaneous fat).

You can easily see this body fat difference by comparing the physical appearance of Olympic swimmers and runners. Swimmers have a lot of subcutaneous fat and a sleek, streamlined look. Runners have very little fat and more obvious muscle definition.

Accclimation reduces the intensity of cold shock

Acclimation does not protect you against incapacitation, swimming failure and hypothermia.

Body fat does not reduce the intensity of cold shock.

Body fat provides insulation, slows heat loss, and delays incapacitation and hypothermia.

A Very Remarkable Swim

An excellent example of how body fat can prolong cold water survival is the remarkable case of Icelandic fisherman Gudlaugur Fridthorsson. On a cold night in March, 1984, Fridthorsson was working on a 75 foot (23 meter) commercial fishing vessel when her nets snagged on the ocean bottom and she capsized three miles off the rugged coast of Heimaey Island.

Although he wasn't a particularly good swimmer, Fridthorsson swam for six hours in 41-43F (5-6C) water before reaching shore. He was the sole survivor of the five-man crew. How in the world did he do it? In a word, he was obese. At 6'4' and 275 lbs, he had a chart-busting BMI in excess of 30. His physique was similar to a seal’s. Scroll