Did you know that being submerged in cold water for just 20 minutes can lead to hypothermia, unconsciousness, and even death?

How long a person can survive in cold water primarily depends on the water’s temperature, exposure time, and the thermal insulation of the victim’s protective clothing. The insulation within clothing is measured with the unit “Clo.” By determining the rate at which heat is lost from the body, as well as the difference in temperature between the skin and the water, predictions can be made of the rate a person’s body temperature will drop when submerged in water.

For example, one “Clo” represents the thermal comfort of an average person in business clothing in a room temperature indoor environment. Fractions of this thermal comfort are what are perceived when worn in the water, because water transfers heat out of the body 25 times faster than air.

This graph indicates random samples of “Clo” values, and the corresponding estimation of survival times in calm water.

Clo Graph

Wearing an approved immersion suit can help to protect yourself from hypothermia and keep you afloat in cold water. The immersion suit keeps your body heat trapped inside the suit, while keeping the water out, and can significantly increase the time you can survive in water.

Remember that hypothermia can happen to anyone – even on warm, sunny days!

The three critical phases of cold-water immersion:
  • First 5 minutes – immediate shock You will experience the ‘gasp reflex,’ which is the sudden gasp of air as a result of the shock. You will be unable to hold your breath, and hypertension and increased cardiac output will result. Most casualties in this phase are due to drowning or a heart attack, even before hypothermia can begin to set in.
  • Next 15 minutes – inhalation of water You won’t be able to keep afloat or swim, and your ability to grasp or climb into/onto things will diminish. Typically, most casualties in this phase are due to drowning from excessive inhalation of water.
  • 30 minutes – onset of hypothermia 37° C is considered normal body core temperature. When your core temperature drops to 36.1° C, muscle tone starts to become affected. Most people have experienced this feeling of tension in their back and neck when they’ve become chilled.

At a core temperature of 35° C, you would be considered mildly hypothermic. At a core temperature of 33.9° C, subjects experience amnesia - but of course, they don’t remember it! Another 1.1° C drop down to 32.8° C, and a lack of sensation or feeling can be experienced.

At 32.2° C you would be considered profoundly hypothermic and would start to lose the ability to shiver. At 31.1° C, shivering ceases. Since shivering is a human’s only method of increasing their internal heat generation, once it stops, core temperature starts falling rapidly. At 30° C, heart arrhythmias occur. Death follows at 25° C; however the majority of people would have drowned before ever getting to this point.

Tips to preventing hypothermia
  • If you’re in cold water, don’t swim unless you can reach a nearby boat, or floating object. Swimming lowers your body temperature, and even excellent swimmers will drown when swimming in cold water.
  • The more of your body that is out of the water, the warmer you’ll be. If you are near a floating object that is large enough, pull yourself up onto it.
  • Keep your head out of the water to lessen heat loss and increase survival time.

For more information on hypothermia and survival in cold weather, please visit the Transport Canada website