Outdoor swimmers in the UK are almost always swimming in cool or cold water outdoors, since the water here rarely warms to more than 20°C (normal body temperature is 37°C). Perhaps it’s our love of the chill that means increasing numbers are embracing winter swimming. You can acclimatise to the cold over time; this section gives some cold water-related information and advice to help you. Water in the UK is generally considered cold year round, so there’s great value in acclimatising.
The primary risk to a swimmer on an outdoor adventure is that you might get cold away from the shore or bank. When the body gets cold, surface blood vessels constrict so that blood shunts to the core. This helps your body to preserve its core temperature, but in the process arms and legs are weakened by reduced blood flow. Muscles become stiff and sensory nerves numb. This is cold incapacitation. Even fit, strong swimmers will lose strength, power and manual dexterity in these circumstances, although it might take some time (between 3 and 30 minutes) for the effects to be noticeable.
The danger of cold incapacitation is that it’s a key cause of swim failure where you will be unable to swim effectively, regardless of your ability and fitness. Thus if you’re a distance from safety (such as in the middle of a lake or out to sea) you won’t be able to make it back. Another consideration is that an egress point which at first appeared to be a very safe and simple place for you climb out of the water, can become an unsurmountable barrier when you’re cold.
If you are unable to touch the tip of your second finger to your thumb, you have cold incapacitation.
The length of time it takes to become cold incapacitated varies widely between swimmers, based on biology and acclimatisation. But while some people are now doing ice miles (a mile in under 6 degree water) it’s worth remembering the standard distance in a winter swimming event is 25m. It’s common for winter swimmers to only be in a few minutes, or for their objective to be 10 or 20 strokes.
If you enter cold water (at a temperature below 15 °C) and you’re not acclimatised, your body reacts with an uncontrollable gasp. This is followed by hyperventilation (rapid breathing which is entirely out of your conscious control). If you’ve jumped or dived in, or you’re in rough water, water can enter the lungs, and you will quickly drown. Always get in to cold water slowly and in a controlled manner. Try splashing water on your face, don’t swim until your breathing’s under control. This will usually take 1-2 minutes.
There is more information about the physiological effects of cold water in the Cold including the OSS’s cold water expert, Dr Mark Harper expert advice. Cold is not all bad: the bite of icy water on exposed flesh is certainly painful at first, yet the resulting endorphin rush is unbeatable.