Cold Water Shock

Cold water shock is the body's involuntary reaction to sudden immersion in water below 15°C: you gasp, lose control of your breathing and movement, and your heart comes under strain. The effects pass in less than a minute, so the RNLI's advice is not to swim straight away but to float until your breathing settles.

What cold water shock is

The RNLI defines anything below 15°C as cold water. When the skin is suddenly cooled, the blood vessels in it close, which increases the resistance to blood flow; the heart rate rises too, so the heart has to work harder and blood pressure goes up. The RNLI warns that this can cause heart attacks, even in the relatively young and healthy, and can lead to cardiac arrest.

The same sudden cooling triggers an involuntary gasp for breath. Breathing rates can change uncontrollably, according to the RNLI sometimes increasing as much as tenfold. All these responses feed a feeling of panic and increase the chance of inhaling water directly into the lungs, which is why cold water shock can be the precursor to drowning.

Why UK waters are a year-round risk

The RNLI puts the average sea temperature around the UK and Ireland at just 12°C, and the Met Office gives a typical range of 6–10°C in winter to 15–20°C in summer, depending on region and year. Rivers, lakes and reservoirs can be colder still, even in summer. In other words, the sea off a British beach is below the cold-shock threshold for most of the year, and it can still be below it on a hot day.

What to do instead of panicking

The instinct is to thrash about and swim hard for safety. That is exactly what wastes breath in the first minute. The RNLI's Float to Live advice is:

• Tilt your head back, submerging your ears.
• Relax and try to control your breathing.
• Spread your arms and legs out to improve stability, and gently move your hands and legs to help you stay afloat.
• It is OK if your legs sink: everyone floats differently.
• Once your breathing is under control, call for help or swim to safety.

The Met Office gives the same advice: fight the urge to thrash around, lean back and extend your arms and legs, float until you can control your breathing, then call 999 or 112 for help or swim to safety. On the coast, ask for the Coastguard. If you see someone else in trouble, do not go in after them: call 999 or 112, ask for the Coastguard, tell them to float and throw them something that floats.

Getting through the first minute is not the end of the danger. The longer you stay in cold water, the more your arms and legs cool, and swimming becomes weaker and less coordinated; hypothermia, when the body's core temperature falls below 35°C, takes longer to develop but follows if you are not rescued. That is why the float is a way to buy time and call for help, not a reason to stay in.

For planned swims and paddles, the RNLI advises checking conditions, including the water temperature, before you go, wearing a wetsuit of suitable thickness for your activity and the time you will spend in the water, and wearing a flotation device, which greatly increases your chances of getting through the initial shock. Practise floating in a supervised place such as a swimming pool, so the position is familiar if you ever need it.