Cold water shock

The effect on the body of entering water 15°C and below is often underestimated. This shock can be the precursor to drowning.
Underwater shot of crew training in the sea survival pool at RNLI College

Photo: RNLI / Nigel Millard

Underwater shot of crew training in the sea survival pool at RNLI College

What's the risk?

Anything below 15°C is defined as cold water and can seriously affect your breathing and movement, so the risk is significant most of the year.

Average UK and Ireland sea temperatures are just 12°C. Rivers such as the Thames are colder - even in the summer.

What happens?

Cold water shock causes the blood vessels in the skin to close, which increases the resistance of blood flow. Heart rate is also increased. As a result the heart has to work harder and your blood pressure goes up. Cold water shock can therefore cause heart attacks, even in the relatively young and healthy.

The sudden cooling of the skin by cold water also causes an involuntary gasp for breath. Breathing rates can change uncontrollably, sometimes increasing as much as tenfold. All these responses contribute to a feeling of panic, increasing the chance of inhaling water directly into the lungs.

This can all happen very quickly: it only takes half a pint of sea water to enter the lungs for a fully grown man to start drowning. You could die if you don't get medical care immediately.

How can you minimise the risk?

If you enter the water unexpectedly:

  • Take a minute. The initial effects of cold water pass in less than a minute so don’t try to swim straight away.
  • Relax and float on your back to catch your breath. Try to get hold of something that will help you float.
  • Keep calm then call for help or swim for safety if you’re able.

If you’re planning on enjoying the water:

  • Check conditions - including water temperature - before heading to the coast. Visit magicseaweed.com for full surf reports in the UK and Ireland.
  • Wear a wetsuit of appropriate thickness for the amount of time you plan to spend in the water and the type of activity you're doing, if entering.
  • Wear a flotation device. It greatly increases your chances of making it through the initial shock. See our guidance on lifejackets and buoyancy aids (PDF 3.3MB).

Our seas and rivers are cold enough to leave you helpless in seconds. Treat water with respect, not everyone can be saved.

Learn more about cold water shock and see its effects in our Magazine article: 

Cold water shock: A bolt from the blue.