Cold water shock: what it is, and what to do in the first minute

Most trouble in cold water starts in the first minute, and it has little to do with how well you swim. The cold takes over your breathing before you've had a chance to settle. That's cold water shock. Know what it is before you get in.

What cold water shock is

When cold water hits your skin, the blood vessels near the surface close up. The RNLI explains that your heart rate rises too, so your heart has to work harder and your blood pressure goes up. This is how cold water shock can cause heart attacks, even in people who are relatively young and healthy.

At the same moment, the sudden cooling makes you gasp. You don't decide to; it's involuntary. The RNLI says your breathing can then change uncontrollably, in some cases speeding up as much as tenfold. Together these responses feel like panic, and they make it more likely you'll breathe water straight into your lungs.

Fitness doesn't switch it off. The Met Office, sharing advice with the RNLI and RLSS UK, says it can affect anyone, whatever their age, fitness or swimming ability.

The 15°C line

UK guidance draws the line at 15°C. The RNLI defines anything below 15°C as cold water, and HM Coastguard and RoSPA both describe cold water shock as a risk at 15°C or lower.

UK water often sits well below that line. The RNLI puts the average sea temperature around the UK and Ireland at just 12°C, and says inland waters such as lakes, rivers and reservoirs can be colder, even in summer. For most of the year, the risk is real.

RLSS UK goes further. It treats cold water as anything from roughly swimming-pool temperature downwards, and says every water around the UK is cold enough to bring on cold shock, even in high summer. Treat 15°C as the point where the risk is well established, not a promise that you're fine above it.

Check the water before you go. Wild's spot pages show sea temperature at the coast and a modelled estimate for lakes. For how that changes through the year, see UK sea temperature by month and why big, deep lakes stay cold below the surface. Watch for cold pockets as well. Inlets can bring sudden drops in temperature that may, in severe cases, trigger cold shock. More on that in inlets and outlets.

The first minute: Float to Live

If you get in too fast, slip or fall in, the RNLI's advice is to take a minute. It says the initial effects of cold water pass in less than a minute, so don't try to swim straight away. The Met Office puts it at around 90 seconds. Rather than counting, wait until your breathing settles.

While you wait, float. The RNLI's Float to Live steps are:

1. Tilt your head back, with your ears under the water.
2. Relax and control your breathing.
3. Move your hands and legs to help you stay afloat.
4. If your legs sink, that's fine. Everyone floats differently.

Spreading your arms and legs out helps you stay stable. Once your breathing is back under control, call for help or swim to safety. The RNLI suggests finding your float in advance, somewhere supervised like a swimming pool, so it's familiar before you ever need it.

The instinct is to thrash for the edge. The RNLI's open water advice is to fight that instinct, turn onto your back and wait for the shock to pass. Panic in the water covers the breathing side in more detail.

Getting in gradually

On a planned swim, you control how fast you meet the cold. The RNLI's advice for acclimatising:

• Walk in slowly. Never jump in or dive, because that increases the risk of cold water shock.
• Try not to hold your breath. Let your breathing settle before you go any deeper.
• Splash some water under your arms and on your neck, and wet your forehead.

HM Coastguard says the same: go in slowly to acclimatise, and never jump straight in. It also says a wetsuit adds buoyancy and reduces the chance of cold water shock.

The RNLI points out that the colder the air and water, the faster you cool, so the colder it is, the shorter your swim should be.

After the first minute

Getting through the shock isn't the end of it. RLSS UK explains that as your muscles cool they lose strength, until you reach what it calls "swim failure": the point where you can't swim any more. Its advice is to use the window once your breathing is back under control to head for an exit.

Hypothermia comes later. RLSS UK says it takes at least 30 minutes to become hypothermic, even in very cold water, and that the risk carries on after you get out. That's covered in hypothermia: the signs. For where all of this sits in your pre-swim routine, see the outdoor swimming safety checklist.

Sources

1. RNLI, Cold water shock
2. RNLI, Float to Live
3. RNLI, Open water swimming
4. RNLI, Top tips: Open water swimming for beginners
5. HM Coastguard, Safety spotlight - Sea swimming and cold water shock
6. RoSPA, Open water swimming
7. Met Office, Cold water shock: what it is and how to stay safe
8. Royal Life Saving Society UK, Cold Water Shock – the Facts

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.