Hypothermia: the signs in yourself and the person next to you

Cold water shock is about the first minute. Hypothermia is slower. It builds over a swim and can keep building after you're out, and it dulls the judgement you'd use to spot it. Watch the person next to you as closely as you watch yourself.

What hypothermia is

The NHS defines hypothermia as a dangerous drop in body temperature below 35C, against a normal of around 37C. It's a medical emergency that needs treatment in hospital.

In the water, it isn't instant. RLSS UK says it takes at least 30 minutes to become hypothermic, even in very cold water, and that cooling muscles can stop you swimming well before that (more on this in cold water shock). But RLSS UK also says hypothermia remains a risk after you get out, unless you get out of the cold and warm up promptly and properly.

The signs in yourself

The Outdoor Swimming Society (OSS) lists the signs swimmers may notice in themselves:

• shivering you can't control, and numbness
• losing simple coordination, so your stroke changes and you swim more upright
• arms and legs that feel weak and sluggish
• a clenched jaw, and finding it hard to talk freely
• hands that go claw-like and are hard to control

Your thinking slows down too, so you may not be able to judge how cold you are. The OSS warns that in more severe cases swimmers can even start to feel warm, which is a big danger sign if they're still in the water. Shivering isn't a reliable guide either: the OSS says that when shivering stops, it can mean you're warming up, or that hypothermia has become severe.

Decide how long you'll swim before you get in, while your judgement is still your own.

The signs in the person next to you

From the bank or alongside, the OSS says to watch for shivering, a change in someone's technique or speed, not being able to speak, swimming more upright, splayed fingers and blue around the lips. Any of these means get them out of the water as quickly as possible.

On dry land, the OSS uses "the umbles":

• Grumbles: a low, negative mood.
• Fumbles: slow reactions, dropping things, poor coordination. Fiddly jobs like buttons go first.
• Mumbles: slurred speech.
• Stumbles: stiffness and lost coordination, and eventually not being able to walk without help.

The NHS lists the symptoms in adults as shivering; pale, cold and dry skin, with lips that may look blue or grey; slurred speech; slow breathing; and tiredness or confusion.

Afterdrop: why you get colder once you're out

During a cold swim your body keeps its warm blood around your core, and your skin and the layers beneath it go cold. When you get out, that cold outer layer goes on cooling your core. The OSS calls this afterdrop, and says you're likely to feel colder ten minutes after getting out of a winter swim than you did the moment you left the water. It can bring shivering, faintness and nausea.

The RNLI's advice is to allow about 30 minutes for warming up before you head for home, and not to drive in that first 30 minutes, until afterdrop has passed.

Warming up properly

• Get out of wet layers and dry off straight away. The OSS says to pat your skin dry rather than rub it.
• Dress quickly in warm layers with a windproof outer layer, as the RNLI suggests, and add a hat and gloves.
• Stand on something while you change, so you're not losing heat through your feet.
• Sip a warm drink and have a bite to eat. The OSS notes that the drink does little for your core temperature, but it may help you feel better.
• Leave the foil blanket at home. The OSS says they don't help swimmers, because you aren't radiating heat for them to trap.
• Go easy on hot water. The OSS calls warm showers and baths contentious: an option only if you're not hypothermic, warm rather than hot, and sit down straight away if you feel dizzy.

Most of this is easier if you set it up before you get in.

The open water kit list covers what to bring for afterwards.

When to call for help

If you think someone has hypothermia, the NHS says to go to A&E or call 999. They shouldn't drive themselves: ask someone to drive them, or call 999. Who to call covers how to raise the alarm from the water's edge.

While you wait, the NHS advice is to:

• move them indoors, or somewhere sheltered, as quickly as possible
• take off any wet clothing and wrap them in a blanket, sleeping bag or dry towel, with their head covered
• give them a warm, non-alcoholic drink and something sugary, if they're fully awake
• keep them talking, and stay with them

And the NHS says don't put them in a hot bath, don't use a hot water bottle or heat lamp, don't rub their arms, legs, hands or feet, and don't give them alcohol. The OSS adds that someone who is really cold is safer lying down than standing.

Before you swim, know what hypothermia is, what cold water shock is and how to deal with it. All three are on the outdoor swimming safety checklist.

Sources

1. NHS, Hypothermia
2. Outdoor Swimming Society, Understanding Hypothermia
3. Outdoor Swimming Society, The Subtle Art of Warming Up
4. Royal Life Saving Society UK, Cold Water Shock – the Facts
5. RNLI, Top tips: Open water swimming for beginners

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.