Understanding Hypothermia – Outdoor Swimming Society (2024)

The lowdown on the fine lines between cold, very cold, and hypothermic

Kate Rew and Imogen Radford

Niall Meehan

In most of the world – anywhere where water is cooler than body temperature – swimmers can achieve hypothermia at any time of year, it’s just a matter of how long it will take. Knowing what hypothermia is, how it develops and how to treat it is will help keep you, and people you swim with, safe, and it all comes down to being able to answer these simple questions.

1. What is the difference between being cold and being hypothermic?

“Cold is feeling cold, hypothermia is defined medically as a core body temperature drops below 35 ˚C, but your swimming performance is likely to be affected when it drops below 36˚C,” says OSS’ expert medical advisor Dr Mark Harper of Brighton and Sussex Medical School.

Hypothermia can be classified as:

mild – 32.3-35 ˚C
moderate – 28-32.2 ˚C
severe – below 28 ˚C

2. How can I recognise if I am becoming hypothermic?

Hypothermia creeps up on people gradually, and “most of the symptoms found in mild hypothermia can be found in any temperature below normal body temperature (36.5 degrees C),” says Harper.

Signs that you could be becoming hypothermic include:

uncontrollable shivering and numbness
loss of simple coordination – swimming stroke changes, swimming position may become more vertical
weakness in arms and legs – may feel sluggish
clenched jaw and some difficulty speaking freely
hands becoming claw-likeand being less able to control them

The only way to recognise your own limits is to get to know them, through experience. Whether it’s summer or winter, start with short immersions close to the shore and build up your swim time. Notice that you are colder 10-40 minutes after you get out than you are when you exit (a phenomenon called ‘afterdrop’, covered below). Notice that your body is not a machine (so don’t use x degrees = x minutes in the water) – on days with wind chill, or when you are tired, hungry, stressed or hungover, you will likely get cold faster.

3. How can I tell if someone else is becoming hypothermic?

Any of the signs of growing cold listed above are possible to witness in another – shivering, a change in swimming technique or speed, being unable to speak, growing more vertical in the water, fingers becoming splayed. When blue appears around the lips that it also a sign someone is cold. These signs suggest someone should get out of the water as quickly as possible.

The Umbles

Marathon swimmers refer to ‘The Umbles’ as a way of remembering what to look for. A hypothermic person shows some or all of the following (and might appear drunk):

Grumbles – negative mental outlook
Fumbles – slow reaction time, drops things, poorly coordinated. Fine motor movement lost first – i.e. cannot do up buttons or move fingers properly when asked
Mumbles – slurs words
Stumbles – appears stiff, loses coordination, eventually unable to walk without help

4. What can I do to help myself or another swimmer who seems to be experiencing hypothermia?

The key is to get warm and dry as fast as possible:

Get out of the water
Remove ALL wet clothes as soon as possible
Stay out of the wind
Pat skin dry, don’t rub
Dress in dry warm clothes, including hat, gloves and thick socks – ideally lay these out in advance so you can do this quickly. Reflective space blankets will not help, except around the outside of these to keep the wind off – use wool ones instead. Merino thermal base layers, wool jumpers, insulated jackets, thermal bottoms, hats, gloves, coats – winter swimmers take all these or similar things to wear post swim.
For really cold people, it is safer to have them lie down than stand up. Sitting down can help also.
Take in warm drinks and food (not alcohol or caffeine, sugary if tolerated).
Do not use warm showers or baths (for someone who is hypothermic).
Shiver (shivering is good as the muscular activity generates heat – as you rewarm you will shiver less).
Covered hot water bottles can help – be careful they don’t burn cold skin (when skin is very cold, it will not be able to judge what is burning hot and what is not). Try placing one between a base layer and jumper, for instance – covered, not directly next to the skin.
Seek out a warm place.
If symptoms are mild, some gentle activity (walking) can help warming.
Keep yourself and others safe by waiting until you have warmed up before driving.

@coldwaterswimming

5. What attention should I pay to shivering?

Shivering is not a reliable sign of whether a cold swimmer is okay. Shivering is a way that a body can warm itself up: the involuntary muscle movement is your body’s natural response to getting colder and trying to warm up.

6. Should I measure my temperature, and if so how?

No, do not try to measure your temperature. Oral and rectal thermometers are the most reliable guides to internal body temperature but are difficult to use effectively in an outdoor situation (it is likely to be difficult to keep a thermometer under the tongue in place long enough whilst shivering). Infrared thermometers will just be reading skin temperature, which will be misleading.

Instead, know and look out for signs of cold and err on the side of caution: put efforts into getting the cold person warm earlier rather than later.

7. Can you get hypothermia in summer?

Yes. Getting cold is an inevitable result of swimming in natural temperature water in temperate climates, and, outside the tropics, it is possible to become hypothermic in summer as well as winter.

Niall Meehan

8. What can swimmers do to avoid hypothermia?

Avoid peer pressure, and avoid self pressure. What is there to prove? Cold talented people may tell you that “cold is in the mind”, but it does also exist in the body. In physiological terms there are fast coolers and slow coolers. Just as there are people who can run marathons in the desert and people who feel unwell when the weather is still in the 20s, people arrive at cold swimming with different baselines.

Your own baseline will also change: lack of sleep, food, hangovers and external factors such as the weather and temperature will all affect how long you can swim before you become hypothermic.

The best way to avoid hypothermia is to be cautious, and when the signs of getting cold appear, get out of the water, get quickly back into clothes and warm up.

9. Is hypothermia dangerous?

Yes. “Exposing ourselves to cold water is a good and healthy thing, if done with care,” says Mark Harper. “I believe it may have health benefits in assisting with anxiety, depression, inflammatory conditions such as arthritis, easing of unwanted menopausal symptoms and maybe even dementia. But the body functions best over a very small range of temperatures, with any deviation impairing physiological functioning, and can – in extreme cases – lead to problems that include unconsciousness, heart attacks and death.”

10. Is it possible to acclimatise without pushing myself into hypothermia?

Acclimatisation is the process of becoming better able to deal with cold, and is seen across sports – for example, in mountaineers and polar explorers, as well as endurance swimmers, channel swimmers and winter swimmers. It is achieved by consistent, increasing exposure to cold. The committed also avoid its opposite – being too warm – by wearing less, sleeping in cooler rooms and so on.

“Acclimatisation just requires exposure to cold, but not so much exposure that you experience hypothermia,” says Harper.

11. How long can I stay in before risking hypothermia?

Timings are so individual that it’s impossible and unhelpful to apply a formula. It’s best to err on the side of caution. “Staying in longer does not bring any extra health benefits,” says Harper. “In fact quite the opposite, it’s more likely to be detrimental to your wellbeing.”

12. Can I wear neoprene gloves and socks or similar, and will this help avoid hypothermia?

Yes, it’s quite safe to wear these if you choose, as they will stop or reduce the pain of cold and protect sensitive areas. They won’t stop you getting hypothermia, though.

13. Will wearing a wetsuit protect me from hypothermia?

Yes and no. It will delay the onset of hypothermia but not prevent it.

Calum Maclean

14. What is afterdrop?

It is continued cooling, after the swimmer exits the water, as heat flows out of the body into the cold water.

Once you exit the water, you continue to cool for up to 30 minutes. This means that your deep body temperature may be cooler 30 minutes after your swim than you were when you got out of the water. This is why warming up immediately after your swim is vital. See

15. Is it okay to use warm showers or baths?

This is contentious. As long as you are not hypothermic, you can use warm (not hot) showers or baths to warm up. Be careful if you feel dizzy – if you do, you should sit on the floor straight away, or you can use a chair in the shower.