Risks of Cold Water

Cold shock, cold incapacitation and hypothermia: a beginners guide to cold risks

©Pete Ice Swimmer

Immersing yourself in cold water (which could be defined as UK water temperatures for most of the year, not just winter) carries risk, and if the second step in becoming an outdoor swimmer is learning to love the cold, the first is appreciating it’s risks.

One of the most surprising things about temperature is how fast swimmers adapt to it – or make room for enjoying it despite that fact it feels cold. It used to take about 3 years for swimmers to go from thinking all UK water is chilly, even in summer, to wanting to swim in winter just for the experience. Then we had Covid and swimmers began their careers in winter.

Here are some key reactions and terms to start out with:

Cold shock

The biggest factor in people getting into trouble and drowning is the cold. People think it’s hypothermia that’s the cause of that but you won’t get hypothermia for 30 minutes or probably longer in the warmer weather.

Water in Britain is lower than 15 degrees much of the year. At these temperatures you experience physiological responses when you get in including cold shock, the response where your body initiates a gasp reflex. If you are under the water or a waves comes over your head when this gasp reflex takes place you may inhale some water, and you don’t need to inhale much for it to cause serious problems with your breathing.  You don’t have to completely engulf your lungs for there to be a problem.

After the initial gasp of cold shock comes a period of hyperventilation that you can’t control. Breathing is really rapid and you can’t control it, which can lead to a feeling of panic. New swimmers also tend to go upright in the water, in this position we are less buoyant, and then panic and can really set in, increasing the chances of drowning.

Getting in gradually and staying in depth until the cold water shock response has passed and breathing is under control is safer.

Cold incapacitation

This happens when you get too cold. You get in, and after a couple of minutes of feeling uncomfortable the water feels pleasant. As a novice swimmer, or even as a strong swimmer with limited experience of swimming outdoors, you then attempt to cross the lake, but half way across start feeling cold again. Your body continues to lose heat, blood shunts to the core to keep organs warm. Your muscles lose power, limbs become slow and heavy, and swimming becomes increasingly difficult.

Swimming becomes slower, ragged and short distances can take a long time to cover.

This is cold incapacitation and it can all-too-easily lead to drowning.

A further effect of the loss of coordination is lack of manual dexterity. At best, this means difficulty putting your clothes back on, at worst the bank that previously seemed a safe exit point might now be difficult or even impossible for you to climb as you struggle to grip with your hands, while your limbs are clumsy and numb.

It is safer to not let yourself get too cold, to stay near the shore so that you have a better chance of getting out if you notice your muscles working less well.


Cramp can strike anywhere, and some people are more prone than others. If you’re cold, cramp is perhaps more likely. If you do cramp, float on your back and call for help.


In some asthmatics, cold can trigger an attack, so asthmatics need to be aware of and cautious of this as a risk.

Cold water urticaria

Allergic urticaria on leg in the form of hives induced by cold. Cold urticaria (essentially meaning “cold hives”) is a disorder where hives (urticaria) or large red welts form on the skin after exposure to a cold stimulus. The welts are usually itchy and often the hands and feet will become itchy and swollen as well.


Contrary to popular opinion hypothermia is unlikely to cause a swimmer to get into difficulties since it will take some time to become truly hypothermic; cold incapacitation and cold shock are the main culprits. Be aware that when you get out of the water and begin to warm up,  warm blood from your core will cool quite fast as the peripheral blood vessels open up. This is the point where swimmers experience sudden coarse shivering.

After drop

Afterdrop is a feature of swimming where you feel colder after you get out of the water than you did when you are in.

It helps to avoid the effects to get dry, dressed and warmed as soon as you get out of the water.

Risks from pre existing health conditions

As a general rule, get expert medical advice before  winter swimming if you have a heart condition, high blood pressure, asthma, or are pregnant.  Many people use water as a way of mitigating health conditions, rather than avoiding it because of them – but this journey is individual, and you are the only one who can appraise and mitigate your own personal risk.

See also:

Kate Rew is the author of The Outdoor Swimmers’ Handbook (Rider, 2022, also available signed in The OSS shop) and Wild Swim (Faber). Instagram: @kate_rew.

Kate Rew