How Cold Is Too Cold? The Risks of Cold Water Swimming
According to the RNLI, anything below 15°C is considered cold water and can have a significant effect on your breathing and movement. Average sea temperatures around the UK and Ireland are approximately 12°C, while rivers, lakes and waterfalls can be even colder.
When entering cold water, your body responds immediately to the sudden change in temperature. Understanding these responses, and how to manage them is an important part of swimming safely in open water.
The Risks of Cold Water Swimming
There are three main physiological responses to cold water exposure: cold shock, physical incapacitation and hypothermia. Although they are connected, each occurs at a different stage of exposure and carries its own risks.
Cold Shock
Cold Shock is an involuntary response to sudden immersion in cold water. It cannot be consciously controlled and is particularly strong in people who are not accustomed to cold-water exposure. Cold shock can cause an involuntary gasp, rapid and uncontrolled breathing, and a significant increase in heart rate. Losing control of your breathing can quickly lead to panic and, ultimately, swimming failure.
The RNLI describes the response in more detail:
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.
The initial cold shock response generally lasts for around 90 seconds, making those first few minutes in the water particularly important.
Physical Incapacitation
Incapacitation is a progressive loss of control and strength in the arms, legs, hands and feet. Unlike cold shock, which happens very quickly, incapacitation develops more gradually as the muscles and nerves cool. Even regular cold-water swimmers are not immune. If you stay in the water for too long, your muscles can become increasingly difficult to control, making swimming harder and eventually leading to swimming failure. This is one reason why feeling comfortable in cold water should not be taken as an indication that it is safe to stay in for longer.
Hypothermia
Hypothermia occurs when your core body temperature falls below 35°C. In an adult in cold water, this generally takes at least 30 minutes to develop, although the exact time varies depending on the individual and the conditions. For swimmers, one of the main concerns is the gradual development of mental disorientation and the difficulty of warming yourself back up after leaving the water.
Each of these responses can become less pronounced with repeated exposure. Research has shown that as few as six three-minute immersions in cold water can reduce the cold shock response by around 50%. This reduction in the response to a repeated stimulus is known as habituation
How the Body Adapts to Cold Water
Acclimation (or acclimatization) is the process by which your body gradually adapts to repeated exposure to cold water.
With consistent and controlled exposure, it is possible to significantly reduce the body's cold shock response. This doesn't make cold water harmless, but it can help your body respond more effectively to the initial immersion.
Gradual Exposure
If you want to increase your tolerance to cold water, gradual exposure is best. Your body is remarkably adaptable, but acclimation takes time. Gradual entry and short exposure of just a few minutes can begin the acclimation process.
Start by increasing the amount of time you spend in the water each session. You don't need to swim immediately. In fact, when you are beginning to acclimate, it can be safer to initially enter only waist or chest deep. The goal is controlled, consistent exposure to become acclimated rather than pushing yourself to stay in for as long as possible as quickly as possible.
Enter the Water Slowly
Your body can react differently from one day to the next depending on a range of factors, so never assume that because you have managed a particular temperature before, you will respond to it in exactly the same way each time you get in.
When entering, take your time. Put your hands into the water up to your wrists and splash some water around your neck and face. The blood vessels in these areas are relatively close to the surface, so exposing them to the colder temperature can help you adjust to the sensation.
Control Your Breathing
Breathing is another important consideration when entering cold water. In a stressful situation, your body activates its fight-or-flight response. One part of this response is an increase in breathing rate, which prepares the body for physical activity. In cold water, this rapid breathing can quickly lead to panic.
Controlling your breathing can help interrupt this cycle. Take deliberate, slow breaths and concentrate on keeping your breathing controlled as you enter the water. This can help you regain control of both your breathing and your response to the cold water.
Check Yourself While You're in the Water
It's a good idea to carry out regular body checks during your swim rather than relying solely on how you feel. One simple exercise is to touch each finger to your thumb in turn. If you begin to find this increasingly difficult, it may be a sign that your hands are becoming cold and your coordination is deteriorating. This is the time to consider getting out.
Warm Up After Your Swim
Getting dry and dressed in warm clothing promptly is important. Layer up and have a hot drink ready. Your body can continue to cool after you leave the water, so don't underestimate the importance of your post-swim routine.
Consistency Is Key
Acclimation works through repeated exposure, so consistency is important.
If you swim regularly, your body has an opportunity to gradually adapt as the water temperature changes. However, if you stop for several weeks, you may notice a significant difference when you return.
For example, water that was 16°C during one swim could be 11°C a couple of weeks later. That five-degree drop can feel very noticeable, particularly if you have had a break from cold-water exposure.
This is why we recommend consistent exposure rather than simply trying to tolerate colder and colder water.
Choosing Your Neoprene
As water temperatures drop, below 10°C, cold can cause numbness and pain, especially in the hands and feet. Neoprene socks and gloves can help protect these extremities and make cold-water swimming more comfortable.
The right equipment should be used alongside sensible exposure times and good judgement.
The Bottom Line
Cold-water swimming can be an enjoyable and rewarding activity, but cold water demands respect — even from experienced swimmers.
Your body will adapt to repeated exposure, but acclimation doesn't make you immune to cold shock, incapacitation or hypothermia. Take your time entering the water, control your breathing, pay attention to how your body is responding and know when it's time to get out.
Start slowly, build gradually, stay consistent and never let familiarity with cold water replace caution.