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Physiological responses to cold water swimming

For a lot of newer cold water swimmers, the experience starts before even stepping into the water. I used to get a little anxious, usually not for any particular reason: I knew I was safe, I knew I’d trained properly, I knew I’d love it once I got in. But in the minutes preceding a swim, sometimes even on the drive to the waterfront, I would get nervous, which naturally resulted in an elevated heart rate.

NBennett St Lawrence Seaway

The elevated heart rate is the true cause of concern, hence seeing a medical doctor for advice before taking part in cold water swimming is wise. For people with underlying heart disease, it increases the chances of cardiac arrest, and general advice is to avoid cold water swimming altogether.

I recently wore my heart rate monitor during a training swim at 2.8C(37F), and forgot to stop my watch. The results were interesting, after an initial blip of high heart rate (160bpm), it took about 42 minutes to return to resting rate:


Swimming about in cold water

After some time swimming around in the water, the muscles in my limbs become affected, something called cold incapacity. Understandably, muscles and nerve fibres don’t work well when cold. Neuromuscular activity slows and body fluids literally begin to congeal in the muscles. As a result, movement becomes increasingly difficult in hands and feet, then arms and legs.

With habituation, cold incapacity has become lessened and I can stay in a lot longer before it comes on, but you can’t outrun the cold. Eventually, it catches up with you. Interestingly, I’ve come out of the water with frozen hands and wobbly legs, but somehow my feet are little affected. I’m careful never to stay in so long that I can’t reasonably function, both for swimming back to shore and when exiting the water, although in very cold water my hands get quite cold, and I often need help getting dressed. Thank goodness for support volunteers…


We’ve been out in the water for awhile, time to tackle the topic of hypothermia…

I struggle with writing about hypothermia; its such an important topic, and yet misunderstood by a lot of people, it’s hard to lay out the facts without sounding glib. But here’s my best efforts:

  • Fact 1: There’s no such thing as “sudden” hypothermia, hypothermia is a condition that develops over time. Sudden death in inexperienced cold water swimmers (i.e., fishermen that get thrown overboard) is most often the result of drowning by inhalation of water during cold shock, not sudden onset of hypothermia. One doesn’t “get” or “catch” hypothermia (like a cold), one develops it over time in exposure to cold. It can take up to 20-30 minutes for the average unacclimatized adult immersed in freezing waters to actually register a decrease in core temperature, and about another 20-30 minutes of useful consciousness left before severe hypothermia is likely to be reached.

  • Fact 2: Hypothermia, by definition, refers to low core body temperature. There are three levels of hypothermia: mild, moderate, severe. However, the core temperature ranges associated with each varies depending on the source, so we’ll follow suit with Loneswimmer’s approach and choose the higher of the scales.

    • normal body temp = 37C(99F)

    • mild: 36/37-35C (95F-89.7F)

    • medium: 34.9-32C (94.8-89.6F),

    • severe: 31.9-23.9C (89.5F-75F), leading to cardiac and respiratory failure

  • Fact 3: The temperature ranges associated with each level of hypothermia matter little when in open water, unless you happen to be sporting a rectal thermometer somehow connected to a computer device that’s tracking core temp while you swim. Not likely. What does matter are knowing the risks and being able to identify the symptoms associated to each level, and learning to recognize them when swimming or supporting other swimmers. You can only achieve this knowledge through repeated practice, noting the effects of cold, and making wise training decisions accordingly.

  • Fact 4: In theory, some level of hypothermia is inevitable in water temperatures below body neutral 37C(98.6C). But the length of time it takes to become hypothermic varies with many factors, including level of acclimatization/adaptation, body mass and body fat index, water temperature,  air temps and wind speed. Ignore charts that assign specific water temps and times to the different levels of hypothermia – it’s a moving target, because it’s based on individual physiology and work done to adapt to cold.

  • Fact 5: Thinking you’ll be awesome overnight is a foolish approach to swimming in cold water, see facts 1-4. You can’t will yourself to be great, your confidence as an athlete and swimmer matters nothing to hypothermia, or to any of the other risks associated with cold water swimming. Cold will eventually catch up to you, so swim accordingly, and swim safe.


Nope, not all in the mind, so train your body…


Here are the basics symptoms for each level of hypothermia, best to consult expert resources for more in-depth information:

  1. Mild hypothermia: sensation of cool or cold, goosebumps, peripheral vasoconstriction ensues, dexterity of hands and feet may be reduced, but otherwise the swimmer is fine. Shivering may occur upon exit from water.

  2. Moderate hypothermia: blood thickens more, as such less oxygen to the brain results in thought process slowing or getting “cloudy”, resulting in mild confusion. As blood cools, bradycardia ensues (slow blood rate). Intense shivering occurs upon exit, and shivering may quite possibly occur while the swimmer is still in the water. Coordination slowed.

  3. Severe hypothermia: shivering stops, muscle coordination very poor, possible loss of awareness of others. “Paradoxical undressing” may occur, in which the muscles necessary for inducing vasoconstriction become exhausted and fail, causing warm blood to rush from the core to the extremities – the swimmer starts to feel warm again (on land, this may result in an individual taking off their clothing, further increasing the loss of heat, hence the term “undressing”). Towards the lower end of the temperature scale: pulmonary edina, cardiac and respiratory failure = death.


Most open water swimmers have experienced mild hypothermia at some point, but for cold water swimmers it’s usually a given. I’ve swum into the beginning stages of moderate hypothermia before, not on purpose, but it happens. I was in an outdoor pool at the time, with very cold ambient temperatures (Hong Kong, record winds), I started shivering slightly each time I stopped at the wall – after a couple of laps, I realized the shivering wasn’t going to stop, so I exited immediately.

Well, I’m ready to get out of the water, what’s next?

Getting out of the water

After-drop is a continued cooling of a person’s core temperature during the initial stages of rewarming from hypothermia. The “drop” in after-drop occurs when the blood that’s been cooling in our extremities starts to move through the body again, about 5-10 minutes after exit from the water.

Next to the initial point of entry into cold water, this has the potential to be the most dangerous time in the span of a cold swim – if a swimmer stays in too long, and the blood in their extremities gets too cold, when it starts to recirculate again it can result in a further drop in core temperature. The swimmer should be monitored after a swim, another reason I never swim alone. As part of my training, I’ve tested my thresholds for staying in cold and recovering afterwards very carefully, I don’t care to take unnecessary chances.

It’s also critical that swimmers not rewarm too quickly, so that the cold blood doesn’t move prematurely to the core. No hot showers or baths, gradual rewarming only. I dress quickly, before after-drop sets in, sip a hot beverage, walk along the beach.


Other things I’m careful of when swimming in cold water…


Exostosis (Surfer’s ear)

The most common cause of exostosis is frequent exposure to cold water: exposure to wind and cold water causes the bone surrounding the ear canal to thicken and constrict the ear canal, sometimes to the point of complete blockage (known as “occlusion”) which can lead to substantial conductive hearing loss. I wear earplugs.


Cold water entering the ear canal can cause disorientation, vertigo and nausea. Very unpleasant, and completely avoidable. Another reason for earplugs.


Frostbite (***warning: gross-out pics of my big toe below***)

Frostbite can occur in three different stages:

  • Frostnip is the first stage and a mild form of frostbite, which irritates the skin, causing redness and a cold feeling followed by numbness. Frostnip doesn’t permanently damage the skin, there’s no cellular damage.

  • Superficial frostbite is the second stage, reddened skin turns white or pale. This may be followed by burning, stinging, swelling and blisters in the coming 24-36 hours. Rewarming too quickly at this stage may cause skin to turn mottled, blue or purple.

  • Severe (deep) frostbite is the third stage, affecting even the tissues that lie below the skin. It is often accompanied by numbness, a loss of any sensation including pain, joints and muscles may no longer work. Large blisters for within 24-48 hours after rewarming, and the skin turns hard and dark as the tissue dies.


Frostnip can happen much more easily than most people realize, even in seemingly innocent water temps. At a recent swim while on visit in Hong Kong, I swam for an hour in balmy 15.5C(59.9F) water, but the air was 3.4C(38.1F), but with wicked wind gusts up to 64km/h(37.8mph). I exited the water at the onset of moderate hypothermia, but more to the point, my feet got much colder than I realized, and I made the mistake of letting hot water run over my frozen toes as I waited to rewarm my body slowly in the shower.


Frostnipped skin under the nail was affected, resulting in a bleed. As mentioned above, sudden rewarming is foolish as it encourages a deeper after-drop (I know this now, but not then), but the point being that the skin under both large toenails was damaged, a purple shadow at first, turning into a black patch, lasting months while slowly growing out. Lesson learned.



Cold skin cuts more easily. And because one’s skin gets numb, especially the feet, it’s possible to get deeper cuts without even realizing it. I’m very careful to watch the bottom while entering/exiting, looking for for sharp rocks, broken glass, jagged twigs. It’s another reason we stick to clean, sandy beach entry points wherever possible.

Swimming alone

It’s a foolish idea, no matter what the temperature. Besides, see how much happier we are when swimming with friends?


I wrote the article below several years ago, when I was first starting to swim in cold water and experiencing things for the first time. A lot has changed since then, it's amazing how the body adapts over time and with practice. But it still serves as a good information for beginners on the basics, so I've recently updated it to reflect those adaptations. Read on!


I don't have a sciences background, but I’ve become passionate about a sport that requires any person performing it to understand at least the basics about what cold water does to the human body. There are risks associated with cold water swimming, and as with many sports, the greatest risk may well be that of ignorance. Knowledge is power, as the saying goes, and in our case, the biggest source of direction for swimming safely. Kindly note, I’m by no means an expert on the science behind exposure to cold, or associated risks. It's best to consult your doctor for a discussion on whether this sport is right for you. 

First stepping into cold water


I learned to deal with it over time, in order to stay relaxed I'd try to focus only on the tasks of arriving and setting up for the swim, I'd go through each step one at a time, mechanically: set up tent, lay out kit, put on cap, and so on. If I were ever worried about something serious, I’d rethink the swim, knowing there would always be another day. But otherwise I never let gitters derail my goals; if the idea of swimming in frigid temps wasn't enough to stop me from heading out to the beach, then I was sure as heck not going to let negative self-talk rob me of a good swim in its waters…

Thermal Touch

Imagine we’re on the shoreline now, and about to step into the water, let's talk about thermal touch. Thermal touch is the perception of the temperature of objects in contact with the skin. To register cold, two completely different systems are functioning at the same time; sensation is the ability of sensory receptors in the skin to detect incoming information, whereas perception is the brain’s ability to take that incoming information and consciously determining it’s meaning.

The role of sensory receptors in this process is to register sensations related to objects the skin feels – in our case, skin registers the cold of water it’s being immersed in, or the chill of air that moves across it. Sensory receptors within our skin register contact with cold, and relay this information to the brain, which then translates it into meaningful thought. In moderately cool water, this may feel somewhat uncomfortable, whereas in very cold water it can be outright painful.

There are lots of different types of sensory receptors in the human body, which work together to relay information about the outside world to the brain. Here’s a couple of the most important ones that help us understand the sensations experienced in cold water swimming;

  • Hot (heat) and cold thermoreceptors: located located in the dermis layer of the skin. There’s a much higher density of cold ones than hot, about three times more, with concentrations found especially in the face, ears, hands and feet. These receptors register radiant and conductive temperatures that would be considered harmless to the body.

  • Thermal nociceptors: located in the epidermis layer, those nociceptors responsive to noxious temperatures (extreme hot or extreme cold) signal the central nervous system of imminent tissue damage and generate the impulse to withdraw the affected part from the thermal source. These receptors generate the sensation of pain, in very cold (ice) water swimming this would be the overall feeling of burning or stinging one gets across the surface of the skin, and after prolonged exposure, from ice crystal formation in the skin.

Temperature adaptation is another important concept for cold water swimming. This kind of adaptation occurs when the frequency of nerve impulses declines when stimulus remains constant. In simpler terms, a sensation may be strong at first, but then fades gradually as time passes. It’s one of the reasons why getting into cold water may have hurt or be uncomfortable at first, but within a few minutes I wouldn’t really notice the cold against my skin nearly as much. Sometimes getting past the first few minutes of pain was all it took to end up with a really enjoyable swim in the end.

However, note that this adaptation doesn’t apply if conditions are too extreme. In very cold water, the sensation of cold is sensed by the nociceptors and registered by the brain as pain, and either lasts much longer or never fades off at all. Defining “very cold” is difficult, it varies by individual and by level of habituation to cold. These days, I usually don’t feel this kind of sharper sensory pain unless the water is below about 2-3C(35.6-37.4F), but it took awhile to get there. Over time and with regular exposure to very cold stimuli, my habituation simply improved. I couldn't always make it to the beach, so things like sticking my hands in a snow bank in -20C(-4F) weather, or gripping a frozen steering wheel was considered to be training. It all added up, from what I can tell.

Cold thermoreceptors are most stimulated when the surface of the skin is below 25C(77F), and very importantly, stop being stimulated altogether below 5C(41F). In other words, if I’m in very cold water to the point that my skin temperature itself lowers to 5C(41F) or lower, I won’t register the cold the same way as the thermoreceptors are no longer working – which means I will no longer be able to tell just how cold I’m getting based on the sensations coming from my skin. Nociceptors will still register the pain of cold, but I won’t be receiving any information specific to how cold, information I would otherwise use to ascertain how cold is affecting me and whether I'm close to needing to get out or risk nerve damage. Dangerous territory, trained with great caution. But training was the only way to know where my safety threshold was, I started out slow, to see how different extreme temperatures affect me. Only once I better understood the effects of a given temperature range - including the impact of cold air and wind in the mix of things too - would I then increase my exposure further. Safety first, always.

Now that I’ve painted a seemingly miserable picture of sensory pain, let me say that with practice, it’s not all that bad. In fact, it's a heck of a lot of fun. When I was first starting out, I would usually stand for a second or two in water up to my ankles, then walk right out to hip-deep water and stop to let my body/brain take it all in. When I was ready, I would drop down slowly to immerse my torso. In freezing cold water, this felt like a sensory explosion, every nerve firing, painful but completely alive. These days, I'm in cold water so often that I can walk in with relative ease. I'll still complain a lot, but that's just kinda part of the fun too.

Immersing the torso in cold water

Alrighty, so we’re now waist-deep in the water…what happens next?

Mammalian diving reflex


The mammalian diving reflex is a reflex that optimizes respiration, so that mammals can stay under water for longer. It’s triggered by cold water making contact with the face. Once the reflex is activated, the following happens:

  1. Bradycardia: immediately upon the face’s contact with cold water, heart rate slows down 10-25%;

  2. Peripheral vasoconstriction (capillaries): capillaries in the extremities then start closing off, so that blood stops flowing in those areas, leaving more blood for vital organs to use. It starts with fingers and toes, then hands and feet, then arms and legs, and can result in cramping (especially in hands/feet) for some swimmers; and

  3. Blood shift: the excess blood is pushed into the thoracic organs, especially the lungs.

Many swimmers gently splash their faces when entering water, I would do this when I was standing waist-deep, just before dropping down to immerse my torso. The reason for this is to signal the vagus nerve, a cerebral nerve centre which transmits sensory information from the body to the brain, that cold is coming. The vagus nerve travels along the neck, like a superhighway connecting the body to the brain. Sudden shock to the vagus nerve can cause cardiac arrhythmia, so the splashing of water on the face gives your body time to register that it’s about to be immersed in cold water. These days, I'm less inclined to splash my face as I'm usually in the water pretty often, and my body likely picks up on other signals to prepare itself. But I always tell newbies not to dive right in head first, give your ticker fair warning!

Capillary vasoconstriction:             Vagus superhighway:


Time to lower down and immerse my entire torso…

Cold shock response

Cold shock is the body’s response to sudden immersion in cold water, and lasts a minute or two, longer for some people. Once cold shock comes on, the following happens:

  1. Gasping reflex: rapid cooling of the skin causes an automatic gasping reflex;

  2. Hyperventilation: rapid breathing, also a natural response to the cold;

  3. Peripheral vasoconstriction (arteries): the arteries that carry blood away from the heart narrow, and the heart has to work harder to pump the same amount of blood throughout the body, resulting in an elevated heart rate.


Swimmers unaccustomed to the gasping reflex could be at risk of drowning if the gasp happens underwater and they take water into their lungs. When first learning to cold water swim, I would always keep my head above water when getting in and let cold shock pass before swimming away. Now, as a more experienced cold swimmer, I have learned to swim through it – one has no choice in a competitive racing environment, where swimmers enter water and start quickly (within 10-12 seconds), before cold shock has had a chance to pass.


Slow immersion into the water helps minimize the shock, but adaptation through repetition has been key in helping me manage cold shock. With continuous expose to cold through training swims, cold shock for me is now fairly minimal at 3-4C(37.4-39.2F), it has to be much lower for cold shock to really come on strong.

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