How To: Understanding hypothermia in swimmers – Moderate & Severe Hypothermia

Previous article: Understanding hypothermia in swimmers: Mild hypothermia.

Moderate hypothermia is obviously more serious than Mild hypothermia which I covered previously in the first part of this series. I’m always a bit bemused by the medical terminology of hypothermia. Many serious open water swimmers will have experienced moderate hypothermia and can tell you there’s nothing moderate about it. Moderate is from 35° C. down to 32.5° or even 32° Celsius. Experienced Sandycove swimmers will regularly swim into or experience Moderate Hypothermia during Channel training, (but they are too tough to call it that, they’s just say it was a bit cold).

Something I’ve noticed is huge variation in defining the differences between mild and moderate hypothermia. Hypothermia itself is body temperature below 35° Celsius. In Part 1, Understanding hypothermia for swimmers, Mild hypothermia, I said mild hypothermia was 35° C. to under 37°C (normathermia). Yet I have seen medical articles that define Mild Hypothermia as 32° to 35° C. and Moderate as 29° to 32° C., with Severe Hypothermia as under 29° C. We could argue this point but here’s my specific point for choosing the higher ranges: hypothermia is dangerous. You want to play around with more risky definitions in cold water? More power to you but that’s your own call.

I made the point in the last article, worth repeating, that you can’t become hypothermic in water instantly.  Your body is a heat reservoir, so hypothermia develops over time.

Take a good look at that chart. 0° C. is a safe zone (assuming you exit, and excluding any cardiac problems). In fact the chart indicates lethality occurs post one hour at freezing temperatures.

At a temperature of 10° Celsius you are in the marginal zone of survival until about three hours. Equate this with moderate to severe hypothermia. Also, that doesn’t mean that you definitely can survive three hours at that temperature, nor that you are definitely going to die. Think of it as a realm of possibility.

Now, if you have one, put on your cold water swimming cap, and look further along the x-axis. We know however that some Sandycove swimmers (Lisa, Finbarr, Rob, Ciaran, myself at least, no disrespect to other SISC swimmers who’ve done the same) have swum six hours at under 11° C. Lisa has swum nine hours at that temperature. So wear  an SISC cap (actually we don’t have those), and the zone of lethality moves. And for these articles I’m talking about hypothermia in the spectrum of swimmers. You can see it’s a moveable target.

Another point from this chart; hypothermia is inevitable at temperatures up to about 15° C. I can’t quantify that from any other sources. Theoretically, at any temperature under body neutral, hypothermia should always result. This would exclude acclimatisation, the amount of bioprene (body-fat), using food to offset the extra calories requires or thermogenesis (heat generation) from swimming. Think of 15° Celsius as an average English Channel solo temperature and we see that there is a variability in resulting hypothermia cases among soloists.

I keep stressing experience, because that means acclimation to cold, which really means as I’ve said previously, we/they are PHYSIOLOGICALLY adapted to tolerate cold easier and for longer. This is NOT just a matter of will-power or a psychological advantage or strength. You cannot think your way into cold adaptation. You may be the world’s most confident person, a 5k per hour swimmer, with no self-doubt, but without physiological adaptation due to repeated cold immersion it will mean little.

You train to swim longer and further. You train to swim faster. And you train to swim colder. The human body responds to physical stress by adapting. This is the Training Effect. Adaptation to cold from training in cold, is another less discussed training effect particularly important to open water swimmers.


One of the most dangerous effects of hypothermia is the slowing of thought processes and the person’s consequent inability to gauge their own condition. As blood cools in the body, bracycardia (slower hear rate) ensues. The blood undergoes partial coagulation, in effect becoming thicker and more viscous, and so less oxygen gets carried to the brain. Therefore, like the previously headlined item above, you cannot think your way out of hypothermia.This makes hypothermia deceptive, it sneaks up on you, those not familiar with cold shock and initial responses imagine hypothermia as a prolonged intense cold whereas it enfolds the sufferer gently, lulling them into a gradually more dangerous state. This is partly what makes hypothermia so dangerous, with thermoceptors overloaded, the sense of cold is reduced or eliminated, the cognitive functions are impaired and the person won’t or can’t realise the danger.

Violent shivering in or more commonly out of water is a symptom of moderate hypothermia. Violent shivering is the bodies attempt to warm up through exercise, by muscular contraction. Almost every distance Sandycove or Irish or UK swimmer has had happen this post-swim. This happens five to ten minutes after exiting the water, when the cold blood in the periphery reaches the core (Afterdrop). How do you tell violent shivering from normal shivering? Normal shivering can be stopped voluntarily, violent shivering can’t. We all know what it’s like to stand talking after a swim with severely chattering teeth, difficulty standing still and straight and pain in the lower back.

When swimming distance in cold water, one point of danger is when the person starts to feel warm, after being cold for a long time. This is not just acclimatization, this is paradoxical undressing, which on land can cause people to shed clothes when they are freezing. The swimmer will have passed through violent shivering to get to this point but it may also arrive for swimmers without violent shivering . Again, many distance swimmers will have experienced getting this feeling of warmth while swimming, but those who understand what they are doing will recognise it as an immediate warning sign to exit the water as soon as possible. As swimmers we have to recognise and deal with the problem that we have the Immediately option, having to swim to a safe exit point. There’s no point getting out where you have no access to clothes. About 25% of the deaths caused by hypothermia arise from poor judgement and decisions made by the hypothermic person.

It is also important to always be cognizant of ambient conditions. Irish and UK open water swimmers training through the winter and swimming in five or six degree water will have similar air temperatures and at least in Ireland, wind is a fairly constant factor. (Ireland in fact is one of the windiest countries in the world and that’s not including all my writing here). High humidity and wind means heat stripped from the body faster, both in the water when shoulders and arms can get cold even more quickly, and especially outside the water when wind can cause violent shivering within minutes. For example, last Saturday Ciarán Byrne & I were swimming at Garrylucas, water temperatures were very variable from about 10 Celsius to 13 Celsius, the day was warm by Irish standards at about 20° C. but at the 90 minute feed on the beach, there was a howling offshore south-easterly and we were both shivering violently within less than two minutes. Swimming in cold water on the Northern European seaboard generally means constant cool or cold air temperatures.

Experience, and I mean significant experience, not just the initial four to six immersions required to blunt cold shock, does allow a swimmer greater awareness of their own hypothermia progression, though this is a variable process with a lot of external factors which vary every time, and therefore is not to be advised. Even the most experienced cold water swimmer can’t tell for sure exactly much more time they might have while swimming before moving further up the scale of hypothermia severity. However we live in a world where we are seemingly perpetually warned about the dangers of everything, and if this is something you choose to do, then you should choose to do it with knowledge and awareness. /End short rant.

Next article: Diagnosing and addressing hypothermia in swimmers.

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18 thoughts on “How To: Understanding hypothermia in swimmers – Moderate & Severe Hypothermia

    • Hi Dylan, Thanks, glad it’s helpful. I felt that there was too much incorrect information out there, usually derived from exposure studies on non-swimmers. I can’t recall the source. I usually tag everything so I can refer back to source, so when I get a chance I’ll have a look and let you know if I find anything.

      Liked by 1 person

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  5. Thx Donal, this helps a lot. The acclimatization process that I am going through is more thinking about possible post-Gibraltar swims, and also because I was intrigued about cold water swimming. Now i am not only intrigued, but also thoroughly enjoying it.

    For Gibraltar, the plan from the very beginning was to use wetsuits. 1 of the 4 swimmers is very sensitive to cold water and is not going through cold water acclimatization. We will be swimming together (not in relays), so for me the option was either the 4 with wetsuit or the 4 without.

    And again, thanks a lot for all your posts on cold water swimming. I have found that they are a fundamental component as I continue to better understand what this is all about. Particularly being in Barcelona, where there is basically no cold water swimming culture. So, you are our link to the crazy and foreign (for us) world of cold water swimming!


  6. Hi Donal. Your cold water posts are helping a lot in my cold water acclimatization. I am still taking baby steps here, but I am trying to get a cold water swim once a week. Yesterday I did 31 minutes at 13.5ºC. I know that this is warm for you and other real cold water swimmers, but it has taken me a few months to get to this level. Swimming in this temperature water is a lonely activity around this part of the world.

    By the end of my recent swims, I start getting a bit light headed inside the water but still feeling totally aware and thinking clearly. When I get out, I start shaking. I am feeling OK and think that I can start pushing for more time, but still, I am being careful because of your comment on hypothermia being deceptive and sneaking up on you. I want to make sure that I get out of the water before getting too cold. Any insights you can share (or post you’ve written that you can guide me to) in order for me to better understand if I am ready to push for more time?

    Thx a lot!


    • Thanks Mauricio. This post went out by accident, and was removed from the front page. (So you’ll getting a copy of it again soon, sorry.)

      You are doing everything right. All the articles are meant to illustrate that it’s a process, one I went through myself, and that almost anyone can do it (excluding very thin people or people with circulatory or cardiac problems).

      One the guys who completed the recent ice-mile, that I didn’t finish, couldn’t keep his face in the water for more than 10 minutes at about 12C, only 3 years ago and I remember him asking me about it. Just keep at it, was the essential advice I gave him. In fact I still notice differences in myself every year,after 5 winters without a wetsuit. I remember when I thought 12C could kill me.

      Every minute you spend in cold water now is an investment, and is a harvest you’ll reap in the summer on the relay when the water isn’t so cold. The goal is that you are comfortable on the relay instead of suffering through it.

      The series on Extreme Cold Adaptation will explain more of the science and how you are mentally and physically adapting, if you are interested. This one is good for actually seeing what is going on in the body, this one on cold preception. BTW, the girls may find it harder to get in, but will be able to stay in longer, ironically, since women have 0.25C higher core temperature.


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  8. Two really interesting articles. Lot of symptoms are very familiar to me as I have been training for the a 2013 English Channel attempt for the last six weeks. Swimming in water as cold as 6 degrees. I look forward to the rest of the series.


    • Thanks Paul, plenty more articles about cold around the site. I remember coming across your blog some time back, using the marathon swimmer’s favourite quote is always noticeable, we all use it all the time. Where are you based?


      • I am originally from Nottingham but have lived in Kelowna BC Canada for the last three years. I only discovered OWS since living here (easy when you live next to the lake I guess)….it has become a true passion for me. Next week we have our qualification swim for our 2013 EC attempt and I am really looking forward to it…

        Happy swimming.


  9. Two very interesting articles and a lot of things that I can relate to right now as I have been training for the English Channel for a the last 6 weeks in temps as low as 6 degrees. I look forward to the rest of the series.


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