How To: Understanding hypothermia in swimmers – Mild Hypothermia

When all fails and I am at a loss for something to write about, I can write about cold, my favourite subject. Especially in the context of Cork Distance Week coming in two weeks, when we had a few people pulled from the water with hypothermia last year.

For anyone involved in open water swimming in Ireland, the UK and other Northern Hemisphere cold water locations, being able to spot and diagnose dangerous hypothermia in a swimmer is an essential skill. To do that properly an understanding of hypothermia is useful.

It’s essential to understand that there is no such thing as sudden hypothermia. Most of us grow up hearing this myth, (for example I remember stories of survivors from the Titanic freezing to death in five degree water within fifteen minutes, and that fifteen minute myth is repeated all the time).

The heat in your body can’t instantly disappear. The Second Law of Thermodynamics is always the Universe’s governing and inviolate principle. Hypothermia is a developing situation over time. Your body has enough stored heat that even in zero degree water, you probably won’t develop severe hypothermia until about thirty minutes, though you will be subject to After-drop and potentially lethal consequences even if you emerge before that time. And Afterdrop itself isn’t a myth, as is sometimes inversely claimed to sudden hypothermia, it does exist.

Cold shock response is an entirely different thing to hypothermia, it’s the bodies response to sudden cold, with gasping reflex, hyperventilation and possible acute pain in hands, feet, face and head, and even cardiac events. The biggest danger in immersion is uncontrolled hyperventilation leading to sudden aspiration of water. You gasp and breathe water into your lungs and drown.

Breathing rate increases for the first 20 seconds in cold water

This is the main reason why a diving or jumping entry into cold water for people not cold-acclimated is absolutely a stupid thing to do, and not tough or macho. This response is attenuated in cold-adapted swimmers.

Definitions of Mild Hypothermia can vary depending on where you look but a core body temperature of between 35° and 36° when body-normal is 37° is a good measure, i.e. a drop of about two degrees is a good indicator. The hormone ADH, (anti-diuretic hormone) which controls urination in suppressed and some blood volume is shunted to the core so there is a decrease in blood volume and some dehydration also. There are no long-lasting effects of mild hypothermia, (such that it can be used as a medical procedure for brain protection during certain operations). Almost every serious open water swimmer in these waters will have experienced it as completely normal, and the body acclimates and adapts as we have seen before, by blunting initial response, reducing stress hormones, and increasing brown adipose tissue.

However, people with any diagnosed cardiac problems should avoid cold water swimming.

And also as we’ve often discussed previously, mild hypothermia leads to peripheral vaso-constriction, the reduction of blood flow in the periphery. With experienced open waters mild hypothermia is the completely normal and usual state, in Irish and UK waters. The swimmer will still be able to talk and will still retain motor control in the fingers, but often with reduced dexterity. Surface temperature will be decreased.

Mild hypothermia will of course lead to more severe hypothermia shgould the swimmer continue to be immersed or unprotected. Hypothermia will eventually result for everyone in temperatures under twenty degree is they stay swimming long enough.

There are no great  concerns in recovering from mild hypothermia, just get dry and dressed quickly, following the usual procedure of dressing the torso and head first, and warming up with a walk. Do NOT vigorously dry the extremities even in mild hypothermia.

In diagnosing mild hypothermia, simply seeing if there is some chattering or shivering out of the water. In the water is more difficult, but the swimmer might have clenched jaws and have a minor difficulty speaking freely, or maybe report lesser claw-like symptoms in the hands (lessening on full hand motor control).

In the next article we look at Moderate Hypothermia.

How we FEEL cold water (

Peripheral Vaso-constriction in pictures (

Where did my CLAW go? (

Extreme Cold Adaptation in Humans Part 1 (


9 thoughts on “How To: Understanding hypothermia in swimmers – Mild Hypothermia

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  2. Yes, very useful. It is confusing! Today I went swimming for close to an hour November 18; the air temperature was 7.222 Celsius, I’m not sure on the water, but it’s fresh at probably around 10 degrees; and I experienced new symptoms from cold for the first time, (I’ve been swimming everyday starting October 27th). I just want to make sure I’m not doing anything permanently damaging. It’s reasurring that mild hypothermia can be recuperated from just fine. I also read your posts on the CLAW. I haven’t had the Claw, but my hands were really cold today, painfully so after about 10 minutes; it’s always hard to zip or unzip when I get out, and today I had a hard time using my hands to get my flippers off; and my mouth began to feel funny about 10 minutes before I got out. The burning cold on the rest of my body was like always. Are any of these symptoms of mild hypothermia? My hands were red and painful, but seem to be fine after I’ve warmed up. I realized I couldn’t check to see if my lips were blue, cause there is no mirror. So I was just wondering if anyone has any more thoughts on this. Your blog on getting used to the cold, maybe my hands will get used to it more? Also, I do the freestyle, not the crawl. Is it better for my hands to stay in the water, or every ten minutes or so have them out for a few minutes, or does that make them colder? Thanks so much for this great blog Donal!
    Gazelle on the Willamette River near the Oregon coast.


    • Hi Gazelle,

      At water temps around 10C, you are unlikely to do any permanent damage like the nerve damage possibilty of Ice Mile swims. Mild hypo is so common here, we don’t think too much about it, and I’m the only one who use the term, because I think it’s important to recognise it for what it is, whereas the others prefer to underplay it. But it is what it is. Yes, those all sound like mild hypo, some slight variations between people, like muscle spasms in the lower back for some. I’ve never checked my own or anyone else’s lips to be honest, I don’t think there’s anything to be learned from it.

      Hands are difficult because in cold the thermoceptors are really affected, second only to the face in pain, but you also have cessation of blood flow. In or out of water is not straightforward. heat is lose more quickly in, but having just your hands out won’t do anything except stop you swimming and allow your whole body to get colder more quickly. Get in, swim, get out, get dressed quickly. Try to avoid stopping.


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