‘Tis The Season Day 6 – Cold Water Effects


Today’s entry refers back to day 4. Open water swimming can be dangerous but if you take the time to understand the risks you can make it safe and hugely enjoyable.

The human body is amazing. A highly evolved piece of biology that can withstand a myriad of things we throw at it on a daily basis. When it comes to dealing with the cold, our body’s metabolism maintains a steady body temperature which, in turn, allows all of the chemical reactions in our bodies to keep on working and keep us alive.

But when we submerge into cold water a few things can happen to that amazing piece of kit we call our body.

Cold water shock

Upon initial submersion, as our skin temperature suddenly drops and we can be hit by cold water shock. Our heart rate, blood pressure and breathing rate increases. We can experience a sudden and uncontrollable intake of breath. We start to hyperventilate. Entirely unable to control our breathing, panic can start to set in which, in turn, increases hyperventilation further.

You may have experienced this when you’ve got into a cool swimming pool or waded into the sea on a hot summer’s day. This sudden shock to the system can be deadly to anyone with underlying heart conditions. The sudden intake of breath really speaks for itself if it happens when your face is under the water.

Despite that rather scary explanation, with careful planning, acclimatisation, being mentally prepared and accepting that you need to enter the water slowly it can be overcome. Cold water shock usually passes within a couple of minutes at which point you’re ready to swim.

I feel I should also say here some people don’t seem to experience cold water shock. The Short One has always had the extraordinary ability to just get straight in, stick her head in and swim regardless of the temperature. I just can’t do it. I’m better than I used to be but over winter I still experience mild cold water shock and have to let my body acclimatise before I can swim properly. She’s the only swimmer I’ve met so far who’s like this. I think she’s come kind of biological anomaly!

Cold incapacitation

When we get cold our body draws heat inward to maintain function of our vital organs in our core. We can’t live without those. We can live without a finger or two. So, when we submerge in cold water, as our body starts to cool, it begins to restrict blood flow to our arms and legs thereby keeping the warm deep inside. Without as much fresh, plentiful, oxygen rich blood coming to the muscles in our limbs they can’t function as well. With reduced function in our limbs we can’t swim as well any more. If we’re caught out in the middle of open water when this happens the outcome is inevitable.

Cold incapacitation can happen quickly, so unless you’re very experienced and confident in your body’s ability to function in cold water it’s always best to stay keep it short and stay very close to shore. The Short One and I never swim too far from shore in the winter and, despite our regular temptation to stay in “just that bit longer” we always get out when we still feel warm (ish) and feel like we can still swim well.

In the past we’ve both felt that being “on the edge” moment, where cold incapacitation might kick in but we’ve always been close enough to shore to not let it become a problem.

Sadly, the two effects above are often the reason behind the tragic stories we see every summer. That young person who loses their life in a reservoir or the sea. It’s rarely because they’re a bad swimmer, rather it’s often that they didn’t understand what cold water could do to them.


Hypothermia. The biggy that everyone has heard of. Hypothermia is a reduction in our core body temperature. If our body temperature drops those basic functions of biology stop working. Organs start to fail, our brain starts to shut down. We can become delusional, even convinced that we’re hot and need to take layers of clothing off to cool down. Hypothermia is a risk but it’s often cold water shock and cold incapacitation that will stop you swimming first. It’s still a risk to open water swimmers, if you start to actively shiver after feeling comparably warm in the water it’s time to get out and get yourself warmed up. Don’t take the risk of getting even colder.

I’ve never come anywhere near hypothermia. Last spring however, the Short One did. Not badly but we’d misjudged a swim, the cold, and the wind and by the last few hundred metres of a swim she was shivering and her stroke had slowed significantly. We were very lucky that day. We were swimming together, keeping an eye on each other and we knew how to warm up properly afterwards. Still, it taught us both an important lesson that, regardless of how experienced we are we still need to assess the risks and adjust our swims accordingly.

Whilst ’tis the season is about winter swimming, each of the above relate equally to winter and summer. For anyone who isn’t used to a cooler water temperature (remember swimming pools are usually around the 26-32C mark) they are likely to feel these effects even in the height of summer.  I certainly did when I first started open water swimming a few years ago.

If you want to read more on this check out the Outdoor Swimming Society’s extensive webpages on cold water swimming.

Outdoor Swimming Society – Cold Water

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