The original “Cold Acclimatisation” article.


Cold-water acclimatisation by ”Wildswimmer” Pete Roberts

Recent years have seen the emergence of the somewhat arduous sport of winter swimming in the UK. The desire to keep swimming outdoors without a wetsuit through typically cold, depressing British winters? Quite likely. And it’s a proven fact that winter swimmers have a higher resistance to infection, a significantly lower incidence of cancer and heart disease, together with an extended life expectancy. But isn’t jumping into cold water dangerous? What about shock and hypothermia? From what I can find, all research on the effects of immersion in cold water is posited on the “victim” having involuntarily entered the water as the result of some mishap. Very little if any of which applies to the situation where a Speedo-clad, cold hardened swimmer enters the water voluntarily. Indeed many authorities hold that water temperatures around 13C (mid-50s F) are particularly dangerous, water temperatures that seem almost tropical to swimmers hardened to the British cold.

Basically we need to put aside such “received wisdom” and start out with a clean sheet. The first point to remember is that hypothermia is not the greatest risk. Far from it. The average adult human body holds enough heat to maintain core temperature during immersion for anything up to thirty minutes before approaching borderline hypothermia. Yes, hypothermia is the silent killer, but not where a swimmer has deliberately entered cold water and in a position to exit when he wishes, dry, dress and have some warm drink or food. The real killer is cold shock, otherwise known as sudden immersion syndrome. Typical symptoms are a sharp gasp followed by uncontrollable hyperventilation, a galloping pulse and dangerously high blood pressure which in extreme cases can lead to cardiac arrest. Hyperventilation can disrupt limb co-ordination leading to the risk of drowning from loss of swimming ability. Pretty nasty. However cold hardening reduces or even suppresses the cold shock response and it must be borne in mind that cold shock is a physiological, rather than psychological process. To avoid confusion I’m going to refer to a winter swimmer’s cold training as “cold hardening” because “cold acclimatisation” has several different meanings.

A winter swimmer can swim unprotected in water so cold it would kill an non-hardened individual in minutes because of two factors:
1.) Suppression of the cold shock response.
2.) Physiological changes to the body which reduce loss of heat to ambient, while increasing the rate of internal heat production.
Cold training involves three major physiological changes. Firstly, the body lays down a layer of subcutaneous fat which acts as an internal “wetsuit”. Its only disadvantage is that you can’t take it off during hot weather! However white fat isn’t the only factor. As infants humans also carry significant amounts of brown fat which mostly disappears as we reach adulthood. However it’s been found that adults regularly exposed to extreme cold regrow their brown fat, and in swimmers the brown fat is thought to be concentrated around the heart and great blood vessels in the chest. Brown fat doesn’t act as insulation, far from it. In fact it’s now been found that brown fat (or brown adipose tissue) isn’t fat at all, but a modified form of muscle tissue whose sole function is to generate heat. When the body is cold stressed brown fat begins to “burn” the body’s stores of white (lardy) fat. Brown fat metabolism needs large amounts of oxygen which will result in respiration rates somewhat above those needed for the accompanying physical effort. The mammalian body has two responses to cold stress, the primary being brown fat metabolism, the “back up” process is involuntary muscle activity: shivering. That’s why many winter swimmers don’t shiver after exiting cold water.

Secondly, the vascular system also undertakes changes primarily intended to protect the body’s core from chilling, endangering vital organs. It develops the ability to very quickly reduce (but not completely close down) circulation to the outermost tissues, hence the cyanotic blue or purplish hue of a cold-water swimmer’s skin after a few minutes’ immersion. It must be be borne in mind that the longer the swimmer is immersed, the deeper the chilling which means that upon exit he has to behave as though he does in fact have hypothermia. The reason? The circulation to outer tissues is much reduced with the accumulation of significant quantities of pooled chilled blood. Should this cold blood suddenly enter the core circulation it can result in cardiac arrest. The phenomenon is well known as “cold drop” or “afterdrop”. To minimise any risk of afterdrop it’s vital that after leaving the water, the swimmer dries and and changes as quickly as possible, then allows the body to warm of its own accord. External heat from car heaters or hot showers can fool the body into prematurely opening up the main circulation, dumping large quantities of chilled blood into the core with possible dire consequences. Don’t go for a brisk run after a cold swim either, as exercise can have the same effect. Best way to warm up is to relax in a coffee bar or local pub with a warm drink or two, but no alcohol until fully warmed up. While on the subject of alcohol it can’t be emphasised too strongly that alcohol and winter swimming don’t mix, so no shot of rum beforehand. As well as its other detrimental effects on swimming ability, alcohol seriously impairs the body’s ability to regulate its internal temperature.

Finally, metabolism. Many winter swimmers have significant hunger after a very cold swim which implies that their metabolism is running faster, burning calories to help maintain the core temperature. In fact cold-hardening appears to adapt the body to prioritise heat production. Given that there is a finite limit to the maximum amount of energy a given individual’s metabolism can produce, it’s important to remember that in very cold water the lion’s share of the body’s resources are put towards maintaining core temperature which results in the inevitable reduction in swimming “power”. The swim muscles have to make do with what’s left over. This is under autonomic control so there’s nothing the swimmer can do about it and attempting to “swim through” the reduced performance could result in cramp or other damage. Best stick to slow recreational swimming. Another odd phenomenon is the rapid reduction in core temperature upon entry into very cold water. This isn’t due to direct cooling but a deliberate response by the body turning down its internal thermostat by up to 1C. There was obviously some evolutionary advantage for this but I can’t see what, unless it’s to reduce the thermal gradient between core and ambient.

So, how do you go about cold training? You will need the following: swimming cozzie (1 off), swim cap (1 off), goggles (optional), a reasonably healthy body (1 off). Oh, nearly forgot – water. Forget cold showers or baths, cold-training takes full immersion so suitable water is the sea, rivers, lakes and unheated outdoor swimming pools. Simply start swimming in open water in summer when it’s warm. Come Autumn, just keep going. The golden rule: don’t overdo it – there’s always tomorrow. When you feel cold, get out, get dressed and warm up. You’ll find your tolerance, both time and temperature, will steadily improve. There’s a well-known rule of thumb: when you feel warm and start enjoying yourself, it’s time to get out! You need to keep your head warm and dry so it’s best to forget the crawl which I consider completely unsuited to cold water swimming. Head-up breast is best by far. It’s very unlikely that a winter swimmer will stay immersed long enough to become hypothermic but typical symptoms are confusion, slurring of speech, lassitude and violent shivering. Hypothermia generally occurs after accidental entry into cold water with no means of exit or warming afterwards.

There’s no need to bathe in cold water every day – once a week is enough, and even then for just a few minutes. Cold-hardening is a long-term process. It took two winter’s training before I could swim all through the winter in water down to 8C. Cold-hardening turns out to be cumulative and after a few years’ winter swimming I can now withstand fairly lengthy immersion in water at 0C, literally freezing. The good news. Swimming in a heated pool between cold-training sessions doesn’t interfere with cold-hardening. Nor does a swimmer lose his cold hardening over summer. Indeed the physiological changes associated with cold-hardening appear to be permanent and irreversible. Just like swimming itself – you never lose the ability.
Don’t start cold training until spring/early summer when the water temperature is at least 15C. Never jump or dive into water below 15C (59F). Always wade or lower yourself in – this gives your body time to switch into “cold mode”. Avoid having cold water enter your nose. There is very little bone between the nasal cavity and your brain, and chilling the base of the brain can result in cold shock. While on the subject of cold shock, after training you will find its effects much reduced or in many instances, absent. Always wear a swim cap, as a surprisingly large amount of body heat is lost through the head. If swimming head-up breast you can’t beat a woolly beanie – very popular among Finnish winter swimmers. Even something like the neoprene balaclava I use in cold water. Winter swimmers do lose most heat through their heads as despite being almost naked their bodies are insulated by that ”swimmer fat” layer that’s every bit as effective as a wetsuit. Extra visibility needed? Wear a brightly coloured cap on top. I’d strongly recommend a pair of 3mm neoprene gloves as not only will this make the swim more comfortable, plunging bare hands into icy water can trigger cold shock. A pair of 5mm neoprene boots will protect toes from cold as well as the feet from injury. It’s best not to train alone and keep within your depth. Cold training puts the body under considerable stress and cold water can also cause cramp.

Health problems needn’t prevent you from cold training, but should you have a health problem you should seek the advice of a health professional. Those suffering from asthma, heart problems, epilepsy and diabetes need to take particular care. Ten years ago I was laid low with congestive heart failure, but my cold training put the condition into remission – I’m still here! However it was done under medical supervision. So come in and join us – the water’s great! The “Feel good buzz” is unbelievable. And very addictive!

© 2008 – 2013 Pete Roberts

How I was taught to swim.

I’ve always liked playing in water but for several reasons I wasn’t taught to swim until 1965 at the age of 14.  One reason was because I was brought up on the Wirral’s Dee side and the only swimming pools on Wirral were on the Mersey side.  At the time I was attending Calday Grange Grammar School in West Kirby (bear in mind during the Sixties  the political and educational situation was very different from now) and the school managed to persuade Cheshire Education Committee to finance an indoor heated swimming pool.  However funding was only given for materials with staff and parents supplying the labour.  Furthermore the pool had to be shared with other local schools.

Winter ’65 was long, cold and snowy with field sports snowed off and only cross country running possible.  Very unpleasant wearing just shorts and Rugby shirt in the teeth of an icy gale.  However one cold, grey February morning those of us who didn’t have to attend morning prayers were called into Assembly – we all thought we in for a bollocking over something or other.  We all sat down with the others, the Head looked us all up and down and said “Stand up all of those who can’t swim.”  The non-swimmers stood up, myself included. “Those who want to learn, keep standing.  The rest, sit down”.  No-one sat down!  I thought all my Sundays had arrived together!  If looks could kill we’d all end up as smoking piles of ash!  Learning indoors in a warm pool rather than running in the wind and  snow!*

My weekly Games period morphed into something that was the highlight of my school week rather something to dread.  The only concern was that when we finished our course we’d be sent back to running in the snow so we took our time learning.  It wasn’t to be – we had become the “elite” and kept on swimming, in my case all the way until 1967 when I finished school.  My parents were also pleased as they were spared the expense of Rugby and cricket kit as from then on my only requirement was a pair of Speedos and a towel.

Calday’s pool is still in use half-a-century later but is now run by a trust and mostly  patronised  by local schools.

*Ironically now I’m cold-hardened I can swim outdoors in the ice and snow in just Speedos however given the state of my heart cross-country running is out of the question.

Yes, I’m officially a cyborg!

I’ve been banned from swimming since April – the reason: after several years of chronic heart failure  I’ve been fitted with an implanted cardioverter/defibrillator by the amazing team at Liverpool Heart and Chest Hospital.  The procedure involves harpooning electrical leads into the muscle of my right atrium, right ventricle and left ventricle and forming a pocket to hold the relatively hefty device (an ICD is larger and runs continually unlike a pacemaker which only paces on demand).  Until the leads have been anchored by fibrosis the arm movements involved in swimming could dislodge them.  For six weeks I couldn’t even lift anything with my left arm.  However come Mon. 10th Aug I attended clinic to have my ICD and leads checked and I got the thumbs up – I can now go swimming.  Fortunately I’m a breaststroker as I’m restricted to breast – the other strokes involve rotation of the shoulders which could compromise the three electrical leads plugged into my heart.  However I gave the undertaking that I would never swim alone, and if I swim outdoors I was to use a tow float.

The main risk is down to the defibrillator function. Should my device detect a possibly dangerous arrhythmia  it will shock me to restore sinus rhythm but the sensation is anything but pleasant – I understand it’s like being kicked hard in the chest, from the inside.  The shock can result in incapacitation for a few minutes which is not a good idea in deep water.  The float will support me until I recover.

My ICD is a remarkable bit of kit.  It continually records my ECG (like a Holter monitor) together with other physiological data.  Every three months or so it uploads the data to my bedside monitor via a variant of WiFi.  The monitor then sends all the data to base via a cellular connection. Should anything go wrong my device will send an alarm as soon as I get within wireless range of my monitor.   LHCH is also a BUPA hospital, and my ICD is top-of-the-range and my treatment was the same as I would’ve had if I went private.  The difference is going private the job would’ve cost me about £50,000 (my device itself costs £20,000) so I’m very grateful for our NHS.

Water safety in winter

Given the numbers now trying bare-skin winter swimming, the rules are quite different from those that apply to open-water swimming in summer.  In winter-cold water emergencies are more likely to happen, and rescue less likely from anybody on the bank, especially as summer swimming spots are likely to deserted in winter.

If you intend to swim in water at single-digit temperatures you should swim with at least one cold-hardened swimmer in the water with you.  Whilst this might conflict with received wisdom, we are talking about an extreme sport with its own rules, many of which are still being written.  In summer, should a swimmer get into difficulties then the usual rescue modes can be used and should there no be any alternative then the would-be rescuer can strip off and wade into the water.  But in winter that’s not an option: should a non-acclimatised person enter winter-cold water their survival can be as little as four minutes, assuming that the initial shock doesn’t kill them.

With two in the water one can rescue the other should things go seriously wrong.  However it must be borne in mind that it’s vital to swim within your depth – should you need to help another to the shore/bank you can only do that with your feet planted on the bed and wade or float them to safety.  Discourage any would-be rescuer from entering the water to help, as they will rapidly succumb to the cold and you’ll have another casualty on your hands.  However ask any bystanders to call for an ambulance as after anyone rescued from freezing water needs immediate attention from paramedics.   If I’m mentoring another swimmer I usually stand up to my neck in the water so I’m getting the benefit of full immersion, but both of my feet remain firmly planted on the bed,

In winter we have to be self-reliant  because emergency services personnel can’t enter winter-cold water as they do in summer – they may have to bring up special equipment such as dry-suits and, possibly, a boat.   It’s best to “think safe” and not allow an emergency to develop.

To recap: If swimming outdoors in winter:

1.) Make sure there are at least two cold-hardened swimmers in the water at all times.

2.)Keep within your depth and keep an eagle eye on any other swimmers.

3.) Only swim in spots where you can wade into the water, and wade out.

4.) If you begin to feel warm, get out, that’s a sign of incipient hypothermia.

5.)Don’t swim in winter-cold water unless you are 100% fit – bare-skin winter swimming puts quite a strain on the system.

In winter if you don’t respect the water, you won’t get a second chance!

Some advice about hypothermia

Extract from a post by Jackie Cobell  on the UK Winter Swimmers Facebook page:

“You should be under no illusion about swimming in very cold / ice water . I hear swimmers desperate to complete their ice mile , as if it’s a right of passage to becoming a super hero . You too can be one , ONLY if you have been swimming / training in cold water for at least 2 or 3 winter seasons , perhaps twice a week , in a safe location with like minded swimmers that have safety / your safely , as well as their safety , and of their club in mind . Please swim with other people / swimmers that know , recognise how to deal with swimmers with all degrees of Hypothermia . Wildswimmer Pete has put up a very informative blog , please read it !
Swimmers training for an ice mile , WILL become hypothermic ! There is no getting away with that ! Depending on the amount of training you have done , you will experience mild , to moderate , after 2 to 3 seasons ….. Moderate to severe after 1 season …. Being pulled , very severe hypothermia , risk of drowning if this is your first season . Don’t think because you are a fast swimmer , you will finish in the time it takes you to do a mile in the pool ….. Add another 10 minutes or so . But for every season you swim your times may be faster as you adapt to swimming in cold water .
Remember , everyone reacts to cold / ice water differently , listen to other swimmers tips , but they are not written in stone . Recognise how you feel , and when to get out . Bad feeling / panic can come on in an instance , as can euphoria , get out quickly , calmly , remain FOCUSED , signal for help . Calmly without rushing , get help in your dressing , DO NOT lie down , or fiddle trying to get socks on ,don’t let your heart go lower than your hips , this moves the cold blood into your core quicker . DO NOT have a hot , or even warm shower , and take great care in the sauna , stay on the lower seats , wrap yourself in a towel to stop some of the heat reacting with your cold body .
If some one is severely hypothermic , unconscious , very confused , blue , again do not lie them down . Call an Ambulance . In the meantime , take off your cloths , cuddle them , and warm their main arteries , while under a blanket , dry robe . If you have access to warm water and towels , wrap warm wet towels , around torso , paying
Attention to under arms and groin area . This procedure leaches the cold away from the body , and warms the blood entering the core . This is practised in Russia , and is an excellent recovery procedure !
You take your car for an MOT every year , replace worn tyres etc , it costs money , but it’s safe motoring for your family . Please get checked over by your doctor , before you begin cold water training , as this extreme sport will show up any health problems !
I apologise for nannying peeps , I have said it all before …. We must keep this sport as safe as we can!”

Many thanks for the info Jackie, hope you don’t mind me pinching it.

I can withstand very long immersion in extremely cold water without any sign of hypothermia nor other signs of distress, however I can’t swim very far in such cold conditions which proves my metabolism is geared up to producing heat rather furnishing energy to my swim muscles.  I’m currently doing some research into the effects of the cold on swimming ability compared with the ability to furnish calories for heat production in others.  For what it’s worth I don’t expose myself to any source of heat whatsoever until I have fully regained my normal skin temperature, which is around 25C under clothing.  However I will take warm food and drink, which doesn’t stimulate my outer circulation to allow chilled blood into my core circulation.

Gloves or no gloves?

Every winter I see “sound advice” aimed at newbie winter swimmers regarding wearing gloves and wetsuit boots, some of it very risky.

If swimming in seriously cold water (the ballpark value is commonly taken as 10C downwards) then precautions may need to taken to prevent damage to the blood vessels in the extremities. Such precautions involve wearing neoprene gloves and boots.  Note the may – it depends on individual physiology.  Some medications, notably beta-blockers together with certain other heart drugs, can also act to further restrict peripheral circulation.   I do suffer from cold hands, always have done even as a young boy, so as water temps fall to 12C or so I have to dig out my gloves, boots and my neoprene 2XU swimmer’s helmet.

How do you find out if you need protection? Simple, as soon your hands and feet  begin hurting, you need gloves and boots – period.  Trying to “tough out” cold extremities isn’t “gnarly” or whatever other term you wish to use, it can be very dangerous.  Cold extremities are a symptom of restricted circulation with blue hands indicating cyanosis,  or putting it another way, oxygen starvation.   Should extremities with already compromised circulation be exposed to the insult of further chilling it can result in permanent damage to blood vessels, which in extremis could lead to a complete failure of circulation.  Everybody will be familiar with the damage caused by frostbite, images of black, gangrenous fingers and toes?  Frostbite is just an extreme example of the failure of circulation to the extremities, but similar damage can happen in water at single digit temperatures.  Such impairment might be irreversible and could lead to necrosis of affected tissues.

My feet are nowhere as sensitive to cold as my hands, but I wear boots for another reason: in cold water your feet aren’t as sensitive to rocks, gravel……..or that broken glass on the bed.  A pair of 5mm wetsuit boots don’t just protect against the cold, they protect the soles of your feet from abrasion and cuts.  Wetsuit boots are also very grippy on wet surfaces.  As for gloves, I find 5mm gloves reduce my “feel” of the water too much,  but wearing my 3mm Gul Titanium gloves let me feel the water without suffering the consequences of cold hands.  They are a pain when I try to use my camera in the water  but that’s another story.

Finally, plunging bare hands in very cold water can trigger cold shock even in those like me whose cold-shock response is fully suppressed.  Bear in mind that what many winter swimmers describe as cold shock  isn’t the same as “sudden immersion syndrome” which generally occurs immediately after sudden, unexpected immersion in cold water.


North Sea, Arbroath, Angus 23rd December ’14

A surprise urgent overnight delivery to Aberdeen, 400 miles and 8 hours.  Delivered around 9:30am and then we had a hire car to return home – a BMW as well!

Rather than returning down the A90 I asked to go via the A92 which runs along the coast to Dundee so I had a choice between Stonehaven,  Montrose,  Arbroath and Broughty Ferry.  Given the time of day and the state of the tide I thought Arbroath was the better choice.  Another factor that influenced my choice was there being a McDonalds right on the prom, for those apre cold-swim calories.

View from the promenade
View from the promenade
Meeting my nemesis, together with my thermometer.
Meeting my nemesis, together with my thermometer.

The long walk down


Nearly there
Nearly there

Getting ready to measure the temperature


Yes, I did
Yes, I did

There you are, fully in, but I didn’t venture out too far because there was a bit of undertow together a quite stiff off-shore breeze that if I wasn’t careful could blow me out into deeper water.  The sea has to be respected.


Did it!  Water temp 6C (43F)

While changing I took care to do so secluded from passers-by, but I forgot the railway line following the coast.  Just after I had  finished changing a train stopped overlooking me, while on the approach to Arbroath station.  Had I been a couple of minutes later the passengers would’ve had a right eyeful.   Note to self: despite it being fairly bulky I should carry my Robi robe to avoid encounters of the embarrassing kind.