OSS cold water information: (Includes a new section, what does cold do to your body? by Dr Mark Harper, OSS Cold Expert)

The OSS pledge to 'embrace the rejuvenating effects of cold water' is a valuable mindset change for any open water swimmer. At first it may seem inconvenient that British waters are not warmer, but you may soon find you're addicted to the fresh feeling of natural water.

The good news is that it's relatively easy to acclimatise yourself to water temperatures in the UK, by nothing more complicated than swimming regularly in them.

The human body acclimatises to cold water so well that hundreds of swimmers cross the channel every year (at around 16 degrees, for 10 hours or more, any untrained swimmer would get hypothermia). Over seventy swimmers have done the Ice Mile (one mile in water at 5ºC and below wearing only a swimsuit, hat and goggles). A few swimmers have trained themselves to swim in the Artic.

Getting yourself into a situation where you enjoy summer swimming, particularly if you're prepared to start out with a wetsuit, should not take long.


First a note of caution; if you have a heart condition or asthma, see your doctor before taking up swimming outdoors. And ensure you read the section by Dr Mark before leaping in. Then...

  1. TAKE THE PLUNGE! Much of the acclimatisation process is mental - knowing the moment of immersion will feel cold, and embracing it anyway. Don't jump into really cold water unless you're acclimatised.
  2. EXHALE AS YOU GET IN. In cold water the ribcage contracts, which leads many swimmers to feeling they can't breath. Exhale and the next breath will come naturally in. Shrieking, grunting and fwaw-fwaw-fwawing for your first strokes are perfectly natural accompaniments to a wild swim.
  3. WAIT 90 SECONDS. The pleasure of open water might not be immediate. Give your body a little time to react, and soon your circulation will start charging around and you'll feel alive. 
  4. FIND A ROCK OR TREE TO SWIM TO. Don't just jump in and think about how it feels, as the answer is likely to be 'cold', even unpleasant (particularly in wetsuits, where the expectation of warmth makes the cold dribble in around the zip particularly cruel). Set your intention (to swim to x), and then get in and do it. You'll feel good once you get moving.


The main safety risk you face as a wild swimmer is getting too cold. 

You get in , and after a few minutes of feeling uncomfortable the water feels pleasant. As a novice swimmer, you then attempt to cross the lake, but half way across start feeling cold again.  Your body continues to lose heat, blood shunts to the core to keep organs warm, muscles slow, arms and legs become weak, and swimming becomes increasingly difficult. i.e. you are "in difficulties" and are in imminent danger of drowning.

The media often report water related deaths as if they are mysterious, as if we can't predict what will happen when we're in it, and therefore we should just stay out. This isn't true. We know how cold impairs swimming, and we can moderate our risk by:

  1. Wearing a wetsuit, silicon hat, maybe even booties and gloves depending on the time of year (see our wetsuit Q&A)
  2. Swimming close to the shore
  3. Acclimatising

While outdoor swimming does have risks, there are relatively few swimming deaths. The 2013 National Water Safety Forum's Water Incident Database (WAID) details 381 water-related deaths in the UK, of which 59 were swimmers while 126 were running or walking by water and somehow fell in. 31 of these deaths were due to natural causes. 

Rob Fryer from the River and Lakes Swimming Association referred us to research carried out by Portsmouth University and the Swedish National Defence Research Establishment, published in the Lancet in 1999, confirms that drownings associated with cold water are usually caused by swimming impairment.


Cold water temperature varies hugely, and while this isn't scientific, anecdotally, open water swimmers experience it in bands: 

0-11 DEGREES: Freezing.

Jumping in likely to impair breathing in the uninitiated, as breath comes in big jolting grasps and it feels like someone has clamped on an ice neck brace. Water has bite, skin smarts and burns. This is winter swimming. Limbs soon become weak - 25 metres can be an achievement - and only takes a minute or two at the lower end of temperatures before skin becomes a lurid purple-orange-red when you exit.

That said, the joy of swimming without a wetsuit at this end of the temperature spectrum is the cold water high: the pure exhileration and rush of endorphins that you get from getting in. Winter swimmers frequently become addicted to it, and it is sufficiently powerful that a 1-2 minute swim can leave you feeling good all day. 

12-16 DEGREES: Fresh.

At this temperature triathlons start operating. In a wetsuit you may find you can swim comfortably for a while; if you're not wearing a wetsuit the water is fresh, doable for the brave, and not a problem for hardened open water lovers.

17-20 DEGREES: Summer swimming. 

Lakes and more mature rivers reach this temperature over summer, during hot spells. Still fresh on entry, but comfortable picnic lazy-hazy summer swimming.


You'd think that'd be a good thing, but on the rare occassions that river pools and shallow lakes reach these temperatures during hot spells, there is the odd sense that there's something missing.... the exhilerated feeling when you get out, that cold water 'tang'. On the plus side, some of you will be able to spend hours swimming without a wetsuit.

 30 DEGREES: Pool temperature.

Arguably unpleasant. Plus, as Rob Fryer comments, 'the sun is not invited'.


The secret to acclimatising is just to swim in it, often - at least once a week, and preferably two or three, gradually extending the time that you stay in the water.

It is easier to start your swimming career in summer at 16 degrees and above, and then keep on swimming as the temperature drops if you want to extend into autumn and winter. However, this is not essential.


Dr Mark Harper answers key questions.

What happens to the body when it's immersed in cold water?

Here I will highlight a number of the body’s responses to cold immersion in someone who does not regularly delight in the joys of cold water swimming.  

The first set of responses are nervous reflexes.  

An example of this (not usually related to swimming) is the knee-jerk: when the tendon below your knee is struck with a hammer (a medical one - do not try this with the kind you use to put in nails and don’t use nails either), a signal is rapidly sent to your brain and then back to the quadriceps muscles which causes your knee to involuntarily extend and kick your assailant.

Similarly, when the body senses intense cold, it sends signals to the brain which result in a ‘gasp’-effectively a (much) larger than usual breath in.  This is followed by hyperventilation - a greatly increased breathing rate.  These responses are involuntary, something that was illustrated to me when I was interviewed by Radio 4 in the River Ouse at Barcombe Mills.  Despite having explained that this would happen to the interviewer, her fast, heavy breathing is clearly audible in the broadcast.

A more obvious nervous response is that of shivering.

The other type of response to cold is neuroendocrine.  This is where chemical signals (the hormones of the endocrine system) and the nervous system (the “neuro” part) interact.

Exposure to cold water induces a stress response in the body and in common with other stress responses this results in the release of the catecholamines, adrenaline and noradrenaline.  The most noticeable effects of this are an increase in heart rate and blood pressure.  

The effect on the kidneys can be divided into two phases.  Initially the raised blood flow and pressure in the kidneys ‘squeezes’ out more fluid resulting in greater volumes of urine.  However, the second phase is an increase in blood levels of aldosterone, a hormone which tends to promote retention of fluid in the body.

The cathecholamines also tend to raise blood sugar levels.  This effect is magnified by increased levels of certain steroid hormones such as cortisol in the blood.  Don’t think that this is likely to help build muscles like anabolic steroids; in fact quite the opposite as these are catabolic steroids which tend to promote tissue breakdown.

Another important response to cold water stress is increased insulin resistance.  This is a rather convoluted term but worth understanding.  Insulin is the only hormone in the body which reduces sugar levels and its absence is the cause of Type 1 diabetes.  Type 2 diabetes, which is strongly (though not exclusively) associated with obesity, is actually the result of the body being unable to respond appropriately to insulin - in other words it becomes insulin-resistant.  This is associated with both increased sugar levels and other effects such as worse outcomes after surgery.

Raised blood sugar is appropriate for a ‘fight or flight’ situation which is the evolutionary origin of these responses but is not good for the body in the long term.

On the subject of sugar and energy levels, there was one interesting study which showed that a cold swim does increase your basal metabolic rate - in other words you could potentially eat more afterwards and not put on weight.  However, you need to be careful as they found that people tended to overcompensate and take significantly more calories than the extra they burned off.

Cold exposure also has a number of other less clearly defined effects which include stimulation of the immune system, rises in markers of oxidative stress, and perhaps even an increase in the thickness and ‘stickiness’ of blood.

How Does the Body Adapt to Cold?

Here I will discuss the ways in which the body adapts when exposed to cold water on a regular basis.

The adaptation process has not just been studied through cold water immersion. Researchers have left people in cold rooms and blasted them with cold air. They have also looked at people who are occupationally and environmentally subjected to the cold such as Quebec mailman and eskimos.  One classic study published over 40 years ago, looked at the women who dive in the waters of the Korean peninsula to harvest plant and animal life. 

As I have mentioned before, the first set of responses are nervous reflexes.  Interestingly, although we have little conscious control over these, both the ‘gasp’ reflex and the hyperventilation that are seen in unadapted individuals are reduced and even abolished with adaptation.  In fact I have noticed when I get in the sea during the winter that quite the opposite happens as I tend to hold my breath rather than increase the rate.

Although shivering still occurs in the cold-adapted, the body’s core temperature has to be significantly lower for it to kick in.

The stress response is also blunted.  This applies to adrenaline/catecholamine secretion as well as steroid levels.  The former is commonly known as the fight-flight response and I suppose the cold-adapted are more likely to go in for a fight - or swim, as we know it - than to take flight back to a warm shower or cup of coffee (we can save that for later).

Changes to the immune system are less clear.  There’s lots of talk among my fellow swimmers about how they get less illness than they did before they discovered the delights of cold water and how they get less problems in the winter than their non-swimming partners.  However there’s no real direct evidence for this.  The closest research seems to come is that the levels of some substances in the blood might suggest that, although the response of the immune system is not altered, it may be more ready to mount a response to infection.  That doesn’t mean to say, of course, that the effect isn’t there - just that it hasn’t been studied.

With regard to the kidneys, the increase in blood pressure and heart rate that ‘squeeze’ water out through the kidneys is reduced with adaptation.  However, the increase in level of the hormone aldosterone also reduces with adaptation so urine output and sodium excretion is increased.  In other words you’ll need to pass urine more frequently.

In the previous article I tried to explain the effect of cold on insulin.  With adaptation it is, again, the level of insulin resistance that is significant.  In fact insulin levels actually decrease but because there is an even more marked reduction in insulin resistance blood sugar levels will tend to be lower.

And the other bits and pieces?  Certain markers of oxidative stress, blood ‘stickiness’ and levels of the stress hormone/steroid cortisol are all reduced in adapted when compared to non-adapted individuals.

General Advice on Swimming in Cold Water in December

It's impossible - and not my place - to give you advice on what you should and shouldn't do. However I can give you some background and maybe even wisdom.

Unless you have a heart problem, have drunk too much alcohol or are over-hydrated, the chances are that nothing untoward will happen if you just get on and jump in.

However, there are things you can do to minimise the effects and the small risks that remain.

Most people who die in cold water do not do so because they have a heart attack.  The thing that does for them is the 'gasp' reflex.  This is a reflex whereby as soon as you dive/jump into cold water you have an irresistible urge to immediately take a huge breath (see above).  If you are underwater or in amongst big waves this means filling up your lungs with water and, thereby, drowning. To add to this is the fact that, unacclimatised, you will not be able to hold your breath.

Wearing a wetsuit may reduce the magnitude of this response.  You could also try 'preconditioning' yourself. It would probably not require more than half-a-dozen practice 5 minute dips to produce this adaptation. If you go down this route, make a conscious effort to note your breathing pattern and how it changes as you become adapted.

Other things that may help include ensuring that you are as warm as possible before getting in, or having a short 'pre-dip'.

But overall an acclimatisation programme would probably be your best bet.

Cold water swimming and high blood pressure

I have been asked by a number of people about the effects of cold-water swimming on high blood pressure.  Although I cannot give individual, medical advice, I can give you some facts and some personal opinions.  First, I will outline the complications of high blood pressure and why we treat it.  I will then describe the effect of cold-water immersion on blood pressure and how long-term acclimatisation can affect the response.  Finally I will try and bring this all together.

Why do we treat high blood pressure?

High blood pressure (hypertension) usually has no obvious symptoms and, unless completely out of control, doesn’t cause any day-to-day problems. The reason we look for it and treat it is because of its long-term effects.  These problems result from damage to your arteries caused by the stress on their walls as a result of the constant high pressure.

Blood, and therefore oxygen and nutrients, is pumped by the heart through the arteries to the vital organs and tissues.  Chronic (i.e. long-term) high pressure in this system can result in weakening of and damage to the arteries themselves.  This damage to the arteries restricts blood flow to the organs and the high pressure can cause damage to their delicate tissues.  The consequences of this include:

  • damage to the heart including heart attacks and heart failure
  • damage to the brain including stroke and dementia
  • damage to your kidneys
  • damage to your eyes
  • sexual dysfunction.

(see: http://www.mayoclinic.org/diseases-conditions/high-blood-pressure/in-depth/high-blood-pressure/art-20045868 for more detail)

So far, so disturbing.  But the good news is that treating high blood pressure can prevent all these problems.

What does cold-water immersion do to your blood pressure?

On immersion in cold water:

  • Your heart rate goes up: In those people not acclimatised to cold water, this occurs within 2-3 seconds and is an increase of around 20 beats/minute.
  • Your blood pressure rises: in one study average blood pressures went from 130/76mmHg (fairly normal) to 175/95mmHg (would need treatment though unlikely to cause any immediate complications)

These responses are significantly reduced in people who regularly swim in cold water.  Although maximal adaptation probably occurs after several immersions in 10-14˚C water, there is significant adaptation even at 18˚C i.e. the temperature of the sea down here in Brighton during the summer.

There is therefore very little risk for someone with a normal blood pressure - and this includes people with well-controlled hypertension.

Problems are only likely to occur in those with coronary artery disease because the increased heart rate means less blood goes to the muscle of the heart itself while, at the same time, more strain is being put on the heart.

The only other group of people at increased risk are those with abnormalities in the arteries that take blood to the brain.  These are very uncommon and you are unlikely to know anything about it even if you do have this condition.

So what are the overall conclusions?

The risk of swimming in the sea to people with treated hypertension is probably no different to that of those with normal blood pressure.

The risks are even lower if people are acclimatised to swimming in the cold.  My advice would therefore be to start your outdoor swimming when the water is at its warmest.

And, of course, it’s important to put it all in perspective.  The chances are that the tiny risks are significantly outweighed by the benefits of exercise and cold-adaptation which include better mood and better metabolism (especially with regard to the way the body handles sugar). There is even evidence to support the theory that cold adaptation-which can be achieved through open water swimming-may actually reduce the overall risk of a heart attack.

Remember that this is only a very general overview so, if you have any concerns about your personal situation, please visit your own doctor before throwing yourself into cold water.  

You can contact Dr Mark with any cold-related questions by email at