Updated: Jan 27
Dr Heather Massey is a researcher and senior lecturer at the University of Portsmouth (Extreme Environments Laboratory, School of Sport, Health and Exercise Science). She is an expert in thermal, altitude and survival physiology. Dr Massey has conducted numerous research projects surrounding the impacts of cold water on the human body, provided physiological research to support to the Royal Navy’s fitness testing programme and is a keen open water swimmer. In this blog, she answers our questions on what happens to the body when it is immersed in cold water.
As an outdoor swimmer for more years than I care to remember, I and my swimming group have seen outdoor swimming slowly grow in popularity. However this year, we have seen an explosion in the numbers of new intrepid outdoor swimmers braving the British coasts and waterways.
This year has been tough for everyone with many facilities, including swimming pools, closed and with people furloughed due to the Coronavirus pandemic. Many who joined the outdoor swimming ranks just after lock down 1.0 have continued to swim outdoors and wish to continue their swimming adventures even now autumn is here and winter nearly upon us. This means the water is cooling quickly.
There are several questions that swimmers ask about swimming outdoors and what they should do especially when the water temperature is in decline. In this blog we will try to answer some of those questions.
At what temperature is water considered cold?
If we were to immerse ourselves in water that would maintain our normal function, that water temperature would be 35°C. There is no strict definition of ‘cold water’ however it’s often regarded as temperatures that result in a physiological response. This is somewhere between 15-20°C. These responses to ‘cold water’ include:
the initial responses (getting in),
the effect of local cooling on the nerves and muscles and therefore your strength and swimming performance,
cooling the deep body
exiting the water and rewarming
What happens to me when I get into cold water?
As mentioned above, we have a number of things happen when we get into cold water. The first is the cold shock response which consists of a gasp and uncontrollable rapid breathing, rapidly beating heart, rapid reduction in skin blood flow and high blood pressure. The response occurs as a consequence of rapid skin cooling and like many other ‘reflex’ responses does not appear to have a beneficial purpose. In fact, these changes result in greater strain on the heart and also increase the likelihood of taking water in to your airway.
The advice here is to ensure you are well, fit and healthy, before attempting to get into the water for the first time. Get in gradually and wait until you can talk in full sentences before starting to swim, as that shows you have your breathing under control.
After skin cooling, which results in a cold shock response, the next tissues to cool are the nerves and muscles. Cooling the nerves and muscles impairs function, making the muscles feel stiffer, fatigue earlier and nerve impulses slowed. The fall in muscle and nerve temperature results in a reduction in the speed of movement, strength and dexterity. The muscles and nerves of the arms are vulnerable to cooling, because they are smaller cylinders, with a large surface area. Many of the muscles which control movements of the hand are located in the forearm; therefore hand function is likely to be affected. There are frequent stories of fingers splaying uncontrollably, of people developing claw hand or having difficulty dressing themselves as they no longer have the dexterity. This is all because of nerve and muscle cooling. However, at its most extreme it can sap strength to prevent exit from the water and reduce coordination to the point where swimming strokes are not coordinated enough to maintain your airway above water.
Try to exit the water slowly, sometimes you may feel a dizzy if you try to get out of the water rapidly, in the same way you may experience if you stand up too quickly from sitting. The cold can dull your blood pressure response and make you feel dizzy and faint.
Now that you are out of the water doesn’t mean you will instantly start to warm up. In fact you will continue to cool, sometimes called the ‘afterdrop’. This period of cooling can potentially last 30-40 minutes after your swim. You can have a lower deep body temperature half an hour after your swim than when you got out the water! You may feel fine after your swim, but the cooling occurring during afterdrop can result in signs and symptoms of hypothermia (a deep body temperature of 35°C [normal temperature would be 36.5 to 37°C]), and can slow your reactions, slur speech, and give you the appearance of being drunk. This is why it is important to swim with others who will be able to spot these changes in your behaviour, as you won’t. The afterdrop is a good reason to get out before you become cold. It is important to seek shelter, dry and dress quickly to help the rewarming process. Some people swear by hot showers or a bath, but they can be problematic. Warm water will stop you shivering, so you may have warm skin and a cold body core. Swimmers will frequently leave a hot tub before they have properly rewarmed and so start to shiver once their skin cools. The combination of a long swim in cold water can dehydrate you and dull the blood pressure response, therefore having a warm shower or exiting a hot tub can result in collapse as blood pressure is not able to respond quickly enough.
Can we acclimatise to being in cold water?
We can acclimatise to being in cold water. Research shows that we can reduce the breathing response to the initial immersion by about 50% with only 5 or 6 head out, body in cold water dips.
We can also reduce the shivering response to cold water too. Repeated exposure to cold water which reduces deep body temperature can reduce shivering. Uncontrollable shivering can hinder your ability to swim, but is a form of exercise and helps to defend the deep body from cooling. By reducing this response, the body will cool more readily. The good news is, as the fall in deep body temperature exceeds that which it has previously experienced shivering will return, but closer to the point of hypothermia. So, if you are a well acclimatised swimmer and you are shivering - it is time to get out, if you haven’t already!
I have missed a few weeks of swimming will I have lost all of my acclimatisation?
The reduction in cold shock responses will remain for some time after your last dip. Research has shown that 50% of the reduction in cold shock response remains for 7-12 months after your last swim. However, if you miss a few weeks remember the water temperature is falling at present, so expect that the cold shock response may be greater and last a little longer when you return to the water. Much less is known about how long the reduction in shivering response will last for.
I haven’t been in for ages but will be taking part in a festive dip, what should I expect?
Firstly, you should only consider doing a festive dip if you are not suffering the effects of alcohol, are well and the weather and water conditions are conducive. And of course, dipping as a group is safer and more fun. If you haven’t been in the water for a while, expect to experience the cold shock response (these are the responses we discussed earlier). Get in slowly, with no diving or jumping in.
Is wearing a wetsuit cheating?
Whether you prefer to wear neoprene gloves, booties and bonnet, swim fully encased in neoprene or just wear a swimming costume or shorts is entirely up to you. It isn’t cheating to wear neoprene. Wear what makes you feel comfortable in the water and allows you to enjoy the experience. Wearing a wetsuit doesn’t stop the cold shock response, as the cold water floods into the suit as you get in. However, it might slow down how quickly you get cold, or make swimming in cold water possible. It has some distinct advantages too; you can float better in the water, swim faster and also stay warmer than skin swimmers can.
How long should I swim for when the water is cold?
There are no hard and fast rules and what works for one person is not appropriate for another. As the water and air temperatures cool and then become warmer again, ‘in water time’ will change, with much shorter swims in the winter months. The important thing is not to set time or distance goals which encourage you to stay in longer than you would want. The best thing to do is to be aware of your body’s responses to cooling: Are you shivering? Can you coordinate your stroke? Is it as efficient as normal? Do you feel vague or unusual? If the answer is yes to any of these questions it is time you were out of the water, even if your companions choose to stay in. It is also important to keep an eye on your fellow swimmers: Have they slowed down? Are they stopping frequently when normally they wouldn’t? Are they struggling to speak, when usually they are very articulate? If any of these are present it is time to take action. Get them out of the water and help them to rewarm.
I seem to take ages to rewarm, is this normal?
Continued cooling aka as the ‘afterdrop’ can potentially last 30-40 minutes after your swim. This means you can have a lower deep body temperature half an hour after your swim than when you got out the water and can take several hours to rewarm fully. The simple answer to the question is yes it can take a while to rewarm. During the initial afterdrop period, it is important that you avoid driving for your own safety and the safety of others. You may be fine when you get out of the water, but (as I’ve mentioned earlier) may develop signs and symptoms of hypothermia, which can make you unsafe to drive. This is a great time to catch up with friends, have a cuppa and cake, and watch each other shiver. If you feel comfortable, you could do some light exercise such as a gentle walk. This can increase the amount of heat you are producing and will speed up your rewarm.
When does the water start to rewarm?
The water temperature will depend on the location, if the water is fresh or salty, the amount of rainfall and also how cold the weather has been. Coastal waters can be as cold as 3°C in February and early March, whereas rivers and lakes may freeze during very cold spells. Smaller bodies of water will cool and rewarm more quickly. So as a general rule, if you have had a patch of cold weather with lots of rain, sleet or snow, expect the water to be very cold. Remember, it is always best to get into the water slowly and sort your breathing out before attempting to swim.
Typically water temperatures do start to raise by early March. However this isn’t the time to start your outdoor swimming adventures. If you have been swimming through winter and have built up acclimatisation and tolerance you will be in the water for a few minutes at most. For those wanting to give it a go, the best time to start is in the summer when the water is warmer (it is still cold, but not as cold as in spring).
In spring the air temperature may be increasing steadily, but the water will still be very cold (increases in water temperature always lag behind the air temperature). Even at the traditional start of the open water season in May, the air temperature can be balmy and the day sunny, but the water temperature still very cold.
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This article was first published to https://www.wildswimmingcornwall.co.uk/blog/what-happens-to-the-body-in-cold-water-with-dr-heather-massey