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Anyone swimming in Outer Banks waters during the winter will be exposed to conditions that will challenge your very soul to cope with cold and wet. The Atlantic is a cruel master but it’s also an excellent environment to build physical and mental skills to understand the processes, and limits, of your body and mind.


First, blood vessels at the skin surface close down, or constrict. This does two things:

  • Less blood goes near the surface of the body so that less heat is lost to the outside.
  • More blood goes to the “core” or the center of the body to keep the brain, heart,lungs, liver, and kidneys warm. This means fingers and toes tend to get cold and are more susceptible to frostbite and injury.

The next step is shivering. Shivering is reflexive regular muscular contractions and causes internal heat production. Shivering can only last for a short time before exhaustion occurs. With shivering you will either warm up, as usually occurs when you build a fire or find shelter, or continue to get colder and start to become hypothermic.

The biggest statistical danger for injury from the cold comes from frostbite and other medical conditions like trench foot but hypothermia is the number one killer of people in a survival situation.

How cold is cold?

Your body temperature right now is somewhere between 96.5F and 100.4F degrees, 98.7F being the norm and regulating that temperature between this fairly narrow range is critical. (When we exercise, the body temperature can quickly rise to about 102F, and can even go to 104F with few ill effects if the conditions are temporary.)

The body regulates temperature in a number of different ways. In heat, its through sweating, sending blood to the skin etc. In the cold, the opposite happens – you shift blood away from the skin, you shiver and release hormones that help keep the temperature up. The body is naturally insulated by skin, muscle and fat, and it’s no coincidence that a lean athlete with low body fat is likely to get colder sooner.

A drop in core body temperature is rare on land, even in very cold conditions, provided you are not forced into prolonged exposure and cannot find shelter. The main factor predicting body temperature is metabolic rate. If you are moving, your body  temperature will rise, regardless of how cold it is…up to a point. Your body temperature will only fall if you lose heat faster than you produce it. When you are in cold conditions, the fact is that you’re usually:

  • Prepared. You are wearing, or have access to, warm clothing and waterproof equipment. You are not likely to lose heat too rapidly and your body temperature is a function of heat production. In this case, cold is a function of perception and you can regulate your body temperature.
  • Not Prepared. You are dressed inadequately, wet or not waterproof, and possibly with skin exposed. Or, you have emerged from the sea/river/snow having just experienced an unforeseen event. In this case, you’ll feel cold immediately or within minutes depending on the temperature and will be unlikely to do anything other than attempt to build a fire or find shelter on the spot to stay alive or not lose body parts.

The point is, the sensation cold, mediated through your skin over time, dependent on temperature, precedes the possibility of hypothermia. Provided you have free-will (and choose to exercise it), you will be able to get warm as long as you have the resources to get dry and can make/find shelter. If you are unlucky enough to be forced to spend a good deal of time exposed to the cold (boat accident, injury on the run/ride, getting lost, evasion, plane crash, etc.), your physiology is going to be challenged. Remember, the more you know about using your environment, the fewer resources you may need.

Wet Vs. Dry

There is a big difference between cold air and cold water exposure. The key difference is that heat losses in water, or when you are wet, are much greater than when dry. You are far less likely to develop hypothermia on land because you can typically “get dry” within a period of time that will prevent lower core temperature. But because water conducts heat about 25 times better than air, especially when it is moving across your skin as in a river or ocean, you are going to have a bad day unless you can get dry quickly.

Perhaps the the most surprising fact about cold water physiology, is that your body has heat stores to prevent hypothermia in water for about 20-30 minutes, no matter how cold the water is. In other words, it is not possible to get so cold that you are in danger unless you are in the water for more than about 30 minutes. The graph below shows this:

Cold Water Exposure

From this graph, you can see that even at water temperatures of 32F degrees, 30 minutes falls within the marginal danger zone, not the lethal zone. Many would probably survive for close to an hour – this is demonstrated by shipwreck victims, who have survived freezing water for this long.

Three critical phases of cold-water immersion:

First 5 minutes – immediate shock You will experience the ‘gasp reflex,’ which is the sudden gasp of air as a result of the shock. You will be unable to hold your breath, and hypertension and increased cardiac output will result. Most casualties in this phase are due to drowning or a heart attack, even before hypothermia can begin to set in.

Next 15 minutes – inhalation of water You won’t be able to keep afloat or swim, and your ability to grasp or climb into/onto things will diminish. Typically, most casualties in this phase are due to drowning from excessive inhalation of water.

30 minutes – onset of hypothermia 98.7° C is considered normal body core temperature. When your core temperature drops to 95.1° C, muscle tone starts to become affected. Most people have experienced this feeling of tension in their back and neck when they’ve become chilled.

The cold-shock response

One of the first things experienced when submerging in cold water is “cold-shock response.” This is characterized by an uncontrollable gasp for air, followed by hyperventilation – rapid breathing. This response is one of the most likely causes of death in most cold-water immersions such as when one falls out of a boat into icy water. If the timing of your “gasp” is wrong, you’ll take in a lungful of water, and one or two gasps while underwater is all it takes to drown.

Another big killer is a heart attack, which can result when the temperature of the blood returning to the heart is suddenly cooled – this can affect the electrical conduction within the heart, causing fibrillation.

Swimming in the cold

Once you’ve overcome cold-shock, the next thing to worry about is swimming. Hyperventilation doesn’t necessarily stop with the initial shock, cold water has a profound effect on the ability to swim in an efficient manner. Breathing goes up from about 16 breaths per minute to 75 breaths per minute within the first 20 seconds. It then stays up at 40 breaths per minute for the next few minutes. It is not difficult to see how that would affect your ability to swim, because your stroke rate would have to change substantially to allow you just to breathe.

The Titanic Problem

When muscle and the skin are cooled, the muscle becomes weaker. So cold water on the skin will make a powerful swimmer incapable of swimming, simply because his skin is cooled. There is evidence from studies that shows that the ability of the muscle to produce force is as much as 25% lower immediately after exposure to water at 10 degrees celsius – this would only drop in even colder water. Then we add to that the fact that as you get cold, your body’s natural response is to shiver. But when you shiver, your co-ordination is affected, making it even more difficult to swim.

This has profound implications on ability to swim. The character played by Leonardo DiCaprio could not swim to safety in the movie Titanic because of this physiological explanation – he simply could not swim; his skin and muscles were too cold to contract normally. The principle remains – a good swimmer in warm water will be an average swimmer in the cold. And a weak swimmer…Sayonara.

Adapting to cold shock

The good news is that humans are adaptable organisms, and just like we make adaptations to things like marathon training, we also make adaptations to stressors such as cold-water immersion. The data show that repeated (as few as six) exposure to cold water as short as three minutes in a 50F shower will attenuate the cold-shock response by as much as 20-30%. If you have even longer exposure to cold water in training, you can bring it down by 50%. That is obviously a significant reduction, and the implication is that swimming will be far easier if you are simply adapted to the cold.

The second important adaptation involves blood flow and heat loss. When at rest your muscle tissue actually acts as in insulator. This changes when you exercise because now you are pumping lots of blood to the working muscles, and it is the blood that transports heat around the body. Therefore when you start to swim in cold water you send more blood to the muscles, and all this does is increase your heat losses as now the blood, and the heat it contains, close to the surface of the body. Since water conducts heat very well, the heat from your body readily moves to the water, and the consequence of this is a decrease in core temperature even though you are producing some heat with your muscle contractions.

Decreases in shivering

Another big change that occurs with repeated cold-water exposure is the lowering of the “shivering threshold,” or the temperature at which we begin to shiver. The bonus of shivering is that we produce heat as our muscles are contracting, although involuntarily. The downside is that when trying to perform a complex movement such as swimming (or motor skills like tying a knot), shivering can really foul things up. If you can adapt by lowering the temperature at which you begin to shiver, the result will be swimming/working longer before being hampered by shaking limbs and uncontrolled movements.

Evidence for non-shivering thermogenesis?

There is evidence that humans can actually increase their core temperature either acutely or chronically in response to repeated cold-water exposures. The net effect of this response is that they can then remain in a cold environment for much longer before suffering any detrimental effects of the exposure, such as decreased nerve conduction velocity and then shivering (and a loss of coordination as a result of that shivering). Simply put, they have more heat in their bodies, and together with the other adaptations we mentioned above it means they reach a critically low temperature much later than someone who is not adapted to the cold.

The message here is that cold-water exposure is just like any other “stressor” or training stimulus. Our physiological response to these stimuli is to make adaptations that allow us to cope better, in this case is cold-water immersion.

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