Body heat loss in cold water, for a non-exercising body, is two to four times greater than the heat loss in cold air at the same temperature. When you exercise in cold water, swimming for example, the heat loss from your skin can be 70 times greater than air of equal temperature.
Yes, you read correctly — 70.
One reason the heat loss is so large during cold-water exercise is that your body loses heat due to both conduction and convection. Conductive losses are direct losses from one molecule to another through a liquid, solid or gas. Convective losses depend on how fast water (or air) moves past the body.
You can do an experiment to experience the two mechanisms for heat loss. It is easiest, and least uncomfortable, to do it with your foot or hand—though you can use your entire body. Fill a tub or bucket with cold water and submerge your hand. Make the water cold enough so that it is uncomfortable when you place your hand in the water.
Once in the water, keep your hand very still, with no movement whatsoever. After a short while the water doesn't seem as cold. There is a very small layer of water next to your body that is transferring body heat to the cold water and that layer right next to your skin is slightly warmer than the big tub of cold water.
Once your hand seems able to comfortably tolerate the cold water, move your hand around in the water and you will find that it feels cold again. In the stationary situation, there were no convective body heat losses. Once you began moving, there were losses due to conduction and convection.
With the extreme body heat losses in cold water, compared to air, it is easy to understand why triathletes and swimmers are so sensitive to small changes in pool temperatures. Experienced swimmers can detect day-to-day changes in pool temperatures as small as a single degree.
The Body's Responses to Cold
In a previous column we learned that the body has several unfortunate responses to cold, including shivering, constricting blood vessels, increasing metabolism to use more fuel, increasing urine volume, increasing lactate production and decreasing VO2max.
All of the body's cold responses are undesirable for triathletes, in that the responses lead to decreased race performance. A reasonable question, then, is can the body acclimatize to cold, in a similar fashion to the way it acclimatizes to heat? Can we adapt to cold and decrease the negative effects?
The answer is yes, but the process isn't nearly as much fun as the heat acclimatization process.
Before getting into strategies for cold acclimatization, let's take a look at people that have acclimatized to cold water temperatures quite nicely.
In history and in current culture, there are people around the world that intentionally take cold baths and swim in cold water because they believe there are health benefits to doing so. For example, the Romans in the first century B.C. would take cold-water baths as a cure for headaches and stomach problems.