The Physiology of Cold Water Immersion

Dealing with changing conditions is one of the unique challenges in open water swimming. Depending upon geographical location, seasonal changes can drastically impact the water temperature. The dangers of swimming in excessively warm water are well known, but what about swimming in cold water?

Immersion in cold water presents several challenges to the body. First, it’s different than exposure to air (consider how different 50-degree air temperature feels compared to 50-degree water temperature). Conductive heat loss (transfer of heat energy) between skin and water is 20 times greater than it is between skin and air, and the potential for heat loss via convection (contact with a moving fluid) while swimming in cold water can be 70 times greater. In addition, the larger the difference between body temperature and the temperature of the water—known as the thermal gradient—the greater the rate of heat loss, affecting the duration of exposure that can be tolerated.

Compounding the potential for heat loss is the fact that much more skin is exposed while swimming than in land activities, where skin is protected with more clothing to aid in insulation. The greater surface area of skin exposed to the water facilitates increased heat transfer. Upon immersion in cold water, blood vessels in the periphery (the limbs, especially hands and feet) constrict. This is the body’s effort to keep more warm blood near the core to protect internal organs. Meanwhile, systemic blood flow continues with blood flowing to the periphery being cooled by the environment and sent back to the core. This can result in an eventual drop in core temperature.

Even a slight drop in core temperature impairs performance by reducing exercise capacity. Many open water swimmers are familiar with the feeling of disorientation after exposure to cold water, a sense of lightheadedness and confusion. These are indicators of a decrease in core temperature that will become more serious with prolonged exposure. This means that the metabolic heat generated by the exercise itself is insufficient to maintain thermal balance, and your core temperature will continue to drop.

How bad can it be? If core temperature falls just three degrees Celsius, amnesia, poor judgment, and behavioral changes will occur. This is a particular danger while in an open water environment, as it’s already easy to become disoriented in the water. The loss of only one additional degree Celsius beyond this point will result in ataxia, or a total loss of motor control. Again, this is an extremely dangerous possibility in open water swimming.

Of course, thermoregulatory responses to cold will vary from person to person, depending upon, especially, age and amount of subcutaneous fat. This will affect the temperature that the swimmer can tolerate and the duration of exposure. Purposely gaining in fat thickness is a typical strategy for swimmers training for the English Channel and other long-distance, cold-water swims for this reason.

The efficacy of repeated bouts of exposure to cold water for acclimatization is not extremely reliable—what might seem like physiological acclimatization might actually be habituation, marked by decreased shivering. To say this is an indication of increased tolerance for long durations of exposure to cold water is dubious.

The bottom line is that it’s dangerous to presume that it’s possible to mentally overcome a drop in core temperature. It’s important to remember that the colder the water, the greater the thermal gradient between the water and your body temperature. A bigger gradient increases the potential for heat loss, which is compounded by duration of exposure. Constant vigilance of physical sensations is key to staying safe when swimming in cold water, and knowing when to get out.

This article was sourced in part by Chapter 23: Physiological Systems and Their Responses to Conditions of Heat and Cold in ACSM’s Advanced Exercise Physiology, authored by M.N. Sawka, J.W. Castellani, S.N. Cheuvront, and A.J. Young, research physiologists with the U.S. Army Research Institute of Environmental Medicine.