How Cold is Cold Water?

Cold water is not as cold as you might think.

How Cold is Cold Water?

Water temperature is critical to an immersed body's ability to stave off the effects of hypothermia. There is disagreement over at what temperature water is considered "cold". Water temperatures near the ocean beaches of Hawaii and the coasts of much of the contiguous U.S. are considered by millions of people to be warm enough to swim in, even with little (or no) clothing. Yet, just offshore these same beaches, water temperatures may be debilitating enough to cause death in just a matter of hours. How cold does water need to be in order to be considered "cold" and thus life threatening?


It is important to consider that the naked human body was not designed to live in much of the world's ambient air temperatures. It can survive in almost none of the world's water temperatures. To make up for this poor design feature, humans devised clothing as insulation that gives the margin needed for survival. The human body will become hypothermic in any water temperature under 91° F (body at rest) and any air temperature under 80° F (body at rest).


So, much of the earth's climate is inherently inhospitable to humans. But we usually make up for it with our primary shelter – clothing. Another way we can compensate for the cold environment is by exercise. The body produces heat by exertion. Thus, if exercising in water, such as swimming or treading water, the human body will not get hypothermic unless the water temperature is less than 72° F (body exercising).


In water temperatures above 72° F, the body can generate enough heat to keep the body warm. The problem with exercise is that the body needs rest and food to keep going. Water will begin to zap the body of heat as soon as a person stops moving. However, swimming or movement can stave off hypothermia for a time in waters between 91° F and 72° F. Also, factors such as body fat, clothing, activity level, will to survive, etc. can significantly influence survival time in the water.


Nearing 70° F, water begins to have effects on the body that accelerate the risk. Cold water shock (sometimes referred to as the the mammalian diving reflex) occurs in any water temperature less than 70° F. Trigeminal nerves in the face, when in contact with cold water, can have a profound effect on the body's metabolism. The result can be a lowering of the body's blood pressure and other physiological changes affecting competence in the water. This is why it is so important to keep the head out of cold water, if possible. In water below 70° F, the use of heat conservation methods such as the H.E.L.P. (Heat Escape Lessening Position) and Huddle position become critical to extend survival time. Exercise, such as swimming, will hasten the hypothermia process at these temperatures.


Unfortunately, regulatory agencies do not consider the above medically-defined temperatures of cold water when they apply them to survival equipment regulations. For example, the U.S. Coast Guard defines cold water as water at or below 59° F. There is no medical basis for this choice of temperature. However, by choosing a temperature of 59° F, the oil industry in the Gulf of Mexico and in other parts of the U.S. is exempted from having to carry survival equipment that would meet standards for colder water. The definition of cold water may depend on what you are doing and who you are talking to. But, for a real emergency, consider the actual risks posed by the temperature of the water you are operating on. Make decisions accordingly about the type of emergency and survival equipment you should carry!