Updated: 27 October 2008
Although conduction and convection constantly remove body heat when the air temperature is lower than your skin temperature, their contribution to your body's total heat loss in air is relatively small - only about 10% -20%. However, the amount of heat dissipated by conduction in cold water is about 30 times greater than in similar air temperature.
However, sweat must evaporate to provide cooling; sweat that drips off the skin or is washed away in the water provides little or no cooling. Thus, whereas radiation and sweat evaporation are the primary mechanisms for heat loss in air, conduction allows the greatest heat transfer during immersion in water. Water has a termal conductivity about 30 times greater than air, which means that heat loss by conduction is about 30 times faster in water than it is in air. When all factors are considered (conduction, convection, radiation, and evaporation), the body generally loses heat four times faster in water than it does in air of the same temperature.
This heat transfer also occurs when immersing in water hotter than body temperature, just in the other direction! Water in hot tubs, whirlpools and Jacuzzis is not only heated to higher than 100oF, jets may create very high speed water movement, which increases the speed of heat transfer from the hot water into the human body. The hot water will raise the body's temperature, which triggers its temperature regulation mechanism. Sweat production increases but is ineffective in the water and effects no cooling through evaporation. Only skin exposed to air like the head will provide some degree of heat dissipation, which is why wearing swim caps or shower caps while in the hot tub is strongly discouraged. Prolonged immersion in hot water that is warmer than normal body temperature can lead to hyperthermia. As body temperature rises the body responds with increased efforts to effect cooling which places high demands on the circulatory system, affects blood pressure, and may cause health problems such as drowsiness, loss of consciousness (and potentially drowning), heat stroke, and cardiac arrest. The recommendation is that hot tubbers take breaks during soaking and leave the hot tub after 15 minutes to allow the body to cool down.
The US Consumer Product Safety Commission recommends keeping the water temperature of a hot tub below 104oF. According to their findings, water temperatures over 110oF have caused a number of deaths. They also advise against soaking in a hot tub before consulting a physician for special risk populations:
Aside from these concerns, many hot tub manufacturers oppose consuming alcoholic beverages before, during, and/or after soaking in a hot tub. Alcohol expands the blood vessels and increases body temperature, as does the hot tub! The combined effect may compound the health risks of a raise in body temperature. Plus, in many cases, alcohol causes impaired performance of mental and motor functions, so an affected hot tubber might lose track of time spent in the hot tub, not recognize the early stages of overheating, and potentially lose the ability to leave the hot tub!
While hot tubbing is a popular activity many people are not aware of the potential for and dangers of hyperthermia. By contrast, most people know about the potential for and dangers of hypothermia, the life-threatening lowering of the core temperature as a result of extended exposure to extreme cold (air or water). Land hypothermia may occur as a result of exposure to cold, wind, rain, snow or freezing precipitation and is usually seen during high risk recreational or sports activities such as mountaineering, climbing, skiing, prolonged exposure during hiking or snow-mobiling. Aquatic hypothermia occurs mainly during prolonged open water swimming, wind surfing, waterskiing, kayaking or boating when sudden and persistent temperature drops occur, or with accidental falls in the water. Common advice to capsizing boat accident victims in cold water is to hold onto their clothes, adopt a crouched position with their arms crossed in front and keep floating. Floating is preferable to swimming, when the person is further than a thousand yards from land because activity in cold water will speed heat loss and promote hypothermia. When there is a group of people it is recommended to hug each other to conserve and acquire heat. Immersion in water temperatures below 10oC (50oF) is of great danger, because severe hypothermia will develop in most people in less than an hour.
Exercise increases the metabolic rate, and thus heat production. During exercise blood is diverted from the working muscles to the skin in an effort to carry the heat from the core to the body surface. The hotter (and more humid) the environment, the more blood is diverted for cooling. As a result, less blood is available to the exercising muscles, which cannot work as long or as hard, and exercise becomes more difficult. When exercising in the water, some of the body's metabolic heat production is offset by heat loss due to conduction (water temperature) and convection (water movement). Less blood needs to be diverted to the skin, exercising muscles are supplied a full complement of blood and exercise seems to be "easier".
A combination of the various qualities of water explored on this and the previous pages means that the heart doesn't have to work as hard in the aquatic environment as on land, even during rest: Fluid pressure facilitates blood circulation and water temperature accelerates cooling of the body. Simultaneously, buoyancy counteracts the pull of gravity which reduces the effort required of the muscles to maintain balance and good posture. Thus, heart rates in the water are generally 12-15 beats per minute lower than on land (while performing the same activity at the same intensity level). This means that a person can expend the same amount of muscle work for exercise in the water and yet keep the heart rate lower than on land, thus making exercise seem less strenuous.
|Go to the next chapter: Aquatic Exercise Equipment
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