The Aquatic Environment  -  Qualities of Water

Water Temperature

• Heat conduction involves the transfer of heat from one material to another through direct molecular contact.  Heat generated deep in the core of the body can be conducted through adjacent tissue until it reaches your body's surface.  It can then be conducted to your clothing, air, or water that is in direct contact with your skin.  The larger the difference in temperature between the two, the more effective the heat transfer.   Conversely, if the air or water around you is hotter than your skin, heat from the air will be conducted to your skin, warming it.

• Convection involves transferring heat from one place to another by the motion of a gas or a liquid across the heated surface.  Though we're not always aware of it, the air around us is in constant motion.  So is the water during exercise.  As it circulates around us, passing over the skin, it sweeps away the air or water molecules that have been warmed by their contact with the skin.  The greater the movement of the air or water the greater the rate of heat removal by convection.  When combined with conduction, convection can also cause the body to gain heat in a very hot environment.

  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.

• At normal room temperature (69^oF to 77^oF) the nude body at rest loses about 60% of its excess heat by radiation which is the primary method for discharging the body's excess heat at rest.   The heat is given off in the form of infrared rays, a type of electromagnetic wave.  Your body constantly radiates heat in all directions to the objects around it, such as clothing, furniture, air, and water.  If the temperature of the surrounding objects is greater than that of your body, you'll experience a net body heat gain via radiation from those surrounding objects.

• During land-based exercise, evaporation is the primary avenue for heat dissipation.  It accounts for about 80% of the total heat loss when you are physically active, as compared to only about 20% heat loss at rest.  Some evaporation occurs without our awareness and is referred to as insensible water loss, which happens wherever body fluid is brought into contact with the external environment, such as in your lungs, at the mucous membranes (e.g., of your mouth or nose), and at your skin.  Insensible water loss removes about 10% of the total metabolic heat produced by the body, and is relatively constant, so when your body needs to lose more heat (e.g., during exercise), an additional mechanism is needed.  Sweat production increases.  As sweat reaches the skin, it is converted from a liquid to a vapor by heat from the skin  (Perspiration).  Sweat evaporation becomes increasingly important as body temperature increases during land-based exercise.

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 100^oF, 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 104^oF.  According to their findings, water temperatures over 110^oF have caused a number of deaths.  They also advise against soaking in a hot tub before consulting a physician for special risk populations:

• Pregnant women  -  since excessive water temperatures have a high potential for causing fetal damage during the early months of pregnancy
• Young children  -  since their bodies do not get used to temperature changes as easily as adults do. (Age limits in public hot tubs vary, but infants, toddlers and "young" children are generally not allowed use of the hot tub.)
• Individuals suffering from obesity or diabetes
• Individuals with a medical history of heart disease, low or high blood pressure, circulatory system problems

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 10^oC (50^oF) 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 Access other interesting Topics related to exercise. Go to Lia's Aquatic Exercise Page . Go to Lia's SplashPage. Created by Lia Kidoguchi.