A Balancing Act
The human body feels most comfortable when the heat produced by metabolism is equal to the sum of the heat dissipated to the surrounding space, (less any heat stored in the body). This phenomenon is represented by this equation:
QM −W = QE ± QR ± QC ± QS
QM =Metabolic heat produced within the body W = Useful rate of working
QM −W = Heat to be dissipated to the atmosphere QE = Heat lost by evaporation,
QR = Heat lost or gained by radiation QC = Heat lost or gained by convection, and
QS = Heat stored in the body.
The graphic below may illustrate the process better by showing how the human body looses heat.
Even when inactive, an adult must lose about the same amount of heat as a 100 watt light bulb to offset heat generated by metabolism. While deep tissue temperature is 98.6°F, the surface our the skin is always a bit cooler. Any time the operative temperature of the living space falls below the skin's temperature, the body looses heat to the surroundings. If the heat loss of the body exceeds the heat loss disipated by metabolism, the person will feel cool
The graphic below illustrates how the human body gains heat.
When the operative temperature of the living space rises below the skin's temperature, the body gains heat from the surroundings. Perspiration provides some cooling. But if the heat gain from the space exceeds the heat loss disipated by metabolism, the person will feel warm.
The body loses heat to surrounding space primarily through four heat transfer mechanisms.
Conduction is the process of losing heat through physical contact with another object or body. For example, if you were sitting on a metal chair, the heat from your body would transfer to the cold metal chair. Under normal conditions, conduction accounts for about 18% of the body's total heat loss.
Convection is the process of losing heat through the movement of air or water molecules across the skin. The use of a fan to cool off the body is one example of convection. Heat loss from convection is not tyically significant without drafts or air movement across the skin.
Radiation is a form of heat loss through infrared rays. This involves the transfer of heat from one object to another, with no physical contact involved. For example, if you are sitting near a large window on a cold day, the body will radiate heat to the cold glass. Radiation is the largest form of heat loss accounting for about 60%.
Evaporation is the process of losing heat through the conversion of water to gas. This occurs when body perspiration evaporates in the surrounding air.
Operative Temperature is what the human body experiences in a space. It represents the combined effects of the radiant surface temperatues from adjacent walls, windows, ceilings, floors and the temperature of the air.