In the human body THREE types of heat generating processes are operative.
1 Conversion of food matter into useful energy is a continuous heat generating process.
2 Muscular activities like even sedentary work or sleeping, are heat generating processes.
3 Lastly, certain infections and dysfunctions within the body, elevate or lower the body temperature by extra ordinary rate of heat generation or weakened heat dissipation mechanism.
Of all the energy produced in the body only 20 % is utilized, rest 80 % is surplus heat. The body must not only release all the excess heat that is generated from within the body, but all the excess heat as gained from the environment. Heat is lost from the body by
Radiation (60 %),
Evaporation (25 %),
Convection and Conduction (15 %).
1 Heat loss through Radiation: Radiation mainly occurs when there is a difference in temperature of opposing surfaces. As long as temperature of the opposite surface or object (sun, fire, radiator) is below skin temperature, the body can lose heat by radiation. But once it reaches an equilibrium occurs, body will rather gain heat by radiation.
2 Heat loss through Evaporation is controlled by the level of humidity in the air. Level of humidity is in turn affected by temperature of the air and human body, rate of air movement and the existing proportion of humidity . Body dissipates heat through evaporation by perspiration, sweat and exhalation of air. Dryer air encourages faster evaporation. Evaporation can occur if air has velocity and appropriate humidity (low). Even in case of very high humidity conditions a high velocity air can remove the humidity.
3 Heat loss through Convection occurs when the air in the vicinity of skin becomes hot, expands, decreases in density, and elevates to allow cooler air in its place. Rate of heat convection from body depends on the difference in temperatures (skin & surrounding air) and rate of air movement. When the convective process is inoperative and radiation heat gain is positive, the body can maintain the thermal balance by evaporation.
4 Heat loss through Conduction depends on the difference between the body temperature and the contact object.
Normal skin temperature is between 31° and 34°C. As the air temperature approaches the skin temperature heat loss from the body gradually decreases. In the circumstances the vasomotor regulation increases the body temperature to 34°C to maintain the heat loss, However if air temperature is higher (such as under the sun or very warm climates, the convective heat loss may not work.