About 90 percent of the human body's heat is produced in the torso area by the major organs and muscle groups. The amount of heat generated is increased as the body works harder. To maintain a constant core temperature, the body must either give up or retain this heat as necessary. How this is accomplished depends greatly on the ambient temperature and humidity around you.
Convective body cooling
Under normal conditions (60° to 80° F ambient temperature), the circulatory system carries core heat toward the skin's surface. Since heat always travels from hot to cold, rather than from cold to hot, the body heat is carried away as the cooler outside air passes over the skin. This process is known as convective cooling, since the heat is removed by the movement of air.
Evaporative body cooling
As the temperature outside begins to rise, the difference between normal skin temperature (90° F) and the ambient temperature narrows. This difference is known as the delta T. As the ambient temperature rises above 80° F, the delta T is not great enough to allow the body's internal heat to flow away from the body by convection.
Instead, the body reacts by cooling itself through a process known as evaporative cooling. When water fluid is exposed to warm, dry air, it will evaporate into water vapor. This change of state is called a phase change and it produces a tremendous cooling effect. The body creates this phase change by secreting perspiration from our sweat glands to the surface of the skin.
When the air surrounding the skin is warm and dry, this is an extremely efficient process. But as the humidity rises, perspiration can no longer evaporate to water vapor, as the air is already saturated. This is a dangerous condition, since the body has no other natural mechanism to give up heat.
Vasoconstriction and the brain
When the outside temperature drops below 60° F, the body needs to reverse the process and retain its internally generated heat. This is accomplished by a process called vasoconstriction. Vasoconstriction is the restriction of blood flow to the skin surface by contraction of blood vessels. Since the body's organs must always have a flow of blood, vasoconstriction is applied only to those vessels carrying heat to the skin's surface.
When the brain is fooled into thinking the temperature is cool – when ice is applied to the body, for example – vasoconstriction occurs in an effort to prevent loss of heat, even though the core temperature is actually rising. This can lead to dizziness and fainting. More dangerous is the fact that the cool skin temperature physiologically feels comfortable, so you may actually work harder, creating an even faster rise in core body temperature and the risk of cardiac arrest.