"If normal body temperature is about 98 degrees Fahrenheit, why do we feel hot at that air temperature?"
—S. Meyer, Melbourne, Fla.
 
Jeffery W. Walker, a physiology professor at the University of Arizona, has a cool explanation:

The human body is like an engine that continuously generates large quantities of heat, and its radiator, so to speak, disperses heat least effectively in hotter climes.

Heat is an unavoidable by-product of the work being done by the tissues of the body. Contracting muscles of the heart, diaphragm and limbs; ion pumps that maintain the electrical properties of nerves; and biochemical reactions that break down food and synthesize new tissues (to name a few) generate body heat continuously. With this gurgling volcano of active internal organs, the body has a critical need to dissipate heat to the surroundings. It does so by circulating blood near the surface of the skin, by exhaling warm, humidified air, and by evaporating sweat.

These processes function best when ambient temperature is around 70 degrees Fahrenheit, where we feel most comfortable, and they serve to maintain core body temperature around 98 degrees F. But when the surroundings match core body temperature, the dispersal mechanisms are not optimal, so we feel hot, especially when humidity is high. Humidity has a significant effect because water on the body absorbs enormous amounts of heat and then dissipates it by evaporation. Anything that interferes with this vaporization of water (humid air, lack of a breeze, heavy clothing, and so on) makes us feel especially hot and uncomfortable.