The greatest energy users, and therefore internal heat producers, are muscles. Hard work in a hot environment contributes to the body's heat load. Conduction - the direct transfer of heat to the body by direct contact with a warm object. Generally, an almost insignificant source of heat when considering heat gain to the body. Convection - the exchange of body heat with the surrounding air that comes into contact with the skin.
Air speed is an important factor in heat loss or gain.
Radiation - the transfer of heat to the body through air from a hot source, for example, a furnace, an oven, or the sun. This is an important factor as, again, heat is lost from the body only if the surrounding air is cooler than the body. Evaporation - evaporation of sweat from the skin cools the body. At very high humidity however, sweat does not evaporate as quickly. High air speed and low humidity increases evaporation.