When the body has to take adaptive measures, it is known to be under thermal stress. Thermal stress equates to discomfort while a minimum of thermal stress provides comfort. The human body can be compared to a machine (in the engineering world) that converts fuel into energy for the purpose of doing work -- the more active the body, the more fuel that is consumed. The rate of heat production within the body is known as the metabolic rate (units=met= 360 Btu/hr) and includes all of the heat given off by all of the chemical reactions taking place in the body. Some examples of typical metabolic rates are as follows:

Figure 1. Like the machine, the conversion of fuel (oxidation of food) into work is not 100% efficient. That energy which is not converted to do work is in the form of heat, and if not needed to maintain a constant body temperature, it is brought to surfaces by blood flow (See Figure 1), then rejected to the body's surrounding environment. This heat is rejected in two forms, sensible and latent heat transfer.

Figure 2. There are three sensible heat loss mechanisms : Radiant loss to cooler surfaces (or gain from warmer surfaces); Convection loss to cooler air (or gain from warmer air) which is heated and rises; and dry respiration heat loss to cooler air that enters the lungs and is exhaled warmer

Figure 3. The latent heat loss mechanisms include: latent respiration heat loss; water diffusion through the skin; and the evaporation of sweat (skin wettedness).