A typical system functions as follows: air is drawn from the room through a return grill and passes through an air filter and into a furnace (or air handler).Inside the air handler is the circulating blower and a heat exchange coil commonly called the evaporator. The evaporator is cold and thus removes heat from air that blows across it. In addition to removing heat, water also deposits on the evaporator surfaces. The cooled air returns to the room via outlet grills. The moisture falls into a drain pan under the evaporator and drains through a pipe out of the system.

The air drawn into a system contains dust, much of which is biological spores and other organic particles. Even the best filters capture only part of this material. Much of the rest falls out of the air stream onto the heat transfer surfaces. It combines with the water and the spores germinate and grow rapidly into a Bio Toxic build up.