Publisher Summary This chapter presents the fundamental principles of air flow within a ventilation system. The purpose of industrial ventilation is essentially the same as that of natural ventilation: to supply fresh air within an area so as replace the air that has undesirable characteristics. One potential method for accomplishing this task is to heat the air in the area, thus causing it to rise and escape through vents in the roof. This air is then replaced with the fresh air that enters through openings in the building. However, quite often such a system does not circulate air fast enough to remove the contaminants before a hazardous level is reached. Thus, it is necessary to provide an artificial means for moving the air, such as the mechanical fan. There are a number of reasons that ventilation systems should be used in a given work environment. First, ventilation can be used to maintain an adequate oxygen supply in the area. A second reason for installing a ventilation system—and one that is of significant importance to the industrial hygiene engineer—is to control hazardous concentrations of toxic materials in the air. A final reason for installing a ventilation system—and one that is also of major importance to the industrial hygiene engineer—is to remove the undesirable contaminants at their source before they enter the workplace air.