Air pollutants consist of numerous particles, fibers, mists, molds, bacteria, and gases. It may be helpful to think of air pollutant sources as fitting into one of the categories in the table shown below. In addition to the number of potential pollutants, indoor air pollutant levels can vary within the school building, or even a single classroom. Pollutants can also vary with time, such as only once each week when floor stripping is done, or continuously such as when fungi is growing in the HVAC system. Not all HVAC systems are designed to do all of these things.
Some buildings rely only on natural ventilation.
Others lack cooling, and many have little or no humidity control. Forces Airflow patterns in buildings are caused by mechanical ventilation systems, human activity, and natural effects such as wind. Air pressure differences created by these forces move airborne pollutants from areas of higher pressure to areas of lower pressure through any available openings in building walls, ceilings, floors, doors, windows, and HVAC system. An inflated balloon is an example of this driving force. Even if the opening is small, air will move until the pressures inside and outside are equal.