Air pollutants consist of numerous particulates, fibers, mists, bioaerosols, 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, another complicating factor is that indoor air pollutant concentration levels can vary by time and location within the school building, or even a single classroom. Pollutants can be emitted from point sources, such as from science store rooms, or from area sources, such as newly painted surfaces, and pollutants can vary with time, such as only once each week when floor stripping is done, or continuously such as fungi growing in the HVAC system. Not all HVAC systems are designed to accomplish all of these functions.
Some buildings rely only on natural ventilation.
Others lack mechanical cooling equipment, and many function with little or no humidity control. The two most common HVAC designs used in schools are unit ventilators and central air handling systems. Both can perform the same HVAC functions, but the central air handling unit serves multiple rooms while the unit ventilator serves a single room. The three building figures in this Backgrounder show typical methods for how outdoor air enters a room, and how exhaust air exits through a vent. If exhaust airflow through door or wall grilles into corridors is sealed due to fire codes, ensure that air has another path to reach the central exhaust.