AFDs consist of a motorised fan, filter(s), and housing, designed to remove airborne contaminates from the incoming air stream. AFDs typically have a series of filters consisting of a pre-filter secondary filter and a primary HEPA filter. The pre-filter and secondary filter prevent premature loading of the HEPA filter by trapping larger particles.
Airborne gases can be removed through the use of sorption filters like activated carbon. The sorption media will be effective on specific gases. Activated carbon filters will absorb water vapour along with other gases and when used in high humidity situations, the carbon media can load with water vapour and lose capacity for other airborne gases.
AFDs are sized based on the cubic feet of air per minute (cfm) they process. They can be installed to create negative, neutral or positive pressure in an area. AFDs can be installed to create negative or positive pressure differentials. When used to create negative pressure differentials they are referred to as negative air machines (NAMS). They also may be used as air scrubbers to recirculate and filter within a space.