Important factors to consider when choosing an air purifier are the type of filter used and the volume of air the unit can clean. While other features may be useful, a unit that passes as much air as possible through a true high-efficiency particulate arrestance filter provides the best performance.
Filters with the HEPA certification are rated to remove 99.97 percent of particles greater than 0.3 microns in size from air that passes through the filter. This is suitable for trapping dust, pollen, mold spores and many other types of particles that can reduce air quality.
Ionizing air purifiers place an electric charge on particles, causing them to stick together or become attracted to surfaces in a room. While this technically helps remove floating particles from the air, it does nothing to trap them. As soon as the charge wears off, or if the charge is disturbed, the particles re-enter the room air.
Ozone-generating air purifiers can oxidize bacteria and other odor-causing substances, removing them from surfaces in a room. However, ozone purification takes a long time to work and usually requires concentrations of ozone that can be harmful to human health. The Environmental Protection Agency recommends against using ozone-generating devices for home air purification.