Filter-based air purifiers are generally much more effective than those that rely only on ionization to remove particles from the air. Without a built-in fan, an air ionizer can only clean the air in its immediate vicinity, leaving dust motes, pollen, and mold spores elsewhere.
An air ionizer charges particles that pass through it, causing them to attract dust, pollen and other particulate matter floating in the air. As these particles clump together, they become heavy enough to fall out of the air column. They also become attracted to solid surfaces in the room, causing them to stick to walls and furniture. However, they are not removed from the air, and if disturbed they can rise back into the air column and recirculate into the room.
An air filter uses a fan to draw room air in and through a physical filter designed to trap airborne contaminants. One of the most common filter specifications is HEPA, which stands for High Efficiency Particulate Air. These filters are capable of removing 99.97 percent of all contaminants greater than 0.3 microns in size, effectively purifying the air that passes through the filtration medium. Some filter-based purifiers also include an air ionizer to help encourage particulate matter to stick to the filter, improving the overall effectiveness of the air cleaner.