Filter-based purification traps airborne particles by size exclusion. Air is forced through a series of different filters and particles are collected by the various filters.
HEPA filters can, by definition, remove at least 99.97% of 0.3-micrometer particles, and are usually more effective for particles which are larger or slightly smaller.
In dusty environments, a HEPA filter may follow an easily cleaned conventional filter (prefilter) which removes coarser impurities so that the HEPA filter does not need to be changed or cleaned frequently.
Activated carbon filters are made up of a highly porous material that adsorb volatile chemicals on a molecular basis, however does not remove larger particles. It is important to note that the adsorption process when using activated carbon must reach equilibrium therefore it may be difficult to completely remove contaminants. Activated carbon is merely a process of changing contaminants from a gaseous phase to a solid phase, when aggravated or disturbed contaminants can be regenerated in indoor air sources. Activated carbon filters can be used at room temperature and have a long history of commercial use. It is normally used in conjunction with other filter technology, especially with HEPA.
Air Ionizer purifiers use charged electrical surfaces or needles to generate electrically charged air or gas ions. These Ions attach to airborne particles which are then electrostatically attracted to a charged collector plate. This mechanism produces trace amounts of ozone and other oxidants as by-products. Most ionizers produce less than 0.05 ppm of ozone, an industrial safety standard. There are two major subdivisions: the fanless ionizer and fan-based ionizer. Fanless ionizers are noiseless and use little power, but are less efficient at air purification. Fan-based ionizers clean and distribute air much faster.