There are basically two different ways to go about cleaning your air using an air purifier. You can either get a whole home air purifier or a room air purifier (also called portable). Home air purifiers are in one way or another tied in with the current air conditioning/heating system already present in your home. In contrast, room air purifiers are stand-alone units that can be moved from room-to-room.
Home air purifiers are designed to clean the air over an entire house or office. Most operate by having some type of filter (i.e. activated carbon) over or inside the air vents. As air is pushed in and out, the filters are designed to remove particles.
There are several reasons why home air purifiers are not always the most efficient method of purifying your air. The filters commonly available do not clean the air as efficiently as the HEPA filters in portable air purifiers. Using a HEPA filter in a central ventilation system would restrict the airflow to unacceptable levels.
Also, the current created by air conditioning/heating units is not sufficiently strong enough to get the air cleaned and back into the filter or unit. This does not apply to all systems of course, but it does to many.
Another drawback with home air purifiers is the fact that they can only remove particles 1 to 1.5 microns in size and larger, and they do nothing to remove gases and odors. They are not designed to remove the smaller particulates, which tend to cause the most health problems.
Another consideration as far as cost in involved, most whole home air purifiers require professional installation. This can run anywhere between $200 to $400.00 and more depending on where you live.
Amaicare make home HEPA filtration units that filter down to 0.3 microns and are separate from the furnace.