HEPA and ULPA filters: There seems to be two definitions for the acronym HEPA; either "high efficiency particulate air,", or "high efficiency particulate absorbing." ULPA means "Ultra Low Particulate Air". Both kinds attempt to strain as many foreign particles from the air as they can and still let air through. ULPAs are tighter filters than HEPAs, though only by a few percents.

Electrostatic Filters: Work the same way the static on your clothes do, only they use an electric charge on the filter to draw particles to the filter. Turn the electricity off, the filtered particles drop, usually into a disposal receptacle. You only need to look at how dust particles tend to stick to your monitor's glass surface (it's generating static electricity, too) to see an example in action. Clever, no?

Electrostatic Precipitators: Not just a tongue-twister, but a similar concept as the filter; here we're using two opposite charges to attract airborne particles to a plate or grid wire. This is how some of the purifiers that don't need a filter to change work; you just wash off the precipitator plate and reuse it. A similar stunt on a larger scale is how a coal-burning power-plant scrubs some of the ash out of the smoke it produces.

Ionizers: Ionizers use static electricity in a different way; they make the pollutant particles stick to each other. This will make a large particle out of many little particles, which then are easier for a filter to trap. Ionization is rumored to have a bunch of fringe benefits, such as improving your mood, helping reduce depression from Seasonal Affective Disorder, and alleviating the symptoms of asthma - but there is some debate as to which of these effects are real. Certainly, I notice that I feel cheerful just after a thunderstorm, and I'm told that's because lightening puts more ions in the air, so the air always feels cleaner afterwards.

Ozone Air Cleaners: In a radical departure from previous methods, these purifiers are not content to sit and wait for the pollutants to come to them. Instead, they release small amounts of ozone, which then go out and find the pollutants and neutralize them.

Ultraviolet: These use radiation from ultraviolet light bulbs to kill biological organisms. From germs to funguses, these purifiers are like a "bug zapper" for microscopic bugs! Frequently seen in hospitals.

Carbon Filters: Exactly as the name suggests, these are the regular old technology where you catch stuff in the filter until it's dirty, then you change it for a new one. Unimaginative compared to all the whiz-bang space-age technology out there; plus they only catch big particles.

Washable Foam: Another large particle-catcher, these are similar to the carbon filters, only they can be washed in warm, soapy water, drip-dried, and reused.

Furthermore, different air purifiers can mix and match two or more of these methods. The scientific quest to perfectly clean the air is a rigorous one, with applications in industrial, hospital, laboratory, and home uses.