Electronic air filters are the most effective filters on the market today, and the most expensive. They are 99.9% effective. They capture particles as small as 1 micron so they are even effective against viruses. Electronic filters create an electrical charge to attract allergens and other irritants. They may also contain collecting plates that capture the particles within the system. Some units require that the collecting plates be cleaned periodically. Some use an ion-type cleaner that may produce an ozone byproduct. Make sure to ask for proof from the manufacturer that their product creates EPA - acceptable levels of ozone. Electronic air filters like the Trane Clean Effects can reduce the levels of particles in the air of a home to 2-3% of original levels. A very effective filtering system

This is a diagram of the typical filtering process:

Diagram showing efficiency level of varies filter mediums


Links: http://www.epa.gov/iaq/pubs/residair.html