They give particulates in the air stream a strong electrical charge, then collect and trap them on oppositely-charged plates.
Incoming air passes through a prefilter to remove larger particles.
Then, particulates are given a strong positive or negative charge as they pass through a series of ionizing wires. In he collection section, the charged particles are attracted to an oppositely-charged plate, where they stay until the unit is cleaned. Some units manufacturers offer may include a charcoal filter to help reduce household odors. Cleaning is simple. Just remove the collection cells three to four times a year, wash with household cleaner to remove contaminants, soak in a laundry tub or place in the dishwasher.