The electrostatic air purifiers and Ionic Air Purifiers function by ionizing the air, imparting a positive or negative charge to particles in the air as they pass through the unit. These ionized particles are then removed from the air by one of two major methods. An ion generator imparts a charge to pollutant particles in the air, which then are attracted to and deposited on walls or furniture, or stick together and fall out of the air. More advanced ionic air purifiers use a principle called electrostatic precipitation to trap particles within the unit, collecting them on an oppositely-charged metal plate or sleeve, which must then be routinely cleaned. Either type of ionic air cleaner may also include a fan to help the air circulate, though many do not.