Air-O-Swiss, 2 Pack, A200, Hydro Cell, For Use With All Air-O-Swiss Ultrasonic Humidifiers & Air Washer, Uses Activated Carbon Technology To Provide Constant Freshness In Air-O-Swiss Units, Helps Reduces Heavy Metals, Organic Impurities, Pesticides & Odor Causing Chemicals, Replace Every 1-2 Months Depending On The Water Quality.