The GA19 vapor pad, sometimes called a water filter or humidifier pad, is a very important part of the humidifier. The purpose of the vapor pad is to act like a sponge, absorbing water which moisturizes the dry air in your home, reducing the negative effects of over-dry air: such as allergy and asthma symptoms, dry nose, itchy skin, and cracked/warped wood floors or furniture. The pad is made of aluminum, and coated with a nontoxic material that evenly distributes water throughout the pad. Over the course of the heating season, minerals build up on the pad, reducing its efficiency. Because of the special coating, the pad is not cleanable, and must be replaced a minimum of once per season. This part fits models 900, 900XM, 1000, 1000XM.