The GeneralAire 990-13 Vapor Pad or humidifier pad is made of expanded aluminum and coated with a hydrophilic material that absorbs and holds water. When warm air flows across the pad, the water is evaporated into the air humidifying the environment.

Minerals contained in the water are left on the pad as the water evaporates, clogging the pad and restricting both water and air flow. For this reason the pad should be changed at least once per heating season.Cleaning the pad is not recommended, as it will reduce performance.

This product is also sold under part numbers 7002 and GF-990-13.