The Lennox X2661 humidifier pad is made of expanded aluminum. The pad is coated with a hydrophilic material that absorbs and holds water flowing through the pad, allowing warm air flowing across the pad to evaporate the water and humidify the air. 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 once per heating season. If you have hard water with higher mineral content, you may need to change the pad more frequently. Cleaning the pad is not recommended, as this removes the hydrophilic coating and will greatly reduce the performance of your humidifier.