The 100 hour PureGuardian Ultrasonic Cool Mist Humidifier Tower enhances comfort, helping you sleep better. It provides quick relief from dry air making life pleasant for those with colds, allergies and dry skin. The space saving tower design, with an easy to fill large 1.5 gallon tank, is ideal for use in medium size rooms. Unlike other humidifiers that grow slimy mold on the water tank surface, Silver Clean Protection is embedded in the tank to fight the growth of mold and mildew on the surface of the water tank. Thanks to ultrasonic technology, it disperses a fine mist into the air, and is ultra quiet making it ideal to have on while you sleep. The humidifier is equipped with a 12 hour timer, a night light and variable mist control. An indicator light lets you know when it s time to refill the water tank. The humidifier shuts off automatically once empty. Note: Run time is based off of running the unit on low speed, also depends on atmospheric conditions.