The Pure Guardian Manual Ultrasonic Humidifier (H-1500) uses silver clean technology to prevent the growth of mold and bacteria in the water tank. By humidifying clean water, you provide a healthier breathing environment by not spreading germs and bacteria into the air. The Manual Humidifier has an option for warm or cold humidification. The unit has a 20 hour run time on its maximum setting and has over a 1 gallon tank capacity with a low water indicator light that turns on when it is time to refill the tank. And since the humidifier is ultrasonic it’s virtually silent when it runs, making it ideal to have on while you sleep. There are no added costs with the unit since there are no evaporating pads, filters or wicks to clean or replace. The variable mist control makes it perfect to use in both small and large rooms.