Use distilled or demineralized water. Tap water contains minerals that can lead to the growth of bacteria inside your humidifier. When released into the air, these minerals often appear as white dust on your furniture. You also might breathe in some minerals that are spread into the air. Distilled or demineralized water has much fewer minerals than does tap water. Also, use demineralization cartridges or filters if the maker of the humidifier suggests doing so.
Change humidifier water often too. Don't let film or other types of buildup form inside a humidifier. Unplug the device first. Then empty the water tank, dry the inside surfaces and refill it with clean water. Do this every day if you can. These steps are very important if you use a cool-mist or ultrasonic humidifier.