Why is white dust accumulating on surfaces near my humidifier, and how can I prevent it?

White dust is caused by minerals suspended in hard water and is not a defect in the humidifier. To reduce white dust, use distilled water and a Homedics demineralization cartridge in the tank. The demineralization cartridge should be replaced every 30–40 fillings, or more frequently if using very hard water. (Page 5, 6, 12)