Quote Originally Posted by cls515
White dust: This is from minerals in the water. These minerals may be released in the mist of the humidifier and settle as fine white dust that may be small enough to enter the lungs. The health effects of this dust would depend on the types and amounts of minerals in the water. Though the U.S. government does not know if these minerals will cause any serious health problems, one should still be safe and use water with a lower mineral content and follow the manufacturer's recommendation for keeping your humidifier clean.
Do not use tap water because it has a high mineral content—especially if you have well water. Even though distilled water may still contain some mineral content, it will probably have fewer minerals than tap water. It is important to know that not all bottled water has had the mineral taken out of it. For example, spring water, artesian water, and mineral water still contain these minerals that can lead to scale build-up and white dust.
Pulled this off a website, maybe that is your problem...minerals in the water....
If he is using distilled water how can the white be a problem with minerals? I would say its more a problem with with variations of temp and humidity.