Humidifiers--why white dust?

Ultrasonic humidifiers are notorious for white dust. I have one in one room, and get white dust on things all over the house, especially staticky surfaces like TV screens, not to mention my reading glasses.

Other types of humidifiers, such as evaporative and vaporizers, do not do this.

Why do they make white dust and why is this peculiar to this type of humidifier?

Most humidifiers get a whole lot of white stuff on their evaporative pads. It’s mostly calcium deposits from the water, but it’s often laced with mold. I haven’t used ultrasonic ones, but I have the impression they produce a very fine mist. When the mist evaporates in the air, the flake of calcium stuff in each droplet falls where it falls. If the droplet evaporates on a surface, the mineral crystallizes on the surface.

Adding humidity to a dry house always gives you tricky byproducts. You may eventually decide, as I have, that it’s simpler to have dry air.