> Hard tap water or mineralized drinking water will coat your work area in chalk-like dust.

Also, y'know, your lungs. Deep inside your lungs.

Running tap water in an ultrasonic humidifer's going to spike the particulate pollution (PM1/2.5/10) throughout your entire house by hundreds of ug/m^3. And it seems that children are particularly prone to inhaling this stuff and having it deposited in their lungs (~2x more particles and ~3.5x more mass).

They really shouldn't be used with anything except distilled water. The things should come with a continuity tester that disables them if the water's conductive or something.