[amluto]

An ultrasonic humidifier turns whatever impure water you feed in into tiny aerosols, from which most of the water quickly evaporates, leaving even smaller aerosols behind. Those likely consist of soluble salts (likely not terribly harmful) and bacteria (possibly quite harmful).

I would avoid using them unless, perhaps, I ran the output through a filter. Or I would use an evaporative humidifier instead.

[leguminous]

From the same site as this air purifier article:

"The case against ultrasonic humidifiers."

https://dynomight.net/humidifiers/

[imp0cat]

This! Don't use an ultrasonic humidifier unless you fill it with distilled water. Tap water will just create a ton of white dust everywhere.

If you absolutely need more humidity in your home, get something like the Venta LW45 which has a half-submerged rotating disk and blows air over the still wet surface. Works with tap water (requires regular cleaning and a special additive).