I think the parent poster is correct that it depends on the water.

I have a couple ultrasonic humidifiers, if tap water is put in them it immediately sends the AQI on my air quality monitor into the "Dangerous" level. I have the monitor upstairs and it detected it when my girlfriend put tap water in the humidifier downstairs.

Purified or distilled water works fine. I bought a counter top water distiller because it was a pain lugging 15 gallons of water into the house every week all winter. You can see the residue of whatever was in the tap water at the bottom of the distiller after it runs and it doesn't look like anything I would want to be breathing.