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I find it hard to believe that some calcium ions constitute a health hazard.

Probably depends on what’s in the water. Quoting the original research paper:

> A phenomenon known as “white dust” is associated with filling ultrasonic humidifiers with hard water; inhaling this white dust caused childhood lung injury (Daftary and Deterding, 2011)

Not entirely convinced this is correct. I've noticed that the humidity from a shower can also make air quality sensors readings spike so it does seem plausible that it's at least partially picking up the water

I had to scrape a thick layer of calcium dust off of screen and furniture after using an ultrasonic humidifier all winter.