Last time I checked (brought my sensor to the office to one room with ultrasonic) it showed 101/105 ug/m3 for pm2.5/pm10

In the next room (where there were none) it was 6.

Depends on the water, I guess.

How large is the space with the humidifier? My space is wide open and that might be the biggest reason for my lack of increased PM2.5 readings.

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.

The ones I had for a bit basically fogged out the apartment immediately and left white (i’m guessing salt) deposits all over everything. I know you are supposed to use distilled but it’s cost prohibitive at the rate these blow through water unless you also have a home distillery.

> any notable increase in PM2.5 particles

What's your PM2.5 baseline, and did you measure TDS in the water?

Since Nov it's been fluctuating between 5-7 ug/mg, with a few spikes hitting 9 and 10. At this moment its a 6.

I haven't checked the TDS but when I used a water test strip for an aquarium early this year it was in the hard water range.

Thanks to this post, I checked my ultrasonic filled with tap water. With it running all night in a bedroom with an open door, morning pm2.5 readings are ~30 and the meter is in the kitchen.