Why does humidifier put measurable PM2.5 in the air?
All tap water contains some dissolved solids, and "hard water" contains more, particularly calcium salts. There are also microorganisms found even in treated tap water, and some organisms can build up a biofilm inside the humidifier. Ultrasonic humidifiers are effective at putting these particulates into the air. However, as others stated, you may just be detecting fine water droplets from the humidifier before they evaporate.

To find out if it's water droplets or particulates from dissolved material, do a comparison test, using distilled, filtered, low-particulate water such as High Performance Liquid Chromatography (HPLC) grade water.


Clean out the humidifier thoroughly, rinsing repeatedly with a dilute acid safe for that machine (perhaps vinegar, but check the humidifier manual).
Rinse repeatedly with distilled water and finally with HPLC water.
Refill with pure HPLC water.


If this does not greatly increase the particulates above background, but tap water does, then you've found the source of the issue. If not, then you're probably detecting mostly water mist.

Caveat: Buying HPLC water is not inexpensive. If you can't easily make your own on-site, perhaps just leave the dehumidifier off.