>I think not using an ultrasonic humidifier would be better stated as only using distilled water with ultrasonic humidifiers
That's a common misconception. They still atomize bacteria from the tank because nobody ever cleans & sterilizes their dehumidifier tank often enough. Distilled water users still report high indoor particle counts when it runs, which is how you know it's an ineffective prevention.The cost of distilled water every year (even making it at home) means ultrasonic humidifiers are the most expensive option too.
Just use an evaporative pad humidifier.
https://www.youtube.com/watch?v=oHeehYYgl28
>while there are lots of studies showing population level correlations between pm2.5 particles and health problems, I don't think there is currently evidence specifically showing that things like using air purifiers actually improve health.
Air purifiers reduce PM2.5 concentration, so if PM2.5 is bad for health then air purifiers are good for health.This is like "there's no evidence parachutes improve survival after jumping from a plane."