Any of the "cool" humidifiers will spray droplets out and create white powder/dust if you have minerals in the water. The ultrasonic types create ultra fine mineral dust that I suspect is bad for electronics and gets everywhere. I used an ultrasonic humidifier for a while, and found nasty, conductive, white mineral dust all over the inside of my TV some time late. I suspect it's bad for electronics. The "cool" humidifiers create dust, but it doesn't go as far as the ultrasonic because the droplets are small.
I suggest a cheap "vaporiser" that evaporates by heat. These shouldn't create dust unless they also have something to spray droplets, too. They tend to leave the minerals inside the water tank, so you need to clean them out more often. Unfortunately, this type of humidifier seems harder to find these days.
Anything that sprays droplets to evaporate water has the potential to blow germs into the air more than a steam humidifier that boils the water. If there's germ soup in your humidifier tank, a "cool mist" or ultrasonic humidifier will be spraying the germs into the air in conveniently inhaled aerosol form.
The Mayo clinic has some recommendations on cleaning.
http://www.mayoclinic.org/diseases-cond ... 48021?pg=2