Though ultrasonic humidifiers used to be popular, they have fallen into disfavor because of the suspicious “white dust” they create. With ultrasonic humidifiers, a thin sheet of water is passed over a transducer that vibrates and excites the water, breaking it into millions of tiny droplets. The problem is that calcium and other minerals in the water become airborne with the moisture, particularly in areas that have hard water. This ends up as a dust that falls on objects in the room and pollutes the air you breathe.
The Environmental Protection Agency has warned that the tiny particles of minerals and microorganisms that may be propelled into the air can cause illness or allergies.
The other three basic types of humidifiers:
