An impeller humidifier (cool mist humidifier) uses a rotating disc to fling water at a diffuser, which breaks the water into fine droplets that float into the air. The water supply must be kept scrupulously clean, or there is a risk of spreading bacteria or mold into the air.
Ultrasonic humidifier [ edit ]
An ultrasonic humidifier uses a metal diaphragm vibrating at an ultrasonic frequency to create water droplets that silently exit the humidifier in the form of a cool fog. Ultrasonic humidifiers use a piezoelectric transducer to create a high frequency mechanical oscillation in a film of water. This forms an extremely fine mist of droplets about one micron in diameter, that is quickly evaporated into the air flow. Unlike the humidifiers that boil water, these water droplets will contain any impurities that are in the reservoir, including minerals from hard water (which then forms a difficult-to-remove sticky white dust on nearby objects and furniture). Any pathogens growing in the stagnant tank will also be dispersed in the air. Ultrasonic humidifiers should be cleaned regularly to prevent bacterial contamination from being spread throughout the air.
Impeller and ultrasonic humidifiers do not selectively put pure water into the air, they also add any suspended material in the water, such as microorganisms and minerals. The amount of minerals and other materials can be greatly reduced by using distilled water , though no water is absolutely pure. Special disposable demineralization cartridges may also reduce the amount of airborne material, but the EPA warns, "the ability of these devices to remove minerals may vary widely." [ 1 ] The mineral dust may have negative health effects [ citation needed ] . Wick humidifiers trap the mineral deposits in the wick; vaporizer types tend to collect minerals on or around the heating element and require regular cleaning with vinegar or citric acid to control buildup.