impeller humidifier

impeller humidifier

Impeller humidifiers (cool mist humidifiers) use a rotating disc to spray water onto a diffuser, which breaks the water down into fine droplets that float in the air. The water supply must be kept strictly clean, otherwise there is a risk of spreading bacteria or mold into the air.
ultrasonic humidifier

ultrasonic humidifier
Ultrasonic humidifiers use a metal diaphragm that vibrates at ultrasonic frequencies to create water droplets that silently leave the humidifier as a cool mist. Ultrasonic humidifiers use piezoelectric transducers to generate high-frequency mechanical oscillations in the water film. This forms extremely fine droplets of about one micron in diameter, which quickly evaporate into the air stream. Unlike humidifiers, which boil the water, these droplets pick up any impurities in the tank, including minerals in hard water (which then form sticky white dust on nearby objects and furniture that's hard to remove). Any pathogens that grow in the stagnant tank will also become airborne.
Impeller humidifiers and ultrasonic humidifiers do not selectively put pure water into the air, they also add any suspended matter in the water, such as microorganisms and minerals. Using distilled water can greatly reduce the amount of minerals and other substances, although no water is absolutely pure. Special disposable demineralizers can also reduce the amount of material in the air, but the EPA warns that "the ability of these devices to remove minerals may vary widely." Mineral dust can have negative health effects. Wick humidifiers deposit minerals in the wick; evaporator types tend to collect minerals on or around the heating element, requiring regular cleaning with vinegar or citric acid to control buildup.