are not dried out completely between fillings, and become saturated with mineral deposits over time. They regularly need rinsing or replacement; if this does not happen, air cannot pass through them, and humidifier stops humidifying the area it is in and the water in the tank remains at the same level. Evaporative humidifiers function similarly to evaporative coolers. Impeller humidifiers Impeller humidifiers (a type of cool mist humidifier) are usually noisier than others. It 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 humidifiers An ultrasonic humidifier uses a ceramic diaphragm vibrating at an ultrasonic frequency to create water droplets that silently exit the humidifier in the form of cool fog. Usually the mist gets forced out by a tiny fan, while some ultra mini models have no fans. The models without fans are meant mainly for personal use. Ultrasonic humidifiers use a piezoelectric transducer to create a high frequency (1-2 MHz) 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. The amount of minerals and other materials can be greatly reduced by using distilled water. 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." The mineral dust may have negative health effects. 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. Tanks The water is usually supplied by manually filling the unit on a periodic basis. Top fill A top fill tank has a hole at the top to make it convenient to refill the tank on