To put moisture into the air, humidifiers use one of four different methods:
Steam. Some humidifiers disperse water vapor into the air the same way a tea kettle does: by boiling water. Steam units are manufactured as tabletop or wall-attached models that spray steam into a living space or into forced-air heating ductwork through a dispersion tube. Except for tabletop models, tap water is piped directly into a steam unit, so that it doesn't require manual filling. Because steam is evaporated water, it doesn't contain bacteria or mineral deposits that might contaminate the air you breathe.
Ductwork systems, which are manufactured for both commercial and residential applications, can be custom-designed to output any amount of steam a house might need--though custom units may be prohibitively expensive for most home applications. And they can be costly to use; steam units employ an electrical heating element to heat the water; when this is on for extended periods, it can rack up energy dollars.
Ultrasonic. Though ultrasonic humidifiers were the rage a few years ago, they've fallen into disfavor because of the suspicious "white dust" that they create. With ultrasonic humidifers, 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 (EPA) has warned that the tiny particles of minerals and microorganisms that may be propelled into the air may cause illness or allergies.
Cool mist impeller. This type of humidifier spins tiny droplets of moisture into the air. Available primarily as tabletop units, these hold from 2 to 3 gallons of water, are filled manually, and are easy to move around. They don't have the same problem with white dust that ultrasonic units have because the droplets produced are larger . Unlike steam humidifiers, cool mist units use electricity only to propel the air--not to provide heat. This means they consume far less power.
Evaporative humidifiers. Evaporative humidifiers, the most popular technology today, capture virtually all minerals and pollutants from even the hardest water, eliminating the white dust ultrasonic humidifiers can create. These work by passing an air stream through a wet medium, such as a sponge or dampened grill. Some have fiber honeycomb-type panels that wick water upward from a reservoir; others employ a foam or cloth drum that spins through a water-filled trough as air passes by. Still others blow air through a woven aluminum pad that is saturated by a constant stream of water. In some cases, filters are treated with a special compound that retards bacterial growth.
With tabletop and console models, a quiet fan blows air through the damp surfaces; some whole-house models use the heating system's blower to do this job. A filter collects the dissolved solids left behind by the evaporating water; this is generally disposable.
Central evaporative humidifiers are made to be attached to the home heating system, too. Their output capacities range up to 25 gallons per day. High-capacity models are fan powered; the others don't require a motor. All are hooked up directly to the plumbing so manual filling isn't needed.
One newer product that Emerson offers has no moving parts, electric cords, hot water or even a motor. Ideal for a child's room, this unit sits directly over a forced-air floor register and can be easily moved from one place to another. It retails for about $20.
