Humidifiers come in all shapes and sizes, from hatbox-sized portable units to furniture-sized consoles. But they all do the same thing: They put moisture in the air. And they do it in one of five ways.

1. Evaporation: Evaporative units have a fan inside them that blows air through a wet pad, and this moisture-laden air then continues on into the room. (“Wicking” humidifiers do the same thing, only the air is blown through a moisturized filter instead of a pad.) Evaporative units don’t produce a spray and are therefore less likely to spread germs than are units that throw a cool mist into the air. The wet pads however, can become a breeding ground for bacteria if not cleaned regularly.
2. Steam:These units, sometimes called vaporizers, boil water and send it into the air as steam. They tend not to put microorganisms into the air; after all, it would take an extraordinarily tough germ to survive the boiling process. Unlike other kinds of humidifiers, they produce little or no “white dust.” Steam units leave the minerals behind when they boil water in their tanks.
3. Warm mist:These units boil water just like steam vaporizers, only the steam is cooled slightly before exiting the unit, resulting in a “mist” of warm water droplets instead of real steam.
4. Cool mist:These units break up water into tiny droplets and spray a cool mist into the air. Because the water isn’t boiled, the cool vapor may contain potentially harmful organisms if the tank is not properly cleaned and sanitized.
5. Ultrasonic:These units use high-frequency vibrations to break up water droplets into an extremely fine mist. The mist produced by these machines contains no molds and comparatively few live bacteria. It is thought that the ultrasonic vibrations may “break up” and shatter living organisms along with the water droplets. However, while these bits of dead microbes may not cause respiratory infections, they may still trigger allergic reactions in sensitive people.