What they do: Does walking on your carpet give you an electric shock? Those sparks are an indicator that the humidity level in your home is too low. A humidifier can counteract this by adding moisture to the air.
How to use them effectively: Most people rely on stand-alone humidifiers to add moisture in individual rooms, which is fine if you can't attach a unit to the furnace. However, according to Dr. Portnoy, the most effective way to humidify the whole house is usually with an evaporative model that can be placed in your central furnace. To give the humidifier enough time to work effectively, you may need to leave the fan running even when the heat is off.
Safety alert: Stand-alone cool-mist humidifiers "can shoot bacteria and mold into the air," Dr. Portnoy says. Be sure to follow the manufacturer instructions for cleaning. Hot humidifiers, or vaporizers, avoid this problem, but can be a scalding risk around toddlers, who may accidentally tip them over.
Upkeep tips: A diluted bleach solution is the best way to clean cool-mist and cool-evaporative humidifiers. For more information, refer to the manufacturer's instructions.
Additional advice: The optimal humidity level is between 35 and 50 percent -- any higher, and you'll end up creating a breeding ground for mold and bacteria, Dr. Portnoy notes. A simple way to measure indoor humidity levels is with a hygrometer (humidity meter), which can be purchased inexpensively at a drugstore.
Originally published on BHG.com, March 2005.