Relative humidity is defined as the amount of water in the air relative to the amount of water in the air when it is saturated at a specific temperature. When that relative humidity moves past 50 percent in your home, dust mites and mold will thrive — possibly triggering asthma symptoms.

There are many sources of indoor humidity including water vapor coming in from the outside, which is usually the top source of humidity during hot weather. Then there is the air drawn through a home's crawl spaces or passed through soil and cracks in your home's substructure. Or, it might be caused by activities in your home — such as bathing, laundering, or cooking — or through evaporation of water that originates from leaks or condensation.

Dehumidifiers

Activities such as using vents in bathrooms and kitchens (see Chapter 15) or repairing leaking appliances or pipes can help lower household humidity. But many individuals also look at dehumidifiers to bring that humidity down, especially with lower floors or basement areas of homes.

Just remember, though, to change the dehumidifier's collector pan of water regularly — to avoid creating new sources for mold on floors or carpets.

Humidifiers

On the flip side, make sure your home is not too dry (below 35 percent relative humidity), which could cause discomfort for your asthmatic child in the form of a dry throat, nose, skin, and lips. In this case, you may want to consider a whole-house humidifier attached to your heating system or a portable room or console unit.

However, make sure you follow the manufacturer's guidelines carefully for use and continuous maintenance of the humidifier. If too much moisture is added to the air, the growth of biological organisms, including dust mites or mold, could be encouraged.

Recent studies by the EPA and the Consumer Product Safety Commission have shown that ultrasonic and impeller (or “cool mist”) humidifiers can disperse microorganisms and minerals from their water tanks into indoor air. Two other portable humidifiers (evaporative and steam vaporizer) can allow for growth of microorganisms if they are equipped with a tank that holds standing water, but they generally disperse less, if any, of the pollutants into the air.

Both government agencies note, though, that only limited information currently is available on the growth and dispersal of microorganisms and minerals by home humidifiers.

The EPA also recommends cleaning portable humidifiers every third day — emptying the tank and using a brush or other scrubber to clean it. In the absence of specific recommendations, cleaning is suggested for all humidifier surfaces that come in contact with water with a 3 percent solution of hydrogen peroxide.

When cleaning and maintaining console and forced-air (furnace-mounted) humidifiers, also carefully follow manufacturer directions. In particular, if a humidifier contains a tank, do not allow water to stand in the tank for extended periods of time, which could also cause a mold problem.