Humidity is the amount of water vapor in the air around us. More correctly called "relative humidity," it tells how much water is in the air relative to air of the same temperature when it is completely saturated. When the relative humidity is completely saturated at 100 percent or more, water droplets form. Outside, the temperature at which the humidity is 100 percent is called the "dew point." Since warm air holds more water than cold air, the humidity rises in the cooling air and water condenses on plants and other surfaces when the dew point is reached. When cold air is warmed, the humidity drops because there is relatively less water vapor as the air warms. A humidifier adds moisture to air that is warmed by a furnace inside a house because cold winter air contains less water vapor than warm air. In the summer, we reverse the process, forcing condensation of water vapor inside a "dehumidifier" using refrigeration to wring extra humidity out of the air and make the air feel cooler.
We manage the humidity in our homes for a number of reasons. Very humid air encourages the growth of mold and bacteria that can cause damage to building materials and infections. But organisms---human and otherwise--need a certain amount of humidity to keep tissues moist and operating efficiently. Furniture, paint and paper all require a certain amount of moisture to minimize cracking and deterioration. Cold winter air holds very little moisture and even when outside air with a relative humidity of 75 percent is brought inside and heated to 70 degrees by your furnace, the relative humidity drops to less than 15 percent. Water from every living thing evaporates more quickly into this super-dry air and the result is itchy skin, dry throats and noses, mysteriously dropping fish tank levels and thirsty plants.
Humidity levels below 30 percent are considered dangerous for people and other household contents. Humidity above 75 percent can encourage the growth of mold and bacteria. The ideal range for indoor humidity is 40 to 60 percent and most automatic humidifiers that are "built-in" to furnaces have a regulating device that will measure humidity. Freestanding humidifiers come in a wide price range and variety of models, from countertop to large floor models. Buy an inexpensive hardware-store hygrometer to measure humidity and keep track of the humidity for several days during different types of weather to give you an idea of how your humidifier should be set to keep the humidity between 40 and 60 percent. Be sure to check different places in your house if you're calibrating a free-standing humidifier; you may need more than one machine to keep the humidity level at the right place throughout your home.