THE PROBLEM Perilously low indoor humidity. Indoor air that is to dry may lead to itchy skin, scratchy throats and cracked nasal membranes, among other discomforts. The cause: As the air in your home is heated, its relative humidity drops. For example, according to the National Weather Service, when the outdoor temperature is 30 degrees and the outdoor relative humidity is 60 percent, the relative humidity of the indoor air in a home heated to 70 degrees is just 14 percent. Compare that to the 25 percent average relative humidity in the Sahara desert! A whole-house humidifier is a good solution, adding moisture - and comfort - when it's needed.