Humidity refers to the amount of moisture present in the air. The Relative Humidity (RH) is the percentage (%) of water vapor in the air at a specific temperature.
Relating it with indoor air pollution: too low humidity increases pollution impact on your body. Some examples of health effects due to low humidity (usually in winter) are dry eyes, dry nose, mucous production in respiratory system, increase of infections and cold, skin irritations (itchy skin).
Threshold: above 60% can lead to mold growth. Low humidity ratio may cause dry eyes, and mucous membranes or itchy skin.