Maintaining good indoor air quality depends on more than just providing adequate outside air and controlling indoor air pollutants. Minding the relative humidity of the air is just as important. ASHRAE recommends keeping the relative humidity between 30 and 60 percent.Above 60 percent, mold and mildew start to become a problem. Below 30 percent, problems with static electricity, skin irritation and dry eyes occur. Also, the mucous membranes start to dry out, impairing the ability to defend against upper respiratory illness. Humidity levels also affect how occupants perceive ambient temperature. For example, in winter at a relative humidity level of 30 percent, a comfortable set-point will fall between 69 and 78 degrees Fahrenheit whereas in summer, at 60 percent relative humidity, the thermostat will fall between 68 and 75 degrees.
