Humidity levels above 20 percent help prevent dry, sore throats and make the air feel warmer and more comfortable. Moist air also eliminates static electricity in the house and helps to protect plants and preserve your furniture. On the other hand, humidity levels over 40 percent can cause frosting and fogging of windows, staining of walls and ceilings, peeling paint, mould growth and odors. When relative humidity is over 50 percent, airborne diseases become more difficult to control. Condensation on your windows can provide a good indication of the relative humidity. You may, however, want to install a humidity sensor or humidistat to keep more accurate measurements of humidity levels.