Maintaining an indoor relative humidity of approximately 45 percent is the ideal level for the summer months or warmer climates. Colder climates or the winter months require even lower levels, approximately 30 to 40 percent. Indoor air quality is regarded as one of the top environmental threats by the EPA.
High humidity during the winter causes condensation on windows and exterior walls, which leads to mold and mildew growth. Keeping relative humidity at less than 50 percent also inhibits dust mite infestations. Both these problems can cause serious health issues for those with asthma and other respiratory conditions. If relative humidity falls too low, the chances of colds, rough, cracked hands and dry nose increase.