In many parts of the U.S., homeowners with basements have to manage rising humidity levels for most of the cooling season. It is not uncommon for such individuals to have a dehumidifer running full-throttle in the basement from April through October. High indoor humidity levels has been associated with the growth of fungi, such as mold, among other irritants. The EPA suggests keeping indoor relative humidity (RH) under 60% in order to effectively minimize mold growth. The EPA cites an ideal relative humidity between 30-50%; however, in reality, it can be a struggle to maintain such a low RH range in spaces susceptible to taking on moisture.
In fact, mold growth is only one of several indoor environmental contaminants whose growth correspond to indoor relative humidity levels. Figure 1 is an adaptation of a frequently cited chart by Anthony V. Arundel et al. (1986) that illustrates the relationship between indoor RH and the effect of several indoor environmental pollutions sources.