Why do you want to keep indoor relative humidity (Rh) between 40-60%?

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends keeping the relative humidity in a home between 40-60%. Outside that range your risk of being adversely affected increases. In addition, the Journal of Allergy and Clinical Immunology recommends keeping indoor relative humidity below 51% in order to deter dust mites and boost overall comfort of the home.