The percentage of water vapor air holds relative to its maximum at that temperature. Homes feel and stay healthiest at 40–55% in summer and 30–45% in winter; sustained readings above 60% invite mold and dust mites.
Air-quality terms map to humidity control in muggy Chesapeake summers, filtration (MERV), ventilation for tighter renovated homes, and duct condition in houses that have seen decades of retrofits.