Heat- and Energy-Recovery Ventilators bring in fresh outdoor air while exchanging heat (and, for ERVs, moisture) with the outgoing stale air. They solve the tight-renovated-house staleness problem without throwing conditioning away.
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.