"Air quality" gets used as a catch-all term, but it really comes down to four controllable factors.
What your system physically captures as air passes through — dust, pollen, pet dander, and (at higher ratings) smoke and some bacteria. See our HEPA vs. MERV guide for the specifics.
Relative humidity above ~55-60% creates conditions where mold and dust mites thrive. Your AC removes some moisture as a side effect of cooling, but dedicated dehumidification is the only reliable way to hold humidity in a target range year-round — especially in coastal Orange County.
Modern, tightly-sealed homes trap stale air, VOCs, and CO2 more than older, leakier construction. Mechanical ventilation (ERV/HRV systems) exchanges indoor air for fresh outdoor air without losing the energy you've spent conditioning it.
Leaky return ducts can pull unconditioned air from attics, crawlspaces, or garages directly into your living space — undermining everything else on this list. Sealing and testing ductwork is often the highest-value, most overlooked fix.
Managing a mold-sensitive or CIRS-recovery home specifically? These four factors matter even more — see our full Mold & CIRS Support guide.