Air filters are often overlooked but make a significant impact. Look for homes with return vents that are sealed, well-placed, and contain properly fitted filters. Filter boxes at the HVAC handler (central location) offer better performance than individual filters at each return grill.

Avoid fiberglass-lined ductwork, which can deteriorate over time and harbor mold. Metal ducts with cleanable interiors are preferred. If you can see inside a vent, look for signs of dust buildup or microbial growth. Duct cleaning may help, but if contamination is extensive, replacement may be necessary.