Air from outside can seep past weather seals and through cracks in the building walls.

Smog from outdoors comes in through these pathways, but the resulting concentration is usually so small that the health effects are negligible. Also, the air filters in a house's central climate-control system (heating/AC) serves to remove the particulates in smog pollutants from the indoor air, which may indirectly remove gas molecules that are sticking to the particles.