Air conditioning systems affect indoor air quality both through the filtration they provide and through the humidity control that happens as a side effect of the cooling process. As air passes over the cold evaporator coil, moisture in the air condenses out, which lowers indoor humidity and can reduce conditions favorable to dust mites and certain mold species that thrive in damp environments.
However, if the system itself is not properly maintained, it can become a source of poor air quality rather than a solution; a coil with standing condensate and organic buildup can host mold growth that then gets circulated into the room every time the fan runs. Standard filters remove larger particles like dust and pollen but are not designed to capture very fine particulate matter or most airborne odors, which require finer filtration media or activated carbon layers to address.
Rooms that are sealed tightly for cooling efficiency also see reduced natural ventilation, which can allow indoor pollutants from cooking, cleaning products, or off-gassing furniture to accumulate over time if there is no separate fresh air exchange mechanism. Regular coil and filter cleaning, alongside periodic drain line maintenance, is therefore relevant not just to cooling performance but to the air quality of the space being conditioned.