In the course of a day, your air conditioning and heating system circulates a lot of air throughout your home. The outdoor unit draws in air from the outside, which is then cooled (or heated, depending on the season) and distributed to different rooms throughout the house via vents and registers. For the most part, any dust from the outside gets caught by the filters that line your HVAC system, maintaining your indoor air quality. Sometimes, however, dust can still find its way indoors.