Your air conditioning system works by taking in air from inside your house, cooling it, and then redistributing it. To make sure that you are not damaging your air conditioning unit or spreading allergens, the intake vent has a filter over it. The filter catches dust and allergens that might be in your house. A filter cannot catch every single particle of dust, however, so over time, dust can build up in your air ducts. That’s even more pronounced if you allow your air filter to get dirty without replacing it. If you haven’t cleaned your air ducts in a while, you could be overdue for an air duct cleaning.