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The only way to remove contaminates from the system is to have a professional duct cleaning service clean the ducts. You can't "polish" the inside of the ducts. Installing a filter designed to remove allergens will help keep the allergen levels in the house down.In a typical AC system the air is only re-circulated. No air comes form outdoors.
The outdoor unit is the refrigeration unit that condenses the refrigerant and dissipates the heat from the house to the outdoors (the purpose of the fan). This is the essential aspect of the cooling system.
Also no outside air from a split system.
They typically use a large vacuum suction machine. It pulls air through the ducts with a lot more force than the blower on the system. So anything that doesn't get sucked out is not going to blow out the vent at much lower air pressure. You can replace the ducts if you want and they are accessible. But it usually isn't necessary. New families move into houses all the time that have allergens that didn't bother the previous occupants. Duct cleaning usually is all that's needed.