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[0] Dust [2] Visual inspection.Duct blaster test to determine leakage of ductwork. [3] Have ductwork professionally cleaned without chemical cleaners,Buy a Merv 13 filter for your furnace slot and change regularly.Clean carpeting thoroughly and regularly with a HEPA vacuum [1] 
Leaky or poorly maintained forced air system, poor cleaning.
Air – What the Nose Knows | Healthy Home Consultants | Paula Baker-Laporte, FAIA - Healthy Home Design
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