Over time, air ducts collect windblown dust and other debris carried along by the airflow of a forced-air heating and cooling system. Large particles, such as mold spores, sometimes fall to the surface. Additionally, rodents and wildlife occasionally access narrow ducts and vents inside buildings and homes. These creatures may deposit feces and urine on the surface of a duct. Additionally, they may leave twigs, leaves, mud, and other nest-building materials behind. Sometimes homeowners don’t even notice debris inside a duct upon visual inspection because much of the dirt remains hidden deep within the interior.