A breach in your ductwork can make it easier for insects and rodents to access your home. Rodents are notorious for chewing into even strong materials, and dander and droppings can infiltrate your airstream as the critters use your ducts as a pathway. Diseases like hantavirus have been linked to contact with rodent droppings, making it a priority to deal with such residues in your ducts promptly. While the issue may be identified through a routine duct inspection, you may want to consider contacting your HVAC contractor if you observe evidence near duct vents or note rodent activity from walls or floors. In fact, the EPA also notes that homeowners should consider air duct cleaning if the ducts are infested with vermin.