Air ducts need to be inspected every now and again. It is a good idea to make this job a part of your spring cleaning or before shutting your house up for winter. Taking a few moments to peek inside your air ducts is a good way to ensure that your family is breathing clean, fresh air.
The picture to the right is what an extremely dirty air duct looks like. This debris can make it's way through the vents and into the air you breath. A pprofessional air duct cleaning service should be called to remove this type of mess.
The epa.gov is an excellent source of air quality information to help you research any environmental issue you may have.