Roughly 50% of duct work is internally lined with fiberglass (Internally lined or fiberboard).  Over time, the fiberglass particulates from the internally lined ductwork breaks down from air abrasion as well as fibers are loosened from improper air duct cleanings by using a more abrasive system then ours. Under these conditions, fiberglass fibers circulate throughout your duct work and ultimately into your home.