For years duct liner was exclusively composed of a fibrous glass mator blanket (fiberglass). In 1988, the International Agency for Research on Cancer (IARC) in Lyon, France, classified fibrous glass containing respiratory-sized fibers as "possibly carcinogenic to humans." Although duct liner fibrous glass materials are almost always of the textile variety, thus having longer and larger diameter fibers and not under the IARC classification, they were inappropriately targeted in engineering specifications. Often an entire HVAC duct system was without duct liner or fiber-filled silencers, resulting in many noisy installations. This still occurs today.