Mastic is used in my area to seal duct board transitions from the inside. An example would be attaching an air handler to the plenum in a closet. Since tape cannot be applied to the back seams. Mastic is applied on the inside of the duct board to seal the transition. Does anyone know of health concerns with mastic on the interior. It is made as an exterior duct sealant. The reason I ask is local companies stopped applying it to the inside of ducts because of a lawsuit.