In a hospital enviroment, there is an old, unused trash shoot to be used as a possibly toxic exhaust vent. The trash shoot/ shaft has a fire rating of 2 hours. Since the newly proposed exhaust duct MAY, on occaision, carry toxic fumes to the roof is the exhaust duct required to have welded joints or is the 2" overlap with a mastic sealer considered to be an ample seal? The fan is on the emergency generator power as well to maintain the negitive pressure inside the exhaust duct. Which atricle number in which code covers this?