Maintain minimum average inflow velocity of 75 ft/min (0.38 m/s) through the work access opening. Have HEPA filtered downflow air that is a portion of the mixed downflow and inflow air from a common plenum (i.e., a plenum from which a portion of the air is exhausted from the cabinet and the remainer supplied to the work area). May exhaust HEPA filtered air back into the laboratory or to the environment through an exhaust canopy; and May have positive pressure contaminated ducts and plenums that are not surrounded by negative pressure plenums. Type Al cabinets are not suitable for work with volatile toxic chemicals and volatile radio- nuclides.