Roof exhaust systems for biological containment cabinets should have: a. a high exhaust velocity at the roof b. a stack extending at least 20 feet (6.1 m) above the roof surface to avoid reentrainment by the building c. a stack extending at least 10 feet (3.0 m) above the roof surface to avoid reentrainment by the building d. a stack extending at least 5 feet (1.5 m) above the roof surface to avoid reentrainment by the building e. a recirculating exhaust system