PSI Class II Bio safety Cabinet Type A1 & A2 are designed for protection of operator, product and environment from pathogenic agents and microorganisms transmitted in respiratory way and also for creation of a bacterial dust free air environment. It has a common plenum from which 30% of air is exhausted, and 70% re-circulated to the work area as the down flow. Type A cabinets exhaust air directly back to the laboratory, and they may contain positive pressure contaminated plenums. When toxic chemicals must be employed as an adjunct to microbiological processes, these cabinets should not be used. Exhaust HEPA filtration only removes airborne aerosols including biohazards, and not chemical fumes.