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. For further details http://www.pureairsystemindia.com/products/bio-safety-cabinets/