The ability of a biosafety cabinet to handle vapor is ultimately determined by what distinguishes the different types from one another. That is airflow patterns and velocities, HEPA air filter positions, ventilation rates and exhaust methods.
Class II Type A2 cabinets are recirculating systems and can be vented either directly to the lab or to the outdoors. A greater percentage of air is recirculated through the cabinet when compared to the volume of air exhausted, and therefore is not well suited for work with hazardous vapors. As long as vapors are not hazardous and will not interfere with the work when recirculated, it is acceptable to use an A2 cabinet with a small amount of volatile chemicals when the cabinet is exhausted to the outdoors for removal of gases. It is important to note that if vapors are used in a cabinet not directly vented to the outside, vapors will pass freely through the exhaust filter into the laboratory.
Class II Type B1 cabinets are partially-recirculating systems. About half of the air is recirculated and the other half is exhausted outside. The recirculated air is pulled through the vents in the front of the work area, and the exhausted air is pulled from the vents at the back of the work area. Therefore it is recommended that if vessels containing hazardous materials which may vaporize are used in a B1 cabinet, that the container should be placed toward the rear third of the work surface, but not so far back as to block the flow of air through the rear grille.
Class II Type B2 cabinets provide no recirculation, and 100% of air is exhausted outside. For this reason, when even minute amounts of hazardous vapors are present, it is recommended that the Type B2, 100% total exhaust cabinet be used.
