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A Class I Biosafety cabinet protects the operator and the environment from exposure to biohazards. It does not prevent samples being handled in the cabinet from being exposed to contaminants that may be present in room air. Naturally, there is a possibility of cross-contamination that may affect experimental consistency. Consequently, the scope and application of Class I cabinets is limited, and it is largely considered obsolete.

Class II Type A2

The Class II Type A2 biological safety cabinet is the most common Class II cabinet. It has a plenum from which 30% of air is exhausted, and 70% re-circulated to the work area as the downflow.

Stated from NSF/ ANSI 49:2010, both the Class II Type A1 and Type A2 must have the positively-pressurized contaminated plenum to be surrounded by negative pressure. In case there is a leakage on the positive plenum, the leaking aerosol will be pulled by the negative pressure back to the positive plenum, and it will not leak out.

In the A2 cabinet, about 70% of air from the positive plenum is recirculated as downflow, and the remaining 30% is discharged to the lab through the exhaust filter.

Class II Type B1

The Class II Type B1 biological safety cabinet has a common plenum from which 70% of air is exhausted, and 30% re-circulated to the work area as the downflow. This cabinet also has a dedicated exhaust feature that eliminates re-circulation when work is performed towards the back within the interior of the cabinet. Toxic chemicals employed as an adjunct to microbiological processes should only be allowed if they do not interfere with work when re-circulated in the downflow.

Class II Type B2

The Class II Type B2 biosafety cabinets are suitable for work with toxic chemicals employed as an adjunct to microbiological processes under all circumstances since no re-circulation occurs. In theory, Type B2 biological safety cabinets may be considered as the safest of all Class II BSCs since the total exhaust feature acts as a fail-safe in the event that the downflow and / or exhaust HEPA filtration systems cease to function normally. However, Class II Type B2 biosafety cabinets require large laboratory spaces due to their installation system and will require elaborate ducting works.