Class II Type A1:

The type A1 cabinets have a minimum inflow velocity of 75ft/min where the contaminated divided just above the work station and mixes with the inflow air.

The mixed air is then drawn through a duct network so that it reaches the back of the cabinet.

After this, air might be either recirculated after passing through the HEPA filters or exhausted out of the cabinet, also through a HEPA filter.

This type of cabinet is not as widely used as it is not safe to work with hazardous chemical substances.

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.

Class II Type C1:

The Class II Type C1 cabinets are similar to Type B cabinets in their working mechanism, but these are designed to reduce operating costs add flexibility to the laboratories.

These cabinets work by using the single-pass airflow system where the cabinets move the air by mixing it with the downflow air separated into columns for recirculation.

The air above the workstation is drawn with a second fan which moves the contaminated air out through the exhaust system with a HEPA filter.

In this way, the cabinets provide protection to the environment, the operator, and the workstation or the biological material.

Type C cabinets are different from Type A cabinets as they use a single-pass airflow mechanism where the air is not circulated.

These differ from Type B cabinets in that they don’t require a dedicated ducted exhaust system, can work for an extended duration to increase operator protection in the case of exhaust failure, and can even run without the exhaust at all.