The Class II Cabinet:
The Class II vertical laminar flow biological safety cabinet is defined as a ventilated unit with an average inward force velocity at the work opening of at least 75 feet per minute. The cabinet has an open front with inward airflow for personnel protection, downward HEPA-filtered laminar airflow for product protection and HEPA – filtered exhaust air for environmental protection.

The vertical laminar airflow and the front access opening are common to all Class II Cabinets. However, their airflow patterns and velocities, HEPA filter positions, ventilation rates, and exhausting methods vary in different Class II designs. For these reasons, the Class II cabinets are differentiated into various types.

Class II, Type A Cabinet:
Maintain a minimum calculated or measured average inflow velocity of 75 fpm through the work area access opening. The access opening minimal face velocity varies between 80 fpm and 100 fpm depending on cabinet sash openings.
Have a HEPA –filtered down flow air from a common plenum (i.e., from which a portion of the air is exhausted from the cabinet and the remaining air is supplied to the work area).
May exhaust HEPA-filtered air back into the laboratory.
May have positive pressure contaminated ducts and plenum.

Class II, Type A cabinets are suitable for work with low to moderate risk biological agents in the absence of volatile toxic chemicals and volatile radionuclide.

Class II, Type B1 Cabinet:

Maintain a minimum calculated or measured velocity of 100 fpm through the work area access opening.
Have HEPA-filtered down flow air composed largely of uncontaminated recirculated inflow air.
Exhaust most of the contaminated down flow air through a dedicated duct exhausted to the atmosphere after passing the HEPA filter.
Have all biologically contaminated ducts and plenums under negative pressure or surrounded by negative pressure ducts and plenums.

Type B1 cabinets are suitable for work with agents treated with no more than a few micrograms of toxic chemicals and trace amounts of radio nuclides required as an adjunct to microbiological studies.

Class II, Type B2 Cabinet:

Type B2 cabinets are sometimes referred to as “total exhaust”. This type of unit.

Maintains minimum average inflow velocity of 100 fpm through the work area access openings.
Has HEPA-filtered down flow air drawn from the laboratory or outside air (i.e., down flow air is not recirculated from the cabinet exhaust air).
Exhaust all inflow and down flow air to the atmosphere after filtration through HEPA filter without recirculation in the cabinet or return to the laboratory room air.
Has all contaminated ducts and plenums under negative pressure, or surrounded by directly exhausted negative pressure ducts and plenums (not recirculated through the work area).

Type B2 cabinets may be used with biological agents treated with toxic chemicals and radio nuclides required as an adjunct to microbiological studies.

Class II Type B3 (or A/B3) Cabinets:

Type B3 Cabinets are sometimes referred to “convertible” type units. Some manufactures of equipments have described their “convertible” unit (from Type A Type B) as having a change in air balance from 70 percent recirculate and 30 percent exhaust to 30 percent recirculate and 70 percent exhaust. The essential differences between Type A and B3 are not changed.

All of the contaminated air flows to a common plenum where part of the air is recirculated and part is exhausted. Essentially, the cabinet Type B3 performs as a Type A with a change in air balances. These types of units are designed to:

Maintain a minimum average inflow velocity of 100 fpm through the work area access opening. (Supply air to the workspace and exhaust air flowing through HEPA filters from a common plenum).
Have HEPA-filtered down flow air from that is a portion of the mixed down flow and inflow air from a common exhaust plenum.
Discharge all exhaust air to the outdoor atmosphere after HEPA filtration.
Have all biological contaminated ducts and plenums under negative pressure or surrounded by negative pressure ducts or plenums. This does lend the “convertible” Type B3 some credence as being different from as type A.

Type A/B3 cabinets are suitable for work with biological agents treated with minute quantities of toxic chemicals and trace quantities of radio nuclides that will not interface with the work if recirculated in the down flow air.

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