There are three classes of biological safety cabinets, designated as Class I, Class II, and Class III. Class I and II cabinets have a protective air barrier across the work opening that separates the laboratory researcher from the work area. Class II cabinets also provide a HEPA-filtered, clean work area to protect the experiment from room contamination. Several variations of Class II cabinets are described below. Class III cabinets have a physical barrier between the operator and the work area. Arm length rubber gloves sealed to glove ports on the cabinet provide the operator with access to the work area. The distinctive features of the three classes of cabinets follow. Refer to Table 16-1 for a comparison of the characteristics and applications for the types of BSCs described below, or view this Centers for Disease Control and Prevention document.

A. Class I Cabinets

Figure 16-1
The Class I BSC (A) front opening; (B) sash; (C) exhaust HEPA filter; (D) exhaust plenum. Note: The cabinet needs to be hard connected to the building exhaust system if toxic vapors are to be used.

  • The Class I cabinet is ventilated for personnel and environmental protection, with an inward airflow away from the operator. It is similar in air movement to a laboratory hood.
  • The minimum average face velocity through the work opening is 75 feet per minute (fpm).
  • The cabinet exhausts air through a HEPA filter to prevent discharge of most particles to the outside atmosphere.
  • This cabinet is suitable for work with low and moderate risk biological agents, where no product protection is required.
  • Because of the popularity of Class II cabinets and the product protection they provide, use of Class I cabinets has declined.

B. Class II Cabinets

  • The Class II cabinet ventilates air for personnel, product, and environmental protection, and has an open front and inward airflow for personnel protection.
  • Product protection comes from HEPA filtered laminar airflow from a diffuser located above the work area. The downflow air splits at the work surface, and exits the work area through grilles located at both the rear and front of the work surface, respectively.
  • The cabinet has HEPA filtered exhausted air for environmental protection.
  • Types of Class II biological safety cabinets are designated A1, A2, B1, and B2.

1. Class II, Type A1 (Formerly Type A) Cabinets

Figure 16-2
The Class II, Type A1 BSC (A) front opening; (B) sash; (C) exhaust HEPA filter; (D) supply HEPA filter; (E) common plenum; (F) blower.

    The work opening is 8 to 10 inches (20-25 cm) high.

  • The type A cabinet may have a fixed work opening, a sliding sash, or a hinged window.
  • A fan located within the unit provides the intake, recirculated supply air, and the exhaust air.
  • The BSC fan maintains a minimum average inflow velocity of 75 fpm through the work area access opening.
  • Approximately 70% of the cabinet air recirculates through a HEPA filter into the work area from a common plenum, while approximately 30% of the air enters through the front opening and an amount equal to the inflow is exhausted from the cabinet through a HEPA filter.
  • The cabinet may exhaust HEPA filtered air back into the laboratory or exhaust to the environment through an exhaust canopy.
  • The cabinets may have positive pressure contaminated plenums. Contaminated plenums under positive pressure must be gas tight.
  • Type A1 cabinets are suitable for work with low to moderate risk biological agents in the absence of volatile toxic chemicals and volatile radionuclides.

2. Class II, Type B1 Cabinets

Figure 16-3
The Class II, Type B1 BSC (classic design) (A) front opening; (B) sash; (C) exhaust HEPA filter; (D) supply HEPA filter; (E) negative pressure dedicated exhaust plenum; (F) blower; (G) additional HEPA filter for supply air. Note: The cabinet exhaust needs to be hard connected to the building exhaust system.

  • The work opening is 8 inches (20 cm) high, with a sliding sash that one can raise for introduction of equipment into the cabinet.
  • Type B1 cabinets have a minimum average inflow velocity of 100 fpm through the work area access opening.
  • The HEPA filtered downflow air is composed largely of uncontaminated recirculated inflow air.
  • Supply fans located in the base of the cabinet, below the work surface, draw air through a grille at the front of the work surface, and supply HEPA filters located directly below the work surface. The fans then force the filtered air through plenums in the sides or the rear of the cabinet and recirculate the air through a diffuser above the work surface. Some cabinets have a secondary supply filter located above the work surface.
  • Approximately 70% of the contaminated downflow air is exhausted through a HEPA filter and a dedicated duct and then discharged outside the building.
  • The remote exhaust fan is generally located on the roof of the building.
  • All biologically contaminated ducts and plenums are under negative pressure or surrounded by negative pressure ducts and plenums. The type B1 cabinet is suitable for work with low to moderate risk biological agents. They are also useful for biological materials treated with minute quantities of toxic chemicals and trace amounts of radionuclides.

3. Class II, Type B2 (“Total Exhaust”) Cabinets

Figure 16-4
The Class II, Type B2 BSC (A) front opening; (B) sash; (C) exhaust HEPA filter; (D) supply HEPA filter; (E) negative pressure exhaust plenum. Note: The carbon filter in the exhaust system is not shown. The cabinet needs to be hard connected to the building exhaust system.

  • The type B2 cabinet has a sliding sash with an 8-inch (20 cm) opening.
  • The type B2 cabinet maintains a minimum average inflow velocity of 100 fpm through the work area access opening.
  • No air recirculates within the cabinet.
  • A supply fan draws air from the laboratory and forces it through a supply HEPA filter located over the work area.
  • A remote exhaust fan, generally located on the roof, pulls all inflow air and supply air through a HEPA filter, and discharges it outside the building. As much as 1200 cubic feet per minute may be exhausted from a 6 ft. cabinet.
  • The cabinet has all contaminated ducts and plenums under negative pressure or surrounded by directly exhausted (not recirculated through work area) negative pressure ducts and plenums.
  • Type B2 cabinets are suitable for work with low to moderate risk biological agents. They are also useful for biological materials treated with toxic chemicals and radionuclides.

4. Class II, Type A2 (Formerly B3) Cabinets

Figure 16-5
The tabletop model of a Class II, Type A2 BSC (A) front opening; (B) sash; (C) exhaust HEPA filter; (D) supply HEPA filter; (E) positive pressure common plenum; (F) negative pressure plenum. The Class II Type A2 BSC is not equivalent to what was formerly called a Class II Type B3 unless it is connected to the laboratory exhaust system. Note: The A2 BSC should be canopy connected to the exhaust system.

  • Type A2 cabinets have a minimum of 100 fpm average inflow velocity.
    • All biologically contaminated ducts and plenums are under negative pressure or surrounded by negative pressure ducts and plenums
    • They may exhaust HEPA filtered air back into the laboratory or to the environment through and exhaust canopy.
  • Type A2 cabinets are suitable for work with low to moderate risk biological agents. Type A2 cabinets used for work with minute quantities of volatile toxic chemical and trace amounts of radionuclides must be exhausted through properly functioning exhaust canopies. If the cabinet is not ducted, you cannot work with chemicals in the cabinet.

C. Class III Cabinets

Figure 16-6
The Class III BSC (A) glove ports with O-ring for attaching arm-length gloves to cabinet; (B) sash; (C) exhaust HEPA filter; (D) supply HEPA filter; (E) double-ended autoclave or pass-through box. Note: A chemical dunk tank may be installed which would be located beneath the work surface of the BSC with access from above. The cabinet exhaust needs to be hard connected to an exhaust system where the fan is generally separate from the exhaust fans of the facility ventilation system. The exhaust air must be double HEPA-filtered or HEPA-filtered and incinerated.

  • The Class III cabinet is totally enclosed, ventilated, and of leak-tight construction.
  • Users conduct operations in the cabinet through attached arm-length rubber gloves, which serve as physical barriers.
  • The cabinet maintains negative air pressure of at least 0.5 inches water gauge (120Pa).
  • The BSC fan draws supply air into the cabinet through HEPA filters.
  • Treatment of exhaust air is by double HEPA filtration, or by HEPA filtration and incineration.
  • Class III cabinets are used in maximum containment laboratories (BSL-3) and may be used with agents of low, moderate, and high risk.