16. Biological Safety Cabinets (BSCs)

Biological safety cabinets (BSCs) are among the most effective, as well as the most commonly used primary containment devices in laboratories working with infectious agents. The three general types available (Class I, II, III) have performance characteristics and applications which are described here.

Properly maintained Class I and II BSCs, when used in conjunction with appropriate microbiological techniques, provide an effective containment system for safe manipulation of moderate and high-risk microorganisms (Biosafety Levels 2 and 3 agents).

• Both Class I and II BSCs have inward face velocities (75-100 linear feet per minute) that provide comparable levels of containment for laboratory workers and the immediate environment from infectious aerosols generated within the cabinet.
• Class II BSCs have the additional advantage of providing protection to the research material by high-efficiency particulate air (HEPA)-filtration of the air flow down across the work surface (vertical laminar flow).
• Class III cabinets offer the maximum protection to laboratory personnel, the community, and the environment because all hazardous materials are contained in a totally enclosed, ventilated cabinet.
• A useful link for information about BSCs is http://www.nih.gov/od/ors/ds/pubs/bsc/section1.html.

Comparison of Biological Safety Cabinets
Cabinets			Applications
Type	Face velocity fpm	Airflow Pattern	Radionuclides/ Toxic Chemicals	Biosafety Level(s)	Product Protection
Class I: Open Front	75	In at front; out rear and top through HEPA filter	NO	2.3	NO
Class II:   Type A	75	70% recirculated through HEPA; exhaust through HEPA	NO	2.3	YES
Type B1	100	30% recirculated through HEPA; exhaust via HEPA and hard-ducted	YES (Low levels volatility)	2.3	YES
Type B2	100	No recirculation: total exhaust via HEPA snd hard-ducted	YES	2.3	YES
Type B3	100	Same as IIA, but plan under negative pressure to room, and exhaust air is ducted	YES	2.3	YES
Class III	NA	Supply air inlets and exhaust through 2 HEPA filters	YES	3.4	YES

16.1 Class I

The Class I Biological Safety Cabinet is a negative-pressure, ventilated cabinet usually operated with an open front and a minimum face velocity at the work opening of at least 75 linear feet per minute (lfpm). All of the air from the cabinet is sent through a HEPA filter, either into the laboratory or to the outside.

The Class I BSC is designed for general microbiological research with low and moderate risk agents, and is useful for containment of mixers, blenders, and other equipment. These cabinets are not appropriate for handling research materials that are vulnerable to airborne contamination, since the inward flow of unfiltered air from the laboratory can carry microbial contaminants into the cabinet.

16.2 Class II

The Class II Biological Safety Cabinet is designed with inward air flow at a velocity to protect personnel (75-100 lfpm), HEPA-filtered vertical laminar airflow for product protection, and HEPA-filtered exhaust air for environmental protection.

Class II BSCs are classified into two types (A and B) based on construction, air flow velocities and patterns, and exhaust systems. Basically, Type A cabinets are suitable for work with microbiological research in the absence of volatile or toxic chemicals and radionuclides, since air is recirculated within the work area. Type A cabinets may be exhausted through HEPA filters into the laboratory, or to the outside via a 'thimble' connection to the exhaust ductwork. Type B cabinets are further sub-typed into types B1 , B2, and B3. Type B cabinets are hard-ducted to the exhaust system, and contain negative pressure planes. These features, plus an increased face velocity of 100 lfpm, allow work to be done with toxic chemicals or radionuclides.

It is imperative that Class I and II biological safety cabinets are tested and certified in situ at the time of installation within the laboratory, at any time the BSC is moved, and at least annually thereafter.

As with any other piece of laboratory equipment, personnel must be trained in the proper use of the biological safety cabinets. Of particular note are those activities which may disrupt the inward directional airflow through the work opening of Class I and II cabinets. Repeated insertion and withdrawal of the workers' arms in and from the work chamber, opening and closing doors to the laboratory or isolation cubicle, improper placement or operation of materials or equipment within the work chamber, or brisk walking past the BSC while it is in use are demonstrated causes of the escape of aerosolized particles from within the cabinet.

Class I and II cabinets should be located away from traffic patterns and doors. Fans, heating and air conditioning registers, and other air handling devices can also disrupt airflow patterns if located adjacent to the BSC. Strict adherence to recommended practices for the use of BSCs and then proper placement in the laboratory are important in attaining the maximal containment capability of the equipment as is the mechanical performance of the equipment itself.

16.3 Class III

The Class III Biological Safety Cabinet is a totally enclosed, ventilated cabinet of gas-tight construction and offers the highest degree of personnel and environmental protection from infectious aerosols, as well as protection of research materials from microbiological contaminants. Class III cabinets are most suitable for work with hazardous agents that require Biosafety Level 3 or 4 containment.

All operations in the work area of the cabinet are performed through the attached rubber gloves. The Class III cabinet is operated under negative pressure. Air supply is HEPA-filtered, and the cabinet exhaust air is filtered by two HEPA filters in series, or HEPA filtration followed by incineration, before discharge outside the facility .

16.4 Horizontal Laminar Flow "Clean Bench"

Horizontal Laminar flow clean air benches are not BSCs. They discharge HEPA-filtered air across the work surface and toward the user. These devices only provide product protection. They can be used for certain clean activities, such as the dust-free assembly of sterile equipment or electronic devices. These benches should never be used when handling cell culture materials or drug formulations, or when manipulating potentially infectious materials. The worker can be exposed to materials (including proteinaceous antigens) being manipulated on the clean bench, which may cause hypersensitivity. Horizontal clean air benches should never be used as a substitute for a biological safety cabinet in research, biomedical or veterinary laboratories and/or applications.

16.5 Vertical Laminar Flow "Clean Bench"

Vertical Laminar flow clean benches are also not BSCs. They may be useful, for example, in hospital pharmacies when a clean area is needed for preparation of intravenous drugs. While these units generally have a sash, the air is usually discharged into the room under the sash, resulting in the same potential problems as the horizontal laminar flow clean benches.

16.6 Utility Services

Utility services needed within a BSC must be planned carefully. The protection of vacuum systems must be addressed. Electrical outlets inside the cabinet must be protected by ground fault circuit interrupters and should be supplied by an independent circuit. When propane gas is provided, a clearly marked emergency gas shut-off valve outside the cabinet must be installed for fire safety. All non-electrical utility services should have exposed, accessible shut-off valves. As a rule, propane gas burners should not be operated in a BSC. They disrupt the air flow pattern and may pose fire hazards.

16.7 Ultraviolet Lamps

Ultraviolet (UV) lamps are not required in BSCs. If installed, UV lamps must be cleaned weekly to remove any dust and dirt that may block the germicidal effectiveness of the ultraviolet light. The lamps should be checked periodically with a meter to ensure that the appropriate intensity of UV light is being emitted. A sign must be placed on the entrance door when a UV lamp is operating. UV lamps must be turned off when the room is occupied to protect eyes and skin from UV exposure, which can burn the cornea and cause skin cancer.

16.8 Biosafety Cabinet Work Practices

1. Plan procedures carefully.
2. Start-up the biosafety cabinet and let it run at least 5-10 minutes before beginning to work.
3. Do not overload cabinet. Separate clean from contaminated items (work from clean to dirty).
4. Wear gloves and long sleeved lab coats.
5. Decontaminate the cabinet with disinfectant before and after work is completed.
6. Protect vacuum system by trap.
7. Avoid disruption of airflow (turbulence); extra items storage and movement in and around the cabinet.
8. Perform work 10-15 cm. back from the front intake grille.
9. Equipment that causes turbulence should be placed in back 1/3 of work surface. Do not block the back grille.
10. Avoid use of flame. Turbulence and filter damage may occur.
11. Avoid use of UV light.
12. Check the periodical test performance sticker affixed on the cabinet.