Primary Containment: Biological Safety Cabinets

Biological Safety Cabinets (BSCs) are among the most effective and 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 in this appendix. Properly maintained Class I and II BSCs, when used in conjunction with good microbiological techniques, provide an effective containment system for safe manipulation of moderate and high-risk microorganisms (Biosafety Level 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 to protect laboratory workers and the immediate environment from infectious aerosols generated within the cabinet. Class II BSCs also protect the research material itself through high-efficiency particulate air filtration (HEPA filtration) of the airflow 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.

1.       Class I

Class I BSCs are currently being manufactured on a limited basis; many have been replaced by Class II BSCs. A class I BSC does not offer product protection, it does offer personal and environmental protection from the sample used.

The Class I Biological Safety Cabinet (Fig. 1) 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 exhausted 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.

The Class I BSC can also be used with an installed front closure panel without gloves, which will increase the inward flow velocity to approximately 150 lfpm. If such equipped cabinets are ducted to the outside exhaust, they may be used for toxic or radiolabelled materials used as an adjunct to microbiological research. Additionally, arm-length rubber gloves may be attached to the front panel with an inlet air pressure release for further protection. In this configuration, it is necessary to install a make-up air inlet fitted with a HEPA filter in the cabinet.

2.       Class II

This Biosafety Cabinet is designed to provide personal, product and environmental protection when used properly.

The Class II Biological Safety Cabinet is designed with inward airflow at a velocity to protect personnel (75-100 lfpm), HEPA-filtered downward vertical laminar airflow for product protection, and HEPA-filtered exhaust air for environmental protection. Design, construction, and performance standards for Class II BSCs, as well as a list of products that meet these standards, have been developed by and are available from the National Sanitation Foundation International, Ann Arbor, Michigan. Utilization of this standard and list should be the first step in selection and procurement of a Class II BSC.

Class II BSCs are classified into two types (A and B) based on construction, airflow velocities and patterns, and exhaust systems. Class II A cabinets are sub-typed into type A1 and A2. Type A1 cabinets are suitable for microbiological research in the absence of volatile or toxic chemicals and radionuclides, since 70% of the air is recirculated within the cabinet. Type A1 cabinets may be exhausted into the laboratory or to the outdoors via a "thimble" connection to the building exhaust system. Type A2 cabinets are similar to A1 but will allow a minimal of radionuclides and volatile or toxic chemical to be used when they are exhausted to the outdoors via a "thimble" connection to the building exhaust system. Type B cabinets are further sub-typed into types B1 and B2. A comparison of the design features and applications are presented in Figures 3.5.4. and 3.5.5, respectively. Type B cabinets are hard-ducted to the building exhaust system and contain negative pressure plena. These features, plus a face velocity of 100 lfpm, allow work to be done with toxic chemicals or radionuclitides. Type B1 will recirculate about 30% air and exhaust 70% through the hard duct. Type B2 cabinets will exhaust 100% of the air.

It is imperative that Class I and II biological safety cabinets be tested and certified at the time of installation within the laboratory, at any time the BSC is moved or maintenance is performed , and at least annually thereafter. Certification at locations other than the final site may attest to the performance capability of the individual cabinet or model but does not supersede the critical certification prior to use in the laboratory.

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 activities that may disrupt the inward directional airflow.

         Sweeping sideways motion of hands in cabinet create eddies in airflow

         Repeated insertion and withdrawal of the workers' arms into and out of 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

         Brisk walking past the BSC while it is in use

         Improper placements of BSC in the lab room.

All above have been demonstrated to cause the escape of aerosolized particles from within the cabinet.

Class I and II cabinets should be located away from traffic patterns and doors. Airflow from fans, room air supply louvers and other air moving devices can disrupt the airflow pattern at the face of the cabinet. Strict adherence to recommended practices for the use of BSCs and their proper placement in the laboratory are as important in attaining the maximum containment capability of the equipment, as is the mechanical performance of the equipment itself.

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 attached arm length rubber gloves or half-suits. The Class III cabinet is operated under negative pressure. Supply air is HEPA-filtered and the cabinet exhaust air is filtered through two HEPA filters in series, or HEPA filtration followed by incineration, before discharge outside of the facility.

All equipment required by the laboratory activity, such as incubators, refrigerators, and centrifuges, must be an integral part of the cabinet system. The Class III cabinet must be connected to a double-doored autoclave and/or chemical dunk tank used to sterilize or disinfect all materials exiting the cabinet, and to allow supplies to enter the cabinet. Several Class III cabinets are therefore typically set up as an interconnected system.

 References

U.S. Department of Health and Human Services. Primary Containment of Biohazards: Selection, Installation and Use of Biological Safety Cabinets. (Washington: GPO, 1995)

National Sanitation Foundation Standard 49. 1983.

Class II (Laminar Flow) Biohazard Cabinetry. Ann Arbor, Michigan.

4.         Table1- Comparison of Biological Safety Cabinets

Type

Face velocity (lfpm)

Airflow Pattern

Radionuclides /

Toxic Chemicals

Biosafety Level(s)

Product Protection

Class I* open front

75

In at front; rear and top through HEPA filter

No

2,3

No

Class II Type A1

75

70% recirculated through HEPA; exhaust through HEPA

No

2,3

Yes

Type A2

100

Same as IIA1

No if not ducted

2,3

Yes

Type A2

100

Same as IIA1, but plena under negative pressure to room and exhaust air is ducted

Yes (Low levels/volatility)

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 and hard ducted

Yes

2,3

Yes

Class III

NA

Supply air inlets and exhaust through 2 HEPA filters

Yes

3,4

Yes

* Glove panels may be added and will increase face velocity to 150 lfpm; gloves may be added with an inlet air pressure release that will allow work with chemicals/radionuclides.