What is a Biosafety Cabinet?
A biosafety cabinet
(BSC) is not a chemical fume hood. Fume hoods are designed to remove chemical fumes and aerosols away from the work area. BSCs are designed to provide both a clean work environment and protection for employees who work with biological hazards. BSCs use vertical laminar airflow to create a barrier to airborne particles, such as microorganisms. They use High Efficiency Particulate Air (HEPA) filters to clean air going into the work area and out to the environment. The air in most BSCs is recirculated over the work area through the HEPA filter. The HEPA filter removes airborne particles from the air, but does not remove chemical fumes.
How does a biological safety cabinet work?
There are three kinds of safety cabinets, Classes I, II, and III. Class II and Class III biological safety cabinets provide personnel, environmental as well as product protection. Whereas Class I safety cabinet, which is the most basic one, provides personnel and environmental protection only. When properly used Biological safety cabinets have been shown to be highly effective in reducing laboratory-acquired infections and cross-contaminations of cultures.
Class 1 Biological Safety Cabinets
Class I BSC is the first designed and simple Biological Safety Cabinet (BSC) which provides personnel and environmental protection but not product production (as unsterilized room air is drawn over the work surface). Class I BSCs are suitable for work with Risk Group 1 (RG1), Risk Group 2 (RG2), and Risk Group 3 (RG3) biological material.
The air enters the cabinet via the front aperture passing through a built-in exhaust fan, HEPA and/or Carbon filter, thus providing operator and environmental protection. The air then exits the cabinet at the rear of the work surface. The escape of any airborne particulates generated within the cabinet are therefore controlled by means of the inward airflow through the front aperture and by filtration/absorption of the exhausted air.
Unlike fume hoods, the HEPA filter in the cabinet protects the environment by filtering the air before it is exhausted.
A Class 1 Safety Cabinet is 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. In these circumstances a Biological Safety Cabinet Class II is more applicable.
Class 2 Biological Safety Cabinets
A Class II Biological Safety Cabinet (BSC) is a ventilated cabinet, which provides personnel, product and environmental protection. It is commonly found in clinical and research laboratories working with infectious agents in Risk Groups 2, 3 and 4 (if positive-pressure suits are used) or with tissue culture.
There are four types (A1, A2, B1, and B2 ) of Class II BSCs. The main differences between the types are the ratio of air exhausted from the BSC to the air that is recirculated within the BSC, and the type of exhaust system present.
About 90% of all biosafety cabinets installed are Type A2 cabinets. There is a limited need for Class II Type B biological safety cabinets. In addition, Class II Type B biological safety cabinets require very specific installation and operating conditions to function correctly.
The Class 2 Biological Safety cabinet must meet the requirements for personnel, environmental and product protection. The air-flow is drawn into the work chamber via the front aperture and continues under the worktop and goes up the back plenum, where 70 % is being recirculated through the main HEPA filter to provide down flow and 30 % exits out through the HEPA filter to exhaust. The vertical laminar flow Biological Safety Cabinet provides operator protection by means of inflow, product protection by means of down flow and environmental protection by means of the filtered exhaust.
The Biological Safety Cabinet is equipped with dual HEPA filters according to the European standard EN 12469 and can be certified as per the standard by TüV. It can also be manufactured with triple filters according to DIN 12980. The triple HEPA filter system ensures the elimination of any possible cross contamination, as the air in the work chamber has to pass through two sets of main HEPA filters. This makes the triple filter cabinet the ideal choice of cabinet for working with hazardous materials such as cytostatics, virus manipulation and category 3 pathogens. The triple filter Biological Safety Cabinet is easier to service as no prior decontamination is needed.
Class 3 Biological Safety Cabinets
The Class III biological safety cabinet was designed for work with biosafety level 4 (BSL-4) microbiological agents, and provides maximum protection to the environment and the worker. It is a gas-tight enclosure with a non-opening, completely sealed, viewing window. Access to the interior of the cabinet is through a double-door pass-through “interchange” box (such as an autoclave) that can be decontaminated between uses and/or a dunk tank that is accessible through the cabinet floor into the floor of a Class I or Class II cabinet. Reversing that process allows for safe removal of materials from the Class III biosafety cabinet.
Both supply and exhaust air are HEPA filtered. Exhaust air must pass through two HEPA filters, or a HEPA filter and an air incinerator, before discharge to the outdoors. Air flow is maintained by a dedicated independent exhaust system exterior to the cabinet, which keeps the cabinet and all associated ducting under negative pressure (usually about 0.5 inches of water pressure).
Long, heavy-duty rubber gloves are attached in a gas-tight manner to ports in the cabinet and should permit replacement without compromising containment. Although these gloves restrict movement for the manipulation of the materials isolated inside the cabinet, they prevent the user’s direct contact with the hazardous materials. The trade-off is clearly on the side of maximizing personal safety.
Depending on the design of the cabinet, the supply HEPA filter provides particulate-free, somewhat turbulent, air flow within the work environment. To minimize interior turbulence, however, the inflow air should be ducted to a distribution manifold located on the rear wall just above the work surface. The distribution manifold also provides the means to house an anti-static bar, bathing plastic materials with both positive and negative ions.
Several Class III cabinets can be joined together in a “line” to provide a larger work area. Such cabinet lines are custom-built; the equipment installed within the cabinet line (e.g., refrigerators, small elevators, shelves to hold small animal cage racks, microscopes, centrifuges, incubators, etc.) is generally custom-built as well. Furthermore, Class III cabinets are usually only installed in maximum containment laboratories that have controlled access and require special ventilation or other support systems (such as steam for autoclaves). The reader should consult more definitive literature on these systems.