Biological Safety Cabinets

BSCs are designed to provide personnel, environmental and product protection when appropriate practices and procedures are followed. Three kinds of biological safety cabinets, designated as Class I, II and III, have been developed to meet varying research and clinical needs.

Proper maintenance of cabinets used for work at all biosafety levels cannot be over emphasized. A BSC must be routinely inspected and tested by trained personnel, following strict protocols, to verify that it is working properly. This process is referred to as certification of the cabinet and should be performed annually.

The Class I BSC

The Class I BSC provides personnel and environmental protection, but no product protection. It is similar in air movement to a chemical fume hood, but has a HEPA filter in the exhaust system to protect the environment.

In the Class I BSC, unfiltered room air is drawn across the work surface. Personnel protection is provided by this inward airflow as long as a minimum velocity of 75 linear feet per minute (lfpm) is maintained through the front opening. The use of the Class I BSC has declined due to lack of product protection but, in many cases, Class I BSCs are used specifically to enclose equipment (e.g., centrifuges, harvesting equipment or small fermenters), or procedures with potential to generate aerosols (e.g. cage dumping, culture aeration or tissue homogenation).

Some Class I models used for animal cage changing are designed to allow recirculation of air into the room after HEPA filtration and may require more frequent filter replacement due to filter loading and odor from organic materials captured on the filter. The re-circulating Class I BSC should be annually certified for sufficient airflow and filter integrity.

The Class II BSC

As biomedical researchers began to use sterile animal tissue and cell culture systems, particularly for the propagation of viruses, cabinets were needed that also provided product protection. In the early 1960s, the “laminar flow” principle evolved. Unidirectional air moving at a fixed velocity along parallel lines was demonstrated to reduce turbulence and aid in the capture and removal of airborne contaminants from the air stream. Biocontainment technology also incorporated this laminar flow principle with the use of the HEPA filter to provide a particulate-free work environment. This combination of technologies serves to protect the laboratory worker from potentially infectious microorganisms or materials being manipulated within the cabinet and provides necessary product protection, as well. Hence Class II BSCs protect personnel, the environment AND the product.

Class II BSCs are partial barrier systems that rely on the laminar movement of air to provide containment. If the air curtain is disrupted (e.g., movement of materials in and out of a cabinet, rapid or sweeping movement of the arms) the potential for contaminant release into the laboratory work environment is increased as is the risk of product contamination.

There are 4 types of Class II BSCs (Types A1, A2, B1 and B2) each with unique characteristics, however, in all four, the airflow is drawn into the front grille of the cabinet, providing personnel protection. In addition, the downward laminar flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination across the work surface of the cabinet. Because cabinet exhaust air is passed through a certified HEPA filter, it is particulate-free (environmental protection), and may be recirculated to the laboratory or discharged from the building via a canopy connection (Type A1 and A2). Exhaust air from Types B1 and B2 BSCs must be discharged to the outdoors via a hard connection. HEPA filters are effective at trapping particulates and thus infectious agents but do not capture volatile chemicals or gases. Only Types A2, B1 and B2 BSCs exhausting to the outside should be used when working with volatile, toxic chemicals, but amounts must be limited.

All Class II cabinets are designed for work involving microorganisms assigned to biosafety levels 1, 2 and 3. Class II BSCs provide the microbe-free work environment necessary for cell culture propagation and also may be used for the formulation of nonvolatile anti-neoplastic or chemotherapeutic drugs.

Horizontal Laminar Flow “Clean Bench”

 Horizontal laminar flow "clean benches" are not BSCs. These pieces of equipment discharge HEPA-filtered air from the back of the cabinet 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. Clean benches should never be used when handling cell culture materials or drug formulations, or when manipulating potentially infectious materials. The worker will be exposed to the materials being manipulated on the clean bench potentially resulting in hypersensitivity, toxicity or infection depending on the materials being handled. Horizontal airflow "clean benches" must never be used as a substitute for a biological safety cabinet. See figure below.

Vertical Laminar Flow “Clean Bench”

Vertical laminar flow clean benches also are not BSCs. They may be useful, for example, in hospital pharmacies when a clean area is needed for preparation of intravenous solutions. While these unit generally have a sash, the air is usually discharged into the room under the sash, resulting in the same potential problems presented by the horizontal laminar flow clean benches. These benches should never be used for the manipulation of potentially infectious or toxic materials. See figure below.

It is extremely important that correct work practices are used during BSC work. It is also crucial to perform annual recertification to ensure proper calibration and working of the unit. The Office for Regulatory Compliance Biosafety Program offers a training module on BSC theory, care and use. Please contact the BSO for further information.

A complete description of the BSC can be found in the 5th edition BMBL:

http://www.cdc.gov/od/ohs/biosfty/bmbl5/Appendix%20A_Greenbook%20for%20BMBL_Final.pdf

A Note on Chemical Fume Hoods:

Chemical fume hoods are designed for worker protection only. They do not provide any protection from room contaminants for the work done in the hood. Chemical fume hoods operate by pulling large quantities of air into the front or face of the hood, through the hood box and out through a series of slots in the rear of the hood box using large fans (normally located on the facility roof). One hundred percent of the air that enters the hood is exhausted from the building. Chemical fume hoods do not provide environmental protection against infectious agents because the exhausted air is not filtered through a HEPA filter. The function of the fume hoods is dependent on the constant flow of air. Objects that obstruct air flow will affect performance. Check for the annual certification date to assure your hood has been tested and is performing as intended. Do not use as a BSC!