The primary purpose of a BSC is to serve as a means to protect the laboratory worker and the surrounding environment from pathogens. All exhaust air In HEPA-filtered as It exits the blosafety cabinet, removing harmful bacteria and viruses. This is in contrast to a laminar flow dean bench, which blows unfiltered exhaust air towards the user and is not safe for work with pathogenic agents. Neither are most BSCs safe for use as fume hoods. Likewise, a fume hood falls to provide the environmental protection that HEPA filtration in a BSC would provide. However, most classes of BSCs have a secondary purposeto maintain thesterilityof matenalsinside (the.product").
The U.S. Centers for Disease Control and Prevention (CDC) classifies BSCs into three classes. These classes and the types of BSCs within them are distinguished M two ways: the level of personnel and environmental protection provided and the level of product protection provided.
Class I cabinets provide personnel and environmental protection but no product protection. In fact, the inward flow of air can contribute to contamination of samples. Inward airflow is maintained eta minimum velocity of 75 ft,/min(0.38 m/s). These BSCs are commonly used to enclose specifici equipment (e.g. centrifuges) or procedures (e.g. aerating cultures) that potentially generate aerosols. BSCs of this class are either ducted (connected to the building exhaustsystem) or unducted (recirculatingfiltered exhaust back intothe laboratory).
Class II cabinets provide both kinds of protection (of the samples and of the environment) since makeup air is also HEPA-filtered. There are four types: Type Al (formertyA). TypeA2 (formellyA/B3),Type Bl, and Type B2. Each type's requirements are defined byNSF International Standard 49, which in 2002 reclassified A/133 cabinets (classified under the latter type if connected to an exhaust duct) as Type A2. About 95% of all blosafety cabinets Jnstal led are Type A2 cabinets.
The principle of operation involves using a fan mounted in the top of the cabinet to draw a curtain of sterile air over the productsthat are being handled. The air is then drawn underneath the work surface and back uptothe top of thecabinet where it passesthrough the HEPAfilters. Theair that is exhausted is made up by air being drawn into the front of the cabinet underneath the work surface. The air being drawn in actsasa barrierto potent allycontamlnated air corningbackouttothe operator. The Type Al cabinet, formerly known as Type A, has a minimum inflow velocity of 75 ft/min. The filtered makeup air is divided equally over the work surface at about two to six inches above the work surface.
Exhaust is drawn at the bottom of the cabinet where d rises to the top. At the tap of the cabinet, 70% of Ne air recirculates through the supply HEPA filter, the other 30e0 of alr exhausted through. exhaust FEW, filter. This isdue to the relative sizes of .etwo filters, and dampers typteally allow the adjustment of this ratio. This type is not..fe for work with hazardous chemicals except when ducted, usually with a 'thimble" or eanopy hood Maur. distureng internal airflow.
The Type A2 cabinet, formerly designated A/B3, has a minimum inflow velocity of 100 ft/min. A negate. air pressure plenum surrounds all contaminated plenums that are under positive pressure. In other respects thespecdr.tionsare identicalto thoseof a TypeAlcabinet.
The Type 31 and B2 cabinets have a minimum Inflow velocity of 100 ft/min, and these cabinets must be hard-ducted to an exhaust system rather than exhausted through a thimble connecter. In contrast to the type Al and A2 cabinets. B1 cabinets split the aidlow so 6056 of air is exhausted and only 40% is recirculated.withtheair collected throughthe reargrille beingexhaLeted, and air throughthe front grille being recirculated. Since exhaust Mr Is drawn from the rear grille, the CDC advises that work with chemicals heconducted In the rear of the oabinet.The Type B2 cabinet is expensiveto operate because no air is reerculated within. Therefore, this type is mainly found In such applications as toxicology es, laboratori where the shady to safely use hazardous chemicals is Important. Additionally,Mere is the riskthatcontamnated airwould flow intothelaboratory If the exhaustsystemfor a Type Bier B2 cabinet wereto fail. To megate ths risk. cabinets of thesetypesgenerally monitor the exhaust flow, shutting Off thesupply blower and sounding an alarm dthemeaustflow Is insufficient.
Class II cabmetsarethepommonly used cabinets in clinical and research laboratories.
The Class Ill cabinet, generally only installed in maximum containment laboratories, is specifically designed for work with BSL-4 pathogenic agents, providing maximum protection. The enclosure is gas- tight, and all materials enter and leave through a dunk tank or double-door autoclave. Gloves attached to the front prevent direct contact with hazardous materials (Class Ill cabinets are sometimes called glove box). These custom-built cabinets often attach into a line, and the lab equipment installed inside is usually custom-built as well.