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The Class I biological safety cabinet is an open-fronted, negative- pressure, ventilated cabinetwith a minimum inward face velocity at the work opening of 75 feet per minute(0.4 m/s). Theexhaust air from the cabinet is filtered by a high efficiency particulate air (HEPA) filter Thiscabinet may be used in three operational modes: with a full-width open front, with an installedfront closure panel not equipped with gloves and with an installed front closure panel equippedwith arm-length rubber gloves.

The Class II vertical laminar-flow biological cabinet is an open- fronted, ventilated cabinet with an average inward face velocity at the work opening of 75 feet per minute (0.4 m/s). This cabinetprovides a HEPA- filtered recirculated mass airflow within the work space. The exhaust air fromthe cabinet is also filtered by HEPA filters.

The Class III cabinet is a totally enclosed ventilated cabinet of gas-tight construction. Operationswithin the Class III cabinet are conducted through attached rubber gloves. When in use, the ClassII cabinet is maintained under negative air pressure of at least 0. 5 inches water (124.5 N/m2 or0.9 mm Hg). Supply air is drawn into the cabinet through HEPA filters. The cabinet exhaust air isfiltered by two HEPA filters installed in series. The exhaust fan for the Class III cabinet isgenerally separate from the exhaust fans of the facility' s ventilation system.

Personnel protection provided by Class I and Class II cabinet is dependent on the inward airflow.Since the face velocities are similar, they generally provide an equivalent level of personnelprotection. The use of these cabinets alone, however, is not appropriate for containment ofhighest-risk infectious agents because aerosols may accidentally escape through the open front.The use of a Class II cabinet in the microbiological laboratory offers the additional capability andadvantage of protecting materials contained within it from extraneous airborne contaminants.This capability is provided by the HEPA-filtered, recirculated mass airflow within the workspace.

The Class III cabinet provides the highest level of personnel and product protection. Thisprotection is provided by the physical isolation of the space in which the infectious agent ismaintained. When these cabinets are required, all procedures involving infectious agents arecontained within them. Several Class III cabinets are therefore typically set up as aninterconnected system. All equipment required by the laboratory activity, such as incubators,refrigerators and centrifuges, must be an integral part of the cabinet system.Double-doored autoclaves and chemical dunk tanks are also attached to the cabinet system toallow safe introduction and removal of supplies and equipment.

Note: All Class III cabinets and those Class II cabinets being used with category 3 agents arerequired to be tested and certified yearly. Contact Environmental Health and Safety forarrangements. All biological safety cabinets are required to be tested and certified prior to initialuse and following relocation.