Office of Health and Safety

Primary Containment for Biohazards

SECTION VI

Whereas biological safety cabinets are considered to be the primary safety barrier for manipulation of infectious materials, the laboratory room itself is considered to be the secondary safety barrier.²² Inward directional air flow is established² by exhausting a greater volume of air than is supplied to a given laboratory and by drawing makeup air from the adjacent space. This is optional at biosafety level 2 but must be maintained at BSL-3.^6,28 The air balance for the entire facility should be established and maintained to ensure that air flow is from areas of least- to greater contamination.

At BSL-3 and BSL-4, exhaust laboratory air must be directly exhausted since it is considered potentially contaminated. This concept is referred to as a dedicated single-pass exhaust system. The exhausted room air can be HEPA-filtered when a high level of aerosol containment is needed, which is always true at BSL-4 and is optional at BSL-3. When the building exhaust system is used to vent a ducted BSC, the system must have a sufficient capacity to maintain the exhaust flow if changes in the static pressure within the system should occur. Otherwise, each cabinet must have a dedicated exhaust system.

The room exhaust system should be sized to handle both the room and all containment devices vented through the system. Adequate supply air must be provided to ensure appropriate function of the exhaust system. The facility engineer should be consulted before locating a new cabinet requiring connection to the building exhaust system. Right angle bends, long horizontal runs, and transitional connectors within the systems will add to the demand on the exhaust fan. The building exhaust air should be discharged away from supply air intakes, to prevent entrainment of exhausted laboratory air back into the building air supply system.

Utility services needed within a BSC must be planned carefully. Protection of vacuum systems must be addressed (Fig. 12). Electrical outlets inside the cabinet must be protected by ground fault circuit interrupters and should be supplied by an independent circuit. When propane gas is provided, a clearly marked emergency gas shut-off valve outside the cabinet must be installed for fire safety. Consider providing a timed shutoff valve for the gas service. All nonelectrical utility services should have exposed, accessible shut-off valves. Compressed air should not be provided because of the potential for aerosol generation in the event a pressurized vessel fails.

Ultraviolet (UV) lamps are not required in BSCs. If installed, UV lamps must be cleaned weekly to remove any dust and dirt that may block the germicidal effectiveness of the ultraviolet light. The lamps should be checked periodically with a meter to ensure that the appropriate intensity of UV light is being emitted. UV lamps must be turned off when the room is occupied to protect eyes and skin from UV exposure, which can burn the cornea and cause skin cancer.

Biological safety cabinets were developed (see Section I) as work stations to provide personnel, product and environmental protection during the manipulation of infectious microorganisms. Certain considerations must be met to ensure maximum effectiveness of these primary barriers. Whenever possible, an adequate clearance should be provided behind and on each side of the cabinet to allow easy access for maintenance, and to ensure that the air return to the laboratory is not hindered. A 12- to 14-inch clearance above the cabinet may be required to provide for accurate air velocity measurement across the exhaust filter surface^14,15 and for exhaust filter changes. When the BSC is hard-ducted or connected by a thimble unit to the ventilation system, adequate space must be provided so that the configuration of the duct work will not interfere with air flow. The thimble unit must provide access to the exhaust filter for testing of the HEPA filter.

The ideal location for the biological safety cabinet is remote from the entry (i.e., the rear of the laboratory away from traffic), since people walking parallel to the face of a BSC can disrupt the air curtain.^5,23,25 The air curtain created at the front of the cabinet is quite fragile, amounting to a nominal inward and downward velocity of 1 mph. Open windows, air supply registers, or laboratory equipment that creates air movement (e.g., centrifuges, vacuum pumps) should not be located near the BSC. Similarly, chemical fume hoods must not be located close to BSCs.

HEPA filters, whether part of a building exhaust system or part of a cabinet, will require replacement when they become loaded to the extent that sufficient air flow can no longer be maintained. Filters must be decontaminated before removal. To contain the formaldehyde gas typically used for microbiological decontamination, exhaust systems containing HEPA filters require airtight dampers to be installed on both the inlet and discharge side of the filter housing. This ensures containment of the gas inside the filter housing during decontamination. Access panel ports in the filter housing also allow for performance testing of the HEPA filter (see Section VII).

A bag-in/bag-out (BIBO) filter assembly^9,21 (Figure 13) can be used in situations where HEPA filtration is necessary for operations involving biohazardous materials and hazardous or toxic chemicals. The BIBO system is used when it is not possible to decontaminate the HEPA filters with formaldehyde gas, or when hazardous toxic chemicals have been used in the BSC, but provides protection against exposure for the maintenance personnel and the environment Note, however, that this requirement must be identified at the time of purchase and installation; a BIBO assembly cannot be added to a cabinet after-the-fact.

This page last reviewed May 17, 2001

Office of the Director/Administrator
Centers for Disease Control and Prevention
and Agency for Toxic Substances and Disease Registry
Atlanta, GA