Crane (1994) points out that
Inadequate attention is often given to proper selection of biological safety cabinets in biomedical laboratories. Many "type B" cabinets are selected because the user feels that an exhausted cabinet is inherently safer than a recirculating model. If they are properly installed and maintained, class II, type B cabinets may provide some degree of flexibility for chemical vapor use (Stuart et al. 1985 ref233). However, many (if not most) BSL1, BSL2 and BSL3 laboratories can be adequately served by Class II, Type A cabinets that exhaust air through a HEPA filter back into the room. This eliminates the need for direct connection to the HVAC system and the more difficult installation that ensues.
Simons (1991) points out that
[T]he Class II, Type A cabinet does not require a separate exhaust system for proper operation. Since the exhaust air from an internal fan in the cabinet is HEPA filtered, the air can be exhausted directly to the laboratory if the facility operator and local codes permit that practice after an adequate risk assessment is performed.
Eagleson (1990) explains that
Class II, Type A minimally impact the building's HVAC system, affecting it only as an additional heat load and in determining the location of supply air diffusers. However, it should be noted that this heat load can be substantial since researchers often group several BSCs into a single laboratory.
Simons adds that the "failure of the building exhaust system does not affect the biohazard protection of the Class II, Type A cabinets since protection is independent of the building exhaust system." [Simons, 1991]
