The NEBB Professional 2024 - Quarter 2

EXHAUST SYSTEM INTEGRATION OF CLASS II TYPE A2 VENTED BIOLOGICAL SAFETY CABINETS By Matt Lemieux

Biological safety cabinets (BSCs) are ubiquitous in laboratories, cleanrooms, and many other research and production venues. BSCs are utilized as primary engineering controls (PEC). They are distinct from fume hoods and likewise from unidirectional (laminar) flow benches, and are tragically, all too often, mistaken as such by facilities planners, engineers, and end-us ers. Biological safety cabinets, designated as Class II by the regulatory standard, NSF/ANSI 49, differ from other primary engineering controls in that they provide three kinds of protection: (1) The personnel protection offered by a chemical fume hood, (2) the sterile work area provided by the unidirectional HEPA flow bench, providing product protection, and (3) the environmen tal protection offered by the HEPA filtered BSC airflow exhaust pathway. This complex, integrated engineered control is accom plished by clever design of the cabinet's internal ple nums, work surfaces, discharge plenums, and ducted connections to building exhaust systems. Work sur faces are exposed strictly to HEPA-filtered sterile air and personnel work access openings are subject only to protective ambient room inflow velocities. These two airstreams only mix beyond the critical work areas in the cabinet’s rear negative pressure plenum and a portion of the internal cabinet blower air–equivalent to the work access opening inflow for Type A cabi nets–is exhaust through the cabinet’s exhaust HEPA filter. BSCs are fixed sash height devices (unlike fume hoods), although they are equipped with slidable or hinged windows for product insertion, removal, and cleaning. When in operation and product manipula

tions are taking place, the hood sash is at a predeter mined fixed elevation. As a rule, Class II BSCs can be either self-contained and free-standing in the laboratory (Type A) or con nected to building exhaust systems (Type B). A vari ation of type A can also connect to the building ex haust system – these are nominated as Class II, Type A2-vented. These BSCs were formerly designated as Class II Type A/B3. In this arrangement, the cabinet’s internal HEPA-filtered exhaust discharges to a re mote exhaust fan to provide additional environmental protection. The reason for opting for this installation configuration is because HEPA filters are limited in their filtering efficiency to discrete particulate mat ter, this building-connected installation is appropriate when volatile hazards or radioisotopes are used in small quantities as part of the product manipulations. Common applications for the Class II A2 Vented cabi nets include isoflurane used for anesthetizing animals, volatile chemotherapeutic agents, combustion prod ucts, and small amounts of radioisotope vapors. Figure 1 illustrates the airflow patterns inside a Class II Type A2-Vented BSC in an elevation cross-section. With this design, the work access opening air quanti ty is discharged out through the exhaust HEPA where it joins with bypass room air to comprise the remote exhaust fan air quantity. A recirculation loop of HEPA filtered air contained in the cabinet provides the sterile work surface conditions. The bypass air is normally in duced into the canopy exhaust duct downstream of the cabinet exhaust and below the finished ceiling of the room. Class II Type A cabinets are colloquially known

The NEBB Professional | Quarter 2 | 2024

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