Legionella Testing / Biosafety Training


Richard W Gilpin PhD Blog

B2 BSC Safety and Operational Issues

Safety Issue:

The B2 BSC alarm activates during significant loss of exhaust airflow caused by exhaust fan failure or high outdoor wind velocities. Loss of exhaust airflow (alarm) results in loss of airflow into the BSC work opening (inflow) while the supply air blower continues to operate for a short time period. This pressurizes the work area resulting in loss of personnel protection with aerosols in the BSC work area flowing out the work opening into the laboratory.

Operational and maintenance issues:

Each B2 BSC must have a direct-connected, dedicated exhaust system with heavy-gauge ductwork and roof top high capacity exhaust fan. The B2 BSC exhaust duct cannot be ganged with exhaust from other B2 BSCs or other exhausted laboratory devices such as chemical fume hoods or canopy-connected BSCs.

B2 BSCs are more expensive to operate because they exhaust to the outside up to 1,200 cubic feet per minute of conditioned room air and their exhaust HEPA filters require more frequent replacement because their exhaust fans operate continuously.

The area directly above the B2 BSCs exhaust ducting should be clear of structural elements, water and utility lines, or other fixed obstructions. There should be enough clearance to allow for the passage of a 12 inch (300 mm) diameter duct. Avoid elbows directly above the cabinet’s exhaust duct or an excessive number of elbows to clear other items.

The roof exhaust system should have a stack that extends straight upward at least 10 ft (3 m) above the roof surface or have a stack with a smaller diameter trailing end to produce higher velocity flow to avoid re-entrainment by the building. The stack elevation should be increased when necessary to avoid the influence of surrounding structures. Rain caps or any other structure that deflects the straight upward flow of the discharged exhaust air should be avoided. No precipitation can enter the stack when air is being exhausted at normal stack velocities. To take care of precipitation during periods when system is shut off, a 1 inch (25 mm) hole can be drilled in the lowest point of the fan casing to allow water to drain onto the roof.

It is recommended that roof exhaust fans be energized by direct-connected electric motors to avoid failures caused by slipping and breaking belts. Another advantage of direct-connected fans is the ability to use the motor non-function to activate the B2 BSC alarm in the laboratory, whereas a malfunctioning belted fan motor continues to operate when the the fan is idle.


This information was derived from NSF/ANSI 49 - 2016 Annex E "Biosafety Cabinet Selection, Installation, Use, Lifespan and Decommissioning." This free (not for sale) document from NSF International, Ann Arbor MI, is the United States’ normative reference for biosafety cabinets (BSCs). It is available at the NSF website:  http://www.nsf.org/newsroom_pdf/NSF_49-2016_Annex_E.pdf