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Fume hoods, pharmacy barrier isolators and some types of biosafety cabinets require a remote exhaust system to remove potentially hazardous particulates and/or gases and vapors from the lab environment.


The following information and links may be useful for those who need to design an HVAC system prior to purchasing and installing new biosafety cabinets.

Construction of Biosafety Cabinet Exhaust Systems


Of course, ductwork construction must be strong enough to withstand the static pressure of the lab equipment in order to prevent distortion or collapse. Additionally, personnel protection depends on a steady flow of air, so it’s important to limit the number of turns or bends between the biosafety cabinet exhaust connection and the roof exhaust blower. Class II Type B1 and B2 cabinets require a bubble tight damper (also known as an air tight damper) at or near the equipment connection. This damper seals off the cabinet from the building exhaust ductwork during gas decontamination and is also used by certifiers when testing the exhaust airflow alarm.

Dedicated Exhaust Systems


NSF/ANSI Standard 49 Annex E recommends that all Class II Type B1 and B2 cabinets have a dedicated exhaust system to minimize potential fluctuations in airflow and pressure that may result in a loss of containment at the cabinet work access opening. However, many constant air volume valves are being used in manifolded design facilities with varying results. These automated valves have been shown to maintain proper exhaust airflow during static pressure fluctuations that would typically cause a loss in cabinet face velocity.

Measuring Biosafety Cabinet Exhaust Airflow


Biosafety manufacturers should provide two exhaust requirements for their biosafety cabinets, fume hoods and pharmacy barrier isolators: exhaust airflow volume and duct static pressure. Maintaining these values is critical for proper cabinet operation, especially for Class II Type B2 cabinets, which require more exhaust airflow and static pressure compared to other ventilation equipment. Each value should be verified prior to equipment installation (unfortunately, this is often neglected).

The exhaust static pressure measurement should be taken near the equipment/duct connection, typically no more than 2 duct diameters away – not at the building exhaust fan. A duct traverse is the traditional method for obtaining the exhaust airflow rate. Per ASHRAE 111-2008 Standards, readings should be taken a minimum of 7.5 duct diameters downstream of any turns or bends and 2.5 duct diameters upstream of any turns or bends, respectively. These measurements are then converted to obtain the exhaust airflow rate.

For Class II Type B1 and B2 biosafety cabinets, manufacturers are required to provide a concurrent balance value, or exhaust flow measurement, after the biosafety cabinet has been airflow certified using a Direct Instrument Measurement (DIM) device and a thermal anemometer.

What Is a Concurrent Balance Value?

The Concurrent Balance Value was implemented by NSF Joint Committee members to minimize exhaust airflow discrepancies occurring in the field. This value is determined using the duct traverse measurement method as outlined above. Prior to determining the concurrent balance value, it must be confirmed that the cabinet is operating at its nominal set points for inflow and downflow velocity within 3 FPM. Inflow velocity is set using a DIM, the accuracy of which must be at least +/- 3% and +/- 7 CFM. The static pressure is also measured approximately two duct diameters from the cabinet exhaust connection, and filter load and tolerance values are added to the base static pressure to account for filter loading (0.3″ W.G. for Type B1 biosafety cabinets and 0.7″ W.G. for Type B2 cabinets).

The resulting values may be used for design and balance of exhaust/supply HVAC requirements.

Additional Resources


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