What type of airflow should be used in a Class III BSC?  Is higher velocity better?

Class II BSC vs. Class III BSC Airflow

Unlike Class II BSC which employs laminar flow to protect personnel, the Class III Biological Safety Cabinet does not have an open sash in the front, hence does not require laminar airflow to provide personnel protection.  The main consideration for laminar air flow in a Class III BSC is for product protection.  Laminar flow could be important if an internal process generates copious amounts of aerosol, when work is conducted with fine powders, or if there is a risk of cross-contamination between different procedures being performed in the cabinet. Laminar air flow has a set mass airflow where clean HEPA filtered air comes from one direction at a given speed to entrain particles and carry them directly to the exhaust HEPA. The velocity can be very low, as low as 30 ft/min. However, note that 30 ft/min may cause extremely fine powders to become aerosolized. 

Generally, Class III Biosafety Cabinets use turbulent air flow designs. In a turbulent airflow design clean HEPA filtered air is continuously supplied to the cabinet where it dilutes the concentration of aerosolized particles by carrying them to the exhaust HEPA. This is a more passive mechanism of particle removal as compared to that of the air current generated when laminar airflow is established. The rate at which the particles are exhausted depends on the supply velocity (which is equal to the exhaust velocity). In reality, work in a Class III BSC is conducted methodically and carefully.  Most activities conducted in a Class III BSC produce minimal aerosols so turbulent airflow is the norm.  Turbulent airflows are easily adjustable and can have lower air velocity than those required to maintain laminar airflow, hence can pose less of a problem when working with fine powders.

Another question that comes up is whether the Biological Safety Cabinet should be operated at high airflow velocity to remove particles more rapidly.  Typically higher velocity airflows are used when working with volatile chemicals, but not with microbiological agents or toxins.  High velocity airflow can inadvertently, and very effectively, cause powders to be disseminated throughout the interior of the BSC.

A decision regarding whether laminar or turbulent airflow is needed, and the velocity of supply air required for operations should be made based on the anticipated work and user needs. Use of laminar air flow in a Class III Biosafety Cabinet will typically increase the volumetric supply and exhaust airflow  as compared to a BSC using turbulent airflow.  Higher velocity airflow will similarly increase volumetric supply and exhaust as compared to maintaining low velocity airflow.  Increased exhaust flow rate from the Class III BSC should be considered during facility and HVAC design if the cabinet is connected to facility supply air and is to be exhausted out of the building.