General Technical Bulletin

Class II Biosafety Cabinet Downflow Velocity Impact on Cross Contamination

Over the last several years, Biosafety Cabinet (BSC) manufacturers have lowered downflow velocity while maintaining inflow velocity to increase energy efficiency, reduce exhaust system requirements and lower BSC Noise and vibration levels. However, the lowering of downflow velocity may come with a consequence of greater lateral airflow movement within the work zone creating more significant potential for cross-contamination. The lowering of the downflow velocity does not compromise personnel and product containment performance. Why? Primarily, by maintaining inflow velocity above a certain level as required by the standard, typically 100 fpm (.51 mps). However, the standard does not require a specific downflow velocity and can be adjusted as needed based on the design parameters of the work zone supply diffuser and grill patterns (airflow distribution) to assure compliance to the personnel and product protection requirements of NSF/ANSI 49. NSF/ANSI 49 test procedures do test for cross-contamination, but only at the work zone sidewalls, which doesn’t necessarily translate into the lateral movement of airflow in the center of the work zone. The result of these lower downflow velocities may be a higher level of sample cross-contamination using traditional work practice guidelines.