In the past, there was no practical means of quantifying the performance of a biological safety cabinet (BSC) in the field. The microbiological test defined by NSF Standard 49 validates performance by using actual organisms, but such a procedure is not suitable for an operational environment. The air flow test, currently used for certification in the field, does not measure performance directly. It assumes that containment and product protection performance are achieved if the cabinet air flows are as originally designed.