Abstract
The operational air flow balance recommended by the Class II biological safety cabinet (BSC) manufacturer is called the nominal setpoint. Few, if any, BSCs actually operate at nominal setpoint under normal laboratory conditions. The purpose of this article is to define statistically the air flow boundaries within which a BSC will perform adequately. Performance is quantified by the microbiological aerosol tracer test as defined in the National Sanitation Foundation Standard #49. The boundaries which delineate passing test results from failing are called the performance envelope. Performance envelopes for three different models of BSCs are reported. They were determined by combining test data from at least five individual cabinets from each model. An ad hoc statistical method for calculating confidence limits around the performance envelope lines was developed to enhance the interpretation of conflicting performance envelope data. Results indicated that each model of BSC has its own unique performance envelope in terms of size and shape. Additionally, the magnitude of the standard deviation of the air flow balance data from the performance envelope boundaries varies from one model to another. This indicates that there is a potential for any single microbiological test to be a misrepresentation of a generalized pass-fail situation, and that potential varies from model to model. Replicate microbiological aerosol tracer testing must be done over a wide range of air flow balance setpoints to ensure BSC performance under laboratory conditions.
