Which subtype of BSC Class II describes a system where most of the work surface air is recirculated?

The subtype of BSC (Biological Safety Cabinet) Class II that describes a system where most of the work surface air is recirculated is A2. This type of cabinet is designed to provide a higher level of protection by recirculating 70% of its air back into the work area while exhausting the remaining 30% outside the cabinet. This balance allows for better airflow dynamics and helps maintain the sterile environment needed for compounding.

In BSC Class II A2 cabinets, the use of HEPA filters ensures that the air being recirculated is free of contaminants, thereby protecting both the operator and the product being handled. This aspect makes A2 cabinets particularly suitable for procedures involving hazardous materials or when sterility is critical.

The other subtypes of BSC Class II, such as A1, B1, and B2, have different airflow patterns and exhaust methods that do not match the criteria specified in the question regarding air recirculation. For instance, A1 cabinets recirculate air but with different parameters, while B1 and B2 cabinets exhaust a larger portion of air outside, thereby not focusing on the recirculation aspect mentioned in the question.