NSF® (formerly the National Sanitation Foundation) has specified three classes of biological safety cabinets that offer effective containment for working with human pathogens. Class I, II, and III are described below, and some may include sub class levels.
Class I: Unrecirculated air flows away from the lab worker. After going through a HEPA filter, this airflow is vented into the room. This class provides the lab worker a medium level of protection but does not protect the samples in the cabinet from contamination. Essentially, this level offers staff and environmental protection, but no product protection.
Class II: This level is best if a lab worker is using microorganisms in containment at level 2, 3, and 4 labs. These cabinets are divided into two types (A and B) determined by construction, airflow velocities, patterns, and exhaust systems. The Class II cabinets protect the lab worker, samples, and the external environment.
Class III: This level offers maximum protection for staff workers and the environment as well as samples and is designed to work with biosafety level 4 microbiological agents. These biosafety cabinets have both a HEPA filtered supply and exhaust air and are completely gas-tight and enclosed. Tests are performed while using long-sleeved gloves. The cabinet is under negative pressure of at least 120 Pa while the exterior exhaust system maintains airflow.