There are three different types of Biological Safety Cabinets which are classified based on effectiveness of personnel, environment and product protection as well as specifics of the airflow dynamics.  Below is a brief description of each classification:

Class I Biological Safety Cabinets are ventilated cabinets which provide protection for the researcher and the environment.  They are normally characterized by a non-recirculated inward airflow away from the operator via a limited fixed access opening.  If this air is re-circulated back into the laboratory the exhausted air must be HEPA filtered.  Class I Biological Safety Cabinets may or may not be vented via the facilities HVAC system.

Class II Biological Safety Cabinets provide personnel, product and environmental protection and are characterized by a limited fixed inward air flow access opening which provides protection to the researcher, a vertical downward HEPA filtered work zone for product protection and a HEPA filtered exhaust for environmental protection.  Class II Biological Safety Cabinets are further classified into types based on the amount of air that is re-circulated in the cabinet.

Class II, Type A2 Biological Safety Cabinets are 70% re-circulated within the cabinet and 30% exhausted air back into the laboratory or vented to the outside.

Class II, Type B1 Biological Safety Cabinets re-circulate approximately 30% of the air within the cabinet and 70% exhausted to the outside via an external ventilation system.

Class II, Type B2 Biological Safety Cabinets are 100% exhausted via an external ventilation system and have no recirculation.