These are bio-enclosures that are commonly used for microbiological analysis. These cabinets are suitable for Biosafety levels I to IV where samples being handled range from non-pathogenic to those with lethal effects.
In a Class II cabinet, air enters the front grille near the operator and then passes through a filter. With this design, none of the unfiltered inflow air enters the work zone of the cabinet making the product being handled safe from contaminated air.
Variations in Class II cabinets comes in the percentage of air that is recirculated and exhausted. Exhaustion may be directly back to the laboratory or through a ducted system to the external environment.