There are four types of Class II cabinets:
Class II Type A1 (formerly designated
Type A): It exhausts approximately 30% air back
into the laboratory and re circulates the remainder.
May have positive pressure contaminated ducts and plenums
that are not surrounded by negative pressure plenums.
Class II Type A2 (formerly designated
Type B3): It exhausts approximately 30% air through
a HEPA filter back into the laboratory or to the environment
through an external exhaust duct. It has all biologically
contaminated ducts and plenums under negative pressure
or surrounded by negative pressure ducts and plenums.
Class II Type B1: It exhausts
70% air through a dedicated duct exhausted to the atmosphere
after passing through a HEPA filter and re circulates
the remainder. It has all contaminated ducts and plenums
under negative pressure or surrounded by negative pressure
ducts and plenums.
Class II Type B2: (sometimes
referred to as "total exhaust"): It exhaust
100% air through an exhaust duct after passing through
a HEPA filter and contaminated positive pressure plenums
must be surrounded by a vacuum.
Class III: The Class III cabinet is
a totally enclosed ventilated cabinet of airtight construction.
Operations within the Class III cabinet are through
attached rubber gloves. When in use, the Class III cabinet
maintains negative air pressure. Supply air is drawn
into the cabinet through HEPA filter. The cabinet exhaust
air is filtered by two HEPA filters installed in series,
before it is discharged outside the facility. The exhaust
fan for the Class III cabinet is generally separate
from the exhaust fans of the facilities ventilation