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Class I Biological Safety Cabinet
The Class I cabinet has the most basic and rudimentary design of all biological safety cabinetry
available today. A stream of inward air moving into the cabinet contains aerosols generated
during microbiological manipulations. It then passes through a filtration system that traps
all airborne particles and contaminants. Finally, clean, decontaminated air is exhausted from
the cabinet. The filtration system usually consists of a pre-filter and a HEPA (High Efficiency
Particulate Air) filter.
Although the Class I cabinet protects the operator and the environment from exposure to
biohazards, it does not prevent samples being handled in the cabinet from coming into
contact with airborne contaminants that may be present in room air. Naturally, there is a
possibility of cross-contamination that may affect experimental consistency. Consequently the
scope and application of Class I cabinets is limited and it is largely considered obsolete.
All Class I biological safety cabinets are suitable for work with microbiological agents assigned
to biosafety levels 1, 2 and 3.
Class II Biological Safety Cabinet
Like Class I safety cabinets, Class II cabinets have a stream of inward air moving into
the cabinet. This is known as the inflow and it prevents the aerosol generated during
microbiological manipulations to escape through the front opening. However, unlike Class I
cabinets, the inflow on Class II cabinets flows through the front inlet grille, near the operator.
None of the unfiltered inflow air enters the work zone of the cabinet, so the product inside
the work zone is not contaminated by the outside air.
A feature unique to Class II cabinets is a vertical laminar (unidirectional) HEPA-filtered air
stream that descends downward from the interior of the cabinet. This continuously flushes
the cabinet interior of airborne contaminants and protects samples being handled within
the cabinet from contamination and is known as the downflow.
The differences between the various Class II cabinets available lie primarily with the
percentage of air exhausted to that of air re-circulated from the common air plenum. In
addition, different Class II cabinets have different means of cabinet exhaust. Some cabinets
may exhaust air directly back to the laboratory, while others may exhaust air through a
dedicated ductwork system to the external environment.
Despite these differences, all Class II cabinets, like Class I cabinets, protect both the operator
and environment from exposure to biohazards. In addition, Class II cabinets also protect
product samples from contamination during microbiological manipulations within the cabinet
interior and are all suitable for work with agents assigned to biosafety levels 1, 2 and 3.
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