Chemical Hygiene Plan - Queensborough Community College ...

materials and toxic chemicals because of the buildup of vapors in the air
recirculated within the cabinet and exhausted out into the laboratory.
o Class IIA2: This cabinet has a face velocity of 100 lfpm. About 70% of the air
directed over the work surface is recirculated through a HEPA supply filter, and
about 30% is exhausted through a HEPA exhaust filter. Exhaust air can be
directed to the room or to a facility exhaust system. Minute amounts of toxic
chemicals and trace amounts of radioisotopes can be used within the hood (if used
with facility exhaust), although activities should be conducted toward the rear of
the cabinet.
o Class IIB1: This cabinet has a face velocity of 100 lfpm. In contrast to the A2
cabinet, approximately 70% of the circulated air passes through a HEPA exhaust
filter, whereas the remaining 30% of the air is recirculated to the work area
through a HEPA supply filter.
Class IIB2: These are total exhaust cabinets (no recirculation of air within the work area), and
are widely used in toxicology laboratories and similar applications where chemical effluent is
present and clean air is essential. Room air enters through a blower/motor in the top of the
cabinet and passes through a HEPA supply filter into the work area as laminar unidirectional
airflow. Descending air is pulled through the base of the cabinet through the perforated front and
rear grilles. All of the air is pulled into a dedicated, hard-ducted exhaust system. Small
quantities of toxic chemicals and radioisotopes can be used within the hood. The exhaust of a
large volume of conditioned room air makes this cabinet very expensive to operate.
Additionally, the cabinet must be running continuously so as not to interfere with room exhaust
9.6 BSC Work Practices and Procedures
The proper use of biological safety cabinets (BSC) can complement good microbiological
practices and result in effective containment and control of biohazardous and infectious agents.
These general guidelines should be followed:
• Locate the BSC “deep” in the laboratory away from air currents produced by ventilation
inlets, opening/closing of the laboratory door(s), and away from areas of heavy traffic. If
possible, close laboratory doors, limiting entry and egress, and walking traffic during
operation. Air currents and movements create turbulence that disrupts the protective
envelope of the cabinet. Additionally, other nearby laboratory equipment such as
centrifuges, vacuum pumps, etc. can affect the performance of the BSC. Cabinets should
not be located directly opposite of each other or opposite a chemical fume hood, as
laminar airflow will be hindered.
• Observe the magnehelic gauge and note its relative position each time you operate the
BSC. The magnehelic gauge measures the pressure drop across the HEPA filters, and
thus indicates filter load and integrity. A significant increase or decrease in the pressure
over a short period of time may indicate clogging or leaking of the filter.
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