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BMBL Appendix A
Primary Containment: Biological Safety Cabinets
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Biological Safety Cabinets (BSCs) are among the most
effective and the most commonly used primary containment devices in laboratories working
with infectious agents.(1)The three general types
available (Class I, II, III) have performance characteristics and applications which are
described in this appendix. Properly maintained Class I and
II BSCs, when used in conjunction with good microbiological techniques, provide an
effective containment system for safe manipulation of moderate and high-risk
microorganisms (Biosafety Level 2 and 3 agents). Both Class I and II BSCs have inward face
velocities (75-100 linear feet per minute) that provide comparable levels of containment
to protect laboratory workers and the immediate environment from infectious aerosols
generated within the cabinet. Class II BSCs also protect the research material itself
through high-efficiency particulate air filtration (HEPA filtration) of the air flow down
across the work surface (vertical laminar flow). Class III cabinets offer the maximum
protection to laboratory personnel, the community, and the environment because all
hazardous materials are contained in a totally enclosed, ventilated cabinet. |
Class I (Note: Class
I BSCs are currently being manufactured on a limited basis; many have been replaced by
Class II BSCs.)
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The Class I Biological Safety Cabinet (Fig. 1)
is a negative-pressure, ventilated cabinet usually operated with an open front and a
minimum face velocity at the work opening of at least 75 linear feet per minute (lfpm).
All of the air from the cabinet is exhausted through a HEPA filter either into the
laboratory or to the outside. The Class I BSC is designed for general microbiological
research with low- and moderate-risk agents, and is useful for containment of mixers,
blenders, and other equipment. These cabinets are not appropriate for handling
research materials that are vulnerable to airborne contamination, since the inward flow of
unfiltered air from the laboratory can carry microbial contaminants into the cabinet. The Class I BSC can also be used with an installed front closure panel
without gloves, which will increase the inward flow velocity to approximately 150 lfpm. If
such equipped cabinets are ducted to the outside exhaust, they may be used for toxic or
radiolabelled materials used as an adjunct to microbiological research. Additionally,
arm-length rubber gloves may be attached to the front panel with an inlet air pressure
release for further protection. In this configuration, it is necessary to install a
make-up air inlet fitted with a HEPA filter in the cabinet |
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The Class II Biological Safety Cabinet (Fig.
2) is designed with inward air flow at a velocity to protect personnel (75-100 lfpm),
HEPA-filtered downward vertical laminar airflow for product protection, and HEPA-filtered
exhaust air for environmental protection. Design, construction, and performance standards
for Class II BSCs, as well as a list of products that meet these standards, have been
developed by and are available from the National Sanitation Foundation International,(2)Ann Arbor, Michigan. Utilization of this standard
and list should be the first step in selection and procurement of a Class II BSC. Class II BSCs are classified into two types (A and B) based on construction,
air flow velocities and patterns, and exhaust systems. Basically, Type A cabinets are
suitable for microbiological research in the absence of volatile or toxic
chemicals and radionuclides, since air is recirculated within the cabinet. Type A cabinets
may be exhausted into the laboratory or to the outdoors via a "thimble"
connection to the building exhaust system.
Type B cabinets are further sub-typed into types B1, B2, and B3. A
comparison of the design features and applications are presented in Figures 2b, 2c, and
2d, respectively. Type B cabinets are hard-ducted to the building exhaust system and
contain negative pressure plena. These features, plus a face velocity of 100 lfpm, allow
work to be done with toxic chemicals or radionuclides.
It is imperative that Class I and II biological safety cabinets be
tested and certified in situ at the time of installation within the laboratory,
at any time the BSC is moved, and at least annually thereafter. Certification at locations
other than the final site may attest to the performance capability of the individual
cabinet or model but does not supersede the critical certification prior to use in the
laboratory.
As with any other piece of laboratory equipment, personnel must be
trained in the proper use of the biological safety cabinets. Of particular note are
activities that may disrupt the inward directional airflow. Repeated insertion and
withdrawal of the workers' arms into and out of the work chamber, opening and closing
doors to the laboratory or isolation cubicle, improper placement or operation of materials
or equipment within the work chamber, or brisk walking past the BSC while it is in use
have been demonstrated to cause the escape of aerosolized particles from within the
cabinet. Class I and II cabinets should be located away from traffic patterns and doors.
Air flow from fans, room air supply louvers and other air moving devices can disrupt the
airflow pattern at the face of the cabinet. Strict adherence to recommended practices for
the use of BSCs and their proper placement in the laboratory are as important in attaining
the maximum containment capability of the equipment as is the mechanical performance of
the equipment itself. |
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The Class III Biological Safety Cabinet (Fig.
3) is a totally enclosed, ventilated cabinet of gas-tight construction and offers the
highest degree of personnel and environmental protection from infectious aerosols, as well
as protection of research materials from microbiological contaminants. Class III cabinets
are most suitable for work with hazardous agents that require Biosafety Level 3 or 4
containment. All operations in the work area of the cabinet
are performed through attached arm length rubber gloves or half-suits. The Class III
cabinet is operated under negative pressure. Supply air is HEPA-filtered and the cabinet
exhaust air is filtered through two HEPA filters in series, or HEPA filtration followed by
incineration, before discharge outside of the facility.
All equipment required by the laboratory activity, such as
incubators, refrigerators, and centrifuges, must be an integral part of the cabinet
system. The Class III cabinet must be connected to a double-doored autoclaves and/or
chemical dunk tank used to sterilize or disinfect all materials exiting the cabinet, and
to allow supplies to enter the cabinet. Several Class III cabinets are therefore typically
set up as an interconnected system. |
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Personnel protection equivalent to that provided by
Class III cabinets can also be obtained with the use of a one-piece, ventilated suit worn
by the laboratory worker when working with Biosafety Level 3 or 4 agents in a "suit
area" and using Class I or II BSCs. The personnel suit is maintained under positive
pressure with a life-support system to prevent leakage into the suit. In this containment
system, the worker is isolated from the work materials. The
personnel suit area must be essentially equivalent to a large Class III cabinet. The area
is entered through an air-lock fitted with airtight doors. A chemical shower is provided
as a "dunk tank" to decontaminate the surfaces of the suit as the worker leaves
the area. The exhaust air from the suit area is filtered through two HEPA filter units
installed in series. The entire area must be under negative pressure.
As with Class III BSCs, the gloves of the personnel suit are the
most vulnerable component of the system, as they are subject to punctures by sharps or
animal bites.
Other Devices: Horizontal laminar flow "clean
benches" are used in clinical, pharmaceutical, and laboratory facilities strictly for
product protection. This equipment must never be used for handling
toxic, infectious, radioactive, or sensitizing materials, since the worker sits in the
immediate downstream exhaust from the "clean bench." Vertical laminar flow
benches may be useful for certain manipulations of clean materials (e.g., pouring agar
plats) but should not be used when working with infectious materials. |
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1. U.S. Department of Health and
Human Services. Primary Containment of Biohazards: Selection, Installation and Use of
Biological Safety Cabinets. (Washington: GPO, 1995) 2. National Sanitation Foundation Standard 49. 1983. Class II (Laminar
Flow) Biohazard Cabinetry. Ann Arbor, Michigan. |
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