Office of Health and Safety

Primary Containment for Biohazards

SECTION IV

Primary containment is an important strategy to minimize exposure to the many chemical, radiological, and biological hazards encountered in the laboratory. An overview is provided in Table 2 of the various classes of BSCs, the level of protection afforded by each and the appropriate risk assessment considerations. Microbiological risks are assigned to biosafety levels 1 through 4 and are addressed in depth in BMBL.⁶ BSCs in which chemical and radiological materials are used require design modifications in the cabinet or building exhaust system to include charcoal filters, since HEPA filters do not retain agents which vaporize or sublimate.

Work with infectious microorganisms often requires the use of various chemical compounds, and many commonly used chemicals vaporize easily. Therefore, evaluation of the inherent hazards of the chemicals must be part of the risk assessment when selecting a BSC. In order to determine the greatest chemical concentration which might be entrained in the air stream following an accident or spill, it is necessary to evaluate the quantities to be used. Mathematical models are available to assist in these determinations.²⁷ The Threshold Limit Values for Chemical Substances¹ also will provide information on the risk of personnel exposure. As detailed in Section III, volatile or toxic chemicals should not be used in Class II, Type A cabinets since vapor buildup inside the cabinet presents a fire hazard.

The electrical systems of Class II cabinets are not spark-proof, so a chemical concentration that would approach the lower explosive limits of the compound is to be prohibited. Furthermore, since Class II, Type A cabinets return chemical vapors to the cabinet work space and the room, they may expose the operator and other room occupants to toxic chemical vapors.

A chemical fume hood, which is designed for work with volatile chemicals, should be used in lieu of a BSC. Chemical fume hoods are connected to an independent exhaust system and operate with single-pass air ducted directly outside the building. They also are used when manipulating chemical carcinogens.¹⁹ Class I and Class II, Type B2 biological safety cabinets which are exhausted to the outdoors can be used when manipulating small quantities of volatile chemicals required in microbiological studies. The Class II, Type B1 cabinet also may be used with minute or tracer quantities of nonvolatile chemicals.²⁴

Caution should be exercised in the use of Class II, Type B3 (ducted Type A) cabinets for work involving volatile toxic chemicals, because a change in the air balance between the cabinet and building exhaust may result in release of chemical vapors to the laboratory. The thimble exhaust connection helps minimize this problem. If minute quantities of volatile toxic chemicals are to be used in the Class II, Type B3 cabinet, then the building exhaust system must be monitored and preferably interlocked with the cabinet blower.

Many liquid chemicals, including nonvolatile antineoplastic and chemotherapeutic drugs and low-level radionuclides, can be safely handled inside a Class II, Type A cabinet.³⁰ Class II BSCs should not be used for labeling of biohazardous materials with radioactive iodine. For this work, ventilated containment devices are needed that may require both HEPA and charcoal filters in exhaust systems that are hard-ducted to the outside (Figure 10).

Many virology and cell culture laboratories use diluted preparations of chemical carcinogens^19,23 and other toxic substances. Prior to maintenance of the cabinet, careful evaluation must be made of potential problems associated with decontaminating the cabinet and the exhaust system. Air treatment systems, such as a charcoal filter in a bag-in/bag-out housing,²¹ (Figure 13) may be required so that effluents meet applicable emission regulations.

Recommendations from the former Office of Research Safety of the National Cancer Institute²⁹ (which are still valid) stated that certain work involving the use of some chemical carcinogens (in vitro procedures) can be performed in a Class II cabinet which meets the following parameters: (1) that the exhaust air flow is sufficient to provide an inward flow of 100 lfpm at the face opening of the cabinet; (2) that contaminated air plenums under positive pressure are leak-tight; (3) that the cabinet air is discharged outdoors; NSF 49²⁴ currently recommends that Class II Type B cabinets have all biologically contaminated ducts and plenums under negative air pressure, or surrounded by negative pressure ducts and plenums.

As indicated above, volatile radionuclides such as I¹²⁵ should not be used within Class II, Type A cabinets (see Table 2). When using nonvolatile radionuclides inside a BSC, the same hazards exist as when working with radioactive materials on the bench top. Work that has the potential for splatter or aerosolization can be done within the BSC. Monitoring for radioactivity must be done and BSCs be decontaminated as needed. When Appropriate, a vertical (not sloping) beta shield may be used inside the BSC to provide worker protection when appropriate.

The potential for untoward events must be evaluated to reduce or eliminate worker exposure to or release of infectious organisms. Agent summary statements detailed in BMBL⁶ provide risk assessment data for microorganisms known to have caused laboratory-associated infections. Through the process of risk assessment, work procedures are evaluated for the potential to cause exposure to the microorganism.

The hierarchy of controls to prevent or minimize exposure to hazardous materials includes engineering controls, administrative and procedural controls, and work practices which may involve use of additional personal protective equipment. A properly operating BSC available is an effective engineering control (see Section VI), and requiring its use is an administrative control. Some suggested work practices and procedures associated with working safely in a BSC are detailed in the next section.

This page last reviewed May 17, 2001

Office of the Director/Administrator
Centers for Disease Control and Prevention
and Agency for Toxic Substances and Disease Registry
Atlanta, GA