Office of Health and Safety

Primary Containment for Biohazards

SECTION II

From the earliest laboratory-acquired typhoid infections to the hazards posed by today's antibiotic-resistant bacteria and rapidly-mutating viruses, threats to worker safety have stimulated the development and refinement of cabinets in which infectious microorganisms could be safely handled. Work with cell cultures, the need to maintain sterile cell lines, and the need to minimize cross-contamination to maintain product integrity were also addressed in the design of cabinets.

The use of proper microbiological procedures, aseptic techniques, and equipment (as described in BMBL)⁶ cannot be overemphasized in providing primary personnel and environmental protection. For example, high-speed blenders designed to reduce aerosol generation, needle-locking syringes, microburners, and safety centrifuge cups or sealed rotors are among the engineering devices that protect the laboratorian. However, the most essential piece of containment equipment is the biological safety cabinet in which manipulations of microorganisms are performed.

Background

Early prototype clean air cubicles were designed to protect the materials being manipulated from contamination (e.g., from the room or from the worker), rather than to protect the worker from the risk of manipulating the materials. Filtered air was blown across the work surface directly at the worker. Therefore, these cubicles could not be used for handling infectious agents, because the worker would be in a contaminated air stream.

To protect the worker during manipulations of infectious agents, a small workstation was needed that could be installed in existing laboratories with a minimum of modification to the room. The earliest designs for primary containment devices were essentially non-ventilated "boxes" built of wood and later of stainless steel, within which simple operations such as weighing materials could be accomplished.¹⁶

Early versions of ventilated cabinets did not have adequate and controlled directional air movement, and were characterized by mass air flow with widely varying air volumes across openings. The feature of mass air flow into the cabinet was added to draw "contaminated" air away from the laboratorian. This was the forerunner to the Class I BSC. However, since it was unfiltered, the room air drawn into to the cabinet contained environmental microorganisms and other undesirable particulate matter.

Control of airborne particulate materials became possible with the development of filters which would efficiently remove microscopic contaminants from the air. The high efficiency particulate air (HEPA) filter was developed to create dust-free work environments (e.g., "clean rooms" and "clean benches") in the 1940's.¹⁶

HEPA filters are generally rated as being effective at removing 0.3Fm-sized particles with an efficiency of at least 99.97%; they are even more effective at removing both smaller and larger particles.^16,24 A detailed explanation of HEPA filter efficiency and the mechanics of particle collection have been well documented^9,18 and only a brief description is included here.

The medium of a typical HEPA filter is a single sheet of borosilicate fibers which has been treated with a wet-strength water-repellant binder. The filter medium is pleated to increase the overall surface area inside the filter frame, and the pleats are often divided by corrugated aluminum separators (Figure 1). These prevent the pleats from collapsing in the air stream and provide a path for air flow. The filter is glued into a wood, metal, or plastic frame. Careless handling of the filter (e.g., improper storage or dropping) can damage the medium at the glue joint and cause tears or shifting of the filter which result in leaks in the medium. This is the primary reason why filter integrity must be certified after a BSC is initially installed and after it has been relocated (see Section VII).

Various types of containment devices incorporate the use of HEPA filters in the exhaust or supply air system to trap airborne particulate material. Depending on the configuration of these filters and the direction of airflow, varying degrees of personnel, environmental and product protection can be achieved.²⁶ Section V describes good practices and procedures to be followed in order to address these safety concerns.

This page last reviewed May 18, 2001

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