Figure 1. The Class I cabinet. (Figure courtesy of Labconco.)

These dynamics will require that our design community, which has been focused mainly on the institutional and commercial markets, acquires the knowledge and develops the skills necessary to function in this new environment. A common feature of the biosafety laboratory is the biological safety or biosafety cabinet (BSC). Although many are familiar with the chemical fume hood, the BSC may be more of an enigma.

Figure 2. The Class II, Type A1 cabinet. (Figure courtesy of Labconco.)

A Brief History Of Biosafety

Another scientist, Dr. Howard Taylor Ricketts, was not as fortunate and died in 1910 from typhus while studying the disease through a primitive safety cabinet. Typhus was a worldwide scourge that killed 30,000 of Napoleon's soldiers during his retreat from Moscow in the winter of 1812, and 62,000 of the pursuing Russian soldiers also fell to the disease.

In 1915, a Polish scientist, Stanislaus von Prowazek, suffered the same fate studying typhus in Turkey. Obituaries for these pioneers appropriately identified them as medical martyrs. Laboratory-acquired infections grew rapidly, and by 1940 as many as 2,456 workers had become infec- ted by the germs that they were manipulating, and 164 of them had died. These lab-acquired diseases included tuberculosis, Q fever, and the bubonic plague.

Advances brought about by World War II led to the ability to contain aerosols and decontaminate exhaust air from containment cabinets. A research contract awarded to the Arthur D. Little Company as part of the Manhattan Project led to the development of the remarkably efficient HEPA filter. This development was necessary to capture aerosolized microscopic particles that had become contaminated with radioactive material. ¹ Without these developments, a safe biosafety cabinet, and hence a safe biosafety laboratory, would not be possible. These early HEPA filters were constructed of spun glass "paper" that was pleated to maximize the surface area of the filter. The filters are now constructed of randomly oriented boron silicate micro fibers, which cause the particles in the airstream to move in a circuitous path, forcing even the smallest particles to collide with and adhere to the filter. The HEPA filter can remove 99.97% of particles as small as 0.3 microns. Most known bacteria range in size from 0.2 to 5 microns in size. However, since most viruses need a host to survive, they normally attach to bacteria or water droplets ranging in size from 0.5 to 5 microns.

Figure 3. The Class II, Type A1 cabinet with canopy connection. (Figure courtesy of Labconco.)

Biological Safety Cabinets

• Class I cabinets are ventilated cabinets used for personnel and environmental protection, where the airflow is not recirculated and is directed away from the operator. Class I cabinets have an airflow pattern similar to a fume hood, except that it has a HEPA filter at the exhaust outlet, and it may or may not be connected to an exhaust duct system. If not connected to an exhaust system, the HEPA-filtered air is sent back into the laboratory space. Class I cabinets do not provide protection to the product from contamination from room air, since it continuously pulls in unfiltered air across the sash. These cabinets should be designed with a face velocity of 75 to 100 fpm.
• Class II cabinets are ventilated cabinets having an open front with inward airflow for personnel protection, downward HEPA-filtered laminar flow for product protection, and HEPA-filtered exhaust airflow for environmental protection. Class II cabinets are differentiated into various types based on their construction, air velocities, and patterns, and by their exhaust system. For instance, Class II Type A1 cabinets may have contaminated plenums under positive pressure that are exposed to the room, while Type A2 and B cabinets must surround all contaminated positive pressure plenums with negative pressure ductwork.

• Class III cabinets are totally enclosed and ventilated of gas-tight construction. Operations are conducted through attached rubber gloves and the cabinet is maintained under negative air pressure of at least 0.5 in. w.g. Supply air is drawn into the cabinet through HEPA filters and exhaust air is treated by double HEPA filtration before leaving the cabinet. The cabinet also has a transfer chamber capable of sterilizing work materials before exiting the glove box containment system.²,³

Figure 4. The Class II, Type B2 cabinet. (Figure courtesy of Labconco.)

Cabinet Application

Biosafety Level 1 involves secondary educational training and teaching laboratories and other laboratory settings where work is done with strains of microorganisms not known to consistently cause disease in healthy adult humans. This type of facility would not require BSCs, but would instead rely on standard microbiological practices.

Biosafety Level 2 applies to clinical, diagnostic, teaching, and other laboratories where work is done with moderate-risk agents that are present in the community and associated with human disease. The agents can be dealt with safely on an open bench, granted that the potential for producing splashes and aerosols is low; otherwise, Class I and II cabinets would be used. Hepatitis B virus, HIV, and salmonellae are representative of the organisms assigned to this containment level.

Biosafety Level 3 facilities include clinical, diagnostic, teaching, research, and production operations where work is done with indigenous or exotic agents with a potential for respiratory transmission and which may cause serious and potentially lethal infection. Tuberculosis and St. Louis encephalitis virus are representative of the microorganisms assigned to this containment level. More emphasis is placed on primary and secondary barriers to protect the community and environment as well as the laboratory personnel. All laboratory manipulations should be performed in Class I or II BSCs or other closed equipment. Controlled access to the space and ventilation requirements that minimize the release of infectious aerosols from the laboratory are also employed as containment strategies.

Biosafety Level 4 involves work with dangerous and exotic agents that pose a high individual risk of life-threatening disease, which may be transmitted by aerosol and for which there is no available vaccine or therapy. Viruses such as Congo-Crimean hemorrhagic fever are manipulated at this biosafety level.

The primary hazard to personnel working with Biosafety Level 4 agents is respiratory exposure to infectious aerosols, mucous membrane, or broken skin exposure to infectious droplets, and autoinoculation. Work with these agents poses a high risk of exposure and infection to laboratory personnel, the community, and the environment. Worker isolation from aerosols is accomplished by working in Class III BSCs or in a full-body, air-supplied positive pressure personnel suit.⁴

In laboratories designed to utilize the positive pressure suits, Class I and II cabinets can be used. The Biosafety Level 4 facility is generally a separate building or completely isolated zone with complex, specialized ventilation requirements and waste management systems to prevent release of viable agents to the environment. Supply air to the cabinet room, and associated decontamination shower and air lock, are all protected by passage through a HEPA filter. Redundant supply fans are recommended, and redundant exhaust fans are required.

Figure 5. The Class III cabinet. (Figure courtesy of Labconco.)

Installation Considerations

Because of decontamination procedures, it is important to specify either vapor-tight or even bubble-tight dampers on ductwork leading in and out of the containment zone. If formaldehyde gas is to be used or if the facility is a high-containment lab, then serious consideration should be given to the use of bubble-tight dampers, which are expensive and were originally developed for the nuclear industry. The environmentally benign hydrogen peroxide vapor lends itself well to the use of less costly vapor-tight dampers for ventilation system isolation. The BSC can also be specified with an integral isolation damper.

BSCs should not be installed as an integral part of a room's ventilation systems in such a manner that fluctuations of the room supply and exhaust air cause the units to operate outside their design parameters for containment. Certain Class II cabinets require what is known as a thimble or canopy connection, which helps to decouple the exhaust system from the cabinet, eliminating issues with fluctuating pressures. The exhaust connection has a space or gap between the cabinet; this allows a small amount of room air to be drawn into the duct along with exhaust air from the cabinet.

To ensure proper operation, a BSC must be tested and certified if it is either relocated or repaired, and at a minimum it must also be certified annually. Personnel certifying the cabinet should be trained and accredited, as there is a battery of important tests required for proper certification. These tests include down flow velocity and volume, inflow volume, airflow smoke pattern, HEPA filter leak, cabinet leak, UV lamp output, vibration, noise level, lighting intensity, electrical leakage, and ground circuit resistance and polarity.

Table 1. Class II Cabinet Types.