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September 2001
Vol. 4, No. 9, pp 57–68, 60.
the toolbox

Performance anxiety

How confident are you about the abilities of your biological safety cabinet?

As far as the protection of laboratory personnel is concerned, a fume hood and a biological safety cabinet (BSC) are functionally identical—they act as a primary barrier for separating and removing hazardous substances from the work environment. The main difference between the two is that a BSC incorporates a high-efficiency particulate arrestance (HEPA) filter upstream of the fan to remove aerosols from the cabinet exhaust. And although all fume hoods, Class II B2 BSCs, and some Class I BSCs exhaust air to the outside, other BSCs recirculate some air into the cabinet or laboratory (see Table 1). As far as standard tests of performance are concerned, however, fume hoods and BSCs inhabit separate worlds.

In the United States, fume hood testing is defined by a standard developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). ASHRAE has produced standards on just about every aspect of building ventilation.BSC testing, however, is the province of a standard promulgated by the National Sanitation Foundation (NSF). As its name implies, the NSF develops standards primarily for appliances and equipment in which water is involved. A glance through both standardsgives no hint that fume hoods and BSCs are in any way related.

Figure 1. Air flow in a Class II, Type B2 biological safety cabinet.
Figure 1. Air flow in a Class II, Type B2 biological safety cabinet. Air enters the cabinet through the cabinet opening (A) below the sash (B) or is pumped in (C) through a HEPA filter (D). It then passes behind the negative-pressure exhaust plenum (E) and is exhausted from the cabinet through another HEPA filter (F). (Adapted from Reference 1.)
Similar but different

Fume hoods and BSCs are basicallyboxes that partially open on one side and have a duct at the top that leads to a fan. The fan draws air from the room into the opening of the box and through the box and duct, and then exhausts the air, preferably over the roof of the building (see Figure 1). The aerodynamic problem of obtaining a uniform laminar flow across the opening, thereby protecting an operator standing at the opening from pollutants released inside the box, is largely the same in fume hoods and BSCs.

Face velocity—the linear velocity of air flowing into the box at the work opening—is a convenient and easy way to characterize the performance of these types of enclosures. Many studies, however, have shown a poor correlation between face velocity and containment of pollutants released in the box. In other words, face velocity alone is not a good measure of the degree of protection afforded someone working at the box opening. This has long been known in the fume hood world, and the ASHRAE standard correspondingly points out that face velocity is not a quantitative measure of performance.Tracer testing must be used for a quantitative evaluation of fume hood containment.

In a tracer test, a known rate of an innocuous tracer gas is released in the work area of the fume hood and simultaneously measured in the breathing zone of an operator standing at the hood sash opening. Tracer gas concentrations in the breathing zone are typically low parts-per-billion to parts-per-million.

By using a tracer test, it is fairly straightforward to determine the expected exposure of an operator working with a hazardous volatile chemical.Assuming the tracer and the chemical of interest similarly release and disperse in and around the cabinet, then the ratio of the concentration of a substance in the operator’s breathing zone to its release rate in the cabinet is the same for the tracer and the pollutant. Knowing the tracer’s concentration in the breathing zone and the tracer and hazardous chemical’s release rates in the hood enables the expected concentration of the hazardous compound in the breathing zone to be estimated. This value is then comparedwith administrative norms to determine whether the work environment outside the cabinet is safe.

Test-imonial
The NSF standard also describes a tracer containment test that uses a biological aerosol, but this test is not included as one of the recommended field tests to be performed periodically (at least yearly) or whenever HEPA filters are changed, the cabinet undergoes maintenance, or is moved or otherwise altered.This is not surprising because the procedure is complicated, requiring a time-consuming, multistep preparation of a bacterial spore suspension. As such, it is intended as a type test—a comprehensive evaluation of one example of a product line—that is performed by a manufacturer or distributor.The standard gets around this complicationby equating tracer containment with face velocity. If a cabinet has the same face velocity as a cabinet of the same model and type that passed the original tracer test, then it also passes by exemption if operated at the same face velocity as the test cabinet. This is convenient, but it is not necessarily correct, because the performance of a cabinet depends not only on face velocity, but also on the physical layout and ventilation characteristics of the laboratory in which it is installed.

Although the mechanics of a tracer test for containment in a BSC are similar to those of a fume hood test,the results can be harder to interpret. Whereas the release rates of volatile chemicals can be estimated either from empirically derived equations or from use rates, this is not generally so with biological or radioactive aerosols. In addition, administrative norms expressed in concentration levels in air are difficult to come by for substances other than chemicals.In the absence of detailed knowledge of dose–response relationships for particular aerosols, the ideal exposure level for an operator working at the cabinet opening would arguably be zero. This would correspond to an undetectable level of tracer in the breathing zone of an operator during a containment test. The determination of containment performance would then be limited by how large a tracer release rate can reasonably be used and by the sensitivity of the tracer analyzer.

A tracer test that is quick and easy to perform (and is also a reasonable representation of pollutant dispersal and exposure in and around a cabinet) would likely be used more often than a complicated and expensive test.And more accurate containment testing that is done more often would mean better personnel protection in the laboratory. An affordable instrument to measure aerosols at sufficiently low concentrations in real time, as well as equipment to produce a constant known release rate, appears to be lacking. The availability of such equipment could make routine aerosol tracer containment testing of BSCs a reality.

In the meantime, chemical tracer testing could be used instead. All but the heaviest aerosols disperse in a similar fashion to low concentrations of a tracer gas. The use of a chemical tracer release to test containment should therefore be equally relevant in a BSC. In recent years, developments in measurement technology have made fast and inexpensive chemical tracer testing possible, and there is no reason that users of fume hoods and BSCs should not benefit from it.

The consultants
Field performance testing of BSCs is commonly performed by external, independent consultants who are trained and certified by the NSF, whereas field-testing of fume hoods is almost always done internally by, for example, the laboratory’s industrial hygienist.This is likely because BSCs, which have HEPA filters and are often designed to provide product protection (protection of the work as well as the worker), are more complicated than fume hoods.The field tests typically counted on to determine level of containment and therefore personnel protection are, however, the same.These include

  • face velocity tests across the work access opening of the cabinet, and
  • smoke releases to verify that flow direction is inward across the sash opening and that there are no leaks from the cabinet into the room.

It is curious that ventilated cabinets where biological or radioactive substances are used are subject to external scrutiny and approval (by NSF-approved consultants for BSCs and by state or national radiation protection authorities for radioactive users). If only hazardous chemicals are used in a cabinet, however, the laboratory is on its own. This practice can have negative consequences in cases when chemistry laboratory users and management are poorly informed about the principles of good laboratory ventilation and industrial hygiene.

The BSC world thus has a well-established culture of paying independent inspectors to come in and evaluate these important safety systems, whereas the fume hood world does not. As a result, although the industrial hygiene literature clearly shows that tracer testing of fume hoods is the most appropriate means of measuring containment, few laboratories are willing to pay someone to do it.Perhaps this is partly because a pool of consultants to provide a service at a reasonable price cannot form without a market in place to support it or in the absence of an enforcement entity that can generate a market by decree. For users of BSCs, who are accustomed to paying for performance certification, the incorporation of routine tracer containment testing into maintenance evaluation of BSCs would, presumably, be less of an obstacle.

Recognizing that fume hoods and BSCs are related in form and function could have positive implications for users in both worlds. The BSC community could benefit from continuing developments in tracer containment testing occurring in the fume hood world. Conversely, the fume hood world could adopt a system of cabinet evaluation by external consultants, similar to that used for BSCs. In both cases, the end result would be a safer and healthier laboratory work environment.

Suggested reading

  • Standard 110-1995: Method of Testing Performance of Laboratory Fume Hoods (ANSI-approved); American Society of Heating, Refrigerating and Air-Conditioning Engineers: Atlanta, 1995.
  • Class II (Laminar Flow) Biohazard Cabinetry; Report NSF 49; National Sanitation Foundation: 1992.
  • Primary Containment of Biohazards: Selection, Installation and Use of Biological Safety Cabinets; U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and National Institutes of Health: Atlanta, 1995. Available at www.cdc.gov/od/ohs/biosfty/bsc/bsc.htm.

Reference

  1. Biosafety in Microbiological and Medical Laboratories, 4th ed.; U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and National Institutes of Health: Atlanta, GA, 1999. Available at www.cdc.gov/od/ohs/biosfty/bmbl4/bmbl4toc.htm.


James P. Rydock is a researcher at the Norwegian Building Research Institute in Oslo, Norway. Send your comments or questions regarding this article to mdd@acs.org or the Editorial Office by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.

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