About TCAW - Subscription Info
June 2001
Vol. 10, No. 06,
pp 19–20.
Instruments & Applications
Reality-Check Your Fume Hoods

Laboratory hoods need to be tested periodically, exactly as they are used.

Fume hoods are the primary barriers for protecting the work atmosphere in a chemistry laboratory. A fume hood and its underlying ventilation system also represent a significant investment and operating cost. How do you know that your fume hoods protect sufficiently against the substances used there? Smell can give an indication of exposure, but the odor threshold of a substance does not necessarily have any relation to the recommended threshold limit value for that compound. Physical symptoms can also indicate exposure to unhealthy concentrations of chemicals, but what about lower concentrations that might not cause symptoms for many years? A good working environment cannot be ensured without a proper performance test of the fume hoods.

Performance Is Key
What does “proper” performance testing actually mean? Installed fume hoods are checked in most cases with face velocity measurements only, partly because this can be done quickly and easily without expensive equipment, but also perhaps because people are not aware that alternative methods are available. Another type of performance test that gives qualitative information is to release smoke in a fume hood and observe whether smoke leaks out into the laboratory. A third type of performance test that gives quantitative information about containment is to release a tracer gas in the hood and measure tracer concentrations outside the hood. A tracer is a harmless gas (at the concentrations used) that can be easily measured in real time at very low concentrations—typically in low parts per billion.

Another strategy is to measure concentrations in the laboratory of the specific chemicals that are used there. The advantage here is that the results can be compared directly with regulatory limit values. For many substances, however, it isn’t easy to find sampling and analysis methods with sufficient sensitivity, and real-time measurements can seldom be performed. In larger laboratories with many fume hoods and many chemicals, it can therefore be difficult to figure out if there is a problem and where the source is.

Performance testing with face velocity measurements is based on the assumption that a velocity of 100 ft/min in the fume hood sash is optimal for containing and removing pollutants from a fume hood. A face velocity much less than 100 ft/min is insufficient to effectively remove contaminants, and a velocity much higher produces excess turbulence, which also hinders effective containment. There are at least two reasons that face velocity measurements cannot be completely relied on. Many studies have shown a low correlation between face velocity and fume hood containment (measured with a tracer gas). Also, work in a fume hood implies release of a pollutant to the environment. Whether a fume hood performs acceptably must therefore also depend on the character of the release: How dangerous is the substance, and how much and in what way is it released? Face velocity measurements do not take this into account.

Performance tests with a tracer gas, on the other hand, give a more accurate picture of pollutant containment and hood weaknesses. Typical problems are poor performance when the sash level is high and leakage caused by the turbulence of people passing by. This test can be used for periodic evaluation and as an educational tool to demonstrate how a user’s movements can influence exposure to substances used in the hood. Real-time measurement of a tracer gas gives immediate and quantitative feedback. This is important because even the most advanced laboratory ventilation system has its weaknesses and cannot give optimal protection if the users do not realize this.

Assuming both the tracer gas and the pollutant disperse similarly, the ratio of the gas concentration to its release rate is the same for both. If the release rate of the pollutant used in the fume hood can be estimated, then an estimate of the concentration of the pollutant in the breathing zone of a person working at the hood can be obtained and compared with the relevant threshold limit value for that substance.

Test, Test, Test
Fume hood performance tests are generally done in three situations: type tests on a product line are financed by fume hood producers or distributors, commissioning tests are done before an installed fume hood is first used, and maintenance tests are done periodically on installed fume hoods in everyday use. A type test is a thorough evaluation with tracer gas, smoke, and face velocity testing. The test is done in a special testing room with an empty hood and controlled ventilation conditions. Performance with different sash heights and face velocities is investigated with tracer release at different points inside the hood and tracer concentration measurements outside the hood. A type test can take many days and be quite expensive, and is most useful when comparing products from different suppliers.

The type test cannot, however, indicate whether the hood is installed correctly and functions well as a part of the laboratory ventilation system. Placement of a fume hood with respect to supply and extract vents, doors, and windows can substantially affect performance.

A commissioning test is performed when a fume hood has been installed and connected to the ventilation system, and for budgetary reasons, it is usually limited to simple face velocity measurements and perhaps smoke testing. The ventilation contractor is generally responsible for performing a commissioning test.

Whether a fume hood gives sufficient protection depends on more than whether it is a good product and is correctly installed. Performance also depends on the types of substances used, how they are used, what else is stored in the hood, a worker’s actions while at the hood, and the traffic around the hood. It is therefore important that installed fume hoods are tested exactly as they are used in a periodic maintenance test. However, if such a test consists solely of face velocity measurements, it does not yield enough information to determine if a fumehood performs adequately.

Standard Bearers
The American Society of Heating, Refrigerating and Air-Conditioning Engineers has developed a comprehensive standard (ASHRAE 110/1995) for performance testing of fume hoods (1). The test procedure does not have any legal authority, but it carries weight by virtue of the society that backs it and the engineers and industrial hygienists who use it. Several factors work against the widespread use of this standard. First, it is based on the use of sulfur hexafluoride (SF6) as the tracer gas, released at approximately 1.5 kg/h. SF6 is one of the most potent greenhouse gases known, with a global warming potential 25,000 times larger than that of CO2. Clearly, a testing procedure that specifies such a substantial release rate of this environmentally damaging gas simply cannot be used for routine testing of the many thousands of fume hoods in laboratories across the United States.

Second, ASHRAE 110/1995 does not take into account the nature of the substances used in a hood or define acceptable performance levels. Rather, it is a detailed description of how such a test should be performed; thus, the interpretation of the results and decisions regarding fume hood acceptability are quite subjective.

Standardized testing methods for evaluating fume hoods have also been developed elsewhere. The Danish standard (DS 457), for example, is much easier to implement than its U.S. counterpart (2). In this case, an operator stands in front of the fume hood and performs rapid work movements with his or her arms. At the same time, a tracer gas is released inside the hood and its concentration in the operator’s breathing zone is measured continuously. A safety factor then is calculated from the tracer’s release rate, its maximum concentration in the operator’s breathing zone in a 5-min period, the pollutant’s estimated release rate, and the regulatory exposure limit value for the most dangerous substance used in the hood. A fume hood that exhibits an unacceptably low safety factor for a substance in a commissioning or maintenance test is not approved for work with that substance. DS 457 is performed by many consulting firms and is used for periodic performance testing of virtually all fume hoods in Denmark.

Laboratory ventilation, which is instrumental to achieving a good work environment, is a complex interplay of sometimes opposing elements. Every ventilation system is unique, and it is not possible to confidently assess whether a fume hood is adequate without sufficient testing. In the long run, it is bad for everyone if laboratory personnel are happily unaware that they are being exposed to hazardous substances. It is therefore essential that all parties in an organization understand the importance of proper performance evaluation of fume hoods. There is no reason that fume hood users in the United States should not have access to the type of testing commercially available in Denmark. Without a demand from users, however, it is unlikely that a market can develop and thereby generate an interest in offering this service on a large scale.

References

  1. American Society of Heating, Refrigerating and Air-Conditioning Engineers. Method for Testing Performance of Laboratory Fume Hoods; ASHRAE 110/1995; Atlanta, 1995.
  2. Code of Practice for Fume Cupboards. Dansk Standard DS 457, Charlottenlund, Denmark, 1993.


James P. Rydock is a researcher at the Norwegian Building Research Institute in Oslo, Norway. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th St N.W., Washington, DC 20036.

Return to Top || Table of Contents


 CASChemPortChemCenterPubs Page