Methods to Verify Proper Functioning 
Fume Hood Performance Tests
People working in laboratories rely on proper performance of fume hoods as their primary means of protection from overexposure to hazardous airborne chemicals generated during scientific activities. Fume hood performance is defined as the ability to protect people through containment, capture, and removal of airborne hazards generated within the fume hood enclosure. Results from thousands of performance tests by my company, 3Flow, indicate that 15 percent to 30 percent of fume hoods do not meet performance criteria described in ANSI/AIHA/ASSP Z9.5-2012, Laboratory Ventilation. (More information is available in my paper “The Unintended Practice of Using Employee Health as an Indicator of Laboratory Hood Performance” in the January/February 2004 issue of Chemical Health and Safety Journal.)

The factors that affect safety performance are numerous and generally associated with the aerodynamic design of the fume hood, the operation of the airflow systems, the configuration of the laboratory environment, and the work practices of the hood user. These factors are further exacerbated as laboratory hood systems become increasingly more complex with the introduction of unique features and variable air volume (VAV) systems that modulate flow to conserve energy. Since fume hood systems are installed for the sole purpose of protecting people, proper testing is critical to ensure proper performance before people place their health at risk using hazardous chemicals. OSHA’s Occupational Exposure to Hazardous Chemicals standard (29 CFR 1910.1450) requires all facilities to ensure the proper functioning of fume hoods through appropriate testing and maintenance. The current standard in the United States for verifying proper functioning of fume hood systems is ANSI/ASHRAE 110-2016, Method of Testing Performance of Laboratory Fume Hoods. ASHRAE 110 provides quantitative and qualitative test methods to evaluate containment performance under prevailing operating conditions. The standard describes a series of tasks that include inspection of the hood, evaluation of the lab environment, airflow measurements, airflow visualization tests, and tracer gas containment tests. The tests are conducted to verify containment performance and document the prevailing operating conditions. TEST METHODS AND APPLICATION The test methods described in the ASHRAE 110 standard can be divided into two sections: evaluation of prevailing operating conditions, and evaluation of containment performance under prevailing operating conditions.

Evaluation of prevailing operating conditions includes the following methods:
  • fume hood and lab inspection
  • face velocity measurements
  • VAV face velocity control test
  • VAV response test
  • cross-draft velocity tests (optional)

Methods for evaluating containment performance under prevailing operating conditions are:
  • flow visualization (smoke) test
  • tracer gas containment test

To conserve energy, fume hoods are being operated at lower average velocities and equipped with complex VAV systems. The VAV systems are intended to modulate flow to a minimum when the sash is closed or when the hood is unoccupied, and increase flow in proportion to the sash opening area or when the hood is occupied with the sash open. Most organizations employ measurement of average face velocity, but the introduction of more complex VAV systems requires additional testing. While face velocity measurements are applicable when the sash is open, special airflow tests are required to evaluate proper operation of the VAV flow control systems. The VAV face velocity control tests and VAV response tests are conducted to verify proper modulation and control of flow across the range of operating modes from minimum to maximum flow.

In the ASHRAE 110 test method, a mannequin is used to simulate a hood user standing in front of the hood opening.
According to ANSI/AIHA/ASSP Z9.5-2012, the VAV controls should be capable of modulating flow in less than 5 seconds following movement of the sash and preventing flow variations in excess of 10 percent from the design specifications at each sash configuration or operating mode. Measurement of face velocity alone will not ensure proper functioning of VAV controls, and performance can be compromised by slow response and unstable flow control. These systems are also susceptible to degradation over time, which may lead to more frequent testing and maintenance efforts. In addition to problems caused by improper modulation of exhaust flow, fume hood performance can be dramatically affected by the velocity and direction of room air currents near the hood opening. Issues can arise if air supply diffusers are located too close to the fume hood or the air supply flow produces detrimental cross-draft velocities near the opening. ANSI/AIHA Z9.5 recommends that cross-draft velocities be maintained at less than 50 percent of the average face velocity. However, the performance of some fume hoods may be more susceptible to cross-drafts than others. High-velocity cross-drafts can also create turbulence at the plane of the sash and affect the accuracy of face velocity measurements. At a minimum, this can lead to improper calibration and tuning of the airflow controls; at worst, cross-drafts can adversely affect fume hood containment and safety performance. For these reasons, cross-draft velocities should be measured across the range of operating modes to evaluate and reduce the impact of air supply on hood performance.  To assess the impact of these operational factors on hood containment performance, ASHRAE 110 describes application of qualitative flow visualization tests and quantitative tracer gas containment tests. During flow visualization tests, a visible source of smoke is generated in the hood to observe airflow patterns and qualitatively assess hood containment. During tracer gas tests, the gas sulfur hexafluoride (SF6) is discharged from a special ejector located inside the fume hood to simulate a hazardous emission, and a mannequin is used to simulate a hood user standing in front of the hood opening. A tracer gas detector is placed in the mannequin’s breathing zone to quantitatively assess containment for use in rating fume hood performance. The tracer gas is generated at a rate of 4 liters per minute through the ejector located in the hood directly downstream of the mannequin. With the sash in the design opening configuration, the detector samples concentrations at a rate of 1 hertz in the mannequin’s breathing zone for a period of 5 minutes and at a minimum of three locations across the opening (that is, at the left, center, and right sides of the opening). Special tests are also conducted with the mannequin placed at the center of the opening to evaluate containment during three cycles of opening and closing the sash. These tests are called sash movement effect tests and are intended to evaluate potential for escape as a function of sash movement and VAV flow response and stability. Improper operation of the VAV controls can dramatically affect hood performance, and tests must be conducted periodically to verify proper functioning of the flow control systems. Refer to Figure 1 for a photo of the test apparatus used during ASHRAE 110 fume hood tests.
Figure 1. Photo of experimental setup during ASHRAE 110 fume hood performance tests.
Tap on the figure to open a larger version in your browser.
Due to the number of factors that can influence hood containment, ASHRAE 110 tests can be conducted “as manufactured” (AM), “as installed” (AI), and “as used” (AU):  AM tests are conducted before purchase of the hood to help evaluate the impact of hood design on performance and establish an operational performance envelope.  AI tests are conducted during commissioning of the newly installed fume hood system to evaluate performance in the actual lab under the prevailing operating conditions. The AI tests should be conducted before occupancy to document the operating conditions and verify safety performance before people are potentially exposed to hazardous airborne chemicals.  AU tests are conducted after installation and setup of experimental apparatus to evaluate the potential impact of the procedures on containment and help determine work practices that may help further mitigate risk of exposure. The need for AU tests is often subject to the level of risk associated with the work in the hood and applied at the discretion of the chemical hygiene officer.  Once operating and in service, all fume hoods should be tested periodically (at least annually) to verify proper operation. Routine operating tests should include, at a  minimum, inspection of the hood and lab, cross-draft tests, face velocity tests, VAV response tests, and qualitative flow visualization tests. It may not be necessary to conduct periodic tracer gas tests where operation is demonstrated to be equivalent to conditions prevailing during the AI commissioning tests. However, should any significant changes be made to the design or operation of the fume hood system or the laboratory environment, the AI tests should be conducted again to validate performance and benchmark the new operating conditions.  RECENT DEVELOPMENTS  Tens of thousands of fume hoods have been tested to challenge and quantify hood containment using the ASHRAE 110 test methods. The results have been used to improve safety and reduce energy consumption by enabling better performance of fume hoods, laboratories, and ventilation systems. In the original ASHRAE standard published in 1985, dichlorodifluoromethane (refrigerant 12) was specified as the tracer gas. Refrigerant 12 was later identified as a potential environmental pollutant that contributed to ozone depletion. To reduce generation of potential airborne pollutants, the ASHRAE standard switched to SF6 in 1995. Unfortunately, SF6 has now been identified as an extremely potent greenhouse gas and reported to be 24,000 times worse than carbon dioxide. Organizations around the world are discouraging use of SF6 as an air tracer, and California has banned it for “as used” fume hood tests. In addition, the high cost, complexity, and limited availability of SF6 tracer gas detectors has limited the application of ASHRAE 110 tests. Even considering the thousands of tests conducted, it is estimated that less than 10 percent of the nearly 1.4 million fume hoods in the U.S. have been subject to quantitative tracer gas tests. 
Fume hood performance can be dramatically affected by the velocity and direction of room air currents near the hood opening.
In keeping with commitments to occupational safety and environmental health, ASHRAE undertook a research project to determine a more environmentally friendly alternative to SF6. The investigation included a review of available background literature; characterization of the ASHRAE 110 ejector for SF6; determination of alternative tracer generation, delivery, and detection systems; and a series of tests to evaluate the similarity of results. The work revealed that isopropyl alcohol (IPA) and other alcohol mixtures can be vaporized, mixed with air, discharged from the ASHRAE 110 outlet diffuser, and detected using simple, low-cost, and readily available photoionization detectors (PIDs). Hundreds of tests revealed that the alternative test method using IPA provided generally comparable results to the current ASHRAE 110 method using SF6; dramatically reduced the impact on the environment; and was simpler and less costly to conduct. As a greenhouse gas, IPA is reported to be 0.5 times better than CO2 and 48,000 times better than SF6. The results indicate that use of IPA or other alcohol mixtures as a tracer together with PIDs may provide a suitable and environmentally attractive alternative to SF6.  Although the alternative test method is still under review by ASHRAE, a new method that uses a more environmentally friendly tracer and is simpler and less costly to conduct may result in broader application of ASHRAE 110 performance tests. This may in turn lead to further improvements in fume hood performance and better protection for people working with airborne chemical hazards. In addition, the new methods may also be applicable to evaluating performance of other types of exposure control systems, leading to greater safety and better protection for all people working in or around labs and critical workspaces. CHALLENGE AND CONFIRM Due to the critical importance of fume hoods as a primary safety device when working with chemicals and the large number of variables that can affect containment, appropriate tests are required to evaluate fume hood performance and ensure proper functioning. Systems are becoming increasingly more complex. Fume hoods must be tested across the range of operating modes (open and closed sashes, occupied and unoccupied operation). The introduction of new operating modes results in even more factors that can affect fume hood performance and user safety.  Face velocity measurements alone are no longer adequate to evaluate modern fume hood systems. Tests such as VAV response and stability tests must be conducted in addition to face velocity tests to verify proper fume hood operation. Furthermore, test procedures and performance criteria must be specified that are appropriate to challenge and confirm performance at all operating modes. When establishing the test methods required to verify proper performance, consider all the components that could influence performance and verify proper function over the range of operating modes.    THOMAS C. SMITH is president of 3Flow in Cary, North Carolina, and a member of the ASHRAE 110, ASHRAE TC9.10, ASSE Z9, and ASSE Z9.5 committees. Send feedback to The Synergist.

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ASHRAE: ANSI/ASHRAE 110-2016, Method of Testing Performance of Laboratory Fume Hoods (2016). Chemical Health and Safety Journal: “The Unintended Practice of Using Employee Health as an Indicator of Laboratory Hood Performance” (January/February 2004).
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Disadvantages of being unacclimatized:
  • Readily show signs of heat stress when exposed to hot environments.
  • Difficulty replacing all of the water lost in sweat.
  • Failure to replace the water lost will slow or prevent acclimatization.
Benefits of acclimatization:
  • Increased sweating efficiency (earlier onset of sweating, greater sweat production, and reduced electrolyte loss in sweat).
  • Stabilization of the circulation.
  • Work is performed with lower core temperature and heart rate.
  • Increased skin blood flow at a given core temperature.
Acclimatization plan:
  • Gradually increase exposure time in hot environmental conditions over a period of 7 to 14 days.
  • For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment on day 1 and a no more than 20% increase on each additional day.
  • For workers who have had previous experience with the job, the acclimatization regimen should be no more than 50% of the usual duration of work in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4.
  • The time required for non–physically fit individuals to develop acclimatization is about 50% greater than for the physically fit.
Level of acclimatization:
  • Relative to the initial level of physical fitness and the total heat stress experienced by the individual.
Maintaining acclimatization:
  • Can be maintained for a few days of non-heat exposure.
  • Absence from work in the heat for a week or more results in a significant loss in the beneficial adaptations leading to an increase likelihood of acute dehydration, illness, or fatigue.
  • Can be regained in 2 to 3 days upon return to a hot job.
  • Appears to be better maintained by those who are physically fit.
  • Seasonal shifts in temperatures may result in difficulties.
  • Working in hot, humid environments provides adaptive benefits that also apply in hot, desert environments, and vice versa.
  • Air conditioning will not affect acclimatization.
Acclimatization in Workers