Laboratory Facilities - NCBI

9.C.1. Risk Assessment

For all materials, the objective is to keep airborne concentrations below established exposure limits (see Chapter 4, section 4.C.2.1). Where there is no established exposure limit, where mixtures are present, or where reactions may result in products that are not completely characterized, prudent practice keeps exposures ALARA (as low as reasonably achievable).

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For chemicals, determine whether the material is flammable or reactive or if it poses a health hazard from inhalation. If no significant risk exists, the work does not likely require any special ventilation. If potential risk does exist, look at the physical properties of the chemical, specifically its vapor pressure and vapor density.

Vapor pressure is usually measured in millimeters of mercury. A low vapor pressure (<10 mmHg) indicates that the chemical does not readily form vapors at room temperature. General laboratory ventilation or an alternative such as the elephant trunk or snorkel may be appropriate, unless the material is heated or in a higher temperature room that might promote vapor formation. High vapor pressure indicates that the material easily forms vapors and may require use of a ventilated enclosure, such as a chemical hood.

Vapor density is compared to that of air, which is 1. A chemical having a vapor density greater than 1 is heavier than air. If the vapors need to be controlled, a chemical hood or a ventilation device that draws air from below, such as a downdraft table or a slot hood or elephant trunk with the exhaust aimed low may be appropriate. Conversely, a chemical with a vapor density less than 1 is lighter than air. Besides a chemical hood, a ventilation device that draws air from above, such as an elephant trunk or snorkel with the exhaust positioned above the source, may work best.

For radioactive or biological materials, consider whether the operations might cause the materials to aerosolize or become airborne and whether inhalation poses a risk to health or the environment. Determine whether filtration or trapping is required or recommended.

For manipulating solid particulates, a chemical hood and similar equipment with higher airflow may be too turbulent. Weighing boxes or ventilated balance enclosures may be a better fit for such work.

For nanomaterials, a laboratory chemical hood might be too turbulent for manipulating the materials. Also, consider whether the exhaust containing these tiny particles should be filtered. Studies have shown that high-efficiency particulate air (HEPA) filters are very effective for nano-size particles. Containment tests for chemical hoods allow for a very minor amount of leakage into the breathing zone of the user. For chemical vapors, such an amount may be insignificant, but in the same volume of nanoparticles, the number of particles may be quite large, and biosafety cabinets, gloveboxes or filtering hoods would be better. (See section 9.E.5 for more information.)

More specialized ventilation systems, such as biosafety cabinets and gloveboxes, may be necessary to control specific types of hazards, as discussed later in this chapter.

9.C.2. Laboratory Chemical Hoods

Laboratory chemical hoods are the most important components used to protect laboratory personnel from exposure to hazardous chemicals and agents. Functionally, a standard chemical hood is a fire- and chemical-resistant enclosure with one opening (face) in the front with a movable window (sash) to allow user access to the interior. Large volumes of air are drawn through the face and out the top into an exhaust duct to contain and remove contaminants from the laboratory. Note that because a substantial amount of energy is required to supply tempered supply air to even a small hood, the use of hoods to store bottles of toxic or corrosive chemicals is a very wasteful practice, which can seriously impair the effectiveness of the hood as a local ventilation device. Thus, it is preferable to provide separate vented cabinets for the storage of toxic or corrosive chemicals. The amount of air exhausted by such cabinets is much less than that exhausted by a properly operating hood.

A well-designed hood, when properly installed and maintained, offers a substantial degree of protection to the user if it is used appropriately and its limitations are understood. Chemical hoods are the best choice, particularly when mixtures or uncharacterized products are present and any time there is a need to manage chemicals using the ALARA principle.

9.C.2.1. Laboratory Chemical Hood Face Velocity

The average velocity of air drawn through the face of the laboratory chemical hood is called the face velocity. The face velocity greatly influences the ability to contain hazardous substances, that is, its containment efficiency. Face velocities that are too low or too high reduce the containment efficiency.

Face velocity is only one indicator of hood performance and one should not rely on it as a sole basis for determining the containment ability of the chemical hood. There are no regulations that specify acceptable face velocity. Indeed, modern hood designs incorporate interior configurations that affect the airflow patterns and are effective at different ranges of face velocity.

For traditional chemical hoods, several professional organizations have recommended that the chemical hood maintain a face velocity between 80 and 100 feet per minute (fpm). Face velocities between 100 and 120 fpm have been recommended in the past for substances of very high toxicity or where outside influences adversely affect hood performance. However, energy costs to operate the chemical hood are directly proportional to the face velocity and there is no consistent evidence that the higher face velocity results in better containment. Face velocities approaching or exceeding 150 fpm should not be used; they may cause turbulence around the periphery of the sash opening and actually reduce the capture efficiency, and may reentrain settled particles into the air.

With the desire for more sustainable laboratory ventilation design, manufacturers are producing high-performance hoods, also known as low-flow hoods, that achieve the same level of containment as traditional ones, but at a lower face velocity. These chemical hoods are designed to operate at 60 or 80 fpm and in some cases even lower. (See section 9.C.2.9.3.6.)

Average face velocity is determined by measuring individual points across the plane of the sash opening and calculating their average. A more robust measure of containment uses tracer gases to provide quantitative data and smoke testing to visualize airflow patterns. ASHRAE/ANSI 110 testing is an example of this technique (see section 9.C.2.8 for more information). This type of testing should be conducted at the time the chemical hood is installed, when substantial changes are made to the ventilation system, including rebalancing and periodically as part of a recommissioning or maintenance program.

Once a chemical hood is tested and determined to be acceptable via the ASHRAE/ANSI 110 method or an equivalent means, the face velocity should be noted and used as the reference point for routine testing. Each chemical hood, laboratory, facility, or site must define the acceptable average face velocity, minimum acceptable point velocity, and maximum standard deviation of velocities, as well as when ASHRAE/ANSI 110 or visualization testing is required. These requirements should be incorporated into the laboratory's Chemical Hygiene Plan and ventilation system management plans (see section 9.H).

When first installed and balanced, a laboratory chemical hood must be subjected to the ASHRAE/ANSI 110 or equivalent test before it is commissioned. When multiple similar chemical hoods are installed at the same time, at least half should be tested, provided the design is standardized relative to location of doors and traffic, and to location and type of air supply diffusers.

9.C.2.2. Factors That Affect Laboratory Chemical Hood Performance

Tracer gas containment testing of chemical hoods reveals that air currents impinging on the face at a velocity exceeding 30 to 50% of the face velocity reduce the containment efficiency by causing turbulence and interfering with the laminar flow of the air entering the chemical hood. Thirty to fifty percent of a face velocity of 100 fpm, for example, is 30 to 50 fpm, which represents a very low velocity that can be produced in many ways. The rate of 20 fpm is considered to be still air because that is the velocity at which most people first begin to sense air movement.

9.C.2.2.1. Proximity to Traffic

Most people walk at approximately 250 fpm (approximately 3 mph [4.8 kph]) and as they walk, vortices exceeding 250 fpm form behind them. If a person walks in front of an open chemical hood, the vortices can overcome the face velocity and pull contaminants into the vortex, and into the laboratory. Therefore, laboratory chemical hoods should not be located on heavily traveled aisles, and those that are should be kept closed when not in use. Foot traffic near these chemical hoods should be avoided when work is being performed.

9.C.2.2.2. Proximity to Supply Air Diffusers

Air is supplied continuously to laboratories to replace the air exhausted through laboratory chemical hoods and other exhaust sources and to provide ventilation and temperature/humidity control. This air usually enters the laboratory through devices called supply air diffusers located in the ceiling. Velocities that exceed 800 fpm are frequently encountered at the face of these diffusers. If air currents from these diffusers reach the face of a chemical hood before they decay to 30 to 50% of the face velocity, they cause the same effect as air currents produced by a person walking in front of the chemical hood. Normally, the effect is not as pronounced as the traffic effect, but it occurs constantly, whereas the traffic effect is transient. Relocating the diffuser, replacing it with another type, or rebalancing the diffuser air volumes in the laboratory can alleviate this problem.

9.C.2.2.3. Proximity to Windows and Doors

Exterior windows with movable sashes are not recommended in laboratories. Wind blowing through the windows and high-velocity vortices caused when doors open can strip contaminants out of the chemical hoods and interfere with laboratory static pressure controls. Place hoods away from doors and heavy traffic aisles to reduce the chance of turbulence reducing the effectiveness of the hood.

9.C.2.3. Prevention of Intentional Release of Hazardous Substances into Chemical Hoods

Laboratory chemical hoods should be regarded as safety devices that can contain and exhaust toxic, offensive, or flammable materials that form as a result of laboratory procedures. Just as you should never flush laboratory waste down a drain, never intentionally send waste up the chemical hood. Do not use the chemical hood as a means of treating or disposing of chemical waste, including intentionally emptying hazardous gases from compressed gas cylinders or allowing waste solvent to evaporate.

For some operations, condensers, traps, and/or scrubbers are recommended or necessary to contain and collect vapors or dusts to prevent the release of harmful concentrations of hazardous materials from the chemical hood exhaust.

9.C.2.4. Laboratory Chemical Hood Performance Checks

When checking if laboratory chemical hoods are performing properly, observe the following guidelines:

• Evaluate each hood before initial use and on a regular basis (at least once a year) to visualize airflow and to verify that the face velocity meets the criteria specified for it in the laboratory's Chemical Hygiene Plan or laboratory ventilation plan.
• Verify the absence of excessive turbulence (see section 9.C.2.6, below).
• Make sure that a continuous performance monitoring device is present, and check it every time the chemical hood is used. (For further information, see section 9.C.2.8 on testing and verification.)

Box 9.1 provides a list of things to do to maximize chemical hood efficiency.

BOX 9.1

Quick Guide for Maximizing Efficiency of Laboratory Chemical Hoods. Many factors can compromise the efficiency of chemical hood operation, and most are avoidable. Be aware of all behavior that can, in some way, modify the chemical hood and its capabilities. (more...)

9.C.2.5. Housekeeping

Keep laboratory chemical hoods and adjacent work areas clean and free of debris at all times. Keep solid objects and materials (such as paper) from entering the exhaust ducts, because they can lodge in the ducts or fans and adversely affect their operation. The chemical hood will have better airflow across its work surface if it contains a minimal number of bottles, beakers, and laboratory apparatus; therefore, prudent practice keeps unnecessary equipment and glassware outside the chemical hood at all times and stores all chemicals in approved storage cans, containers, or cabinets. Furthermore, keep the workspace neat and clean in all laboratory operations, particularly those involving the use of chemical hoods, so that any procedure or experiment can be undertaken without the possibility of disturbing, or even destroying, what is being done.

9.C.2.6. Sash Operation

Except when adjustments to the apparatus are being made, keep the chemical hood closed, with vertical sashes down and horizontal sashes closed, to help prevent the spread of a fire, spill, or other hazard into the laboratory. Horizontal sliding sashes should not be removed. The face opening should be kept small to improve the overall performance of the hood. If the face velocity becomes excessive, the facility engineers should make adjustments or corrections.

For chemical hoods without face velocity controls (see section 9.C.4.1), the sash should be positioned to produce the recommended face velocity, which often occurs only over a limited range of sash positions. This range should be determined and marked during laboratory chemical hood testing. Do not raise the sash above the working height for which it has been tested to maintain adequate face velocity. Doing so may allow the release of contaminants from the chemical hood into the laboratory environment.

Chemical hood sashes may move vertically (sash moves up and down), horizontally (sash is divided in panes that move side to side to provide the opening to the hood interior), or a combination of both. Although both types of sash offer protection from the materials within the hood and help control or maintain airflow, consider the following:

• Some experimentation requires the lab personnel to access equipment or materials toward the upper portion of the chemical hood. If the chemical hood is equipped with a vertical sash, it may be necessary to raise the sash completely in order to conduct the procedure.

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  The laboratory chemical hood must provide adequate containment at that sash height. Thus, the chemical hood must be tested in that position.

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  With the sash completely raised, it no longer provides a barrier between the chemical hood user and the materials within the hood.

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  If the only way to keep the sash in a fully raised position requires the use of a sash stop, the laboratory personnel may get into the habit of leaving the sash in this position, potentially reducing the safety and energy efficiency of the chemical hood.

• The standard operating position for the vertical sash may be comfortable for the majority of users. However, shorter laboratory personnel may find that this position does not provide an adequate barrier from the materials within the chemical hood and may need to adjust downward. Taller laboratory personnel may need to raise the sash more in order to comfortably work in the chemical hood.

For chemical hoods with horizontal sashes, the intended operating configuration is to open the panes in such a way that at least one pane is between both arms, providing a barrier between the user and the contents of the chemical hood. In addition,

• Do not remove panes. Permanently removing panes may decrease the safety afforded by the sash barrier and negatively affect containment and waste energy.
• Working with all panes moved to one side or through an opening in the center of the laboratory chemical hood provides no barrier between the user and the materials within the chemical hood. The chemical hood is not intended to be used in this configuration.

Sash panes should be equal width with a maximum of 15 in. (375 mm) to accommodate use of the sash pane as a protective barrier with operator arm on either side.

Conventional glass or plastic sashes are not designed to provide explosion protection per ANSI/NFPA (ANSI, ; NFPA, ). Sash panes and viewing panes constructed of composite material (safety glass backed by polycarbonate, with the safety glass toward the explosion hazard) are recommended for chemical hoods used when there is the possibility of explosion or violent overpressurization (e.g., hydrogenation, perchloric acid).

For all laboratory chemical hoods, the sash should be kept closed when the hood is not actively attended. Lowering or closing the sash not only provides additional personal protection but also results in significant energy conservation. Some chemical hoods may be equipped with automatic sash-positioning systems with counterweighting or electronic controls (see section 9.H.2).

9.C.2.7. Constant Operation of Laboratory Chemical Hoods

Although turning laboratory chemical hoods off when not in use saves energy, keeping them on at all times is safer, especially if they are connected directly to a single fan. Because most laboratory facilities are under negative pressure, air may be drawn backward through the nonoperating fan, down the duct, and into the laboratory unless an ultralow-leakage backdraft damper is used in the duct. If the air is cold, it may freeze liquids in the hood. The ducts are rarely insulated; therefore, condensation and ice may form in cold weather. When the chemical hood is turned on again and the duct temperature rises, the ice will melt, and water will run down the ductwork, drip into the hood, and possibly react with chemicals in the hood.

Chemical hoods connected to a common exhaust manifold offer the advantage that the main exhaust system is rarely shut down. Hence, positive ventilation is available on the system at all times. In a constant air volume (CAV) system (see section 9.C.4.1), install shutoff dampers to each chemical hood, allowing passage of enough air to prevent fumes from leaking into the laboratory when the sash is closed. Prudent practice allows 10 to 20% of the full volume of flow to be drawn through the laboratory chemical hood in the off position to prevent excessive corrosion.

Some laboratory chemical hoods on variable air volume (VAV) systems (see section 9.C.4.2) have automatic setback controls that adjust the airflow to a lower face velocity when not in use. The setback may be triggered by occupancy sensors, a light switch, or a timer or a completely lowered sash. Understand what triggers the setback and ensure that the chemical hood is not used for hazardous operations when in setback mode.

Some chemical hoods do have on/off switches and may be turned off for energy conservation reasons. They should only be turned off when they are empty of hazardous materials. An example of an acceptable operation would be a teaching laboratory where the empty chemical hoods are turned off when the laboratory is not in use.

9.C.2.8. Testing and Verification

The OSHA lab standard includes a provision regarding laboratory chemical hoods, including a requirement for some type of continuous monitoring device on each chemical hood to allow the user to verify performance and routine testing of the hood. It does not specify a test protocol.

Laboratory chemical hoods should be tested at least as follows:

• containment test by manufacturer;
• containment test after installation and prior to initial use (commissioning);
• annual or more frequent face velocity and airflow visualization;
• performance test any time a potential problem is reported; and
• containment test after significant changes to the ventilation system, including rebalancing or recommissioning.

9.C.2.8.1. Initial Testing

All laboratory chemical hoods should be tested before they leave the manufacturer according to ANSI/ASHRAE Standard 110- or equivalent, Methods of Testing Performance of Laboratory Fume Hoods (ANSI, ). They should pass the low- and high-volume smoke challenges with no leakage or flow reversals and have a control level of 0.05 ppm or less on the tracer gas test. It is highly recommended that chemical hoods be retested by trained personnel after installation in their final location, using ANSI/ASHRAE 110- or equivalent testing. The control level of tracer gas for an “as installed” or “as used” test via the ANSI/ASHRAE 110- method should not exceed 0.1 ppm.

The ANSI/ASHRAE 110- test is the most practical way to determine chemical hood capture efficiency quantitatively. The test includes several components, which may be used together or separately, including face velocity testing, flow visualization, face velocity controller response testing, and tracer gas containment testing. These tests are much more accurate than face velocity and smoke testing alone. Respectively, ASHRAE and ANSI found that 28% or 38% of chemical hoods tested using this method did not meet the pass criteria, even though face velocity testing alone found them to be in an acceptable face velocity range.

Performance should be evaluated against the design specifications for uniform airflow across the chemical hood face as well as for the total exhaust air volume. Equally important is the evaluation of operator exposure. The first step in the evaluation of hood performance is the use of a smoke tube or similar device to determine that the laboratory chemical hood is on and exhausting air. The second step is to measure the velocity of the airflow at the face of the hood. The third step is to determine the uniformity of air delivery to the hood face by making a series of face velocity measurements taken in a grid pattern.

Leak testing is normally conducted using a mannequin equipped with sensors for the test gas. As an alternative, a person wearing the sensors or collectors may follow a sequence of movements to simulate common activities, such as transferring chemicals. It is most accurate to perform the in-place tests with the chemical hood at least partially loaded with common materials (e.g., chemical containers filled with water, equipment normally used in the chemical hood), in order to be more representative of operating conditions.

For the ASHRAE 110- leak testing, the method calls for a release rate for the test gas of 4 liters per minute (Lpm), but suggests that higher rates may be used. One-liter per minute release rate approximates pouring a volatile solvent from one beaker to another. Eight liters per minute approximates boiling water on a 500-W hot plate. The 4-Lpm rate is an intermediate of these two conditions. If there is a possibility that the chemical hood will be used for volatile materials under heating conditions, consider a higher release rate of up to 8 Lpm for worst-case conditions.

The total volume of air exhausted by a laboratory chemical hood is the sum of the face volume (average face velocity times face area of the hood) plus air leakage, which averages about 5 to 15% of the face volume. If the laboratory chemical hood and the general ventilating system are properly designed, face velocities in the range of the design criteria will provide a laminar flow of air over the work surface and sides of the hood. Higher face velocities (150 fpm or more), which exhaust the general laboratory air at a greater rate, waste energy and are likely to degrade hood performance by creating air turbulence at the face and within the chemical hood, causing vapors to spill out into the laboratory ( ).

FIGURE 9.3

Laminar versus turbulent velocity profile. Velocity data are from a single traverse point on two separate hoods. The light line represents a hood where supply air interference caused large variations in velocity, a “typical” turbulent (more...)

An additional method for containment testing is the EN , which is the standard adopted by the European Union and replaces several other procedures that were in place for individual countries. Parts 3 (Type tests) and 4 (On-site tests) of this standard address methods for “as manufactured” and “as installed/used” systems, respectively.

9.C.2.8.2. Routine Testing

At least annually, the following test procedures should be conducted for all chemical hoods:

• Analyze face velocity using the method and criteria described in section 9.C.2.8.4.
• Visualize airflow using smoke tubes, bombs, or fog generators.
• Verify that continuous flow monitoring devices are working properly.
• Verify that other controls, including automatic sash positioners, alarm systems, etc. are functioning properly.
• Check the sash to ensure that it is in good condition, moves easily, is unobstructed, and has adequate clarity to see inside the laboratory chemical hood.
• Ensure that the laboratory chemical hood is being used as intended (e.g., no evidence of perchloric acid in a chemical hood not designed for it, not using it as a chemical storage device).
• Note any conditions that could affect laboratory chemical hood performance, such as large equipment, excessive storage, etc.
• Take corrective actions where necessary and retest.

Provide information and test results to the chemical hood users and/or supervisors. Document the results in order to maintain a log showing the history of chemical hood performance.

9.C.2.8.3. Additional Testing

Laboratory personnel should request a chemical hood performance evaluation any time there is a change in any aspect of the ventilation system. Thus, changes in the total volume of supply air, changes in the locations of supply air diffusers, or the addition of other auxiliary local ventilation devices (e.g., more chemical hoods, vented cabinets, and snorkels) all call for reevaluation of the performance of all chemical hoods in the laboratory.

9.C.2.8.4. Face Velocity Testing

Visually divide the face opening of a laboratory chemical hood into an imaginary grid, with each grid space being approximately 1 ft2 in area. Using an anemometer, velometer, or similar device, take a measurement at the center of each grid space. Face velocity readings should be integrated for at least 10 seconds (20 is preferable) because of the fluctuations in flow. The measured velocity will likely fluctuate for several seconds; record the reading once it has stabilized. Calculate the average of the velocity for every grid space. The resulting number is the average face velocity. Analyze the results to determine if any one measurement is 20% or more above or below the average. Such readings indicate the possibility of turbulent or nonlaminar airflow. Smoke tests will help confirm whether this is problematic.

Traditional handheld instruments are subject to probe movement and positioning errors as well as reading errors owing to the optimistic bias of the investigator. Also, the traditional method yields only a snapshot of the velocity data, and no measure of variation over time is possible. To overcome this limitation, take velocity data while using a velocity transducer connected to a data acquisition system and read continuously by a computer for approximately 30 seconds at each traverse point. If the transducer is fixed in place, using a ring stand or similar apparatus, and is properly positioned and oriented, this method overcomes the errors and drawbacks associated with the traditional method. The variation in data for a traverse point can be used as an indicator of turbulence, an important additional performance indicator that has been almost completely overlooked in the past.

If the standard deviation of the average velocity profile at each point exceeds 20% of the mean, or the average standard deviation of velocities at each traverse point (turbulence) exceeds 15% of the mean face velocity, corrections should be made by adjusting the interior baffles and, if necessary, by altering the path of the supply air flowing into the room (see ). Most laboratory chemical hoods are equipped with a baffle that has movable slot openings at both the top and the bottom, which should be moved until the airflow is essentially uniform. Larger chemical hoods may require additional slots in the baffle to achieve uniform airflow across the face. These adjustments should be made by an experienced laboratory ventilation engineer or technician using proper instrumentation.

FIGURE 9.4

Effect of baffles on face velocity profile in a laboratory chemical hood.

9.C.2.8.5. Testing Criteria

Prior to the initial tests, determine the acceptance criteria for the ANSI/ASHRAE 110- leak test, face velocity (based on the results of the ANSI/ASHRAE testing and the design of the laboratory chemical hood), and visual airflow tests.

One important factor to consider is acceptable sash position. It is common to set the acceptance criteria as an acceptable level of containment and/or face velocity range at the standard operating position of the sash, often 18 in. However, one must understand how the chemical hood will be used to determine the range of sash positions needed. For example, if the users will need to sometimes use the hood with vertical sash fully open, then the test criteria should be for 100% sash opening.

It may be prudent to set the acceptance criteria with the sash 100% open and 80% open, ensuring adequate containment at both of these positions.

9.C.2.8.6. Instrumentation

Anemometers and other instruments used to measure face velocity must be accurate in order to supply meaningful data. Instruments should be calibrated at least once a year and the calibration should be National Institute of Standards and Technology traceable.

9.C.2.8.7. Additional Exposure Monitoring

If there is any concern that a laboratory chemical hood or other ventilation device may not provide enough protection to the trained laboratory personnel, it is prudent to measure worker exposure while the hood is being used for its intended purpose. By conducting personal air-sampling using traditional industrial hygiene techniques, worker exposure (both excursion peak and time-weighted average) can be measured. The criterion for evaluating the hood should be the desired performance (i.e., does the hood contain vapors and gases at the desired worker-exposure level?). A sufficient number of measurements should be made to define a statistically significant maximum exposure based on worst-case operating conditions. Direct-reading instruments may be available for determining the short-term concentration excursions that may occur in chemical hood use.

9.C.2.9. Laboratory Chemical Hood Design and Construction

When specifying a laboratory chemical hood for use in a particular activity, laboratory personnel should be aware of the design features. Assistance from an industrial hygienist, ventilation engineer, or laboratory consultant is recommended when deciding to purchase a chemical hood.

9.C.2.9.1. General Design Recommendations

Construct laboratory chemical hoods and the associated exhaust ducts of nonflammable materials. Equip them with vertical, horizontal, or combination vertical/horizontal sashes that can be closed. For the glass within the sash, use either laminated safety glass that is at least 7/32-in. thick or other equally safe material that will not shatter if there is an explosion inside. Locate the utility control valves, electrical receptacles, and other fixtures outside the chemical hood to minimize the need to reach within the chemical hood proper. Other specifications regarding the construction materials, plumbing requirements, and interior design vary, depending on the intended use. (See Chapter 7, sections 7.C.1.1 and 7.C.1.2.) Information regarding the minimum flow rate through hoods can be found in ANSI Z9.5.

Although chemical hoods are most commonly used to control concentrations of toxic vapors, they can also serve to dilute and exhaust flammable vapors. Although theoretically possible, it is extremely unlikely (even under worst-case scenarios) that the concentration of flammable vapors will reach the lower explosive limit (LEL) in the exhaust duct. However, somewhere between the source and the exhaust outlet of the chemical hood, the concentration will pass through the upper explosive limit and the LEL before being fully diluted at the outlet. Both the designer and the user should recognize this hazard and eliminate possible sources of ignition within the chemical hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory hood ductwork is not necessary or desirable.

9.C.2.9.2. Special Design Features

Since the invention of the chemical hood, two major improvements have been made in the design—airfoils and baffles. Include both features on any new purchases.

Airfoils built into the bottom and sides of the sash opening significantly reduce boundary turbulence and improve capture performance. Fit new chemical hoods with airfoils and retrofit any hoods without airfoils

When air is drawn through a laboratory chemical hood without a baffle (see ), most of the air is drawn through the upper part of the opening, producing an uneven velocity distribution across the face opening. All chemical hoods should have baffles. When baffles are installed, the velocity distribution is greatly improved. Adjustable baffles can improve hood performance and are desirable if the adjustments are made by an experienced industrial hygienist, consultant, or technician.

9.C.2.9.3. Laboratory Chemical Hood Airflow Types

The first chemical hoods were simply boxes that were open on one side and connected to an exhaust duct. Since they were first introduced, many variations on this basic design have been made. Six of the major variants in airflow design are listed below with their characteristics. Conventional laboratory chemical hoods are the most common and include benchtop, distillation, and walk-in hoods of the CAV, CAV bypass, nonbypass, and VAV, with or without airfoils. Auxiliary air hoods and ductless chemical hoods are not considered conventional and are used less often. Trained laboratory personnel should know what kind they are using and what its advantages and limitations are. In general, the initial cost of a CAV system may be less than VAV, but the life-cycle cost of the VAV will almost always be lower than a CAV system.

9.C.2.9.3.1. Constant Air Volume Laboratory Chemical Hoods

A CAV chemical hood draws a constant exhaust volume regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position. The laboratory chemical hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and the chemical hood should be operated at this height. (See section 9.C.4.)

9.C.2.9.3.2. Constant Air Volume Nonbypass Laboratory Chemical Hoods

A nonbypass chemical hood has only one major opening through which the air may pass, that is, the sash opening. The airflow pattern is shown in . A CAV nonbypass chemical hood has the undesirable characteristic of producing very large face velocities at small sash openings. As the sash is lowered, face velocities may exceed 1,000 fpm near the bottom. Face velocities are limited by the leakage through cracks and under the airfoil and by the increasing pressure drop as the sash is closed.

FIGURE 9.5

Effect of sash placement on airflow in a nonbypass laboratory chemical hood.

A common misconception is that the volume of air exhausted by this type of chemical hood decreases when the sash is closed. Although the pressure drop increases slightly as the sash is closed, no appreciable change in volume occurs. All chemical hoods should be closed when not in use, because they provide a primary barrier to the spread of a fire or chemical release.

Many trained laboratory personnel are reluctant to close their CAV nonbypass chemical hoods because of the increase in air velocity and noise that occurs when the sash is lowered. This high-velocity air jet sweeping over the work surface often disturbs gravimetric measurements, causes undesired cooling of heated vessels and glassware, and can blow sample trays, gloves, and paper towels to the back of the laboratory chemical hood, where they may be drawn into the exhaust system. Exercise care to prevent materials from entering the exhaust system where they can lodge in the ductwork, reducing airflow, or can be conveyed through the system and drawn into the exhaust fan and damage the fan or cause sparks.

Because of numerous operational problems with the design of nonbypass hoods, their installation in new facilities is discouraged. If present in existing facilities, their replacement should be considered. In many instances, the cost of replacement can be recouped from the resulting reduction in energy costs.

9.C.2.9.3.3. Bypass Laboratory Chemical Hoods

A bypass chemical hood is shown in . It is similar to the nonbypass design but has an opening above the sash through which air may pass at low sash positions. Because the opening is usually 20 to 30% of the maximum open area of the sash, this hood will still exhibit the increasing velocity characteristic of the nonbypass chemical hood as the sash is lowered. But the face velocity stops increasing as the sash is lowered to the position where the bypass opening is exposed by the falling sash. The terminal face velocity of these types of hoods depends on the bypass area but is usually in the range of 300 to 500 fpm—significantly higher than the recommended operating face velocity. Therefore, the air volume for bypass laboratory chemical hoods should also be adjusted to achieve the desired face velocity at the desired sash height, and the hood should be operated at this position. This arrangement is usually found in combination with a vertical sash, because this is the simplest arrangement for opening the bypass. Varieties are available for horizontal sashes, but the bypass mechanisms are complicated and may cause maintenance problems. For a well-designed bypass hood, the face velocity will stay relatively constant until open about 12 in., then increases rapidly.

FIGURE 9.6

Effect of sash placement on airflow in a bypass laboratory chemical hood.

9.C.2.9.3.4. Variable Air Volume Laboratory Chemical Hoods

A VAV chemical hood, also known as a constant velocity hood, is one that has been fitted with a face velocity control, which varies the amount of air exhausted from the chemical hood in response to the sash opening to maintain a constant face velocity. In addition to providing an acceptable face velocity over a relatively large sash opening (compared to a CAV hood), VAV hoods also provide significant energy savings by reducing the flow rate when it is closed. These types of hoods are usually of the nonbypass design to reduce air volume (see below). Even though the face velocity responds to the position of the sash, the face velocity may drop off as the sash height increases, depending on the design. As a result, there is a maximum sash height above which the chemical hood becomes less effective.

9.C.2.9.3.5. Auxiliary Air Laboratory Chemical Hoods

Quantitative tracer gas testing of many auxiliary air laboratory chemical hoods has revealed that, even when adjusted properly and with the supply air properly conditioned, significantly higher personnel exposure to the materials used may occur than with conventional (non-auxiliary air) chemical hoods. They should not be purchased for new installations, and existing ones should be replaced or modified to eliminate the supply air feature. This feature causes a disturbance of the velocity profile and leakage of fumes into the personnel breathing zone.

The auxiliary air chemical hood was developed in the s primarily to reduce laboratory energy consumption and is a combination of a bypass hood and a supply air diffuser located at the top of the sash. They were intended to introduce unconditioned or tempered air, as much as 70% of the air exhausted, directly to the front of the chemical hood. Ideally, this unconditioned air bypasses the laboratory and significantly reduces air-conditioning and heating costs. In practice, however, many problems are caused by introducing unconditioned or slightly conditioned air above the sash, all of which may produce a loss of containment.

9.C.2.9.3.6. Low-Flow or High-Performance Laboratory Chemical Hoods

With rising energy costs and high interest in sustainable laboratory design, manufacturers are producing low-flow, “high-performance” hoods that are able to meet the performance criteria of the ANSI/ASHRAE 110- tests at a lower face velocity. They tend to be deeper than the traditional laboratory chemical hood and some have altered air front airfoils, internal or external auxiliary air devices, and/or automatic baffle controls. There are other design differences from a traditional chemical hood; thus, it is usually not possible to simply reduce the flow of a traditional hood to a lower face velocity and expect it to meet the same performance criteria as these specially designed hoods.

Like any other chemical hood, the design criteria and limitations need to be fully understood before one is selected for the laboratory. For example, if the chemical hood is designed to meet performance criteria at a sash height of 18 in., but users must operate it at a sash height of 24 in., the hood may not be effective at 24 in., creating a potentially hazardous situation.

Reviews by users have been mixed. For best results, be sure that the engineers, trained laboratory personnel, and the vendors understand how the chemical hoods are intended to be used. Their design and function continue to improve.

9.C.2.9.3.7. Ductless Laboratory Chemical Hoods

Ductless laboratory chemical hoods are ventilated enclosures that have their own fan, which draws air out and through filters and ultimately recirculates it into the laboratory. The filters are designed to trap vapors generated in the chemical hood and exhaust clean air back into the laboratory. They frequently use activated carbon filters, HEPA filters, or a combination of the two. Newer filter materials on the market claim that they capture a larger variety of chemicals.

These ventilated enclosures do not necessarily achieve the same level of capture and containment as a chemical hood. Unlike a conventional laboratory chemical hood, it is not possible to conduct tracer gas studies to measure containment even with the newer technology ductless hoods. Because the collection efficiency of the filters decreases over time, the filters must be monitored and replaced routinely. Depending on the materials and the laboratory environment, chemicals can desorb from the filter and reenter the laboratory over time. They do not control fire hazards and National Fire Protection Association standard 45 states “Ductless chemical fume hoods that pass air from the hood interior through an absorption filter and then discharge the air into the laboratory are only applicable for use with nuisance vapors and dusts that do not present a fire or toxicity hazard” (NFPA, ).

Ductless chemical hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in it are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with its serious limitations. If these limitations cannot be accommodated, do not use this type of device.

The benefits of recirculating chemical hoods are that they are much more energy efficient than a ducted chemical hood and they do not require a ventilation system that relies on a fan on the roof or upper levels. Some urban buildings retrofitted with laboratories on lower floors, buildings with limitations on the ventilation system or laboratories with minor chemical use have successfully used these ductless hoods, under the limited conditions cited above and with rigorous filter maintenance programs. They can also be used for control of particulate material where a chemical hood or even Class 1 or 2 biosafety cabinets provide too much turbulent air (see section 9.E.4.1).

For more information, please visit Laboratory Ventilation Solutions.

To determine whether recirculating hoods are appropriate, an industrial hygienist or safety professional should conduct a risk assessment that includes

• an analysis of the chemicals that will be used, the hazards they pose, and the materials they generate as byproducts;
• the frequency and duration of use of these chemicals; and
• the nature of the materials that must be controlled compared to the filter media provided with the recirculating hood.

Individuals using recirculating hoods need training on the use and limitations of the recirculating hood. Each ductless chemical hood should have signage explaining the limitations, how to detect whether the filter media are working, and the filter maintenance schedule.

9.C.2.10. Laboratory Chemical Hood Configurations

9.C.2.10.1. Benchtop Laboratory Chemical Hoods

As the name implies, a benchtop chemical hood sits on a laboratory bench with the work surface at bench height. It can be of the CAV or VAV variety and can have a bypass or nonbypass design. The sash can be a vertical-rising or a horizontal-sliding type or a combination of the two. Normally, the work surface is dished or has a raised lip around the periphery to contain spills. Sinks in chemical hoods are not recommended because they encourage laboratory personnel to dispose of chemicals in them. If they must be used, to drain cooling water from a condenser, for instance, they should be fitted with a standpipe to prevent chemical spills from entering the drain. The condenser water drain can be run into the standpipe. Spills will be caught in the cupsink by the standpipe for later cleanup and disposal. A lip on the cupsink could be used as an alternative to a standpipe to prevent spills from getting into the sink. A typical benchtop chemical hood is shown in .

FIGURE 9.7

Diagram of a typical benchtop laboratory chemical hood.

9.C.2.10.2. Distillation (Knee-High or Low-Boy) Chemical Fume Hoods

The distillation hood is similar to the benchtop hood except that the work surface is closer to the floor to allow more vertical space inside for tall apparatuses such as distillation columns. A typical distillation hood is shown in .

FIGURE 9.8

Diagram of a typical distillation hood.

9.C.2.10.3. Walk-In Laboratory Chemical Hoods

A walk-in hood stands on the floor of the laboratory and is used for very tall or large apparatus. The sash can be either horizontal or double- or triple-hung vertical. These hoods are usually of the nonbypass type. The word “walk-in” is a misnomer; one should never actually walk into a chemical hood when it is operating and contains hazardous chemicals. Once past the plane of the sash, the personnel are inside with the chemicals. If the personnel are required to enter the hood during operations where hazardous chemicals are present, they should wear PPE appropriate for the hazard. It may include respirators, chemical splash goggles, rubber gloves, boots, suits, and self-contained breathing apparatus. A typical walk-in chemical hood is shown in .

FIGURE 9.9

Diagram of a typical walk-in laboratory chemical hood.

9.C.2.10.4. California Laboratory Chemical Fume Hoods and Ventilated Enclosures

The California chemical fume hood is a ventilated enclosure with a movable sash on more than one side. They are usually accessed through a horizontal sliding sash from the front and rear. They may also have a sash on the ends. Because their configuration precludes the use of baffles and airfoils, they may not provide a suitable face velocity distribution across their many openings.

A ventilated enclosure is any site-fabricated chemical hood designed primarily for containing processes such as scale-up or pilot plant equipment. Most do not have baffles or airfoils, and most designs have not had the rigorous testing and design refinement that conventional mass-produced chemical hoods enjoy. Working at the opening of the devices, even when the plane of the opening has not been broken, may expose personnel to higher concentrations of hazardous materials than if a conventional hood were used.

9.C.2.10.5. Perchloric Acid Laboratory Chemical Hoods

The perchloric acid laboratory chemical hood, with its associated ductwork, exhaust fan, and support systems, is designed especially for use with perchloric acid and other materials that can deposit shock-sensitive crystalline materials in the hood and exhaust system. These materials become pyrophoric when they dry or dehydrate (see also Chapter 6, section 6.G.6). Special water spray systems are used to wash down all interior surfaces of the hood, duct, fan, and stack, and special drains are necessary to handle the effluent from the washdown. The liner and work surface are usually stainless steel with welded seams. Perchloric acid hoods have drains in their work surface. Water spray heads are usually installed in the top, behind the baffles, and in the interior. The water spray should be turned on whenever perchloric acid is being heated in the chemical fume hood. The ductwork should be fabricated of plastic, glass, or stainless steel and fitted with spray heads approximately every 10 ft on vertical runs and at each change in direction. The fan and stack should be fabricated of plastic, fiberglass, or stainless steel. Welded or flanged and gasketed fittings to provide airtight and watertight connections are recommended. Avoid horizontal runs because they inhibit drainage, and the spray action is not as effective on the top and sides of the duct. Any washdown piping, which is located outside must be protected from freezing. A drain and waste valve on the water supply piping that allows it to drain when not in use is helpful. Route the drain lines carefully to prevent the creation of traps that retain water. Write special operating procedures to cover the washdown procedure for these types of hoods. The exhaust from a perchloric acid hood should not be manifolded with that from other types of chemical hoods.

9.C.2.10.6. Radioisotope Laboratory Chemical Hoods

Design chemical hoods used for work with radioactive sources or materials so that they can be decontaminated completely on a regular basis. A usual feature is a one-piece stainless steel welded liner with smooth curved corners that can be cleaned easily and completely. The superstructure of radioisotope hoods is usually made stronger than that of a conventional hood to support lead bricks and other shielding that may be required. Special treatment of the exhaust from radioisotope hoods may be required by government regulations to prevent the release of radioactive material into the environment. This treatment usually involves the use of HEPA filters (see section 9.C.4.2).

Another practical way to handle radioactive materials that require special exhaust treatment is to use a containment chamber within a traditional chemical hood. Several safety supply companies offer portable disposable glovebag containment chambers with sufficient space to conduct the work and then dispose of them in accordance with applicable nuclear regulatory standards.

9.C.2.10.7. Clean Room Laboratory Chemical Hoods

Chemical hoods in clean rooms are generally no different than traditional chemical hoods, except that they are usually made of polypropylene or thermoplastics. Some have hinged sashes rather than sliding sashes. Most require separate chemical hoods for acid work and solvent work.

Polypropylene hoods burn easily, melt quickly, and may become fully involved in a fire. There are fire-retardant polypropylene and other thermoplastics available, but they cost more. Alternatively, an automatic fire extinguisher may be installed inside.

9.C.2.11. Laboratory Chemical Hood Exhaust Treatment

Until recently, treatment of laboratory chemical hood exhausts has been limited. Because effluent quantities and concentrations are relatively low compared to those of other industrial air emission sources, their removal is technologically challenging. And the chemistry for a given chemical hood effluent can be difficult to predict and may change over time.

Nevertheless, legislation and regulations increasingly recognize that certain materials in laboratory chemical hoods may be sufficiently hazardous that they can no longer be expelled directly into the air. Therefore, the practice of removing these materials from exhaust streams will become increasingly more prevalent.

9.C.2.11.1. Laboratory Chemical Hood Scrubbers and Contaminant Removal Systems

A number of technologies are evolving for treating chemical hood exhaust by means of scrubbers and containment removal systems. Whenever possible, experiments involving toxic materials should be designed so that they are collected in traps or scrubbers rather than released. If for some reason collection is impossible, HEPA filters are recommended for highly toxic particulates. Liquid scrubbers may also be used to remove particulates, vapors, and gases from the exhaust system. None of these methods, however, is completely effective, and all trade an air pollution problem for a solid or liquid waste disposal problem. Incineration may be the ultimate method for destroying combustible compounds in exhaust air, but adequate temperature and dwell time are required to ensure complete combustion.

Incinerators require considerable capital to build and energy to operate; hence, other methods should be studied before resorting to their use. Determine the optimal system for collecting or destroying toxic materials in exhaust air on a case-by-case basis. Treatment of exhaust air should be considered only if it is not practical to pass the gases or vapors through a scrubber or adsorption train before they enter the exhaust airstream. Also, if an exhaust system treatment device is added to an existing chemical hood, carefully evaluate the impact on the fan and other exhaust system components. These devices require significant additional energy to overcome the pressure drop they add to the system. (See also Chapter 8, section 8.B.6.1.)

9.C.2.11.2. Liquid Scrubbers

A laboratory chemical hood scrubber is a laboratory-scale version of a typical packed-bed liquid scrubber used for industrial air pollution control. shows a schematic of a typical chemical hood scrubber.

FIGURE 9.10

Schematic of a typical laboratory chemical hood scrubber.

Contaminated air from the chemical hood enters the unit and passes through the packed-bed, liquid spray section, and mist eliminator and into the exhaust system for release up the stack. The air and the scrubbing liquor pass in a countercurrent fashion for efficient gas-liquid contact. The scrubbing liquor is recirculated from the sump and back to the top of the system using a pump. Water-soluble gases, vapors, and aerosols are dissolved into the scrubbing liquor. Particulates are also captured quite effectively by this type of scrubber. Removal efficiencies for most water-soluble acid- and base-laden airstreams are usually between 95 and 98%.

Scrubber units are typically configured vertically and are located next to the chemical hood as shown in . They are also produced in a top-mount version, in which the packing, spray manifold, and mist eliminator sections are located on top of the chemical hood and the sump and liquid-handling portion are underneath for a compact arrangement taking up no more floor area than the hood itself. Most hoods do not require a scrubber unit, assuming the exhaust stack is designed properly and chemical quantities of volatile materials are low.

9.C.2.11.3. Other Gas-Phase Filters

Another basic type of gas-phase filtration is available for chemical hoods in addition to liquid scrubbers. These are inert adsorbents and chemically active adsorbents. The inert variety includes activated carbon, activated alumina, and molecular sieves. These substances typically come in bulk form for use in a deep bed and are available also as cartridges and as panels for use in housings similar to particulate filter housings. They are usually manufactured in the form of beads, but they may take many forms. The beads are porous and have extremely large surface areas with sites onto which gas and vapor molecules are trapped or adsorbed as they pass through. Chemically active adsorbents are simply inert adsorbents impregnated with a strong oxidizer, such as potassium permanganate (purple media), which reacts with and destroys the organic vapors. Although there are other oxidizers targeted to specific compounds, the permanganates are the most popular. Adsorbents can handle hundreds of compounds, including most volatile organic components but also have an affinity for harmless species such as water vapor.

As the air passes through the adsorbent bed, gases are removed in a section of the bed. (For this discussion, gas means gases and vapors.) As the bed loads with gases, and if the adsorbent is not regenerated or replaced, eventually contaminants will break through the end of the bed. After breakthrough occurs, gases will pass through the bed at higher and higher concentrations at a steady state until the upstream and downstream levels are almost identical. To prevent breakthrough, the adsorbent must be either changed or regenerated on a regular basis. Downstream monitoring to detect breakthrough or sampling of the media to determine the remaining capacity of the bed should be performed regularly.

An undesirable characteristic of these types of scrubbers is that if high concentrations of organics or hydrocarbons are carried into the bed, as would occur if a liquid were spilled inside the hood, a large exotherm occurs in the reaction zone of the bed. This exotherm may cause a fire in the scrubber. Place these scrubbers and other downstream devices such as particulate filters in locations where the effects of a fire would be minimized. Fires can start in these devices at surprisingly low temperatures because of the catalytic action of the adsorbent matrix. Therefore, use and operate such devices with care.

9.C.2.11.4. High-Efficiency Filters

Air from laboratory chemical hoods and biological safety cabinets (BSCs) in which some radioactive or biologically active particulates are used should be properly filtered to remove these agents and prevent their release into the atmosphere. Other hazardous particulates may require this type of treatment as well. The most popular method of removal is a HEPA filter. These HEPA filters trap 99.97% of all particulates greater than 0.3 μm in diameter and may be just as effective with smaller particle sizes. Studies have shown that HEPA filters can be quite effective at trapping nanoparticles, due to Brownian motion and electrostatic capture. Before any filtration system is installed, a risk assessment should be performed to determine the need and the appropriate level of filtration required.

Ultra-low penetration air (ULPA) filters are an alternative to HEPA filters. These filters are 99.% efficient in removing particles greater than 0.12 μm. However, ULPA filters are more expensive than HEPA filters, and they increase the system static pressure. Note that any system designed to provide protection against radioactive particles can be expected to be effective against nanoparticles, and studies have confirmed that HEPA filters provide sufficient capture for nanoparticles (HHS/CDC/NIOSH, a) making ULPA unnecessary.

These systems must be specified, purchased, and installed so that the filters can be changed without exposing the personnel or the environment to the agents trapped in the filter. Sterilizing the filter bank is prudent before changing filters that may contain etiologic agents.

The bag-in, bag-out method of replacing filters is a popular way to prevent personnel exposure. This method separates the contaminated filter and housing from the personnel and the environment by using a special plastic barrier bag and special procedures to prevent exposure to or release of the hazardous agent.

9.C.2.11.5. Thermal Oxidizers and Incinerators

Thermal oxidizers and incinerators are extremely expensive to purchase, install, operate, and maintain. However, they are one of the most effective methods of handling toxic and etiologic agents. The operational aspects of these devices are beyond the scope of this book. Also, their application to chemical hoods has historically been rare. When considering this method of pollution control, call an expert for assistance.

9.C.3. Other Local Exhaust Systems

Many types of laboratory equipment and apparatus that generate vapors and gases should not be used in a conventional laboratory chemical hood. Some examples are gas chromatographs, atomic absorption spectrophotometers, mixers, vacuum pumps, and ovens. If the vapors or gases emitted by these types of equipment are hazardous or noxious, or if it is undesirable to release them into the laboratory because of odor or heat, contain and remove them using local exhaust equipment. Local capture equipment and systems should be designed only by an experienced engineer or industrial hygienist. Also, users of these devices must have appropriate training.

Whether the emission source is a vacuum-pump discharge vent, a gas chromatograph exit port, or the top of a fractional distillation column, the local exhaust requirements are similar. The total airflow should be high enough to transport the volume of gases or vapors being emitted, and the capture velocity should be sufficient to collect the gases or vapors.

Despite limitations, specific ventilation capture systems provide effective control of emissions of toxic vapors or dusts if installed and used correctly and, in some cases, can result in energy savings. A separate dedicated exhaust system is recommended. Do not attach the capture system to an existing laboratory chemical hood duct unless fan capacity is increased and airflow to both hoods is properly balanced. One important consideration is the effect that such added local exhaust systems will have on the ventilation for the rest of the laboratory. Each additional capture hood will be a new exhaust port in the laboratory and will compete with the existing exhaust sources for air supply.

Downdraft ventilation has been used effectively to contain dusts and other dense particulates and high concentrations of heavy vapors that, because of their density, tend to fall. Such systems require special engineering considerations to ensure that the particulates are transported in the airstream. Here again, consult a ventilation engineer or industrial hygienist if this type of system is deemed suitable for a particular laboratory operation.

9.C.3.1. Elephant Trunks, Snorkels, or Extractors

An elephant trunk, or snorkel, is a piece of flexible duct or hose connected to an exhaust system. To capture contaminants effectively, it must be closer than approximately one-half a diameter of the hood from the end of the hose. An elephant trunk is particularly effective for capturing discharges from gas chromatographs, pipe nipples, and pieces of tubing if the hose is placed directly on top of the discharge with the end of the discharge protruding to the hose. Note that unless the intake for the snorkel is placed very close to the point source, it will be susceptible to inefficient capture. Newer designs mount the intake on an articulated arm, which tends to make the systems more effective and convenient to use. (See .) The volume flow rate of the hose must be at least 110 to 150% of the flow rate of the discharge.

FIGURE 9.11

Fume extractor or snorkel.

The face velocity for a snorkel or elephant trunk is usually 150–200 fpm. The velocity and the capture efficiency drop sharply with distance from the intake. As a result, efficient capture of contaminants is generally adequate when the discharge source is 2 in. away, but inadequate if it is 3 in. away. In cases where there is a question about efficacy of capture, perform a smoke test to determine if the flow rate is adequate (ACGIH, ).

9.C.3.2. Slot Hoods

Slot hoods are local exhaust ventilation hoods specially designed to capture contaminants generated according to a specific rate, distance in front of the hood, and release velocity for specific ambient airflow. In general, if designed properly, these hoods are more effective and operate using much less air than either elephant trunks or canopy hoods. To be effective, however, the geometry, flow rate, and static pressure must all be correct.

Typical slot hoods are shown in . Each type has different capture characteristics and applications. If laboratory personnel believe that one of these devices is necessary, a qualified ventilation engineer should design the hood and exhaust system.

FIGURE 9.12

Diagrams of typical slot hoods.

9.C.3.3. Canopy Hoods

The canopy hood is not only the most common local exhaust system but also probably the most misunderstood piece of industrial ventilation equipment. Industrial ventilation experts estimate that as many as 95% of the canopy hoods in use (other than in homes and restaurants) are misapplied and ineffective. The capture range of a canopy hood is extremely limited, and a large volume of air is needed for it to operate effectively. Thus, a canopy hood works best when thermal or buoyant forces exist that move the contaminant up to the hood capture zone (a few inches below the opening). However, because canopy hoods are generally placed well above a contaminant source so that laboratory personnel can operate underneath them, they draw contaminants past the breathing zone and into the exhaust system. If a canopy hood exists in a laboratory, use it only for nonhazardous service, such as capturing heated air or water vapor from ovens or autoclaves. For design advice, consult the American Conference of Governmental Industrial Hygienists ventilation manual (ACGIH, ) and ANSI Z9.2.

9.C.3.4. Downdraft Hoods

Downdraft hoods or necropsy tables are specially designed work areas with ventilation slots on the sides of the work area. This type of system is useful for animal perfusions, gross anatomy laboratories, and other uses of chemicals where there is a need to have full access over and around the materials (which would be obstructed by the three sides of a chemical hood) and the chemicals in use have vapor densities that are heavier than air.

9.C.3.5. Clean Benches or Laminar Flow Hoods

A clean bench or laminar flow hood resembles a chemical hood but is not intended to provide protection to the user. A clean bench is generally closed on three sides and either is fully open in the front or has a partial opening. Some have hinged or sliding sashes. On the top or back of the clean bench, HEPA filters pull room air through the filters and pass that air across the work surface, providing clean air. The clean bench is for product protection, not personal protection, and is not connected to the ventilation system. Mark such equipment “not for use with hazardous materials” to remind laboratory personnel not to use anything in it that they would not use on the benchtop.

9.C.3.6. Ventilated Balance Enclosures

Ventilated balance enclosures are commonly used in laboratories to weigh toxic particulates. These devices are installed with different specifications for face velocity than the standard laboratory chemical hood and are well suited for locating sensitive balances that might be disturbed if placed in a laboratory chemical hood. The average face velocity is specified at 75 fpm plus or minus 10 fpm (0.40 ± 0.05 m/s). Individual face velocity at each grid point should be within a tolerance of plus or minus 20 fpm (0.10 m/s). Ventilated balance enclosures are typically equipped with HEPA filters to remove hazardous particulates captured within the device prior to exhaust. They can be either the recirculating type or 100% exhausted to the exterior.

Housings for ventilated balance enclosures are generally constructed of minimum 3/8-in.-thick (10-mm-thick) clear acrylic. Edges of the vertical sides are beveled, rounded, or otherwise aerodynamically designed to reduce turbulence at the perimeter of the face. Ventilated balance enclosures consist of an integrated dished base that facilitates cleaning at the interface of the vertical and horizontal surfaces. Airfoil sills have an ergonomic radius on the front edge. Sash configuration consists of a hinged single sash pane for cabinet widths and provides a full, clear, and unobstructed side-to-side view of the entire cabinet interior. Sash openings are usually at a fixed height of 8 to 12 in. (200 to 300 mm) above the work surface.

9.C.3.7. Gas Cabinets

Whenever possible, minimize use of highly toxic or hazardous gases and restrict them to lecture bottles that are placed on stands and used within the confines of a chemical hood.

Use and store containers for highly toxic or hazardous gases, such as diborane, phosgene, or arsine, that are too large to be used within a chemical hood in ventilated gas cabinets. In the event of a leak or rupture, a gas cabinet prevents the gas from contaminating the laboratory. Consult the standards developed by SEMI for specific, recommended exhaust rates for gas cabinets.

Connect gas cabinets to laboratory exhaust ventilation using metal ductwork, rather than flexible tubing, because such tubing is more apt to develop leaks. Use coaxial tubing for delivering gas from the cylinder to the apparatus. Coaxial tubing consists of an internal tube containing the toxic gas, inside another tube. Nitrogen, which is maintained at a pressure higher than the delivery pressure of the toxic gas, is between the two sets of tubing, ensuring that, in the event of a leak in the inner tubing, the gas will not leak into the room.

9.C.3.8. Flammable-Liquid Storage Cabinets

Store flammable and combustible liquids only in approved flammable-liquid storage cabinets, not in a chemical hood, on the bench, or in an unapproved storage cabinet. These cabinets are designed to prevent the temperature inside the cabinet from rising quickly in the event of a fire directly outside of the cabinet. These cabinets may be ventilated or unventilated. Ventilating flammable-liquid storage cabinets is a matter for debate. One view is that all such cabinets should be vented by using an approved exhaust system, because it reduces the concentration of flammable vapors below the LEL inside the cabinet. A properly designed cabinet ventilation system does this under most circumstances and results in a situation in which no fuel is rich enough in vapor to support combustion. However, with liquid in the cabinet and a source of fresh air provided by the ventilation system, all that is needed is an ignition source. The other view is that in most circumstances flammable-liquid storage cabinets should not be ventilated.

Both opinions are valid, depending on the conditions. Ventilation is prudent when the liquids stored in the cabinet are highly toxic or extremely odoriferous. Particularly odoriferous substances such as mercaptans have such a low odor threshold that even with meticulous housekeeping the odors persist; and, ventilation may be desired. Local authorities may have specific regulations regarding the need for ventilation within the fire cabinet.

If a ventilated flammable-liquid storage cabinet is used under a chemical hood, do not vent it into the chemical hood above it. It should have a separate exhaust duct connected to the exhaust system. Fires occur most frequently in chemical hoods and may propagate into a flammable-liquid storage cabinet that is directly vented into it.

If a specially designed flammable storage cabinet ventilation system is installed, use an Air Movement Control Association C-type spark-resistant fan and an explosion-proof motor. Most fractional horsepower fans commonly used for this purpose do not meet this criterion and should not be used. If the building has a common laboratory chemical hood exhaust system, hook a flammable-liquid storage cabinet up to it for ventilation.

9.C.3.9. Benchtop Enclosures

Many laboratory ventilation system manufacturers offer ventilated enclosures that can be sized to fit equipment that would normally be placed in a chemical hood, such as rotovaps and microwave ovens. They can be made of metal or plastic and could have doors or sashes for access. The velocity of air will vary depending on the material being ventilated. The enclosure may be fitted with a filtration system for nanomaterials. By placing larger equipment in a ventilated enclosure rather than a hood, the amount of space in the hood in maximized and smaller hoods may be acceptable, resulting in energy and space savings.

9.C.4. General Laboratory Ventilation and Environmental Control Systems

General ventilation systems control the quantity and quality of the air supplied to and exhausted from the laboratory. The general ventilation system should ensure that the air is continuously replaced so that concentrations of odoriferous or toxic substances do not increase during the workday and are not recirculated from laboratory to laboratory.

Exhaust systems fall into two main categories: general and specific. General systems serve the whole laboratory and include devices such as chemical hoods and snorkels, as codes and good design practices allow. Specific systems serve isotope hoods, perchloric acid hoods, or other high-hazard sources that require isolation from the general laboratory exhaust systems.

General laboratory ventilation is typically set to provide 6 to 12 room air changes per hour. However, there is no specific requirement for ventilation rates. More airflow may be required to cool laboratories with high internal heat loads, such as those with analytical equipment, or to service laboratories with large specific exhaust system requirements or those with high densities of chemical hoods or other local exhaust ventilation devices. The ACGIH industrial ventilation manual states that “‘Air changes per hour’ or ‘air changes per minute’ is a poor basis for ventilation criteria where environmental control of hazards, heat and/or odors is required. The required ventilation depends on the problem, not the size of the room in which it occurs” (ACGIH, ). Where dilution ventilation will be the primary means of controlling exposure, the ventilation rate is dependent upon the materials in use. Standard industrial hygiene calculations may help to determine the required rate. Computational fluid dynamics models are often utilized to determine minimal rates when the lab is occupied and unoccupied.

Air should always flow from the offices, corridors, and support spaces into the laboratories. Exhaust all air from chemical laboratories outdoors and do not recirculate it. Thus, the air pressure in chemical laboratories should be negative with respect to the rest of the building unless the laboratory is also a clean room (see section 9.E.2). The outside air intakes for a laboratory building should be in a location that reduces the possibility of reentrainment of laboratory exhaust or contaminants from other sources such as waste disposal areas and loading docks.

Although the supply system provides dilution of toxic gases, vapors, aerosols, and dust, it gives only modest protection, especially if these impurities are released into the laboratory in any significant quantity. Perform operations that release these toxins, such as running reactions, heating or evaporating solvents, and transfer of chemicals from one container to another, in a laboratory chemical hood where possible. Vent laboratory apparatus that may discharge toxic vapors, such as vacuum pump exhausts, gas chromatograph exit ports, liquid chromatographs, and distillation columns to an exhaust device such as an elephant trunk.

The steady increase in the cost of energy, coupled with a greater awareness of the risks associated with the use of chemicals in the laboratory, has caused a conflict between the desire to minimize the costs of heating, cooling, humidifying, and dehumidifying laboratory air and the need to provide laboratory personnel with adequate ventilation. However, cost considerations should never take precedence over ensuring that personnel are protected from hazardous concentrations of airborne toxic substances.

9.C.4.1. Constant Air Volume Systems

CAV air systems assume constant exhaust and supply airflow rates throughout the laboratory. Although such systems are the easiest to design, and sometimes are the easiest to operate, they have significant drawbacks due to their high energy consumption and limited flexibility. Classical CAV design assumes that all chemical hoods operate 24 hours per day, 7 days per week, and at constant maximum volume. Adding, changing, or removing chemical hoods or other exhaust sources for CAV systems requires rebalancing the entire system to accommodate the changes. Most CAV systems in operation today are unbalanced and operate under significant negative pressure. These conditions are caused by the inherent inflexibility of this design type, coupled with the addition of chemical hoods not originally in the plan.

9.C.4.2. Variable Air Volume Systems

VAV systems are based on laboratory chemical hoods with face velocity controls. As users operate the chemical hoods, the exhaust volume from the laboratory changes and the supply air volume must adapt to maintain a volume balance and room pressure control. Consult an experienced laboratory ventilation engineer to design these systems, because the systems and controls are complex and must be designed, sized, and matched to operate effectively together.

VAV systems provide many opportunities for increased safety and energy conservation that cannot be accomplished with a CAV system. Individual laboratory chemical hoods, groups, or all chemical hoods on the same system can be adjusted to a lower airflow when not in use through timers, occupancy sensors, or other means (see section 9.H.3). Similarly, exhaust may be automatically increased to purge the room in the event of a spill or release (see section 9.C.6.4).

9.E.3. Environmental Rooms and Special Testing Laboratories

Environmental rooms, either refrigeration cold rooms or warm rooms, for growth of organisms and cells, are designed and built to be closed air circulation systems. Thus, the release of any toxic substance into these rooms poses potential dangers. Their contained atmosphere creates significant potential for the formation of aerosols and for cross-contamination of research projects. Control for these problems by preventing the release of aerosols or gases into the room. Special ventilation systems can be designed, but they will almost always degrade the temperature and humidity stability of the room. Special environmentally controlled cabinets are available to condition or store smaller quantities of materials at a much lower cost than in an environmental room.

Because environmental rooms have contained atmospheres, personnel who work inside them must be able to escape rapidly. Doors for these rooms should have magnetic latches (preferable) or breakaway handles to allow easy escape. These rooms should have emergency lighting so that a person will not be confined in the dark if the main power fails. Because these rooms are often missed when evaluating building alarm systems, be sure that the fire alarm or other alarm systems are audible and/or visible from inside the room.

As is the case for other refrigerators, do not use volatile flammable solvents in cold rooms (see Chapter 7, section 7.C.3). The exposed motors for the circulation fans can serve as a source of ignition and initiate an explosion.

Avoid the use of volatile acids in these rooms, because such acids can corrode the cooling coils in the refrigeration system, which can lead to leaks of refrigerants. Also avoid other asphyxiants such as nitrogen gas in enclosed spaces. Oxygen monitors and flammable gas detectors are recommended when the possibility of a low oxygen or flammable atmosphere exists in the room.

Box 9.2 provides some basic guidelines for working in environmental rooms.

BOX 9.2

Quick Guide for Working in Environmental Rooms. Mold growth can cause problems for an experiment and affect personnel health. To avoid mold: Report any leaks or condensation to maintenance personnel for repair.

9.E.3.1. Alternatives to Environmental Rooms

Shaker boxes may be a viable alternative to environmental rooms. These boxes come in a variety of shapes and sizes and may be stackable. They use less electricity, take up much less space, and have just as much control over the environment.

A shaker box is a sealed cabinet with a pull-out work surface. The user may control the environment within the cabinet, including the temperature, humidity, carbon dioxide level, lighting, and vibration. Shaker boxes may be used as incubators or for cooling, giving a full range of options.

9.H.9. How to Choose a Ventilation System

There is no one choice that is right for every laboratory. The designers, the laboratory users, and the facilities staff must discuss the possibilities. EHS professionals and laboratory managers are helpful in these discussions as well. The individuals who decide which systems to install must understand the needs of the users, and the users must understand how the systems work, the capabilities and limitations of the systems, and what to expect from them. The facilities staff must understand how the systems need to be maintained, and those who are choosing the system need to know whether there is in-house expertise to maintain them.

Check local, state, and federal codes and regulations before choosing a new system. Only a few actual regulations cover ventilation systems, but more and more municipalities are adopting international building and mechanical codes. These codes impose limitations on manifolding ductwork and may require detection or sprinklers within ducts.

When considering a new technology, benchmarking is usually helpful. Find someone who is using a similar system and discuss their experience. Ask for samples. Visit laboratories that use similar products. Find the systems that work best for your applications. Continue communications between the users and the installers and the maintenance staff to ensure that the systems are working as intended.

Remember that even if all the chemical hoods are removed, ventilation is still needed in the laboratory.

9.I.2. Removal, Cleaning, and Decontamination

The second step in decommissioning is to remove all hazards from the space. Be sure that all chemicals, radioactive materials, and biologicals have been removed from use and storage areas, including refrigerators and freezers. Movable equipment should be appropriately cleaned and/or disinfected, and removed from the lab.

Residual perchloric acid and mercury contamination are common concerns for laboratory decommissioning. If perchloric acid was used outside of a hood designed for that purpose, hoods and ductwork can become contaminated with explosive metal perchlorates. (See section 9.C.2.10.5 for information about the hazards of perchloric acid in laboratory hoods and ventilation.)

Mercury is used in most laboratories, and mercury spills are common. Unless it is certain that no mercury was used, laboratory decommissioning should include testing of floors, sinks, cupboards, and molding around furniture and walls. Be sure to check and clean sink p-traps. Visual inspection alone is inadequate as historic spills may reach beneath floor tiles and furniture, and behind walls. As described in the ANSI Laboratory Decommissioning Standard, modern mercury testing utilizes a portable atomic absorption spectrophotometer with a sensitivity of 2 ng/m3. Decommissioning clearance levels consider the U.S. Agency for Toxic Substances and Disease Registry's Minimal Risk Level (MRL) of 200 ng/m3 for non-occupationally exposed individuals. Chapter 6, section 6.C.10.8, includes information on dealing with mercury contamination. Additional mercury testing may be necessary as furniture, floors, walls, and plumbing are removed during renovation.

After hazardous materials and movable equipment have been removed, areas known to be contaminated (e.g., stained floors and cupboards) should be cleaned appropriately, or destructively removed and disposed of. Chemical decontamination can be done using appropriate surfactant soaps, solvents, neutralizing agents, or other cleaners.

Unless is it known that no biological materials were used in the space, the furniture, equipment, and other surfaces should be cleaned with an appropriate disinfectant. Sophisticated biological decontamination technologies are available for areas where high-risk pathogens have been used.

As a precautionary measure, it is a standard practice to remove dusts and other settled particulates via a thorough final wet-cleaning of floors, vertical surfaces and furniture using commercial cleaning products.

The Cost of Clean Air

Traditional ducted fume hoods are well known in the industry to be energy hogs. As a matter of fact, they can represent one of the largest expenditures for laboratories due to the ongoing cost of power and HVAC loads.

Q: How can I effectively manage lab ventilation costs beyond the initial installation of my hood?

A: Start by learning the ventilation system that your lab requires.

Your lab’s ventilation system will determine how your fume hood functions in your unique space. Understanding the benefits and shortcomings of your laboratory ventilation system will enable you to make informed decisions that contribute to long term energy savings and reduced maintenance costs.

There are two main types of ventilation systems on the market: Variable Air Volume (VAV) and Constant Air Volume (CAV). Let’s take a look:

Constant Air Volume (CAV) systems

Variable Air Volume (VAV) systems

• Exhausts the same amount of air at all times, regardless of sash height.

• Varies the amount of air exhausted, based on sash height, to maintain a constant face velocity.

• Exhaust uniormity can simplify the control and maintenance of the system.

• Dynamically adjusts the volume of air being exhausted from the fume hood based on the specific requirements of the laboratory space.

• CAV systems are the most common and are often seen in older laboratories and smaller spaces, or spaces with a low quantity of hoods.

• Exhaust changes are made in response to the sash position. These systems automatically respond to changes in the airflow needs, ensuring a more energy-efficient operation.

CAV PROS

CAV CONS

VAV PROS

VAV CONS

Simplicity: CAV systems are known for their straightforward design and ease of control, making them a reliable and easy-to-maintain choice.Energy Consumption: consumes more energy than VAV as they operate at a constant volume, even during periods of low demandEnergy Efficiency:  excel in energy conservation as they adapt to the actual demand, reducing unnecessary power consumption during low-activity periods.Initial Cost: The upfront cost of installing VAV systems may be higher compared to CAV systems.Lower Initial Cost: CAV systems typically have a lower upfront cost compared to VAV systems.Limited Adaptability: not as responsive to changes in laboratory activities, potentially leading to less efficient energy utilization.Cost Savings: results in lower energy bills, making them a cost-effective choice in the long run.Maintenance Complexity: may require more intricate maintenance due to their sophisticated design.Enhanced Safety: maintain a constant face velocity even when sash positions change, ensuring a consistent level of protection for laboratory personnel.

AMS Solution Series: Designed for Both CAV and VAV Systems

To simplify your fume hood purchasing process, opt for a hood that can handle both CAV and VAV systems such as our customizable AMS Solution Series hoods. These hoods are custom built to your space and are engineered with the evolution of your lab in mind. For example, if your lab is equipped with a CAV system, and you have an AMS solution series hood, you have the capability to switch to a VAV system without switching your hood. Once you have switched to a VAV system, simply install your VAV monitor to your Solution Series fume hood and your hood is ready to go.

Canadian Scientific supplies and installs a dynamic range of fume hoods engineered to work with both CAV and VAV systems without any mechanical alterations. We strongly recommend Solution Series hoods for maximum safety, protection, and efficiency.

Bottom line: The choice between a CAV or VAV system depends on the duration of your hood operation and budget. Working with a company that can customize your fume hood to CAV or VAV will increase your options for flexibility in the lab.

We’ll assist you with selection and installation!

Canadian Scientific is here to guide you through the decision-making and installation process, considering factors such as your lab's size, activities, and budget constraints. Our commitment to quality and customer satisfaction ensures that you receive not only top-notch products but also expert guidance in designing and implementing the most effective ventilation system for your fume hoods.

Contact us today to explore how we can elevate your laboratory environment with cutting-edge ventilation systems, fume hoods, and accessories.

Are you interested in learning more about Chemistry Fume Hood? Contact us today to secure an expert consultation!

Sources:

Labconco Corp. 3 Main Steps to Fume Hood Mechanical System Selection. https://www.labconco.com/articles/3-main-steps-to-fume-hood-mechanical-system-sele