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Fume Hood Protocol Comparisons


A Brief Discussion on the Current Status
of Laboratory Fume Hood Evaluation Protocols

Caoimhín P. Connell
Forensic Industrial Hygienist

Recently, I have noted that Chemical Hygiene Officers, industrial hygienists, ventilation firms and contracting departments of firms who use fume hoods have fallen into the trap of specifying invalid or obsolete laboratory fume hood evaluation protocol when seeking hood evaluation services. This trend has been continued by laboratory supply companies who suggest to the buyer to use old protocol. Worst of all, are the specifications which erroneously rely on the measurement of face velocities to determine hood performance.

We have placed videos of examples of reverse vector flow in hoods that otherwise meet the manufacturer's specifications; click here to view the videos. This discussion will focus on the most current evaluation protocol (the ASHRAE 110) and present a simple, inexpensive and rapid performance protocol.

Overview of Evaluation Protocols

ASHRAE
Currently, the most commonly requested fume hood evaluation protocol is the ASHRAE 110 method. ASHRAE is the American Society of Heating, Refrigeration and Air-conditioning Engineers. ASHRAE is a non regulatory, profit making organization. Although it publishes "standards" none of the standards are binding on U.S. businesses. The ASHRAE 110 standard is an excellent procedure and, in my opinion, achieves its stated goal.

The ASHRAE protocol is a rather complicated three parted evaluation processes which includes placing a gas detector in a mannequin's mouth in front of the hood and injecting a tracer gas into the hood. Although the most elaborate and certainly the only national consensus quantitative evaluation proposed by a U.S. organization, this method is not considered to be capable of ensuring optimal performance of the hood in an "as used" fashion (even by ASHRAE itself).

However, there are considerable misconceptions regarding the ASHRAE standard. It was not an attempt to provide what would constitute a performance criteria, but rather, an attempt to standardize the way in which hoods could be evaluated. Essentially the ASHRAE standard is based on a test procedure designed by K.J. Caplan and G.W. Knutson and published in the Journal of the American Industrial Hygiene Association in 1982. Contrary to common belief, the standard does not make specifications as to what is considered to be acceptable and an hood cannot be “certified” according to the ASHRAE standard.

The ASHRAE 110-1995 is not an engineering investigation into what causes poor performance, nor of ways to improve performance. Indeed, the standard is not even capable of identifying where within the hood the worst performance is exhibited.

The ASHRAE protocol requires elaborate (and expensive) equipment; purpose built tracer gas ejectors, electron capture instrumentation, and mannequins. In the following protocol, the inspector takes the place of the mannequin and the human experience takes the place of the electron capture instrumentation. This change alone results in a reduction of initial costs and rental fees. The down-side to the protocol, is that the protocol is subjective, but no less subjective than the ASHRAE fume challenge.

To my knowledge, where an organization has the equipment necessary for the ASHRAE protocol, that equipment has been “home-made” by that organization. Some years ago, I contacted the original researcher who developed the method and designed the original test equipment (Mr. Knowlton Caplan), and at the time, he was not aware of anyone in the country who manufactured, sold or rented the necessary equipment.

The ASHRAE protocol necessarily requires set-up time to place the test apparatus in place. In this protocol, the set-up time is complete when the inspector steps up to the hood.

It has been my experience, that where fume hoods fail, they fail grossly. The failure is usually sufficient to be detected by a carefully conducted visual fume test. The ASHRAE fume test, however, is NOT a detailed challenge; checking only the perimeter of an hood interior. Therefore, the ASHRAE 110-1995 fume test is incapable of identifying even a grossly inadequate hood where the reversal is in the central portion of the hood face. I have seen many hoods which can "pass" an ASHRAE fume test, but will utterly fail to protect the hood user. It is for this reason, that when the ASHRAE test is used, the hood owner should follow the ENTIRE ASHRAE test and not just the visualization fume challenge.

Although the ASHRAE test quantifies the amount by which the fume hood fails, I believe that under most laboratory situations, it is not necessary to determine the actual amount of failure but rather to document that the hood has failed and attempt to ascertain why.

AIHA
Jointly published by the AIHA and ANSI, Standard Z9.5-2003 is an American National Standard for Laboratory Ventilation. The AIHA standard is not binding on US industry.

SAMA
SEFA and SAMA are furniture and apparatus manufacturer associations. As with ASHRAE, their protocols are not binding, and are not recognized by NIOSH, the EPA, OSHA or other regulatory agencies. They are instead an attempt to share with the general community their experience in fume hood performance in the spirit of disseminating information. In fact, SEFA and SAMA never sought to copyright their protocol and allows anyone to photocopy or quote the material without restriction.

The SAMA LF10 protocol was originally developed incorporating considerable technical input provided by SEFA. In 1988, SEFA separated from SAMA and SAMA separated itself from fume hood evaluations. SEFA took up the task of keeping the protocol current. The SEFA-1 protocol supersedes the old SAMA LF10 protocol. Indeed, if one were to attempt to obtain a copy of SAMA LF10 from SAMA, SAMA would recommend calling SEFA, since SAMA has since disposed of all SAMA LF10 copies. There is no significant difference between the SAMA and SEFA field testing protocol.

It is generally recognized by most researchers and industrial hygienists involved with fume hoods, that the SEFA and SAMA protocol are not adequate to determine the ability of the fume hood to protect the worker. Although the new SEFA-1 does address field testing of fume hoods, it is primarily a design protocol. The SEFA protocol contains several inconsistencies. For example, the evacutory volume (Q) is determined from face velocities, yet the supply Q is determined by drilling holes in the supply duct and measuring dynamic pressure even though measuring the velocities of the supply would be much faster (only seconds) and non-destructive. Additionally, the SEFA protocol suggests disengaging the auxiliary air while measuring the face velocities. I believe that since the auxiliary air will be on when the hood is used and the auxiliary will effect the face velocities, the auxiliary air should be operating in its normal fashion.

I have encountered specifications which require hoods to be certified according to the SAMA LF-10 "Standard." The SAMA protocol is not a standard and has no certification criteria; that is to say that there is no such thing as a SAMA certification; thus, hoods cannot be certified according to SAMA. Additionally, some specifications require that hoods be balanced according to the SAMA protocol. The SAMA protocol does not address balancing of a fume hood.

US Regulators
The Occupational Safety and Health Administration (OSHA), and the US Environmental Protection Agency (EPA) do not have regulatory standards for the performance of laboratory fume hoods. However, OSHA and the EPA both have addressed fume hoods, and more information can be found by clicking here.

The Centers for Disease Control/NIOSH pamphlet entitled "Biosafety In Microbiological And Biomedical Laboratories" does not provide performance criteria for control devices.

NSF
The NSF has several standards which describe many aspects of biological safety cabinets. NSF 49 is a comprehensive performance standard for biological safety cabinets, but does not address laboratory fume hoods.

Deutsches Institut für Normung
The Deutsches Institut für Normung DIN 12 924 standard entitled "Laboreinrichtungen Anforderungen an Abzüge, Abzüge für allgemeinen Gebrauch" is binding on German industry. The standard is considered state of the art, but does not necessarily reflect the highest standard of care. Additional information on the DIN can be found by clicking here.

British Standard 7258
British Standard DD 191 has been replaced by BS 7258. Again, the standard is state of the art, but does not necessarily reflect the highest standard of care.

National Fire Protection Association, Inc.
NFPA 45- Standard on Fire Protection for Laboratories Using Chemicals (2000 Edition). This standard is an excellent reference for fire protection. However, it also underscores the problems that can occur when an organization whose primary focus is in one arena (such as fire protection) but who attempts to make standards in another arena (such as laboratory ventilation). The result is that poor advice may be given, which tends to lower the quality of the standard as a whole.

General Observations

The evaluation protocol described below was developed following extensive review of studies and guidelines published by (but not limited to) ASHRAE, SEFA, SAMA, EPA, OSHA, School of Public Health, (University of Minnesota), and Oak Ridge National Laboratory (Department of Energy) and numerous articles in the scientific literature. In addition to these various organizations, I have incorporated my personal experience of over 25 years working in and around analytical and research laboratories with chemical fume hoods.

The protocol I use in my private consulting business (Forensic Applications Consulting Technologies, Inc. FACTs) gathers more information than is needed for a performance evaluation; it gathers data regarding the hood’s engineering performance, the state of repair of the hood, and a commentary on work practices. And, like the ASHRAE 110-1995 standard, and the SEFA and SAMA protocols, it uses a subjective performance based test to evaluate air flow characteristics. Unlike the ASHRAE standard, this protocol does not quantify the actual concentration of spill-out. This one item, is the only significant issue which this protocol does not address but that the ASHRAE standard does. Another small item is that the ASHRAE standard requires a sketch of the lab to be made; this protocol does not.

This protocol covers all aspects that are covered in the SEFA and SAMA protocol and, in addition, covers those aspects listed in the following table. Below is a table which compares the issues addressed by the various protocols. Where a "Y" appears in the table, the protocol addresses the issue, where a blank appears, the protocol does not explicitly address the issue.

Table of Protocol Comparison
ISSUESAMASEFAASHRAEFACTs
Effects of cross drafting---Y
Efficacy of by-pass baffle---Y
Function of auxiliary air---Y
Face VelocityYYYY
Noise---Y
Determine protection factor--Y-
Function of the utilities---Y
Sketch of lab--Y-
Diagnostic air flowYYYY
Functioning sashYY-Y
Perchloric hoods---Y
Washdown---Y
Equipment in hood---Y
Bottom Foil--YY
Lights---Y
Balancing----
Spill control---Y
Categorize (pass/fail criteria)---Y
QualitativeYY-Y
Quantitative--Y-



Evaluation Groundwork
The protocol I use is not a cookbook method. It requires a patient, intelligent and conscientious evaluator. However, in unpublished work, approximately 10 years ago, Mr. Tom Smith, (Exposure Control Technologies, Inc.) and I performed blind ASHRAE –100 comparisons with this protocol. What we found was when diligently performed, this protocol identified each hood that had been found to be substandard by the full ASHRAE 110 method. Similarly, each hood initially identified by this method as deficient was subsequently identified by the ASHRAE 110 method as deficient.

Since, as a private consultant, we live in the real world of marketing and perceptions, the evaluation described here is sensitive to the pressures of marketing and public relations. I am cognizant of the fact that many contracting officers and lab managers simply would not hire a consultant to perform an hood evaluation if the evaluation protocol did not include face velocities. For this reason, the form displays expected albeit occasional, unnecessary information, including face velocities. However, once the full evaluation has been performed, just the “Performance Characteristics” section of the evaluation needs to be conducted to ensure the continued performance of the hood.

Although an IH with no lab experience can certainly use the protocol, the realities are that if the IH doesn’t understand laboratory settings and the work that is taking place therein, the performance test will be limited by his own knowledge. This is because the protocol begins with simply understanding the work that is to be performed in the hood and determining if a laboratory hood is even the most appropriate type of control. If, for example, work involves substances such as bis-choromethyl ether, materials potentially contaminated with bloodborne pathogens or large pieces of equipment, then perhaps a laboratory fume hood is not the correct control device regardless of how well the hood works.

Similarly, if the hood is used to control relatively innocuous substances such as MeOH, acetone, toluene, etc, then a lower performance may be tolerated since even a lower performance may provide adequate protection. The evaluator should be given the ability to ensure that the hood is providing an appropriate level of control.

In general, hoods that pass this protocol may be expected to give a protection factor on the order of 1E5 to 1E6.

Evaluation Form
A standardized form is used to ensure that consistent evaluation of each hood is achieved, and that a record of performance is established. The form contains a large square in the centre depicting a graphic of the face of the hood. The depiction permits the evaluator to note areas of reversal, static zones, face velocities, and disruptive influences (such as large pieces of equipment in the hood).

The form I use to record the performance of each hood supplies more information than is needed to satisfy the objectives of a performance evaluation. The intent is to provide sufficient information to facilitate further investigation in the event that an hood performs poorly.

The complete evaluation, minus the sound criteria, takes approximately 15 minutes per hood. Typically, I carry a small tape recorder and dictate the evaluations onto tape, and later transcribe the information onto the forms. In this fashion, as many as 20 hoods can be evaluated in a single day allowing for time to make minor adjustments.

Hoods normally are evaluated either "as manufactured" or "as used." Obviously the "as used" is the better of the two choices since even the very best and properly designed and built hood may utterly fail if poorly placed within a lab, or the lab employee exhibits poor work practices (such as hanging a lab coat on the utility knobs). Therefore, hoods are evaluated as I find them. Where necessary (such as to lower the sash) I will move equipment, but otherwise the hood is evaluated as it is found.

Face Area: Face area is measured using a standard tape measure. The unit's dimensions are square feet.

By-pass Baffle: During the evaluation, the efficacy of the by-pass baffle is evaluated. During this determination, the hood sash is lowered to an opening of about five and a quarter (5.25) inches, and the face velocity is measured at three equidistant points using an hot wire anemometer. The "mean closed velocities" is the arithmetic mean of nine tenths of the altitude corrected individual readings.

The closed velocities serve to evaluate the effectiveness of the by-pass baffle in the hood. As mentioned earlier, the function of the by-pass is to allow make-up air, circumventing the sash, into the hood as the sash is lowered, thus maintaining a steady-state hood static pressure (SPh) without creating excessive face velocities. The open-to-closed velocity ratios provide the best measurement of how well the by-pass baffle is working.

In a properly operating by-pass hood, the "closed to open" ratio (C:O) should be no greater than 3 and no less than unity. 2 C:O ratios greater than 3 are indicative of a dysfunctional by-pass baffle (or a hood that is a simple cabinet). Hoods displaying C:O ratios greater than 3 should not be used for powders or open flames. A hood with a C:O ratio less than unity may indicate a gross leak somewhere in the casework of the hood.

Evacutory Volume: For most laboratory fume hoods, the manufacturer designs the device to evacuate a specific volume of air (Q) per unit time, per unit face area. For non-specific laboratory fume hoods, the design criteria is incumbent on several parameters which must be evaluated by the manufacturer. 1 For most hoods, the design criteria for Q is 100 cubic feet of air per minute (cfm) per square foot of maximum open face area.

To ensure that the design criteria has been met, it is often convenient to measure the velocity of the air at the face of the hood, since the velocity of the air at the face will be a function of the face area and Q. Traditionally, too much faith has been placed in the value of face velocities as an indication of hood performance. Face velocities are useful as diagnostic indicators in the event that the hood does not perform well, but should not be used as the sole method for determining the effectiveness of the hood. A discussion on face velocities can be found by clicking here.

Although there are several variations on the theme, by-pass hoods have a variable inlet baffle which permits more air to enter the hood via a special inlet as the sash is lowered. The effect is to maintain a non-linear increase of face velocity as the sash is lowered, to avoid excessive face velocities when the sash is only opened by a foot or so; again the velocities change but the Q remains essentially constant. It is for this reason that the results of the face velocity measurements taken have been calculated as evacutory Q and compared to the evacutory design criteria of the manufacturer. The value of Evacutory Q reported on the form is the product derived from the arithmetic mean of the barometric corrected individual face velocities and the face area.

Face Velocities: Although I believe that traditionally, too much weight has been placed in face velocities, this protocol does measure face velocities. In fact, this protocol more precisely measures the face velocities than does the ASHRAE 110-1995, SAMA and SEFA protocols. In the FACTs protocol, face velocities are measured at one square foot intervals using a 16-point pressure differential grid per square foot. The air velocity is integrated over each sampling area. In this way, hundreds of face velocity measurements are taken across a standard hood face. The velocities are corrected for local altitude and temperature. The instrument calibration is traceable to NIST (formerly the National Bureau of Standards, Department of Commerce).

The sampling points were determined by "drawing" an imaginary equidistant grid in the face of the hood. For a typical laboratory chemical fume hood, the centre of the second or third reading position would roughly be at the nose of a “model man” user, when standing at the face of the hood.




Performance Characteristics: This section contains the true essence of the evaluation. It is this test which, independent of all engineering aspects, will determine the performance of the hood. It is also this section that can be completed in a matter of minutes on a regular basis (weekly or monthly) to determine the continued performance of the hood.

To evaluate the practical capture and flow characteristics of each of the hoods, each hood is challenged with a fume of titanium tetrachloride (TiCl4). TiCl4 is selected because as water vapour in the air reacts with TiCl4, it creates a highly visible fume without requiring a heat source. The smoke test is performed by standing directly in front of the central portion of the hood, (emulating the area most likely to be occupied by the user of the hood) and carefully generating a stream of smoke in a manner which traverses the face of the hood from the top right, across the top to the left, then from the left to right... et cetera, until the face of the hood has been completely checked from left to right and top to bottom.

Static cells and cells of reverse vector flow can quickly be observed. In an optimally operating hood, the fume is captured and quickly cleared from the hood, moving in a laminar fashion toward the back of the hood. Hoods can exhibit several manifestations of poor performance, including reverse vector flow and static cells. In reverse vector flow, the air from the hood does not flow toward the back of the hood, but rather spills out into the surrounding ambient air, or migrates into the users face. Where this occurs, I note the boundary of the reverse flow on the grid on the form with a line and mark the area with the letter "R." Static cells are identified on the grid with an “S.”

Although subjective, a thoroughly performed fume challenge can quickly and accurate identify poorly performing hoods. Occasionally, due to the subjective nature of the visual fume challenge, lab personnel can be skeptical when the evaluation reveals a poorly operating hood. However, often, the reversal is clearly visible to a casual observer. In the following video, the face challenge is demonstrated and the reversals are clearly apparent.

Video of Fume Challenge at Face

Where a more objective demonstration of the poor performance of a hood is needed, a simple objective test can be quickly performed. Typically, to do this, I use an ultra fine particle counter and inject the challenge fume into the hood while allowing the lab personnel to read the UFP count. The wand of the counter can be directed toward reverse vector flow cells to dramatically demonstrate the escape of contaminant at that point. Similarly, common lab solvents such as acetone can be manipulated in the hood while using an FID or PID to detect the leakage.

Auxiliary Supply: When a person stands in front of a hood, they can create a negative pressure zone between their body and the face of the hood. The result of the negative pressure system is that air can be drawn from the interior of the hood, and spilled out into the laboratory. To mitigate this possibility, some hoods incorporate a make-up panel to fill in this low pressure area; such hoods are referred to as auxiliary hoods. In some hoods, the auxiliary supply air is supplied from the top of the hood above the user; in others, the auxiliary is supplied from a trapezoidal vent which is inserted in the hood, behind the plane of the sash. For the hoods which have a trapezoidal auxiliary insert, the total evacutory volume of the hood must be calculated from the summation of the volume as derived from the face area and the face velocities and the volume derived from velocities and the area of the auxiliary.

Exhaust Termini : Hood stacks are frequently poorly installed or designed. When an exhausting material needs to be removed from a building, the material should be exhausted such that the gases “puncture” the building envelope. If materials are exhausted inside the envelope, they can be reintrained into the building.

Therefore, one should never exhaust any emissions through the side of a building since the building envelope can extend for hundreds of feet or even several blocks. Rather, the emissions should always be directed upward, through the roof, and above the parapet. The duct should be sufficiently elevated above the roof to penetrate the envelope, or the exhaust terminal velocity should be sufficiently high enough to eject the emissions through the building envelope (as can be achieved with jets or Venturi sacks).

The exhaust stacks should never have weather caps. In spite of their common use, weather caps are inconsistent with good ventilation practices and induce reintrainment of exhausts. Instead, to control for precipitation, the exhausts should be fitted with zero-static sleeves.

Perchloric Wash-down: To ensure that explosive perchloric salts do not accumulate, the protocol determines the efficacy of the wash-down, the frequency of use, and, importantly, the system employed to alert roof occupants or outdoor bystanders of an impending washdown.

Sound: Although not directly related to the efficacy of a fume hood, as any lab worker will attest, noise can degrade the general working environment and laboratory fume hoods are often unnecessarily too noisy.

For all practical purposes, we may assume that none of the air in the hoods is truly laminar (i.e. the critical Reynolds number has been exceeded). Therefore, turbulence is inevitable and the turbulent air can give rise to sound in a variety of ways. During the flow of air, continuous and discrete sound frequencies may be generated during the shedding of vortices by an obstruction or turn resulting in Strouhal frequencies.

Most of the continuous band sound will most likely be from areas in the system where turbulent air interacts with a rigid structure; the abrupt change in velocity is accompanied by a change in air density and, at this point, sound is radiated as acoustical energy. Therefore, wherever air changes direction abruptly, turbulence can be encountered; the greater the curvature of the duct (the smaller the aspect ratio of the bend) the more turbulent the airflow, and the greater the potential for the radiation of energy as sound. Strouhal frequencies and certain turbulence related acoustical radiation's are capable of producing pure-tone pitches.

Regulations notwithstanding, maximum recommended sound pressure levels for various types of environments due to ventilation systems have been suggested.6 For example, the sounds created by a ventilation system for a private office is 40-45 dBA. However, laboratories occupy an unusual niche in the working environment being at once part industrial and part administration. Maximum sound pressure levels (SPLs) have not provided for a laboratory per se, and nuisance sound criteria can be difficult to set. Although the U.S. does not have SPL criteria for fume hoods, the German hood standard (DIN 12 924 "Laboreinrichtungen Anforderungen an Abzüge, Abzüge für allgemeinen Gebrauch") states that the maximum SPL for a laboratory fume hood must not exceed 55 dBA. The standard also states that disturbing noises caused by pure-tone pitches or narrow-band tones are not permitted. Where pure tone pitches are encountered, the corresponding SPLs are not to exceed the NR-Curves 50. (The German standard actually states that "Die zugehörigen Grenzwerte des Schalldruckpegels je Oktave dürfen dann die NR-Kurven 50 nicht überschreiten.")

Nevertheless, because the human ear selectively amplifies and attenuates various frequencies, each octave center should be looked at individually. Therefore, a spectral analysis should be performed when evaluating ventilation noise. Each octave center has a maximum recommended pressure level. Using the German standard of 55 dBA, the evaluator should determine the maximum recommend SPLs for each frequency center at the fume hood. Based on the referenced criteria, the protocol compares the recorded SPLs with the recommended maximum SPL spectra for a worker standing in front of a fume hood.

Baffles: Some hoods carry a statement that the slot positions should be varied if "heavier than air" or "lighter than air" gases and vapours are expected. This statement has been investigated 3 with the conclusion that no evidence supports the validity of slot adjustment based on vapour density of the compounds present. It has been demonstrated quantitatively (and to my satisfaction in the field) that in a properly installed and operating hood, there is no difference in capture and evacutory characteristics between for example, helium and carbon tetrachloride. Certain investigators have concluded that the "heavier than air" claim has more basis in sales and marketing than in practical application.

I recommend that when the fans and baffles in the hoods have been properly adjusted for the correct flow, the hoods should be re-evaluated and the baffles set to their optimal positions. Once this has been done, I recommend that the knob adjustment be disabled by locking out or even cutting off the lever with a hacksaw to physically disallow further adjustment of the hoods. Allowing the hood user to play with the baffles ensures poorly working hoods.

Sash Performance: An important, often over-looked point, is the ability of the sash of the hood to hold all positions. A sash that cannot hold all positions will greatly limit the use and protective ability of the hood. Also, a hood sash should be capable of being lowered quickly to contain an explosion, fire, or implosion.

Although the sash should provide adequate protection to the face and the eyes when lowered, each employee should don a face-shield and eyewear whenever performing tasks which warrant these items and not rely on the sash.

Importantly, the performance of the hood should not be incumbent on the position of the sash. A properly operating fume hood should perform equally well with the sash in any position. Hoods which perform adequately only when the sash is in a particular position are deemed to be operating inadequately.

Furthermore, any “stops” on the sash should be removed. If the stop can be disabled by the employee, it should be removed. If the stops are in place to ensure the sash is not opened to a degree greater than the “100 fpm” mark, the hood is operating in a substandard manner.

Hood Utilities: The utilities (water, air, gas, etc.) are checked.

Spill Control: Spill control is also indicated on the form. Common spill control is a built-in lip within the hood or even a tray placed in the hood. All work within a hood involving liquids should be conducted in the spill control basin. Liquid reagents should never be placed on the lip outside the spill control area.

If, upon approaching the hood, the evaluator observes items stored on the lower foil, the hood should be challenged at that plane. This is consistent with the ASHRAE standard in that the ASHRAE standard states:

The final test would be an "as used" (AU) test in which the investigator accepts the hood and the condition in which the user has established the hood (sic). This includes obstructions within the hood, maladjustments of the baffles, thermal challenges within the hood, and other factors.

If the hood users place items on the bottom foil, those items become the point sources for potential exposure. If the hood cannot control those point sources, then the hood is inadequate (that the hood user’s poor work practices is proximal to the exposure is not important.) I have found that the practice of storing or cooling items on the bottom air-foil can be thwarted by installing sloping foils.

Hood Equipment: Equipment in the hood can significantly alter the capture and evacutory characteristics of the hood. Raised equipment will allow the bottom foil to fill in a naturally occurring low pressure area at the bottom centre of the hood. If the equipment is not raised, the efficacy of the bottom foil is greatly reduced. All equipment in the hood should be elevated such that it is approximately one inch off the working surface of the hood.

Hoods cluttered with excessive pieces of equipment can further reduce the laminar flow of the air within the hood. Fume hoods are too often thought of as storage areas. Equipment in a hood should be kept to a minimum. If large convoluted pieces of equipment are used, then perhaps a bench-top fume hood is not the appropriate engineering control.

Foils : Bottom and side foils are checked to ensure that they are not obstructed.

Cross Drafting : Several publications 4 stress the importance of controlled make-up air supplying the hoods to avoid cross drafting. These publications recommend that cross drafting in the laboratory at the location of the hood should be kept to an absolute minimum, never exceeding 25 linear feet per minute (fpm). To put this into perspective, a person walking directly in front of a hood at 1 mph will produce a cross draft of 88 fpm. Some publications 5 suggest that for carefully designed ceiling diffusers, reasonable control of cross-drafting can be achieved if the terminal velocity of the diffuser is less than or equal to the face velocity of the hood.

The effect of ceiling vents on escape of fume from the hood has been found5 to be on the same order of magnitude as the effect of hood face velocity; increasing the face velocity (with a subsequent increase in the challenge from the ceiling vent) had little effect on escape of fume from the hood. Some authors5 have concluded that:

"Terminal velocity of supply air jets is at least as important as hood face velocity in controlling spillage of contaminant, in the range of 0.25 to 0.75 m/s face velocity."

The range of concern expressed by the above author is 50 to 150 linear feet per minute, which is the design range of the hoods at most facilities.

Traffic can create unacceptable cross-drafts and is noted on the form. Traffic is determined based on observations made on the day of the visit and on the expected occupancy of the room. Even medium traffic can create cross-drafting which can result in poor hood performance.

Windows and vents (if any) are indicated since these can be the source of disturbing air currents.

Cross draft information is measured a few inches from the plane of the face with an hot wire anemometer and is given in two directions; vertical to the plane of the sash and horizontal with the plane of the sash.

Performance Classifications: Based primarily on the fume challenge, the hoods are grouped into one of three categories.

Category I In the first category, if the challenge fume is adequately captured at the face of the hood and progressed into the hood where it is exhausted, the hood is considered to be operating in a manner which indicates adequate protection is provided to the user (bearing in mind the toxicological properties of the material being handled).

Category II If the challenge fume reverses flow and migrates toward the user but is recaptured before reaching the user's breathing zone; or static cells are present which also result in gradual migration of contaminant toward the user but are then recaptured and exhausted, the hood is placed in the second category. The second category states that the hood should not be used for extremely toxic materials or carcinogens; but may still be used for compounds with relatively high TLVs/PELs, odour control or are relatively non-toxic. It is important to note that some compounds (such as 1,3-butadiene, MEK, n-hexane, n-heptane, etc have occupational exposure limits which do not necessarily reflect their toxic potential.)

Category III In the third category, the challenge fume migrates directly to the user's breathing zone or completely escapes from the interior of the hood, spilling into the surrounding air. In this case, the recommendation is that the hood not be used for the protection against any harmful contaminant.

References:

1 Laboratory Ventilation Work Book, Burton, Jeff D., Pub. IVE, Inc. SLC Utah, 1991

2 Laboratory Fume Hood Standards Recommendations for the US Environmental Protection Agency, R.I. Chamberlin and J.E. Leahy, 1/15/78. Contract No. 68-01-4661

3 Effect of slot position on laboratory fume hood performance. G.W. Knutson, Ph.D., Heating / Piping / Air Conditioning, February, 1984

4 ASHRAE Report Number 2438 RP 70, K.J. Caplan and G.W. Knutson, 1978

5 Laboratory Fume Hood Standards Recommendations for the US Environmental Protection Agency, R.I. Chamberlin and J.E. Leahy, 1/15/78. Contract No. 68-01-4661

6 Cyril M. Harris Ph.D. Editor, Handbook of Noise Control, (McGrath-Hill), P. 27-3, Reference section in Heating and Ventilation System Noise


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This page was created on April 20, 2004 and since then there have been visitors. I would be happy to answer any questions concerning this protocol or prepare a scope of work or bid for evaluations.

By-pass section was revised 4/21/04.
Visitors to this page generally have an interest in scientific issues and other discussions of mine may be of interest as well. To visit my discussion concerning health effects of moulds, (mostly debunking the irresponsible hype found in the media) click here. For a discussion concerning monitoring for airborne moulds, click here. A discussion concerning myths surrounding duct cleaning, can be found by clicking here.

For a discussion concerning indoor air quality, click here.


Issues surrounding the history and cause of carpal tunnel syndrome are discussed here.

A discussion of indoor radon, can be found here.




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