m f e t y in the chenkcrl laboratory
edited by MALCOLM M. RENFREW University Of Idaho
MOSCOW. Idaho 83843
Fume Hood Testing and Evaluation G. Thomas Saunders Geneva Research, Inc., 7 Scarsdale Place, Durham, NC 27707 Major industrial research laboratories have ongoing programs to help their employees use fume hoods safely and effectively. I t is not a common practice to do the same for collegeand universitystudentsand facultv members. In academia. as in industry, there is a recognition of fume hood protection against inhalation of toxic substances but lessof arealization that even the best of hoods and hood systems do not guarantee complete containment. If total containment is required, the systems must change from hoods to glove boxes. To secure the maximum level of personnel. moduct.. and environmental safetv the ~~~~. aurkings o i the howl system, a v r k r r d i s pline, and hood e\nluatlm mu51 br undrrstood and promoted as rrc,uirrd training. This paper paints out common and correet-
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able hood-system defects and offers acceptable methodology tocheck on hood containment.
Overvlew I t has often been my wish that the authors of "Who-Done-Its" would write the last chapter first so that I could admire their finesse in developing the plot rather than evaluate each character or action as a suspect or a clue. In this vein I first am going t o summarized this article, outline the conclusions and then tell why and how the various components fall into a particular pattern. If you agree with my conclusions, you may be excused from reading past these statements. If you disagree, then please read on, and we can settle our difference a t a future fume hood symposium.
Summary Good fume hood oerformance deoends on these five factors that are listed intheir order of descending importance: 1. Room air patterns 2. Hood design 3. System design 4. Exhaust volume (proportional to face velocity) 5. User discipline Fume hood testing can be accomplished t o the minimum level of acceptable safety performance using these procedures: 1. Smoke bombs andlor sticks 2. Measuring face velocity 3. Using a dry icelwater challenge For quantitative evaluation, and after the hood has passed the three tests above, the only recognized procedure a t this time is ASHRAE 110-1985.' This is a containment protocol developed by the American Society of Heating, Refrigeration and Air Conditioning Engineers and accepted by t h e American Conference of Governmental Industrial Hygienists. Room Alr Patterns Fume hoods remove air from the laboratory space, and that air must he replaced. People work in fume hoods so, abviously, they have access t o the laboratory. We have now defined the two major potential disturbances to good hood performance: 1. Doorways and traffic patterns 2. Air supply systems Let us stop for a moment and quantify these two happenings. At the hood, with a face velocity of 100 fpm, you have a very light breeze going into a moderately sized rectangular opening a t slightly over 1mph, a velocity that is hardly noticeable. If you have a doorway close to or adjacent to this hood, adoor swing will be in the 5-mph range. A person walking through this door will walk from 3 to 5 mph. As the door opens and closes, it acts like a large fan. I t does a fine job of creating some relatively major air disturbances. If your doorways, by location, do not cause major problems, then inspect the ceilings or space above the hood line in case you do not have defined ceilings. Locate those 12- x 12- or 24- X 24-in. louvered air-supply grills (or conventional circular diffusers), and realize that through these two or three openings you must supply all of the air being exhausted by the hood. If the hood face velocitv is 100 fom. iust imaeine what the dis-
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volume the "bottom quadrant" of exhaust action for lack of a better name. Now for top slot "A": this is located adjacent to the exhaust outlet and has the greater amount of suction available to it as compared to slot "C". If "A" is opened too wide, slat "C" has greatly reduced exhaust volume, and the hood has poor surface sweep. With "A" opened too wide, the air rushes toward the slot proper, goes right past the slot, continues in a circular pattern, and tries to go back again. This "try and not make it" routine causes a big circulating pattern in the top portion of the hood, and we refer to it as the "vortex". Again Let us name this section of the hood superstructure as the "top quadrant". For optimum hood performance slot "C" should he full open (2 to 2% in.), and slot "A" should be set and locked, screwed, ete., a t % t o :% in. open. Please do not scoff a t these dimensions, for the improvement in an ASHRAE 110-1985 rating (we will describe this test later) is a n improvement of three orders of magnitude. Another very important part that adds rmmeaiurablv to performance is the huttom fnmt airfoil at the u o r k surface. Here i* the reawnin*. Air is law. Withmt the airfuil, the air Chat enters i h e hood a t the work surface comes partially from below the level of the work surface and has a verticalvector. The air coming from the front plane has a horizontal vector, and when you have the two intersect you get a massive turbulent roll a t the front edge of the work surface. The addition of the airfoil eliminates the verticalvector, turns it into an additive hor-
charge velocity from each diffuser must he. These room-air diffusers will be found t o have discharge velocities from 250 ta 700 fpm or roughly 2% to 8 mph. Unless located far away, they will cause drafts a t the hood face. I t is now time for the question: how can this hood, that is being abused by room-air currents, function effectively? The obvious answer is that it cannot do so. When you realize that this is a major cause for poor hood performance, you have started on your understanding of hood systems.
Hood Dedgn The fume hood, as we see it advertised today, has a front glass-sashed opening, a top exhaust outlet, a rear adjustable baffle system, and a hottom, front, horizontal airfoil a t the work surface (see Fig. 1). Air goes into this hood chamber, past the experimental process, to and through the baffle system, and out through the exhaust outlet. Question: what do the hood-chamher air patterns look like during this process? At the hood face, disregarding room-air patterns, the air is reasonably horizontal in orientation. Once the plane of the sash is passed some of the air heads upward toward the top slot "A", and some goes toward the bottom slot "C". The relationship of the opening sizes a t "A" and "C" is critical in securing good hood performance. Refer to Figure 1. Let us examine slot "C"as our first priority. I t shoud be fixed (not adjustable) and set permanently a t from 2 to 2% in. open. This causes the air t o sweep across the work surface and that area approximately 10 in. above the work surface. Let us call this work
Figure 1. Basic components of a rncdern hood. A mp slot; B, bmtorn slot.
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izontal force and the turbulence disapp e a r s p a o f , just like magic. Refer to Figure 2. .
Lastly, let us consider the glass sash. Well, maybe we do not have to review this important part of a hood because too few people actually use i t to any degree. A majority of sashes are raised 100% of the time. If they had hroken glass or broken cables, a good many hood users would never know it. But this sash isarecognized protectionin case of an explosion or a fire. I t really should be closed whenever possible: open for set up and adjustments and closed far process procedures.
Exhaust Volume (Face Velocity) Since people do not live by bread alme. neitherdo iump hoods live by air alone (fare velority~'l'nr face velocity of a hoad is perhaps the easiest parameter t o measure with some degree of accepted accuracy (plus or minus 20%). For years the scientific communit" was told hv the "exoerts" that hood sat&). w a s achir;cd with &eypeciric h i ~ h ince d w i t y and that all uther !actor9 were less signifirnnt. Mechanical engineers hy training look a t hood systems as requiring so much rxhnuxt and su much makeupau The exhaust isdetermined by thel'arr velocity oi the s p e r ~ f ~ lrnrth c of hood. The b m e r the hood: the moreair sucked from theroom. The days of "the higher the face velocity the safer the hood" started into their sunset ~
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Figure 2. Air flow into a hood (lefl) wlthout an airfoil and (right)wilh an airfoil. in 1978, and with the publication of the new laboratory safety regulations by OSHA in January 1990 the "more is better" days are over. Exhaustive and quantitative experimentation over the past 10 years shows that face velocities in the 60-100 fpm range are adequate and that there are other parameters that contribute substantially to safe hoad operation. Increasing the face velocity above 100 fnm effects little.. if anv. .. imorove, ment ercedr w add to the building operating cost, and ahwe, say. 125 fpm turbulence actunll) w ~ ~ e the n u containment.
Discipline "Mv hood is iust no mod" is not a n une o m m h remari. ~ h e n ' u go e to the hood to dra~nueethe prohlem, we find good room. air patterns, ncceptatdr room location. ade. ~
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quate face velocity-and then we look into the hoad itself, I t appears as though the Kane County Flea Market had been transported into this hood. There is equipment piled on top of equipment, used and useless items stored from last year. Consequently it is so cluttered it makes little difference where slots "A" and "C" are set-you cannot even find them. ~ ~ ~ ~ ~ For effective service clean out (or up) the equipment and materials in the hood. If items are inuse (vacpumps, hot plates, mixers, etc.) and are resting directly on the work surface, slot "C" will be effectively closed. To correct this, use some larger rubber stoppers, baby-food jars, or any compatible and readilv available item to raise the eouioment a t least 1or 2 in. off the work surface. The hood performance will be greatly im~~~
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When equipment is stacked and used all the way to the front edge of the work surface, i t may heeasier (convenient) touse but a t the expknse of causing poor hood performance. DuPontZfound hoods much safer to use when all work is performed a t least 6 in. back into the hood chamber. They have a line painted on the work surface and insist, as good safety practice, that all work he done behind this line. You should follow this practice.
Hood Testing How do you find out whether your hood is functioning properly?
Smoke First we shall qualitatively view the air oetterns with smoke bombs or smoke sticks. smoke bomb' set off i n a hoad can wakeup your *arc-ty senses in a hurry. If you have prohlcms,you w~llnorice them quickly with smoke coming out into the laboratory. The bomb can show cross drafts and traffic problems. Smoke sticks3can show room-air patterns. This is all qualitative and subjective, but i t does show a t what general level of performance your hoods are operating. ~
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Face Velocity There is a multiplicity of instruments for measuring fume hood face velocities. Some are great, some good, and some not so good, but they are all better than nothing. Regardless of the instrument make or model you need to know two things up front: 1. Does the manufacturer provide goad operating instructions?
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2. When was the instrument last calibrated? Have the instrument calibrated, or check it with another calibrated instrument. Read the instructions carefully. Take a nine- or 12-paint survey across the total hood (face) opening. In general, your velocities from top to bottom and side to side should average plus or minus 20%; the average should be an absolute minimum of 60 fpm and a maximum of 100 fpm.
DIY Ice and Water Challenge Take a 10-in. pie or cake pan, fill it threequarters full of warm water, and place it on the front portion of the work surface. Now add a dozen or so small roek-sized pieces of dry ice. This COz- and- water vapor is quite dense, and whether it goes completely out the back baffle (slot "C") or pours down the front of the hood does tell you something about performance. If i t all exhausts through slot "C", you are doing things right. If it pours down the front, you should start lookine for the slot "A"/"C" balance: the airioil ma" not he installed. or the hottomquaarnnt 3neep may be blocked hy equipmmr on the work surince. Fur detailed instructions see ad am^.^ ASHRAE 110-1985 All of the testing that we have described so far has been subjectively visual and qualitative in deoth. ASHRAE 110-1985' orotor d , drvelcried in 1378 bv Caplan and son'and published as a standard in 1985, is a precise and nrruratr quantitative test uf hood performance. In a simplified explanation the protocol is as follows: A known volume of a test gas (usually a t 4 Llmin) is nonturbulently diffused inside the hood chamber. The highly engineered diffuser is located 6 in. back into the h w d and is located laterally according to preestablished coordinates. A department-store mannequin is placed at the hood face, and a collection chamber is located a t its nose area. The collectionchamber volume is taken to a detecting instrument capable of analysis of the test gas at a level of 0.01 ppm. The test gas is turned an. the manneauin breathes awav.. and the ens- concentration is recorded for a nredetera ~ ~ ~ . ~ ~ ~ ~ mined test period. The ~ m e r i c a dconference of Governmental Industrial Hygienists (ACGIH) established in 1982 that an average reading, over a 10-min test period, should not exceed 0.1 ppm for hoods in an "as used" condition and location. If in a smoke test you see the smoke, realize the concentration is at least 200ppmand a usual evaluation of smoke density is pure guess work.Notso with ASHRAE 110-1985. There is no guessing. The results are a quantitative evaluation based on the restrictive protocol itself and the reliability of the instrumentation. Due to the location of the sampling probe the ASHRAE 110-1985 protocol explores mainlythe "topquadrant" action within the h w d chamber. The mannequin, however, causes negative pressure vortices as the air enters the hood chamber. This can then cause some test gas to leak from the work surface as, in effect, there is bottom-qusdrant malfunction. This ASHRAE protocol is expensive from a time and instrument involvement. I t (Continued on page A230)
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would be virtually an impossibletask to test,
at once, all of the hoods in the United States (estimated numbers exceed 600,000 hoods) with this protocol. It is not unreasonable, however, to have selected hoods, perhaps at least 10%per laboratory facility, tested in thismanner after they have passed the visual reviews. This would be especially true when working with materials of known or suspect high toxicity. One more aspect of ASHRAE 110-1985is that it also presents a test for exhaust reentry into the building either through the makeup air system, open windows or same other reentry route. The ASHRAE 110-1985 procedure has some critics-do not we allbut ~ o u n dfor pound it vastly outshines all of the other procedures combined. I have tested hundreds of fume hoods, and I have never seen one passing the ASHRAE test fail by some other method. Hoods are successful safety devices when all sections work in harmony: top quadrant, bottom ouadrant. face. and user. If vou do not get all the pieces to cooperate, you will not have a safe hood.
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ASHRAE 110-1985standard is available from ASHRAE, 1791 Tullie Circle. N.E.. Atlanta. GA
30329. Fuller. F. H.; Etchells. A. W. ASHRAEJ. 1979, (October), 49. Mikeil, W. G.: Hobbs. L. R. J. Chem. Educ. 1981, 58, A 1 6 5 Smoke bombs and sticks available from E. Vernon Hill. Inc.. P.O. Box 7053. CarteMadera. CA
94925.
Adams. J. 8.. Jr. J. Chem. Educ.. in press. 6Capian. K. J.; Knutsan. C. W. RP70 ASHRAE Trans. 1978, (March). 84.
G. Thomas Saundem, president of GP neva Research, a consulting firm, is a graduate of St. Mary's College (California) in physics. He has served university and industrial firms as a health chemist in radiation safety and especially has been concerned with laboratory ventilation. Prior to his present position he was director of research and development for St. Charles1 Whirlpool Corporation. He is a charter member of the American Nuclear Society and maintains membership in the Health Physics Smiety and in ASHRAE.
Control of Peroxldizable Compounds: A Subtractum To the Editor:
Although the recent publication by Mirafzal and Baumgarten ( I ) contains much new information on trace peroxide formation in old alcohol samples, I think that the emphasis put on the nature of the hydroxyl group (as primary, secondary, or tertiary alcohols), rather than on the nature of the carhon and hydrogen structure to which the functional group is attached, serves to divert attention to the least important parts of the total structure. I t is widely recognized that the essential structural feature for autoxidation processes to occur
is the presence of a susceptihle hydrogen atom attached to an activated carbon atom. Such susceptihle hydrogen atoms, as clearly recognized by Jackson et al. in their original paper ( 2 ) , are those on ether-linked C, on the central C of an isopropyl-type grouping, on a vinyl group, particularly if further activated by attached halogen, phenyl, carhonyl, or another vinyl function, in an aldehyde group, and in an ally1 or henzyl position (i.e., on C adjacent to vinyl or ~ h e n v l .res~ectivelv).Manv of t h e &xtu;es 0; the samples oialcohols found to he significantly peroxidized (1) contain susceptihle H atoms in addition to the hvdroxvl function. Among their group ofprimary alcohols are included 2-methylpropanol, 3-methylhutanol, 2-ethylbutanol, and cyclopropyl carhinol, which all contain an isoproovl-tvne ... H a t which autoxidation occurs. Similarly, the henzylic H in 2~henvlethanoland the H on the etheriinkeh C2 in 2-methoxyethanol will he the autoxidation sites. Then in the group of secondary alcohols are included 3-methvl-2-hutanol. 4-methyl-2-pentanol, i n d 3,4-dimethyl-3-pentanol, all of which contain susceptihle isopropyl groups, though the third compound is in fact a tertiary alcohol. I t has been known for some years that 2-propanol (3)and 2hutanol(4) will peroxidize extensively, peroxide concentrations of 4.2 and 12.0 wt %, respectively, being determined. The idea that it is the tertiary H and not the secondary OH that is primarily involved in the autoxidation process is stronelv supported by the isolation of 2-hydroxy-2-hydroperb~~ro~ane from t h e photocatalvzed oxidation of 2prop-anol (5). The inclusion (1) of highly susceptible allylic and henzylic alcohols in the same group as simple unbranched tertiary alcohols with no tertiary H and incapable of autoxidation seems likely to lead to confusion rather than clarification. In the lieht of these facts, the suenestion ( I ) t i a t primary alcohols pi; se should now be classed as ~eroxidizahle compounds is clearly invalid. L. Bretherlck Bridport Domet DT6 6AE, England
Literature Cited 1. Miralrs1.G. S.;Baurngarten, H.E. J . Chem.Educ. 1988. 65. A226 2. Jack~on.H.L.:McCorma~k,W.B.;Rond~vstedt.C.S.: Smelt?,. K. C.: Viele, I. E. J . Chem. Educ. 1979, 47.
A175. 3. Redemann, C. E. J . A m . Chern.Soc. 1942,64,3049
4.
Peterson,D.Chem.Eng. N e m 1981,59(19l,3.
5. Schlenk. W. 0. Angeu. Chem. 1958.70.504.
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