mfety in the chemical loboratory
edited by MALCOLM M. RENFREW University of Idaho Moscow, idaha 83843
Toxic Vapors and Ventilation Parameters Evaluating the Laboratory Atmosphere Richard Bayer Carroll College, Waukesha, WI 53186
Everyone agrees that a basic objective in the area of chemical health and safety in the science Laboratoryis to operate at "acceptahle risk" ( I ) . The definition of "acceptable risk" and the implementation of practices and policies to meet that objective are the problems of the day. When the phrase "acceptahle risk" is apd i e d to the atmosoheres in science laboratones, there 15an implication that wmewhw a roncent ration or range of concentrations of chemicals can be identified and described and that some health problems mav dewlop fur sume people if concentrations are tderated o b o ~ ethis level for a period of time. Relating health effects to concentrations of
RicW E. Bayer received his B.S. degee hom Carroll College in 1954 and his PI1.D. degree in anaiyiical chemisby horn Indiana University in 1959. From 1959to Uw pesent he has b n associated with the Chemistry Department of Carroll College, being Baychairman fmm 1969-1976. Rotis also aesident of Bionomics Corooration. an indenendent laboratorv concerned with sol" ng problems walv ng ehemlrtry and boology For the last eleven years scrence students bve had Uw oppataniIy to b m employees of a company that solves real problems. cants Iran NSF (LOCI Rsgam)and the S. C. Johnson Wax Fund have sponsored chemical safety research for ths last four years. This wwk b s been described and published in )oumaisand symposia associated wilh the ACS Division of Chemical Education and Chemical Health and Safety.
chemicals is not an easy job, and there has been an increasing amount of effort directed toward this goal. Thir work requires that a number of people examine available information, experiences of people, and some laboratory animal test data and arrive at a decision as to what constitutes an acceptable level (2). These decisions are known as Permiarible Exposure Levels (PEL) or Threshold [ h i t Values (TLVI. As in many other area? IPscience the awilable infnrmatiun and data are not complete and include same questionable data and some conelusions based on unverified extrapolations. Thus, the final decisions must be under constant review and must be revised as more information becomes available. Federal laws were written wrth the chemrcal ~ n d w t r yin mind (eight-how eapnrure to personnel, etc.), and it is impossible to gain a specific interpretation of the laws that apply directly to academic laboratory situations. Even though the decisions (PEL values) are subject to change and present taws are not directly applicable, it is important for science departments to develop policies that can be used to guide experimental practice which takes these decisions into account. The following example of a science department policy governing levels of toxic chemicals in their laboratory atmosphere is given (3): ~~
~~
The next step is to implement the policy and introduce simple practical procedures that assure compliance with it.
Evaluation ot Ventllatlon Parameters-Basic Principles The levels of toxic chemicals in the laboratory atmosphere are dependent upon the rate at which chemicals are introduced into the atmosphere and the rate at which they are removed. Many experimental operations allow chemicals to escape into the atmo(Continued on page A386) Resented at Uw American Chemical Society Meeting, New York. 1981
It is not acceptable to have laboratory erperienrea raise the level of chemicals in the atmosphere above the OSHA Permissible Exposure Level (PEL) values for more than 15 minutes. I t must be recognized that Ceiling Values and STEL (Short T e r m E x p u r e Values) are above the PEL values and will also be considered, and it is =sumpd that PEL values are interchangeable with Threshold Limit Values (TLV). In addition, although laboratory work is conducted in both laboratory hoods and in thelaboratory proper, inmany situations a significant fraction of the work is done outside of the hoods. Therefore, it would be exoected that the statement on lev& of toxw materials in the ntm,rxpheres would apply primarily to the general l a h a tory. This policy statement, which emerged as the consensus opinion among the science faculty, then forms one basis for all future plans regarding experimental work in that department. ~~
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Volume 59
Malcolm
M.
Renlreu
(rr3vla
or# . # r e d
n a m n and acanem c e w e * encer n nos a~oroacnto oooc r n f m mat! ces After giaduate study at the uk&rsity of Minne-
~~
~
University of idaho, his Alma Mater. He is active in the American Chemical Society. including service with the Comminee on Safety and the new division of Chemical Health and Safety. He now is profesSor emeritus of chemisby and is patent director Of his University's Idaho Research Foundation. Inc.
Number 12 December 1982
A385
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sphere and these can he identified. Some of these results have been reported previously (4). Examples are given below: Operations Loss 1. Solvent extraction ether acetone methanol isopropanol methylene chloride 2. Pouring ether acetone methanol isopropanol 3. Distillation-Acetone 4. Filtration (cold)-Acetone 5. Filtration (hot)-Acetone -
6.0% 2.6% 1.6% 2.0% 4.5% 1.7% 0.7% 0.4% 0.2% 1.1% 12% 21%
It is possthle to look at a new experiment and ident~fythe unlt operations that d~spel chemicals ~ n t othe atmusphtw. When those data are used in conjunction with the number of students in the lab section and the volume of the room, an estimate of the rate of the increase in the concentration of the chemical into the laboratory atmosphere can be made. Toxic chemicals in the atmosphere leave a lahoratory principally by an air dilution process called ventilation. Air is supplied to the laboratory and the laboratory hood is
A386
Journal of Chemical Education
frequently the exhaust system for the lahoratory room. Normally a laboratory is designed so that there are four to 12 room air changes per hour. The new publication of the National Academy of Science, Prudent Practices for Handling Hazardous Chemicals inLaborafuries, ( 5 ) describes the following procedure for evaluating the ventilation. The average time required fot a ventilation system to change air within a laboratory can he estimated from the total volume of the lahoratory and the rate at which input air is introduced or exhaust air is removed (cfm). The latter is usually determined by measuring the average face velocity for each laboratory exhaust port, such as hoods or other local ventilation systems. The sum of these rates for all exhausted ports in the laboratory will give the total rate at which air is being exhausted from the lahoratory." Also, it states that "The measurement of air flow rates requires special instruments and personnel trained to use them" (fi).
~ h e s & ; d a r d reference, Parry'' Indu\trio1 Hjgtene ond 'ILr~twl~w (7). derive5 [he general equatiun appluble for ventilation by air dilution. c z -Q' ln-=-(tzc1 V
t,) = K (tz- t,)
(1)
or the general ease of coneentration versus time: In C = -Kt in which
+ constant
(2)
CZ
= final concentration
C
Q;
= initial concentration = effective rate of ventilation
V
= volume of the rwm
(ft3/min)
tl = initial time tp =final time The summation of all face velocities (ft3/ min) for dl exhaust ports can he converted to Q' in this equation if certain design assumptions are made. Once Q' is obtained, it can he combined with the volume ( V ) of the rwm to yield a new constant, K. Several different kinds of calculations are now possible. For example, the ventilation equation can determine the time required for a given ventilation system in a particular roam (Q'lV = K ) to reduce toxic levels from an initial concentration, C1, to a final concentration Cz. The problem with this calculation, however, is that few people have faith in the calculations using the ventilation equation when Q' is evaluated in the described manner. The &aim cannot he indeoendentlv -..~ e~~awmotions --evaluated by experiment. This concern is reenforced by the authors of Prudent Practices for Handling Hazardous Chemicals in Laboratories when they suggest that this type of calculation does not, in general, produce useful results, especially for the average science teacher in the lahoratorv. Their suaeestion is that a laboratorv worker shouid wear a portable air-sampling device, parked with a suitable adsurhent, and mounted in the breathing zone for a fixed time period. Analysis then can provide an accurate mea~
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sure of chemical concentrations in the atmosphere. Industrial hygienim use thix approach w evaluate the work environment. However, this approach has limited value in the academic environment, because every experiment would need to be evaluated and the problem uf standards for many different chemicals is difficult w solve. Furthermore. it takes good analytical chemistry to obtain meaningful results. An alternative approach is suggested by re-examining eqns. (1) and (2). This ventilation eqn. (1) looks much like the expression used to describe radioactive decay, where the slope describes the decay constant. When the decay constant is known, for example, it is possible to calculate the radioactivity level at a specific time in the future. The decay constant can be calculated by measuring changeg in concentration with time. In the same manner the most direct way that one can think of to evaluate the rate constant in the ventihtion equation is tofd a laboratory with a chemical and record the decrease in concentration as a function of time. Plotting In C versus t will vield K as the slooe. This aoproaeh was foliowed. A search was made to locate a chemical that (a) was reasonable in cost, (b) had commercial standards available, (c) could he easily distributed in alaboratory, (d) was not toxic at levels used in the experiment, and (e) could he quantitatively identified using available instrumentation. Isobutane was found to be a convenient and safe material to use for this purpose. Stewart and Newton (8) have established that 1OOO ppm isobutane for as long as 8 hrs is not harmful to humans, and if the explosion limits are avoided (safety factor of lo), theehemical meets the desired requirements. Evaluatlon of Ventilation Parameters-Experimental
Detalls
In principle the experiment is very simple. An iwbuulne standard can be purchased from R c
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2)
3)
gas is shut OR and the time recorded. The ventilation under test (some comhination of exhawt and hood fans) is turned on, and air samples are taken every minute in the same manner and location as before until little or no gas remains in the lahoratory.
Evaluation of Ventllatlon Parameters-Results .,-.2
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4)
Figm2.PbtofdatainFigxe 1 tomrrespondtothe ventilation equation.
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1
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Figure 1. Plot of isobutane concentration (ppm)vs time for two differentventilation levels.
Figure 1is a plot of the concentration of isobutane (ppm) versus time. Two different levels of ventilation (A and B) in one room are shown.
A388
Journal
of
Chemical Education
Figure 2 is such a plot of the data for two different levels of ventilation in one room, and the slope of the lines produce corresponding K values. When the ventilation constants (K values) have k e n obtained for a laboratory in a science building under differing ventilation conditions, the ventilation eharacteristics have been established, and useful calculations can begin. The calculations, furthermore, can be baaed on data obtained a t exact loeations in the laboratory (student lab bench, for example). The following observations have been made from such experiments: 1) Frequently, two ventilation constants (Kvalues) can be evaluated during one experiment. If more ventilation is turned on half-way through the exper-
5) 6)
7)
8)
9)
iment, two diffeent slopes in the final plot are obtained. Isohutane is a fairly dense material,but there is little or no difference noted when values near the ceiling are compared to values near the floor. Since many experiments involve Liquids rather than gases, weighed amounts of acetone were placed in evaporating dishes on hot plates (CAUTION)in various places in the laboratory in order to add a known amount of acetone to the laboratory. When isobutane was added, the GC traces followed both isobutane and acetone. There was little difference between the two K values determined for the two eases. T ~ KPvalueobtamd fo&eaperiment that follows isobutsne concentrations from 2000 ppm to 1lKl ppm is identical to an experiment that follows isobutane from 200 ppm to 10 ppm. When one laboratory is characterized, many of the other laboratories in the same buildine have similar values. Studenwcan easdy run the experiment as part of a lvboratury axiignmenr and gain a new perspertivr of chem~cal health and safety. By taking data points more frequently, rrveral locat~oniin the laboratury can he studied during one experiment. Special loeations may have significantly different ventilation characteristics. it was observed that asp311 in one lwa. tlon in the laboratory produred a unlform cunrentratwn throughout the laboratory in a very short period of time. Some work was done to locate "dead
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spots" as far as ventilation is concerned in our lahoratories. The problem areas were fewer in number than expected.
Application ol Ventilation Constants After the ventilation characteristics of a laboratom have been determined. manv difBv ferent kidds of ouestions ,~~~ ean he -wered. comllinint: data fur the rate of removak of chemicalr in the stmusphere with data that estimate the rate a t which chemicals contaminate the atmosphere, it is possible to predict problem situations and consider modifications of the specific experiments. For example, consider the hazard evaluation of a particular experiment. Suppose a class of 30 students conducts an experiment in which a solvent extraction is involved, and methylene chloride (TLV = 75 ppm) is the solvent. For this solvent, it has been determined that a 4.5% solvent loss can he expected when performing an extraction. If the specific gravity is 1.3, the weight of solvent lost to the environm'ent by 30 students is: ~
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~~~~~
If the room volume is 380m3,the concentration becomes 703 mg/m3 or 200 ppm. Assumine that the extraction omration is one of the Zrst oracedures in the (ah exoeriment. first half hour of the lab period. The lahoratory policy must now he considered2It is not acceptable to have laboratory experiences raise the level of chemicals in the atmosphere above the TLV values for more than 15 min." The question is-can the available ventilation hring the expected concentration below the stated risk levels? The ventilation equation can he used to answer the question. How long does it take to reduce the concentration to the TLV concentration? If very little ventilation is available. we have found in our laboratories that the ventilation constant K. is fairlv, small ~~. (0.0050).Therefore, the time rAY', required to reduce the concentratwn of methglme chloride from 200 ppm to 75 ppm can be calculated as below: 200 log -= -0.0050 (AT) 75 ~~
~~~
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AT = 85 min Obviously this is not acceptable, and if another solvent was used that has a TLV of 10, the time required to reduce the concentration to this level would be increased to 4.3 hr. If the ventilation is imoroved. ~OWRYPI. . .-,.hv , turning on more exhaust fans, the improved ventilation constant in our lahoratory hecomes 0.13. The time required to reduce the methylene chloride concentration in the atmosphere to 75 ppm in this system is about 3 min and the corresponding time for a solvent having a TLV of 10 ppm is about 10 min. Some students may spill more and some may spill less and some students may work more quickly than others, hut on the average it is possible for the room atmosphere toreach the 200 ppm level; 4.3 hr is a long time and lOmin is acceptable. Clearly, this part oftheexperiment can proceed when the conditions are such that the ventilation constant is 0.13. It should he further noted that if students are pmvided the volume ofthe room, ventilation cmstantn. T1.V \,slues, numher of students ~
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.~~~ ~-.~-.
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in the lab section. and their exoerimental instructions, they can produce answers to questions regarding proper lahoratory eonditians. It is important that all parts of all science experiments he evaluated and hazards be identified. A f o m has been developed at our college that makes it possible for science teachen to identify most hazards in an experiment and includes the described hazard evaluation of the laboratow atnosohere (9). T h ~evaluat~on s shamld be rrqulred of all erlatlng exper~menralwork w d all new experiments. Presently, the amount and kind of activity that takes place in a science lahoratory is dependent on factors such as the enrollment in a course, the availability of faculty, the amount of eauioment available. and the availability oitc'xts and lab man'uali. Frequentlv, the chem~calhealth and safety aspecm of the laboratory are not taken into consideration. It may not, for example, he possibleto have 30 students conduct certain experiments with the available ventilation and remain below TLV levels. It may be necessary to reduce the enrollment to 10 or 12 students per lab section in order to operate a t acceptable risk. If increasing energy costs cause administrations to decrease available ventilation, the methods discussed abave can clearly suggest the resultant impact on the curriculum of science departments.
Summary A significant amount of scientific work done at academic institutions is done in rooms called lahoratories with limited ventilation capabilities. In order to operate a t "acceotable risk." it is imnortant to have an unders~andingoi what the ventilation system can and cannot do in the u r a of rtmwal of toxic chemicals from the atmchphere. Simple experiments involving available equipment can provide an accurate value for the ventilation constant, and the resultant ventilation expression can be used to answer many different questions regardingthe rate of removal of toxic materials. The laboratory measurements can he done by students as part of a regular laboratory course and can provide an excellent introduction to principles of chemical health and safety in addition to assessingthe conditions under which specific experiments should be performed.
Literature Clted 11) Lawem, Wlam W., "OfAmptabIe Rlk:Scien~. and tbeDeterminstionof Safety," W. Ksufmann, Inc,Lo. Altos, CA 1976. 12) Green.Michael E.,andTbrk, Amoa, "Safewin Working
withChemicals."MsemillanPublishinpCo.,New~ork, SO"* ...",w
"".
" ce
131 "f Clrmll . . Poliev statement of Cbcmiatrv ne~rt.ma~+. ....... ~ o l l e & Wsukeshs, , WI 53186 ~ . (1980). 14) Bayer. Richard, J. CREM.E D U C . , ~A287, 15) National Research Council Committee on Hazardom Substances in the Laboratory, "Prudent Practicpa far Handling Hazardous Chemicals in Laboratories: National Academy Press, Washington, D.C., 1981, p. 195. 16) Ibid, p. 198. 17) Clayton.GeorgeD.,andClayton.FlorenceE.."Patty'a Industrid Hyglenesnd Toi'mlogy,"3rdd.. JohoWiley and Sons,New York, 1978,Vol. 1, p. 784. IS) Stewsrt. Richard D., snd Newton, P a d E.. "Physiological Response to Aeroeal Propellanr.," Envimnmental Health Perspective, 26.278. (1978). 19) Watkina, R., Hudaan.E.,Bsyer, R,"A Method for Aas~ssineHazards in Academic Leborston. Exmriments." ~~
Volume 59
Number 12
December 1982
A389
