mfety in the chemical loborotory
edited bv University of Idaho MOSCOW, Idaho 83843
Laboratory Air Quality Part I. A Concentration Model Samuel S. Butcher, Dana W. Mayo, and Sandra M. Heberl Bowdoln College. Brunswick, ME 04011 Ronald M. Pike Merrimack College, North Andover, MA 01845
The hazards associated with gas-phase pollutants in instructional Laboratories have been the subject of several articles in THIS JOURNAL (i-4).Other sources also provide guidance for chemistry teachers (5, 6).The present paper offers a simple model for estimating vapor concentrations in instructional laboratories. Three methods are described for measuring ventilation rates, and the results of measurements in six lahoratories are presented. Several considerations oromot . the anproach described here. Flrst, the restrrctron of processes that emlt hazardous vapors to hoods is now a polrcy in msny institutions and is an explicit recommendation of the Committee on Hazardous Substances in the Laboratory (5). Nonetheless, many institutions lack adequate hood facilities for all students and are faced with the choice of oerformine these orocesses in the onen lahoratory or eliminating the use of certain materials. Furthermore, there is no clear means of determining which materials are toxic enough to be eliminated from use in the open laboratory. Does one draw the line at bromine, or benzene, or methylene chloride, or ether? The answer clearly depends on msny factors. A significant reduction in experimental scale such as that offered by the
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Unlverslry ana Indiana Jn~rersny,respectively. and are protsssm ol chemlsw at Bowdain College. Ronald M. Pike received his PhD ham Massachusetts lnstnute of Technology and is professor of chemisby at Merrimack College. Pike has also spew three semasten at Bawdoln College as Visiting Pickard Professor of Chemistry. Sandra M. Hebert is a 1984 chemisby graduate of Bowdoin College. This 18 Uw farth paper in a serles dealing with hmicroscale approach.
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Journal of Chemlcal Education
microscale approach ( 7 , 8 ) offers processes that emit very low levels of vapors and permits the use of more hazardous chemicals in the open laboratory. Second. accentahle concentration levels (defined by toxicities) and observed roncentratiunr can both vary over several order* of magnitude. Being able to estimate ronrentrations accurate to within an order of magnitude can thus be helpful in planning experiments conducted on any scale. Emission rates and ventilation rates play important roles in determinine the vanor coneentration in the laboratory. Even though the enact relationdhip between these factor* and the observed concentration involved other unknown parameters, just being able to make rough estimates of the ventilation characteristics will place limits on experiments that are acceptable risks and point the way toward the improvement of air quality by reducing emissions or improving ventilation. This approach bases the assessment of air quality on a miring model rather than on actual measurement of laboratory concentrations. Although monitoring will provide the most accurate means of evaluating exposures, a great deal of monitoring may be reouired to establish the worst-case level of exposure. To our knowledge, little is k n o w about the variability of personal exposures in laboratory programs. The model prcrmted here should provide a simple sceening tool for evaluating worst-case personal exposures.
In this equation the term an the left, Vdcldt, expresses the overall rate of change of the total amount of material in the room. V is the volume of the room (in cubic meters), and c is the concentration (in milligrams per cubic meter). The first term on the right, G, is the rate a t which material is added to the room (in milligrams per minute). The second term, Qe, is the rate at which material is removed. Q is the ventilation rate for the room (in cubic meters per minute). While eqn. (1)is not used in this form hy our students, it is introduced to give a sense of the mass balance principle which is also the hasis of more refined models. Thinking about this equation and those which follow should help to impart a feeling for the role of careful laboratory practice. We feel that it is particularly important that students have a sense of the risks involved in this sort of laboratory work. Using eqn. (1) as a starting point, three other important factors are now considered. The first, and most important, is an empirical factor which describes the degree of incomplete mixing in the laboratory. This is called a mixing factor and is symbolized here by k. The second factor accounts for the possibility that the pollutant may also be present at a concentration e, in the air brought into the building. The third factor allows for recycling of air hack into the lahoratory a t a rate Q,, with a Loss of pollutant in the building air handling system (by filters or sorbants) described by an efficiency factor E. The mass balance equation now takes the form given in eqn. (2) (also see Esmen (9)). Vdeldt = G - kQc - kQ,Ec
Ventllatlon Model The concentration of a pollutant in an indoor space may he expressed by the following mass balance equation for the case in which t h e concentration is uniform throughout the room. Vdcldt = G - Qc
(1)
+ kQc,
(2)
While this equation represents the most general case, a much simpler form is applicable for moat laboratory situations. We make use of two approximations which may
(Continued on page 240)
oratory period, hut this assumption represents the worst case and the resultine expression i i implrfied. The eoneentrction expusurc averaged w e r the laboratory perig x l is then grven hg the followmg rxpression.
The factor A is defined by the following ex. pression. not apply in all circumstances. The first approximation is that there is no recycling of air exhausted from the lahoratory. That is Q,and, thus, the third term on the right side of eqn. (2) are zero. Second, the concentration of the pollutant being considered in the outdoor air is zero, and thus thelast term on the right-hand side of eqn. (2) is also zero. Where these approximations do not apply, it is necessary to have additional information on the ventilation equipment and on ambient air quality in order to use eqn. (2). Another assumption made in the evaluation of the average concentration is that the emissionsfrom each student occur as apulse in which the total amount of pollutant for each student, m, is emitted over a time period tl, which may he much shorter than the lahoratory period. The emission rate for each student is then given by mltl. We also assume that the pulse is emitted a t the heginning of the laboratory period. The concentration builds up as the material is emitted and decays as the ventilation system removes the pollutant from the time at which the pulse ends to the end of the lahoratory period. It is not necessary to have the pulse occur at the beginning of the lah-
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Journal of Chemical Education
where to is the length of the laboratory period; f is the fraction of the laboratory period over which emissions occur (f=tllto); t, is the characteristic ventilation time for the well-mixed room (t, = VIQ); and n is the number of students in the lahoratorv section The further arsumptiuu has been made that, for a given receptor, b is identical for all sources or, alternatively, that k repre. sents an average for all sources. This assumption is discussed further below. The function A approaches 1as kt& hecomes large and as f heeomes small. For many laboratories t, is of the order of ten minutes. If to is 180 min, f = 1, and k = 0.3, the factor A is about 0.8. For present purposes we assume A = l. This is quite a good approximation for laboratories with high ventilation rates (kt&, > 10) and errs on the side of predicting higher concentrations far laboratories with lower ventilation rates. This approximation for A then leads to the following expression for concentration.
The numerator, m, on the right-hand side is called the emission factor; it has the units milligrams per student. The denominator, kQtoln, represents the volume of air available to dilute the emissions of each student. We refer lo this quantity as the dilution factor. I t has the units cubic meters per student. As the number of emission sources incremes, Mrmatmg an appropriate sdlue fur the mixing fwtur h e r ~ m e smore difficult. It a receptor is very elose to a particular source, a small fraction of the total amount of ventilation air is available to dilute the emissions before they reach the receptor and k will he less than one. For receptors on the opposite side of the room from the source. a lamer amount of air dilutes the ernissivns and the miamg fartor becomes greater than m e . For egrwn student recrptor in a real lahoratory. k wrll gcnrrallg he less than one for his or her own emissions and greater than one for emissions on the opposite side ofthe lab. For present purposes, we assume a value of 0.3 for k. This value is close to a worstcase value far a variety of studies (Ishizu ( l o ) ,Drivas, et al. (11))for cases in which the receptor is elose to the source.This value is likely to lead to a conservative estimate of the concentration expected for the student, hut this factor may he justifiedwhen considering exposures of lahoratory instructors who move about in the lahoratory, are exoosed to a varietv of chemicals. and mavalso he exposed for mure than one lnhoratory rc,wrn each week It seems likely that as more experience is gained with the relation~~
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ship between emissions and conrrntrations in rhe laboratory, refinements may be mndp to the value of k. Many institutions may wish t o compare the concentration estimate obtained using expression (5) with the Permissible Exposure Levels (PEL) values established by OSHA (12). It should he noted that the OSHA values are designed t o protect health n the industrial workplace with eight-hour exposures five times per week. The measurement of ventilation rates and andications of the model will be discussed in the final part of the paper.
Acknowledgment The authors acknowledge the generous support of the Surdna Foundation and the ARC0 Foundation for the development of this laboratory program.
Literature Cited (1) Bayer, R., J. CHeM.Eouc., 59.A385 (1982). (2) Hertiein, F., J. CHEM. EnUC., 56, A199. (1979) we also letters,J. CHEM. EDUC.,57,910 11980). (3) fieifold, M.J. CHEM EDUC.,59, A351 (1982). (0 Melnikow. J., Keffe, J. R.. and Bernetein, R. L.. S. CHEM.EDUC.. 58. A l l (1981). (5) committee on Hazardous Substances in the Laboretory, National Research couneii. "Prudent Practice for Handling Hazardous Chemicals inLebaratorios? National Academy Press, Washingtion. DC. 1981. (6) Green, M.E., and Turk, A.. '"Safetyin Working 6th Chemicals? Maemillan, New York. 1978. (7)Butcher. S. S., Mayo, D. W.,Pike,R. M..Fwte, C.M.. Hotham, J. R., and Page, D. S.,J. CHEM.EDUC.,in p.ess.
(8) Mayo,D. W.,Butehe~S. S.,Pike,R. M.,Fwto.C. M., . S.. J. CmM. Eouc.. in ~ o t h a m J. . R.. and ~ s z eD
mn. Sci. Tochnoi., 6,609 (1972). (12) "Code of Federal Regulation8 Title 29." Part 1910.1WO. US. Go*. Printing Office, Washington, DC. 1984.
Volume 62
Number 9
September 1985
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