edited by
MALCOLM M. RENFREW University of Idaho Idaho 83843
MOSCOW.
Evaluation of Chemical Atmospheres in Science Laboratories Richard E. Bayer Chemisby Deparlment, University of Wisconsin- Waukesha, Waukesha. W153 186
Science teachers in academic institutions conduct experiments in rooms called lahoratories. These rwms have rows of tables, some electrical, water and gas facilities and perhaps a few hwds. Frequently the ventilation in many parts of the lahoratory is no better than the ventilation in an ordinary lecture classroom. Most chemistry teachers would agree that a t times things are not right in their laboratories. The smell is had, some students complain about odors and "don't feel too well." I t is difficult t o determine the cause of the headaches, giddiness, irritability, numbness, tingling in the limbs and respiratory problems, so we tend to "grin and bear i t for the sake of the profession." Perhaps a good night's sleep or a weekend a t home will remove the effeds. It is difficult to complain because the symptoms are not clear, and many things can cause the same symptoms. Chemists tend t o accept this environment. One of the problems associated with the evaluation of chemicd environments is a lack of understanding and acceptance of toxicity measurements. When we use melting points or equilibrium constants of compounds, we have a feeling for and can identify with the work that was required to produce such data. When NIOSH (National Institute for Oeeupational Safety and Health) publishes a TWA (time-weighted average) exposure limit for a compound, we are not sure how to interpret this value or use it in our work. Some teachen have read page one of the pertinent NIOSH criteria document for a compound and see that the TWA work is associated with workers in industry with an eight-hour exposure and quickly arrive a t the conclusion that the data and concerns are irrelevant t o them because students are only in the lab for 3-5 hours per day. Others will read page two of the NIOSH criteria document and reject the conclusions because of conflicts with animal tests and variations in experimental conditions. Some physical scientists do not accept epidemiological studies. Others will partially accept publishedtoxicity levels, hut wonder how this information can be translated for use in the academic laboratory. Let us cite one eaample-methylene chloride. The current criteria document for methylene chlaride ( 1 ) is a 167 page hook that explains why the present reeommended TWA is 75 ppm. I t is important to note that
the recommended TWA in 1969 was 500 ppm. and as more information became available, the accepted value decreased. A significant proportion of the experimental work involved human volunteers; hence i t is not possihle to dismiss all of the results if one rejects the validity of animal testing for humans. In addition, the bulk of the experimental work was done in established research t'acihtirs by well-rstahlished sricntiiu wing m w h n rerhniquca and imtrummtatim. Srvrral dr~rriptionso i experiments are provided in which humans were expmed for ahuur one hour to mrthylene chiwide at 300-700 ppm levels. The people invdved in the test 'didn't tee1 [MI hndlv." olthoueh same "lightheadednkss" was Gperienc:d. The corresponding medical tests did show significant changes which included central nervous system depression. Stewart, e t al. (2) in 1972 described an interesting "accidental" discovery that methylene chloride is easily converted to carbon monoxide in the blood. If the level of earboxyhemoglobinin the blwd is measured, there is adirect correlation with methylene chloride exposure, and all of the known reactions involving CO in the body were confirmed. The maximum carboxyhemoglobin is noted several hours after exposure, and there is an additive factor with existing CO in the blood. Thus, smokers can tolerate less methylene chloride than nonsmokers. Methylene chloride a t 100 ppm produces some effect and 50 ppm produces very little effect (carboxyhemoglobin) and so the level was set a t 75 ppm for non-smokers. Additional work was done to show that methylene chloride can move quickly through the skin. There is a direct relation between the time during which the skin is exposed and carboxyhemoglobin in the blood. The data available for this solvent (methylene chloride) provide some useful guidelines for understanding the effects of chemicals on the human hody. 1) Many chemical reactions can occur in the hody before a person "feels badly." 2) Animal testing does produce useful guidelines for reactions of chemicals on human tissue. 3) Frequently there is a lag in time between exposure t o a chemical and the toxic reaction to it.
4) Exposure time of three hours, as during a typical laboratory period, is significant, and problems can develop. 5) In general, lab situations involve mixtures of chemicals, and minor components can act as multipliers in evaluating the toxic effect. The "multipliers" may he non-lab related as, for example, smoking. 6) The NIOSH reeommended values (TWA) are being revised and are moving t o lower valuee 7) There is no ten-fold safety factor. (For methylene chloride there was an effect noted a t 100 ppm and very little noted a t 50 ppm.) The discussion so far indicates that a t this time NIOSH recommendations are probably the best available guidelines for chemistry teachers interested in evaluating chemical problems in the lahoratory. For example, if an organic laboratory would approach the 75 ppm level for methylene chloride, alternatives or modifications in the experiment should be considered. How likely is it, however, to reach this level given a normal chemical operation in a student lab? Let us make some rough calculations. A TWA of 75 ppm is also expressed as 262 mg/m3. If an organic lab has 384 m3, then the amount of solvent needed to hring the lab to the limit is: 261 mg/m3 (384 m3) = 100 g. If a lab section has 20 students. each student can add no more rhon 5 g (3.8 rill, of methylrnerhluride w the environment. T h ~assumes s rhrw is no venrilation in thr mum. 11 should br noted that the Odorl'hrcshcdd fur thmac,lvenr (the pomt when n d~stmetd o r is rrwgnlaed, 1s 220 ppm (31. If the student9 brain to smell mrthvlenc rhlurde. the nmrentmrion in the lab atmosphere is about three times the recommended NIOSH euideline.
assumed that an organic chemistry experiment can he considered to he aseries of unit operations and the loss of solvent per unit operation can be determined. If twenty students are in the lab section, the total amount of solvent introduced into the environment can he determined. (Continued on page AZ88) Volume 57, Number 10. October 1980 / A287
Safety
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A hypothetical organic experiment was assumed to involve a reflux operation and a solvent extraction. The solvent loss associated when methylene chloride is refluxed for one hour, and the solvent loss involved i n a n extraction using 100 ml of methylene chloride was determined. The laboratory space was 384 m3.
150 mlnw placed in a5ilO.ml boiling flark equipped uith a cundenrer. I t a c a r h n nlwrption rube is placed nt rhe top of the condenser t o trap escaping solvent and the usual analytical procedures are followed with regard t o gas chromatography and analysis ( 5 ) ,we have found that 50 pl are lost durine the first 15 minutes and that 11 rrl nrr losr each suhseqtwnr 15min peric9d. Ahmr 83 my areadded to the environmmt each hour fur a totnl of 1.66 g for 20 students ~~~
B. Solvent Extraction If 100 ml methylene chloride are used in a normal extraction sequence, we have found that about 5.5% or 5.5 ml of solvent are lost to the environment. (This does not inrludr rhr solvent I h ~ d u tt, e thesoluldity in orhrr solvrnt~.,Twenty sruden13can add 110 ml or 145 y, ofa,l\,ent to the atmosphere ~
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At this point we must conclude that if the students in the described laboratory section proceed with the assigned experiment in a laboratow that has essentiallv no ventilation. thereisaitmng powbility t i a t theTWA tb; methylme rhluridr will Ire ercepded during Ihe ruurse uf rhe laboralory p e r i d Smce the final answer depends upon the mass balance between the amount of material introduced into and the amount leaving the laboratory, the more ventilation that is available, the more solvent can be tolerated in the environment. There is some ventilation availahle in every laboratory, hut we are also aware that peculiar air patterns can reduce the ventilation to near zero a t some laboratory stations in the room. The hest estimate of available laboratory ventilation should include a combination of theoretical principles (6, 7) and empirical measurements. A reasonable estimate of the ventilation is then combined with some h n ~ i rintormation reganling rhr laburatw, experiment-such a9 toxicity trf the compuundw, a m w n u usrd, and unit
