wastewater discharges, sometime previous to the time of sampling, or to surface runoff in the vicinity. Water and bottom sludge samples were taken from the sanitary sewer system serving the industrial park. Grab samples of the water were taken three times from a manhole access. As shown in Table IV, PCB’s and PCT’s were not detected in waters sampled a t 1530 and 1815,but when sampled a t 1145 the following day, PCB’s and PCT’s were present a t a concentration of 11.1and 7.5 fig/l., respectively. Sludge deposits within the sewer contained 0.14 ppm PCB and 5.0 ppm P C T on a dry weight basis. I t appears that intermittent discharge of both PCB and P C T to the san’itary sewer system is occurring.
Conclusions PCB contamination of the environment has occurred near the domestic production facility and two user facilities. The spatial distribution of soil contamination suggests airborne transport of the contaminant from the manufacturer and one of the two users surveyed. At the manufacturing site, Aroclor 1260 and decachlorobiphenyl were distributed in such a manner as to indicate the production facility to be the source. In urban Chicago, decachlorobiphenyl and P C T contamination appears to be superimposed on a rather random background of Aroclor 1260 contamination in such a manner to suggest the investment casting facility is the source of the decachlorobiphenyl and the PCT, but not the Aroclor 1260. In suburban Detroit, the background soil level of Aroclor 1260 shows no distribution pattern that would indicate the source to be the facility surveyed. Influent and effluent wastewater samples from Sauget, Ill., attest to the removal. albeit incomplete, of PCB in the biological waste treatment system. Water and sediment samples from the suburban Detroit location indicate PCB and P C T
discharges or contaminated runoff into surface drainageways sometime in the past. Discharges to the sanitary sewer system are apparently continuing. Voluntary control on the release of PCB’s and PCT’s to the environment by industrial sources cannot be considered entirely effective. I t is not possible to determine from soil contamination levels a t the manufacturing site, whether or not the contamination has occurred since voluntary controls have been instituted (ca. 1970). At the urban Chicago location, however, it appears soil contamination has occurred since 1972 when Fenclor DK came into use. Contamination of wastewaters is still evident.
Literature Cited ( 1 ) Selikoff, I. J., Ed., Enuiron. Res., 5, 3 (1972). ( 2 ) US. Government Interdepartmental Task Force on PCB’s, “PCB’s and the Environment”, NTIS Document COM-72-10419, 1972. (3) Hutzinger, O., Safe, S., Zitko, V., “The Chemistry of PCB’s”, CRC Press, Cleveland, Ohio, 1974. (4) Environmental Protection Agency, “Analysis of Pesticide Residues in Human and Environmental Samples”, Pesticides and Toxic Substances Effects Laboratory, Research Triangle Park, N.C., 1974. ( 5 ) Environmental Protection Agency, “Method for Polychlorinated Biphenyls (PCB’s) in Industrial Effluents”, NERC, Cincinnati, Ohio, 1973. ( 6 ) Snyder, D., Reinert, R., Bull. Environ. Contam. Toxicol., 6, 5 (1971). (7) Murphy, P. G., J . Assoc. Olf. Anal. Chem., 55,6 (1972). ( 8 ) Papageorge, W. B., Mopsanto Industrial Chemicals Co., St. Louis, Mo., private communication, 1975.
Received for review March 4 , 1976. Accepted J u n e 3, 1976. Work conducted under Project 68-01-2978 with the Environmental Protection Agency Office of Toxic Substances. Vincent J . DeCarlo, chief, Monitoring and Information Systems Branch, served as project officer.
Use of Permeation Tubes for Calibration of Vinyl Chloride Analyses William R. Burg*, Shelton R. Birch, John E. Cuddeback, and Bernard E. Saltzman Department of Environmental Health, University of Cincinnati, Cincinnati, Ohio 45267
A study was conducted to determine the performance of vinyl chloride permeation tubes when used to prepare lowlevel gaseous calibration standards which are convenient for the analysis of samples in the gas or liquid phase. Permeation rates were determined gravimetrically a t three temperatures including extended periods a t 25 “C. The rates a t 25 “C for F E P and T F E Teflon tubes (0.250 in. 0.d. X 0.030 in. wall thickness) were, respectively, 0.259 and 1.98 fig/cm min, and the activation energies of permeation were determined to be 15.0 and 11.9 kcal/g mol. Gas samples prepared in a flow dilution system containing a permeation tube were used to calibrate a gas chromatograph equipped with a gas sampling valve. Calibration data, accuracy, and limit of detection are presented. Although the permeation tubes were convenient, periodic weighing is recommended for highest accuracy since slightly decreasing rates were observed in four tubes and some polymerization of the reservoir liquid occurred in one tube.
Recent recognition of the carcinogenic hazards associated with vinyl chloride exposure generated the need to monitor low levels in work areas and ambient air. The federal Occupational Safety and Health Administration set an occupational exposure limit of 1 ppm for an 8-h time-weighted average with a ceiling of 5 ppm for a period not exceeding 15 min ( I ) . An “action level” of 0.5 ppm and above was established for employers for implementing certain required control procedures. Because of important economic and legal implications, accurate analysis of airborne vinyl chloride is essential. The Environmental Protection Agency recently published a tentative method of analysis for ambient air concentrations ( 2 ) ,similar to that recommended by the National Institute for Occupational Safety and Health for occupational areas ( 3 ) . Both methods were based on adsorption of vinyl chloride on charcoal and desorption into carbon disulfide followed by gas chromatographic determination using a flame ionization detector. The accuracy of the method was thus dependent upon Volume 10, Number 13, December 1976
1233
the accuracy of the standards as well as the sampling procedures. Calibration standards were prepared by gravimetric and volumetric procedures. Gravimetric techniques were used to determine the weight of vinyl chloride absorbed when it was bubbled through toluene; aliquots of this solution were diluted with carbon disulfide. Standards were also prepared volumetrically using measured volumes of the vinyl chloride gas which were injected directly into carbon disulfide. Both of these methods were inconvenient and were subject to errors from evaporation. Permeation tubes are useful in preparing continuous, lowlevel concentrations of a variety of compounds including SO2, NOz, HzS, Clz, "3, propane, and other hydrocarbons (4-6). Tube behavior with each individual compound must be determined experimentally since a stable permeation rate cannot be achieved if the tubing is affected by the permeant gas or if the contents polymerize, decompose, or react with the dilution air. The purpose of this work was to evaluate permeation tubes for preparing accurate gaseous vinyl chloride standards.
Experimental Permeation tubes were prepared from 0.250 in. 0.d. X 0.030 in. wall thickness tubing of both F E P (fluorinated ethylene propylene copolymer) and TFE (tetrafluoroethylene polymer) Teflon. A long-lived tube was constructed by attaching a TFE Teflon tube to an 8-ml stainless steel bottle. The tubes were constructed and filled, and the ends plugged with a Teflon rod secured with Swagelok ferrules in the manner previously described ( 5 ) .The vinyl chloride was purified by passage through a cold trap to remove traces of phenol which are sometimes added to the cylinder gas as stabilizers. The tubes after standing in the hood for approximately 2.5 days were main-
tained a t a constant temperature under a continuous flow of dry, purified air. Two sets of permeation tubes were studied. Permeation rates from the first set of tubes were determined gravimetrically over periods up to 8 months, during which the tubes were held at three temperatures in a cyclical pattern as shown in Table I. Several weighings were made a t each temperature, as noted, and the slopes of plots of weights vs. age were used to determine the rates. Least-squares plots of the data are shown in Figure 1,which is an Arrhenius-type plot. The abscissa scale is not linear but is proportional to the negative reciprocal of absolute temperature, and the ordinate is a logarithmic scale of permeation rates. The rates a t 25 O C for FEP and TFE Teflon were, respectively, 0.259 and 1.98 pg/cm min, and from the slopes, the activation energies of permeation were calculated to be 15.0 and 11.9 kcal/g mol. The higher rate for TFE Teflon was more convenient for instrument calibration. The observed permeation rates were corrected to 25 O C by means of the Arrhenius equation and are given in the last column of Table I. These corrected rates were plotted against the final ages, and a line was fitted to the data for each tube by the method of least squares. The rates for the FEP Teflon tube (I) were essentially constant. Those for TFE showed very slight downward trends of 0.14 and 0.03%/day of the initial rates for the 20-cm tube (11) and the 11-cm tube with a stainless steel reservoir (III), respectively. The monomeric liquid vinyl chloride appeared stable in the first set of permeation tubes which were exposed to flows of dry dilution air for periods as long as eight months. A stable permeation rate was maintained when the tube was kept at a constant temperature. For greatest accuracy, it was found desirable after a temperature cycle to determine the new permeation rate. The relative standard deviations of the ~
Table 1. Permeation Rates of Vinyl Chloride Through Teflon Tubing (0.250 in. 0.d. X 0.030 in. Wall Thickness) Mean a temp, OC
25.0 30.0 25.0 35.0 30.0 25.0 35.0 25.0 25.0 25.0 25.0 30.0 25.0 35.0 30.0 25.0 35.0 30.0 25.0 35.0 25.0 25.0 25.0 25.0 a
Holdlng period, days
Final age, days
YO Re1 SD 01
slope
I. FEP Teflon (14-cm effective length) 21 9 0.35 34 8 0.11 47 10 0.16 54 6 0.14 80 11 0.08 107 8 0.07 115 6 0.09 123 8 0.26 160 2 ... 198 2 ... 235 2 1.1 II. TFE Teflon (20-cm effective length) 8.9 12 7 0.042 13.0 25 10 0.10 8.1 33 6 0.26 5.8 39 7 0.21 111. TFE Teflon (1 1-cm effective length with 8-ml stainless steel reservoir) 10.1 13 9 0.11 7.1 20 5 0.09 25.9 46 11 0.31 26.6 73 8 0.05 4.8 78 6 0.26 6.9 84 8 0.25 122 2 ... 37.2 38.0 160 2 ... 196 2 1.6 36.7 16.1 12.7 13.0 7.1 25.9 26.6 4.8 6.9 37.2 38.0 36.7
Thermostatic control f0.02 OC. Coefficient of variation X 100.
1234
No. 01 welghlngs
Environmental Science & Technology
By means of Arrhenius equation.
Permeation rate, Mgfcm mln Obsd Corr to 25 O C
0.244 0.373 0.256 0.593 0.399 0.265 0.597 0.268 0.262 0.265 0.257
0.244 0.246 0.256 0.261 0.263 0.265 0.262 0.268 0.262 0.265 0.257
2.766 1.999 3.710 2.671
1.987 1.999 1.934 1.918
2.018 3.958 2.817 1.994 3.762 2.004 1.967 1.975 1.915
2.018 2.064 2.023 1.994 1.962 2.004 1.967 1.975 1.915
permeation rates for the final 4-month periods (at 25 "C) were 1.8%for tube I and 1.9% for tube 111. Gaseous standards were prepared in a flow dilution system containing one of these permeation tubes. The vinyl chloride concentrations were calculated from the gravimetrically determined permeation rates, and air dilution rates were measured with appropriate rotameters. These standards were used directly to calibrate the gas chromatograph which was equipped with a gas sampling valve. The flame ionization detector is sensitive enough that a portable gas chromatograph with this sampling device can be used to directly measure the ambient air levels of vinyl chloride in the range of 0.05 ppm. No calibration difficulties arise in this situation since both the standard and the sample are gaseous and are injected into the column in an identical manner. Similarly, liquid standards and liquid samples pose no problem. However, if the peak area from a liquid injection is to be compared with the peak area from a gaseous standard, the absolute volume of the gas sampling loop must be known. Since it was desired to introduce either liquid or gaseous samples into the same column without shutdown, it was replumbed so that the carrier gas flowed through the gas sampling valve and then the injection port in series before entering the column. Note that the retention times for gaseous and liquid injections may differ due to the time involved for the gas sample to travel through the line connecting the gas sampling valve to the injection port. The gas chromatograph utilized was a Tracor Model M T 160 DPTFF, equipped with a gas sampling valve and dual flame detectors, of Micro Tek DuoCone design. The effective volume of the sample loop and connections to the sampling valve was determined to be 3.4 ml by the method of standard additions ( 7 ) .A 4 f t X Ys in. stainless steel column packed with Chromosorb 102 maintained a t 125 "C was employed. The helium carrier gas flow rate was 25 ml/min. The FID was operated at the flows recommended by the manufacturer, 70 ml/min of hydrogen and 300 ml/min of air. The calibration data for gaseous samples are given in Table I1 and plotted in Figure 2 to show the line fitted by the method of least squares. Statistical tests indicated that a slightly
curved line would be a better fit. However, the more convenient linear model fitted the data adequately. In a second experiment, two more permeation tubes were prepared using T F E Teflon tubing. One tube (IV) was constructed entirely of Teflon, and one tube (V) was constructed with an 8-ml stainless steel reservoir. These tubes were maintained at a single temperature of 25 "C. The gravimetrically determined permeation rates are listed in Table 111. The initial rates were about 10%lower than those of the previous set, and the values declined at 0.27 and O.lO%/day for tubes IV and V, respectively. An examination of tube I\' after 40 days revealed about 300 mg of a semisolid residue inside, indicating that some polymerization had occurred. No other tubes showed a noticeable residue.
Discussion The additional tubing from the gas sampling valye to the liquid injection port increases the retention time of vinyl chloride in a sample injected by the gas sampling valve as compared to that in a liquid sample injected by a syringe. However, the adjusted retention times calculated by subtracting the appropriate retention times for the air peaks showed no significant difference. The peak widths for gaseous samples are broader because the vinyl chloride is distributed in a larger sample volume, which flows through the detector in a longer time. The ratio of the peak heights from gaseous samples to those from liquid samples containing the same quantity of vinyl chloride varied from 0.8 to 50 ppm to 0.95 at 2 ppm. However, the calibration lines for peak areas vs. quantity showed less than a 0.5%difference between gaseous and liquid samples.
Table 11. Calibration Data for Gaseous Vinyl Chloride Concentrations Concn, pprn
0.5c
1.oc
5.0
1
2.0c
5.0d
10.0d
._
3 .70-
15.0d
25.0d
35.0e
50.0 e Figure 1. Permeation rates of vinyl chloride in Teflon tubes of 0.250 in. 0.d. X 0.03 in. wall thickness FEP Teflon, 14 cm long X TFE Teflon, 20 cm long 0 TFE Teflon, 11 cm long, with 8-ml stainless steel bottle
+
Peak area
960 800 880 1920 1920 1760 3008 3600 3120 9120 9440 9280 17600 16960 16960 23680 24320 24960 42880 42880 44 160 6 1440 58880 60 160 83200 83200 83200
Mean
Re1 SD
880
9.1
1866
5.0
3243
9.7
9280
1.7
17173
2.2
24320
2.6
43306
1.7
60 160
2: 1
83200
0.0
a Disc integrator counts X attenuation factor. Coefficient of variation X 100. Using the FEP Teflon permeation tube (I). Using the TFE Teflon permeation tube with reservoir (Ill). e Using both tubes.
Volume 10, Number 13, December 1976
1235
80
cc
6ol t
70
50
40 I
301 20
CONCENTRATION, p p m Figure 2. Calibration curve for gaseous concentrations of vinyl chloride. Ordinate is integrator counts X attenuation factor X
Table 111. Permeation Rates of Vinyl Chloride Through TFE Teflon Tubing at 25.0 OC (0.250 in. 0.d. X 0.030 in. Wall Thickness) Final age, days
Permeation rate, wg/cm min
IV. (20.65-cm effective length) 1.o 1.861 5.0 1.852 18.3 1.848 28.2 1.782 33.0 1.713 36.0 1.664 V. (6.98-cm effective length with 8-ml stainless steel reservoir) 4.0 6.0
1.813 1.834
10.8 12.8 15.8 17.8 31.1 40.8 45.8 48.8
1.793 1.860 1.775 1.802 1.787 1.755 1.716 1.775
The decreases in the permeation rates in the four tubes constructed of T F E indicated a gradual change in permeability of the Teflon. Moreover, an additional cause of the decline was noted in one tube in which the permeant 1 isibly polymerized and thus lowered the vapor pressure of the liquid monomer. The FEP permeation tube did not exhibit this type of declining permeation rate. I t seems reasonable to assume that backward permeation of atmospheric oxygen into the tubes was associated with the polymerization and decline of permeation rates. This would explain the smaller rates of decline for the TFE tubes with stainless steel bottles, and also for FEP Teflon which is much less porous and permeable than T F E Teflon. The polymerization could have occurred within the Teflon walls as well as inside the tubes. Similar phenomena have been observed (4-6) for hydrogen sulfide tubes, the 1236
Environmental Science & Technology
walls of which turn white upon exposure to air because of precipitation of sulfur and for nitrogen dioxide tubes, which blister upon exposure to atmospheric humidity due to formation of nitric acid within the walls. The remaining phenomenon to be explained was the higher rate of decline observed for the second batch of T F E tubes. The same lecture bottle of vinyl chloride was used for both batches. However, the effectiveness of the cold trap used for removing the phenol added as a stabilizer to the commercial gas (and for removing impurities) may have been different for the two batches. It may have been better if the phenol had not been removed a t all. Another difference was that the TFE Teflon for the two batches came from different manufacturers, although the dimensions were the same. Teflon, like other linear noncross-linked polymers, does not have a single reproducible physical state but may have varying degrees of crystallinity. The void content depends on the manufacturing process, cooling rate, previous heat treatment, and molecular weight. Cross-linking is not believed to occur in Teflon, so that less than 100%crystallinity results mainly from the entanglement of polymer chains. Teflon tubing, commonly called spaghetti, is manufactured by one of two processes: the polymer of tetrafluoroethylene (TFE) by a powder paste-extrusion process, or the copolymer of hexafluoropropylene and tetrafluoroethylene (FEP) by a melt (or free) extrusion technique. The FEP process produces a smaller void content which accounts for the approximate ratio of permeation rates of 1 : l O for F E P to T F E tubing. Like crystallinity, the void content of T F E Teflon depends upon its manufacturing preform pressure, sintering temperature, and sintering time. In addition, the particle size, shape, porosity, and molecular weight of the resin also affect the void content. Since it is the higher void content of the T F E Teflon that accounts for its higher permeation rate compared to F E P Teflon, the slow closing or blocking of these voids can be responsible for the observed decreases. This could be caused by polymerization of vinyl chloride initiated by oxygen, impurities in the gas or in the Teflon, or by ultraviolet irradiation. There also could be physical changes due to the familiar tendency of Teflon to cold flow under stress, caused by the high internal gas pressures (3.7 atm). Teflon tubing has been manufactured with sufficient uniformity for construction of permeation tubes for many other compounds. No special difficulty appears likely in its application to vinyl chloride. The permeation device is one of the safer ways to handle this toxic agent. In the very unlikely event of an accidental rupture of the permeation tube, only about 10 g of the gas would be released.
Conclusions The vinyl chloride permeation tube was a convenient and accurate device for making low-level mixtures of vinyl chloride in air. The tubes should be periodically weighed since polymerization can alter the permeation rates. With care, vinyl chloride permeation tubes can be used to prepare gaseous standards for comparison with the liquid samples prepared in the charcoal tube technique. Application of the permeation tube system is not limited to generation of standard samples. The device can also be used for animal exposure studies since the tube lifetimes are on the order of two-thirds of a year. Literature Cited (1) Department of Labor, Occupational Safety and Health Admin-
istration, Fed. Regist., Part II,39 (194), 33890 (Oct. 4, 1974). (2) Methods Standardization and Performance Evaluation Branch, EPA, “Tentative Method for the Determination of Vinyl Chloride
in the Atmosphere (24-Hour Integrated Sampling)”, Research Triangle Park, N.C. (3) National Institute for Occupational Safety and Health, Method PBiCAM # 178, in “NIOSH Manual of Analytical Methods”, HEW Publication No. (NIOSH) 75-121, Cincinnati, Ohio, 1974. (4) O’Keeffe, A. E., Ortman, G. C., Anal. Chem., 38,760 (1966). (5) Saltzman, B. E., Burg, W. R., Ramaswamy, G., Enuiron. Sci. Techno/., 5, 1121 (1971). (6) Saltzman, B. E., “Permeation Tubes as Primary Gaseous Standards”, pp 64-8 in “International Symposium on Identification and
Measurement of Environmental Pollutants”, National Research Council of Canada, Ottawa, Ont., Canada, June 1971. ( 7 ) Cuddeback, J. E., Birch, S. R., Burg, Mi. R., Anal. Chem., 47,355 ( 197 5) I
Rcceiwd f o r reuieu August 28, 1975. Accepted June 7, 1976. Work supported partly by the Enuironmental Protection Agency, Grant R-800869, and p a r t l y by the N a t i o n a l Institute f o r Enuzronmental Health Sciences, Grant E S 00259.
Effect of Photochemical Air Pollution on Two Varieties of Alfalfa C. Ray Thompson* and Gerrit Kats Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92502
Eldon L. Pippen Western Regional Research Laboratory, U.S. Dept. of Agriculture, 800 Buchanan St., Berkeley, Calif. 94710
William H. lsom Cooperative Extension, University of California, Riverside, Calif. 92502
The effect of air pollution on two varieties of alfalfa (Medicago sativa L ) was measured by growing the alfalfa in pots in a carbon-filtered atmosphere (CF) and nonfiltered (NF) air. Green forage weights, leaf-to-stem ratios, numbers of stems, and numbers of surviving plants were determined. Green forage yields in N F air were 42.2 and 33.3% less than in CF air for the varieties Hayden and Eldorado, respectively. Reduced yields paralleled increases in concentrations of oxidants in the atmosphere a t the experimental site for the first four of the seven harvests made. At the conclusion of the experiment, plant counts in the N F air showed stand losses of 36.40/0for Hayden and 38.1% for Eldorado when compared to CF air. Chemical analyses of dried forage samples showed higher crude fiber, beta-carotene, and vitamin C in both varieties grown in CF air. Niacin was significantly decreased. A determination of protein efficiency ratio with rats and an in vitro nitrogen digestibility test showed no varietal or treatment effects.
Alfalfa (Medicago sativa L ) is injured by ozone, the major component of photochemical smog ( I , 2 ) . Hill et al. ( 3 )recognized the problem and suggested that ozone sensitivity may influence crop selection in some areas. Howell et al. ( 4 ) evaluated 14 strains of alfalfa by exposing them for 4-h periods to 20 parts per hundred million (pphm) of ozone under greenhouse conditions. Two separate exposures were made separated by a 4-week time lag. After each exposure, they rated the alfalfa strains for degree of leaf injury. They found that by selecting plants that showed no leaf injury, tolerance to ozone could be significantly improved over the original parental lines. Injury from oxidant air pollutants reduces the sugar content of grapes ( 5 ) ,and perhaps other plant constituents are also affected. The major phytotoxic component of photochemical smog is ozone, but peroxyacyl nitrates, oxides of nitrogen, and particulate matter are all present. Each may cause plant injury (6). The present work was designed to compare two alfalfa varieties of different ozone sensitivity when grown under controlled conditions. They have been observed to differ in sen-
sitivity to leaf injury to air pollution when grown in field plots a t Riverside, Calif. They are classified as “susceptible” and “tolerant” to photochemical smog. Open-topped chambers similar to those developed by Mandl et al. ( 7 ) and Heagle et al. (8)were used because they most nearly approximated field conditions and permitted control of the pollutant concentrations. All tests were developed to show differences between CF and NF environments for each variety. Tests were designed to determine if these comparisons would yield data which were quantitative enough to detect differences in varietal susceptibility to photochemical smog as measured by total yield and nutritive value of the forage, and enable growers and seed producers to evaluate economiclosses which occur presently by extrapolating the results to field conditions. Procedure Two certified varieties of alfalfa (Medicago satiua L ) were selected for comparison of their growth, leaf retention, yield, and nutritive quality when grown in carbon-filtered air (CF) or nonfiltered (NF) air at Riverside, Calif. This area has high persistent levels of photochemical oxidants during late spring, summer, and fall. The variety Hayden was used as an example of one “susceptible” to oxidant, and Eldorado was selected as a more tolerant type. For each variety, seeds were planted a t 12 mm depth in 30 pots (30 cm diam X 40 cm height) in a 1:1:1 mixture of peat moss: redwood shavings and silt; superphosphate, KNO3, K2S04, and oyster shell lime were added to the soil mixture. Irrigation was with one-half strength Hoagland’s solution. Electrical conductivity of leachate from pots was monitored periodically to detect and avoid an excessive accumulation of salts during the experimental period. Four seeds each were placed in 16 holes in each pot to obtain an adequate plant population for study. No thinning was done, and plant competition was relied on for the final establishment of the stand. The seedlings were grown in open-topped cylindrical chambers 2.4 m in height and 3.0 m diam with 15 pots of each of the t w o varieties. The vertical sides of the chambers were corrugated translucent “Filon” economy grade 440 (Vistron Corp., Hawthorne, Calif.) fiberglass panels. Translucent plastic screening (20%shade) was secured over the top of the Volume 10, Number 13, December 1976
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