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American Industrial Hygiene Association Journal Volume 27 Sepfember-Oc+ober, 1966 Number 5 : Fate of Arenes Incorporated with Airborne Soot............................................................. 415 Bernard D. Tebbens, Sc.D., Jerome F. Thomas, Ph.D., and | Mitsugi Mukai, M.P.H. I Cutting and Grinding Fluids and Their Effectson the Skin...................................... 423 ' Marcus M. Key, M.D., Edmond J. Ritter, and Kenneth A. Arndt, M.D. Z The Density of Uranine Aerosol Particles........................................................................... 428 " Felix Stein, M.S., Nurtan Esmen, M.S., and Morton Corn, Ph.D. Measurement of Dust Exposures in the Asbestos Textile Industry.............. 431 . Jeremiah R. Lynch and Howard E. Ayer - Radionuclides in Selected Human Tissues............................................................................. 438 ; E. ]. Baratta and E. S. Ferri C Uses and Safety Aspects of the Low-Energy SourceYtterbium-169................... 444 > Farno L. Green S. Quantitative X-Ray Diffraction Analysis of Crocidolite and Amosite in Bulk or Settled Dust Samples.......................................................................................... 449 John V. Crable and Marta J. Knott Chronic Neurological Disease in Two Manganese Steel Workers..................... 454 L C. M. Whitlock, Jr., M.D., S. J. Amuso, M.D., and J. B. Bittenbender, M.D. Solving Air Contamination Problems Through Diagnostic Air Sampling...... 460 i A. J. Breslin ' The Pittsburgh Air Pollution Control Story..................................................................... 469 . Edward L. Stockton Industrial X-Ray Study in Jefferson County, Alabama........................................... 475 Edgar F. Seagle, Kelly G. Sauer, and Frank E. Russey j Establishment of the Analytical Methods Evaluation Service........................... 480 J Bernard E. Saltzman President's Page ......................................................................................................................................A-2 V Meetings in Prospect ..............................................................................................................................A-4 American Industrial Hyoiene Association Journal, published bi-monthly by the American Industrial Hygiene Association. Dohrman H. Byers, Editor in chief; Richard F. Scherberger, Editorial Manager; Kenneth W. Nelson, Kathleen Kumler, Dorothy B. Hood, Allen E. Dooley, and Thomas T. Mercer, Associate Editors. Editorial offices, 1014 Broadway, Cincinnati, Ohio 45202. George D. Clayton, Publications Manager. Business office 14125 Prevost, Detroit, Michigan 48227. The subscription price is $15 per year in U.S.A., and $16 elsewhere'. Single copies, when available, may be purchased from the business office at $3 per copy. Copyright 1966, by the American Industrial Hygiene Association. The Association reserves the right to edit all advertisements 'and to refuse advertising copy when it does not meet the high standards adopted by the Association. Library of Congress Catalogue No. 57-3191. Second class postage paid at St. Paul, Minnesota Printed for the American Industrial Hygiene Association by THE BRUCE PUBLISHING COMPANY, SAINT PAUL, MINNESOTA, 55114 * ' A-3 PLAINTIFF'S EXHIBIT Measurement of Dust Exposures in the Asbestos Textile Industry JEREMIAH R. LYNCH and HOWARD E. AYER Division of Occupational Health, U. S. Public Health Service, 1014 Broadway, Cincinnati, Ohio (g Data obtained from environmental surveys of nine asbestos textile mills, which represent the baseline for the textile segment of the Public Health Service epi demiological study of asbestos processing industries, are presented. From these data concentration ranges are derived which yield significant differences between typical sample groups. Variance ratio tests of different methods of counting and analysis were made and count weight ratios based on magnesium analyses for asbestos were calculated. Introduction characterizing the exposure had to be studied THE DIVISION of Occupational Health simultaneously. In order to accomplish this, of the U.S. Public Health Service has a large number of samples were required been conducting an epidemiological study (Table I). of the asbestos products industry since Janu ary of 1964. This study was prompted by the need to refine and update earlier work done in these industries in relation to asbestosis1,2 and to appraise the association of asbestos with other health hazards suggested by a number of investigators.3" The pro cedures and objectives of this study have been discussed in earlier papers2,8 and the relationships between impinger counts and fiber concentrations on membrane filters have The analysis of these data by manual methods would have involved an excessive burden and, consequently, the major por tion of the data were transcribed onto punch cards for computer processing. In addition to facilitating routine calculations of dust concentrations, computer processing made possible detailed statistical analyses which would otherwise have been prohibitively time-consuming. been presented.9 The environmental field study phase com menced with an asbestos textile plant in January, 1964, and the last of the textile plants to be included in the study was sur veyed in June, 1965, thus completing the asbestos textile baseline. The nine plants surveyed account for over 80% of the work This paper summarizes the baseline as bestos textile dust count data and gives some observations on sampling methods and strategy and count-weight relationships. Oth er data, resulting from analyses for poly nuclear aromatic hydrocarbons, trace metals, etc., will be included in later papers. ers in the industry and provided a cohort of over 2500 employees exposed to asbestos dust in varying degrees. The conduct of this study was somewhat more complex than previous studies of dusty trades by the Public Health Service in that both the exposure itself and methods of Table i Numbers of Samples Collected in Asbestos Textile Plants Breathing zone Personal General air Total Membrane Filter 608 810 478 1.896 Impinger 605 2 508 1,115 431 432 September-October, 1966 Table ii Mean Dust Concentration by Plant and Operation Impinger, mppcf Operation Plant A B C D E F G H I All* Fiber prep. Carding Spinning Twisting Winding Weaving All* 5.0 1.5 2.6 2.5 0.5 1.7 2.5 0.6 2.2 1.2 2.7 1.8 0.6 0.7 3.2 0.3 2.2 0.7 3.4 3.0 0.5 2.0 1.3 1.1 I 3 2.3 2.5 5.4 0.4 3.9 1.5 0.4 1.2 0.9 2.0 1.7 0.4 3.0 0.4 0.4 i.3 0.5 0.4 3.7 0.6 0.3 1.1 0.4 0.3 2.7 1.3 2.2 2.8 0.6 0.5 1.7 1.1 o.s 2.3 1.2 1.2 1.9 1.0 0.8 1.3 The <4Ali*' concentrations are the grand mean for all the samples in a plant or operation. Dust Concentrations Tables II and III present the mean dust concentrations by plant and principal opera tion in all the plants surveyed. In all cases, the median was also calculated, and was usually in close agreement with the mean. The impinger concentrations include both grains and fibers, and the counts were made in accordance with the American Confer ence of Governmental Industrial Hygienists (ACGIH) procedure. Thus, these values may be compared to the existing Threshold Limit Value of 5 million particles per cubic foot (mppcf). The membrane filter (MF) samples were counted using a procedure de scribed in detail by Edwards.10 Early workers in the study of asbestos ex posure elected to count grains as well as fibers since too few fibers were seen in im- Table rv Ratios of Fiber Counts by Method Membrane Filter >Impinger Optical Electron Microscope Total Total 3 U Total Fibers Fibers Impinger MF Total Fibers Electron Microscope >3/2 Electron Microscope Total 1 s (32) (100) 14 13 13 1 pinger samples. That fibers were not seen was not because they were absent, but be cause the 100X light field counting method used with the impinger did not reveal them. The ratios of fiber counts by different meth ods are shown in Table IV. The ratios given in Table IV were ob tained by dividing the concentration of fibers determined by the method given in the head ing, by the fiber concentration as determined by the method listed at the side for a large number of the same or comparable samples. For example, the total fiber concentration as determined by optical membrane filter counts at 430X phase contrast was eight times that found on comparable impinger samples counted at 100X light field. These ratios are not necessarily meant to indicate any degree of equivalence, but only to show the relative magnitude of the numbers of Table iii Mean Dust Concentration by Plant and Operation Membrane Filter -- fibers/cc Operation Plant A B c D EF Fiber Prep. Carding Spinning Twisting Winding Weaving All w (C) (A) (B) (C) (A) (B> (C) (A) (B) (C) (A) (B (C) (A) C (A) <B (C) 38.1 15.0 7.1 18.1 10.2 4.3 9.6 6.6 3.2 9.3 6.4 3.7 11.7 7.5 4.7 7.7 4.8 2.9 17.3 8.5 4.4 12.3 10.0 3.4 13.6 9.2 4.1 4.1 3.2 1.9 6.9 5.2 3.3 4.4 3.9 1.7 7.0 3.1 2.8 10.2 7.5 3.4 23.3 13.3 5.8 20.6 13.3 5.9 20.2 18.9 6.4 15.8 7.5 4.7 9.6 8.9 3.4 2.9 2.3 1.1 15.7 9.4 4.7 34.0 18.3 8.2 32.9 15.2 8.4 29.8 15.7 10.3 51.4 22.4 18.8 28.6 17.5 11.9 33.8 17.8 9.8 34.7 12.1 10.5 4.5 3.9 1.4 4.5 3.9 1.4 8.1 3.0 t.l 6.0 3.5 1.6 5.1 3.5 1.8 4.8 3.3 1.9 4.5 3.2 2.0 2.9 2.2 1.2 4.6 3.0 1.5 Note: A = total fiber*; B = > 5 fiber*; C = > 10 fl fiber* G 7.6 4.5 2.0 17.2 8.1 3.2 24.8 10.8 6.3 25.9 12.9 7.8 25.7 11.7 7.3 9.5 5.7 3.3 16.2 8.1 4.4 H 35.5 17.0 9.3 28.2 13.4 7.1 20.8 10.5 6.1 16.7 7.2 4.1 7.9 2.7 1.6 8.1 3.0 1.8 14.4 6.3 3.6 I 11.8 2.6 1.2 8.3 2.0 0.9 7.4 1.8 11 3.1 1.1 0.7 3.6 01..39 2.9 1.5 0.7 5.9 1.7 0.9 All 21 .2 7.6 4.2 14.9 7.0 3.7 12.3 6.2 14.0 6.7 4.7 9.9 4.6 3.4 8.0 3.6 12.5 5.6 3.5 American Industrial Hygiene Association Journal 433 Table v Significantly Different Dust Concentration Groups Group 1 2 3 Impinger isppcf Grains & Fibers .1- .5 .5-2.0 2.0-8.0 Membrane Filter Total Fibers >5 U Fibers >10// Fibers l- 5 s-zo 20-80 .7- 3 3.0-12 12.0-48 .3- 3.5 1.5- 6.0 6.0-24.0 fibers detected by the different methods. The ratios enclosed in parentheses were obtained by indirect comparisons since electron micro graphs of impinger samples were not pos sible. It is evident* that most of the fibers, even those longer than 3 /x, are not counted by either the impinger or the optical membrane filter procedure, probably because they are too thin to be detected by light microscopy. However, enough fibers are found on mem brane filters at 430X phase contrast to be a statistically useful index of the number of fibers present. Since it is probable that the significant biological property of asbestos is its morphology, only fiber counts on mem brane filters are presented. Regarding the interpretation of these data, some caution must be used to avoid assign ing excessive significance to small differences. Some method of categorizing relative dusti ness is needed for eventual correlation with health data to arrive at safe levels of ex posure. Given the inherent variability of the environment and the variability in dust counting, broad ranges or classes of dust concentrations will be required. From the standard deviations calculated for each plant and operation group of samples, it was de termined that a four fold difference in con centration was required to obtain a signi ficance level of p = 0.05 on typical sample sets. Based on this, the data fell rather conveniently into three arbitrary groups such that the probability of any value in a group not being significantly different from the middle value of an adjacent group does not exceed p=0.05 (see Table V). The ratios between corresponding groups by different count methods are based on the equivalence between impinger and filter count. The groups are designated by number rather than low, medium and high, to avoid the implication that they can be compared with any existing standard. Using the con centration ranges given in Table V, a chart of the group numbers by plant and opera tion was prepared (Figure 1). The upper left triangle contains the group number as determined by impinger and the lower right by membrane filter total fiber counts. In 75% of the cases, the two methods lead to the same estimate of relative dustiness, and in cases where the groups are different, the differences never exceed one group and show no consistent trend. Since our concern is with exposure, or body burden rather than with dust concen trations, a different grouping may be de rived by the method of Roach.11 It can be shown that: Coef. of Var. of Body Burden ----------------------------- ------------ < 0.59 Coef. of Var. of Concentration where t is the duration of the samples used in determining the concentration and T is the biological half-time of the contaminant. Since t never exceeded two hours and T is greater than six months, the ratio of co efficients of variation cannot be greater than 0.026. If the Coefficient of Variation of Body Burden obtained in this manner is used to estimate the standard error of the differ ence, ranges of concentration can be cal culated such that the body burdens caused by sample means falling within one range are significantly different (p<0.01) from those caused by concentrations at the mid point of an adjacent range (Table VI). The same procedure illustrated by Figure 1 was applied using the body burden groups. As would be expected from the narrower limits of the classes, the agreement was not as good, with only 50% of the sets of samples yielding the same group by both methods. However, in less than 10% of the sets did the appraisal of dustiness differ by more than one group, in no case by more than two groups, and again these differences showed no consistent trend. The general agreement between the groups gives one confidence that both methods are 434 September-October, 1966 Figure 1. Comparison of results by concentration groups. measuring approximately the same thing in asbestos textile plants. However, if in the case of either the body burden or concentra tion method of grouping, the membrane filter total fiber count arrived at exactly the same decision as to relative dustiness as the impinger count, there would be no basis for distinguishing between these dust measure ments by their relevance to disease param eters. Both would correlate, or not correlate, equally well with medical findings. Thus, the larger range of disagreement between methods in the body burden groups may be useful in establishing the relevance of the methods. By either the concentration or body burd en method, the results appear to be more uniform for different operations within a Table vi Significantly Different Dust Concentration Groups Body Burden Method Group 2 3 4 5 6 Impinger mppcf Graini & Fibers 0.1-0.2 0.2-0.4 0.4-0.8 0.8--1.6 1.6-3.2 3.2-6.4 Total Fibers Membrane Filter fibers /cc > sa Fibers > io n Fibers 1- 2 2- 4 4- 8 8-16 16-32 32-64 .6-- 1.2 1.2- 2.5 2.5- S.O 5.0-10.0 10.0-20.0 20.0--40.0 .3- .5 .6- 1.2 1.2- 2.5 2.S- S.O 5.0-10.0 10.0-20.0 plant than for the same operation in differ ent plants indicating that the engineering effort made toward good dust control within a plant is more of a determining factor than the difference between operations. Sampling Methods and Strategy All of the usual and some of the less fre quently used methods of sampling for min eral dusts in industrial environments were used in this study, and the large numbers of samples of each kind permitted com parison of these methods. One question which could be answered from these data is the relative variability of different count methods of analysis. To ex amine this factor, the variance ratio test12 was applied to the sample groups by different count methods in each plant and operation. These methods included impinger grains and fibers versus membrane filter total fibers, total fibers versus >5 p. fibers and total fibers versus >10 p fibers. In all cases, the variance ratio test indicated that the difference be tween the standard deviations by the differ ent methods was not significant (p>0.05) and that none of the methods could be con sidered less variable than the others. This finding is interesting in that it had been American Industrial Hygiene Association Journal 435 assumed that membrane filter counts under phase contrast might be less variable than light field impinger counts, and that long (>10 fi.) fiber counts might be less variable than total fiber counts since the long fibers should be more visible. To investigate this matter further, groups of samples on which magnesium analysis for asbestos were performed (to be discussed later in this paper) were compared with total fiber counts made on the same samples. Since the precision of the magnesium analyses (<5% error) is very good, com pared with dust counts, it was assumed that the variance ratio test would show less vari ability in the asbestos concentrations deter mined by magnesium analysis. However, this was not the case, and again the differences between standard deviations were not signi ficant. From this outcome, it may be as sumed that the sample variances are inde pendent estimates of the same environmental variance. Thus, the variability observed is actually the variability of the environment; that is, the variability of the exposures of a group of workers doing nominally the same job at different times and places. The vari ability in dust counting, large as it is, may not be significant when compared to the actual variability of the workers' exposure. Count-Weight Relationships Chrysotile asbestos, which accounts for over 90% of the asbestos used in this coun try, is a hydrated magnesium silicate and has a relatively constant magnesium content of about 25%. With the aid of recently de veloped atomic absorption techniques, it is possible to analyze for magnesium with a sensitivity in the order of 0.1 micrograms (/xgm). Consequently, magnesium analyses may be performed on membrane filter samples which are light enough to count, and this magnesium may be related to the weight of asbestos present on the filter. From the count and weight on the same sample, a direct count-weight ratio may be made. Several potential sources of error are im mediately apparent. Serpentine, the nonfibrous parent mineral from which chryso tile asbestos developed, is usually present in Figure 2. Distribution of count-weight ratios. varying amounts with the fibrous asbestos and will contribute some magnesium but will not contribute to the fiber count. A second source of error is the ubiquitousness of mag nesium and magnesium-bearing minerals. Magnesium is found in general air pollution, and significant amounts could be present in industrial dust from other magnesium-rich process materials. Little can be done to eliminate the first source of error, and a tendency to over estimate the mass of fibrous asbestos is un avoidable. Regarding the second source of error, air pollution survey data13 revealed that the usual atmospheric concentrations of magnesium are several orders of magnitude lower than in-plant concentrations. Other magnesium-bearing process materials can be eliminated as a source in textile plants since the other materials do not contain signifi cant amounts of magnesium. Membrane filters from which a small wedge had been removed for counting were sliced in half with a sharp blade against a 436 September-October, 1966 Table vii Count-Weight Ratios Type Count Total fiber* > 5 U. fiber* y id fJL fibers Fibers per ptgm 11.000 5.000 3,000 brass die. From analyses of magnesium on these filter halves, the count-weight ratios in terms of weight of asbestos, for each type count, were calculated for each of the 250 samples analyzed. The frequency distribu tions of these sets of ratios are given in Figure 2. Given the sources of error discussed above, and the natural variability of dust counts, a considerable spread in the count-weight ratios would be expected. This spread did occur, in fact, but a central tendency in each of the groups of data is apparent, and yield the approximate values shown in Table VII. It would appear from Figure 2 that the count-weight ratios of the longer fibers ex hibited less spread. However, the ratios were found to follow a log normal distribution with the same standard geometric deviation of 2.3 for all three type counts. Thus, each of the groups of ratios is equally disperse, and no one type of count is to be preferred as an estimate of mass. An independent estimate of the countweight ratio was obtained from size distribu tions on electron micrographs. Since the number of large fibers visible on these slides is small, though their contribution to the weight is very large, it was necessary to pool a number of electron micrographs of dif ferent samples, to obtain a statistically use ful size distribution. From these data a typi cal mass median fiber volume of 50 cubic microns (/a3) was obtained, which yields a count-weight ratio of 8000 fibers//rgm based on a density of 2.5 for chrysotile asbestos. This result is of the same order of magni tude as fiber ratios obtained from the mag nesium analyses. By applying the impinger-membrane filter equivalents obtained earlier to the weight count ratios given in Table VII, it was found that 1 mg/m3 of asbestos by magnesi um analysis as approximately equivalent to 1 mppcf by impinger. It should be empha sized that this relationship was developed by a very indirect method. However, it does indicate that asbestos mass concentrations may be higher than previously assumed. A number of sets of simultaneous samples were taken with and without size selective presamplers. Thirty-minute samples on a pair of membrane filters, one of which was pre ceded by a horizontal elutriator with a 7.1 fi unit density sphere cut-off, were taken in each operation in each plant. While not all of the magnesium analyses are completed, preliminary results indicate that the respir able fraction of asbestos as determined by this elutriation is approximately 50 to 60%. Ad ditional samples were taken in each plant using two pairs of membrane filters run for five to eight hours, with cyclones conform ing to the Atomic Energy Commission cri teria attached to one pair of filters. From these samples an estimate of respirable frac tion of from 30 to 40% was obtained. In both cases the figures given are approximate; however, they do follow the usual perform ance of these presamplers, in that the cyclone passed a smaller fraction. If these respirable percentages are applied to the gross airborne asbestos by magnesium results, 1 mg/m3 of the total dust would probably contain 0.5-0.6 mg/m3 of dust passing an elutriator and 0.3-0.4 mg/m3 passing a cyclone. Additional magnesium analyses are being performed so that mean mass concentrations by plant and operation can be compared with the groups established for impinger and membrane filter counts, to determine if mass concentrations yield the same relative hazard estimates. Conclusions The results of environmental surveys of nine asbestos textile plants reveal that dust concentrations as measured by impinger are generally below the Threshold Limit Value. Due to the natural variability of the environ ment, categories of significantly different dust measurements must be broad. Three signifi cant groups were obtained from concentra tion data and six significant groups were de rived from body burden considerations. Es American Industrial Hygiene Association Journal 437 timates of relative dustiness from membrane filter samples were in good agreement with those made from impinger samples by either concentration or body burden methods of grouping. Variance ratio tests of different systems of measurement indicate that most of the variance seen is due to the variance of the population and that none of the methods of counting exhibit any significant superiority in terms of a lessened variance. Count-weight ratios obtained by magnesium analyses of membrane filters were 3000 fibers > 10 fc/figm or 5000 fibers > n//igm or 11,000 total fibers/^gm. None of the sets of ratios exhibited any less dispersion than the others. A mass median fiber volume of 50 fj.3, obtained from electron micrographs, yielded a count-weight ratio of 8000 fibers/ figm which was of the same order of magni tude as the ratio obtained by magnesium analysis. Based on these ratios, a gross air borne mass concentration of asbestos equiva lent to one mppcf would be on the order of 1 mg/m3, and respirable mass concentra tions, as determined by elutriator and cy clone, would be in the order of 0.5-0.6 mg/ m3 and 0.3-0.4 mg/m3, respectively. Hygienic criteria should be relevant to the disease-producing mechanisms, capable of ac curate assessment of the environment, and convenient to use. The impinger is as con venient as any other method and in view of the large variance found in the environment, no less accurate than other measurements. How'ever, it is difficult to reconcile the bas ing of the hygienic criteria on counts dom inated by grains while the fibers are thought to be the causes of disease. In this respect the counting of fibers on membrane filters appears to have an advantage, especially in industries where asbestos is only a small frac tion of the airborne dust. The relevance of magnesium analysis for asbestos is unknown and the method involves the use of sophisti cated and expensive laboratory equipment not generally available in state, local and plant hygiene laboratories. However, it is hoped that the intensive study of total and respirable mass concentrations, made pos sible by this method, will lead to the develop ment of a relevant and convenient mass sampling method. Acknowledgments The authors wish to acknowledge the valuable contribution made by Mr. Richard E. Rinser who developed the method for magnesium analysis and by Mr. Thomas A. Brown, who performed a large part of the dust counting. References 1. Fulton, W. B., A. Dooley, J. L. Matthews, and R. L. Houtz: Asbestosis, Pennsylvania Department of Labor and Industry, Special Bulletin 42 (1935). 2. Dreesen, W. C., J. M. Dalla Valle, T. 1. Edwards, j. M. Miller, and R. R. Sayers: A Study of As . bestosis in the Asbestos Textile Industry. Public Health Bulletin No. 241, U.S. Government Printing Office, Washington, D. C. (1938). 3. Lynch, K. M., and W. A. Smith. Carcinoma of Lung in Asbestos-Silicosis. Amer. }. Cancer 24: 56 <1935). 4. Doll, R.: Mortality from Lung Cancer in Asbestos Workers, Brit. J. ihd. Med. 12: 81 (1955). 5. Mancuso. T. F.. and E. J. Coulter: Methodology in Industrial Health Studies. AMA Arch. Environ. Health 6: 36 0963). 6. Enterune, P. E.: Mortality Among Asbestos Products Workers in the United States. Annals of Sew York Acad. Sci. 132: 156 (Dec. 31, 1965). 7. Cralley, L. J.: Objectives and General Plan for Oc cupational Health Study of the Asbestos Products In dustry. Division of Occupational Health, Public Health Service, Department of Health, Education, and Wel fare (1962). 8. Lynch, J. R.t Asbestos Study--Procedures and Find ings. Transactions of the 27th Annual Meeting of the American Conference of Governmental Industrial Hy gienists (1965). 9. Ayer, H. E., G. H. Edwards, J. .f. Fanney, .Tr., and J. R. Lynch: Relationship of Impinger Counts to Fiber Concentrations by Membrane Filter in Asbestos Textile Plants. Delivered at the annual meeting of the American Industrial Hvgiene Association, Houston, Texas (1965). 10. Edwards, G. H.. and J. R. Lynch: The U.S. Public Health Service Method for Membrane Filter Enumera tion of Asbestos Dust. Unpublished (1966). 11. Roach, S. A.: A More Rational Basis for Air Sampling Programmes. Amer. Ind. Hyg. Assoc. J. 27: 1 (Jan.Feb. 1966). 12. Snedecor. G. W.: Statistical Methods. The Iowa State College Press. Ames, Iowa (1956). ,13. Division of Air Pollution: Air Pollution Measurements of the National Air Sampling Network 1953-1957. U.S. Department of Health. Education, and Welfare, Public Health Service. Cincinnati. Ohio (1958).