Document Em0VB5rqyGg7JyV2XZGJVv3Xn

............. ...... NjOJEJHS D jiMLHIHiD.__ NOT COM??HM.-RgS-F!LE$ -C_x t - ^ Reprinted from AMERICAN INDUSTRIAL HYGIENE ASSOCIATION JOURNAL Volume 27, Septcmber*Octot>er, 1966 Measurement of Dust Exposures in tlie Asbestos Textile Industry j JEREMIAH !L LYNCH and HOWARD TL AYER | Ditrmom of Occupational HtaUh, U* S. Public Health Strait*, 1014- Broadway, Cincinnati, Ohio Date obtained from environmental survey! ol nine asbestos textile milk, which i * represent the baseline for the textile segment of the Public Health Service epi <t demiological study of asbestos procming industries, are pteseuted. From these data concentration ranges are derived which yield significant differences between typical sample groups. Variance redo tests ol different methods of counting and analysis were made and count weight ratios based on magnesium analyses for asbestoa were calculated. Introduction characterizing the exposure had to be studied THE DIVISION of Occupational Health simultaneously. In order to accomplish this, at the U.S. Public Health Sendee has a large number of samples were required t beta conducting an epidemiological study(Table I). of the asbestos products industry since Janu The analysis of these data by manual * ary of 1964. This study was prompted by methods would have involved an excessive i the need to refine and update earlier work burden and, consequently, the major por f done in these industries in relation to as- tion of the data were transcribed onto punch bestosis1** and to appraise the association of cards for computer processing. In addition asbestos with other health hazards suggested to facilitating routine calculations of dust by a number at investigators.** The pro concentrations, computer processing made endures and objectives of this study have possible detailed statistical analyses which been discussed in earlier papers*** and the would otherwise have beat prohibitively i relationships between tmpinger counts and rime-consuming. fiber concentrations on membrane filters have J been presented.* This paper summarizes the baseline as The environmental field study phase com bestos textile dust count data and gives menced with an asbestos textile plant in some observations on sampling methods and January, 1964, and the last of the tactile strategy and count-weight relationships. Oth plants to be included in the study was sur er data, resulting from analyses for poly veyed in June, 1965, thus completing the nuclear aromatic hydrocarbons, trace metals, mjhmmtrm textile basejine. The nine plants etc, will be included in later papers. surveyed account for over 80% of the work 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 t Haatun of Stapltt Collected in Asbestos TexUlo Plants Or**OU<* m P*r**e*r Gaaarsl air Trial ii------ - nitfr Qft *10 47ft l,IH 40ft 2 soi I.Ilf TWS DOCUMENT WAS NOT A BFr-m ,nc- 431 f- 4 4 -i" ~r- 44 -? 1- BB 0020723 I 3215 NOTE: THIS DOCUMENT P'O NOT COM^ MiL:;Vi rr-J 432 SepUmber-October, 1966 Table n Ma*fi Dtuit Ceacratrotion by plant and Optratiaa Impmcvr* tnppcf Ob*i*tl* Plant A B e O K F G H 1 All* Fiber prep. CasdiAf folmung Twisting VLndmr Weaving Ail* 5.0 1J 2.4 2.5 0.5 1.7 2.5 0.4 2.2 1:2 2.7 1.3 0.4 0.7 3.2 0.3 2*2 0.7 J.4 3.0 0.5 2.0 1.3 1.1 1 3 2.3 2-3 5.4 0.4 J.9 1.5 0.4 1.2 0.4 2.0 1.7 0.4 3.0 0.4 0.4 1.3 0.5 0.4 3.7 0.5 0.3 1.1 0.4 0.3 2.7 l.J 2.2 2.3 0.4 1.7 1.1 0.5 2.3 1.2 1.2 1.9 1.0 0.3 l.J Hi* "AU" rnctiilrttittfti trt ih irimt Mn far *11 lb* unpl* in plini or p*rtiM. 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* enfce 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.1* 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 Ratio* of Fiber Count*, by Itethod flHn fcapiatar Optical Utroa Micreicop* To<i Total > JU TouU Flbin Fibor* Impinger btr Total Fibor* letri Mlcre*cnpa >3fl Electron Micro* cop* Total 1 1 (32} 4 1 Cl 29} 12 3 1 piiiger samples. That fibers were not seen was not because they were absent, but be cause the 103X 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 umber of the same or comparable samples. For example, the total fiber concentration as determined by optical membrane filter counts at 43QX 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 m Mm Dost Concentration by Pleat sad Operation Mnbtu Filter -- fibers /ce Tbw Prep. "--"i * Tvistlaf Winding 1*1 nil* All (4? <> <c> SSIi (Cl & (5) lc (A) (B) CC) (A) (1 ?C) (A) a5 } U) CB) c{ A 31.1 15.0 7.1 10.1 10.2 43 9-0 4.0 3.2 9.3 0.4 3.7 11.7 73 4.7 7.7 4.3 2.9 17,3 1-5 4.< B 12.3 10.0 3.4 13.3 9.9 4.1 4.1 3.2 1.9 4.9 JJ 4.4 3.9 1.7 jT!.i i.i 10.2 7J 3.4 e 23*3 13.3 5.3 20.4 113 5.9 20-3 13.9 4.4 15.3 7.5 4.7 9.4 4.9 3.4 2.9 21..13 19.7 9.4 4.7 D 34.0 13.3 3.2 32.9 15.2 3.4 29.3 15.7 10.3 51.4 22.4 13.3 28.0 17.5 11.9 53.8 17.3 9.3 34.7 12.1 10.5 Plm kr 8.1 3.0 1*1 0.0 3.5 1,4 5*1 JJ 1.1 4.3 5*3 1.9 43 3.2 2.0 4.1 2.9 3.9 3.2 1.4 1.2 4.5 4.4 2.9 *4 3 .0 a 7.4 4-f 2.0 17.2 8.1 3.2 24.1 10.3 3*3 25,9 12.9 7.8 25.7 11.7 7.J 93 5,7 1.3 14.2 f.l 4.4 H 35.5 17.0 9-3 28.2 13.4 7.1 20.1 10.5 4.1 14.7 7.2 4.1 7.9 2.7 1.4 4.1 5.0 1.8 14.4 4.3 54 t 11.3 2.8 1.2 3.3 2.0 0.9 7.4 1.0 l.L 3.1 1*1 0.7 3*4 1.3 0.9 2.4 1.5 0.7 5.9 1.7 0.9 All 21-2 7.4 4.2 14.9 7.0 3.7 13J 4.2 3.4 14.0 4.7 4,7 9.9 4.4 3.4 3.6 i.e 3-J 13J 5.4 3.5 THIS DpC.UMENT WAS NOT A RECORD OF PPG INDUSTRIES, INC. DID NOT COME FROM IT'S FILES AND CANNOT BE AUTHENTICATED BY. PPG INDUSTRIES, INC, BB 002QT24 i 3216 ftQIt: ihit? nrini M'-f. l\Oi COME FROM FPQ Fli$ American Industrial Hygiene Association Journal 433 Table v Significantly Dilfr*m Dust Concentration Croups The groups are designated by number rather than low, medium and high, to avoid the implication that they can be compared Oraap 1 3 3 awfeww Oral*. a r than .1* -3 .3-3.0 3,0--4.0 mmkrtnf Pitt** Total Flb*r > *^ no*** > ;a 4 Fiber* I- 5 3-30 30-40 .3- 3 3.0-13 13.0-44 .3- 1-3 t-S-- 4.0 4,0--34-0 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 "5 Co 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. 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 It is evident that most of the fibers, even body burden rather than with dust concen those longer than 3 p~, are not counted by trations, a different grouping may be de either the impinger or the optical membrane rived by the method of Roach." It can be filter procedure, probably because they arc shown that: too thin to be detected by light microscopy. However, enough fibers are found on mem brane filters at 43QX phase contrast to be a statistically useful index of the number of Coef. of Vr. of Body Burden /1 \ H* 1 " ^ 0-S9(--) Coef. of Var. of Concentration " 'T' 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. where t is the duration of the samples used in determining the concentration anti T is the biological half-time of the contaminant. Since t never exceeded two hours and T is Regarding the interpretation of these data, greater than six months, the ratio of co some caution must be used to avoid assign efficients of variation cannot be greater than ing excessive significance to small differences. 0.026. If the Coefficient of Variation of Some method of categorizing relative dusti Body Burden obtained in this manner is used ness is needed for eventual correlation with to estimate the standard error of the differ health data to arrive at safe levels of ex ence, ranges of concentration can be cal posure. Given the inherent variability of the culated such that the body burdens caused environment and the variability in dust by sample means falling within one range counting, broad ranges or classes of dust are significantly different (p<0.01) from erwmtT\um% will be required. From the those caused by concentrations at the mid standard deviations calculated for each plant point of an adjacent range (Table VT). and operation group of samples, it was de The same procedure illustrated by Figure a termined that a four fold difference in con 1 was applied using the body burden groups. centration was required to obtain a signi As would be expected from the narrower ficance level of p=0.05 on typical sample limits of the classes, the agreement was not sets. Based on this, the data fell rather as good, with only 50Jr of the sets of samples conveniently into three arbitrary groups such yielding the same group by both methods. that the probability of any value in a group However, in les than 10$o of the sets did not being significantly different Horn the the appraisal of dustiness differ by more middle value of an adjacent group does not than one group, in no case by more than exceed p"0.Q5 (see Table V). The ratios two groups, and again these differences between eonesponding groups by different showed no consistent, trend. count methods are based on the equivalence The general agreement between the groups between impinger and filter count. gives one confidence that both methods are THIS'DOCUMENT WAS NOT A RECORD OF PPG' INDUSTR E5, INC. D!U wO f COME FROM IT'S FILES AND CANNOi BE AUTHENTICATED BY PPG INDUSTRIES, INC. | BB 0020 725_| w . 3217 NOTE: THIS D0C`l?-'r''~ ' 0 ___ .__ _ _ NOLCODFiS i 434 Stpumber-OctobeT, 1966 * ! Fbvu 1, Comparison ol mult* by concentration jroupt. measuring approximately the same thing in plant than for the same operation in differ ; asbestos textile plants. However, if in the ent plants indicating that the engineering case of either the body burden or concentra effort made toward good dust control within tion method of grouping, the membrane filter a plant is more of a determining factor than total fiber count arrived at exactly the same the difference between operations. r derision as to relative dustiness as the im- pinger count, there would be no basis for Sampling Methods and Strategy distinguishing between these dust measures All of the usual and some of the less fre j ments by their relevance to disease param quently used methods of sampling for min eters. Both would correlate, or not correlate, equally well with medical findings. Thus, the eral dusts in industrial environments were used in this study, and the large numbers ; large- range of disagreement between methods in the body burden groups may be useful in wtahliAlng 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 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 test1* was applied to the sample groups by different Table vi count methods in each plant and operation. These methods included impinger grains and - IlC&ificaaUr Different Dost CooCentraticm Croups fibers versus membrane filter total fibers, total -t J Body Burden Unhod fibers versus >5 fi fibers and total fibers 0n* ml OnlM fc Ptftvra Tan rtwr. Film rihwrw />e >sp rtb*r ->10/4 Flbr* versus > 10 n fibers. In all cases, the variance ratio test indicated that the difference be tween' the standard deviations by the differ 1 *>4-3 i- J .4- IJ J- .4 ent methods was not significant (p>0.05) 7 7 OJ-4.4 0.--U.S 3- 4 4-- | ij- :.s 3.J-- *.0 .6- 1.3 2,3 and that none ef the m-thods could be con 4 s 0.4-1.4 1.4-1.3 9-14 14-13 5.0-10*0 io.o-:t.o 3.5-- 5.0 5.0-10. * sidered less variable tHa.-i tW others. This 4 J.3-4.4 13-44 30.0--40,0 ic.o-:o.o finding is interesting in. that it had been V7 THIS &OCUMENT WAS NOT A RECORD OF PPG INDUSTRIES, INC. DID NOT COME FROM | BB 00 20726 | --------------------------~ 3218 i IT'S FILES AND CANNOT BE AUTHENTICATED f BY PPG INDUSTRIES, INC. NOTEJUIS noOJMFWT-rvi NOT COME hiOfy. FrGc;,r American Industrial Hygiene Association Journal 435 assumed that membrane filter counts under phase contrast might be less v_' 'At than light field impingcr counts, and that long .(>10 ft) 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 i the variance ratio test would show, less vari j ability in the asbestos concentrations deter mined by magnesium analysis. However, this ; i was not the case, and again the differences I between standard deviations were not signi i ficant. From this outcome, it may be as sumed that the sample variances are inde i / pendent estimates of the same environmental variance. Thus, the variability observed is actually the variability of the environment; roc */** that is, the variability of the exposures of a group of workers doing nominally the same Fiouke 2. Distribution of count-weight ratios. job at different times and places. The vari ability in dust counting, large as it is, may varying amounts with the fibrous asbestos not be significant when compared to the and will contribute some magnesium but will t actual variability of the workers' exposure. not contribute to the fiber count. A second t source of error is the ubiquitousness of mag Cowt-Weight Relationships nesium and magnesium-bearing minerals. Magnesium is found in general air pollution, Chrysodle asbestos,' which accounts for and significant amounts could be present in ow 90% of the asbestos used in this coun industrial dust from other magnesium-rich i try, is a hydrated magnesium silicate' and process materials. bi has a relatively constant magnesium content of about 25%. With the aid of recently de Little can be done to eliminate the first vii veloped atomic absorption techniques, it is source of error, and a tendency to over pomible to analyze for magnesium with a estimate the mass of fibrous asbestos is un j sensitivity in the order of 0.1 mierograms avoidable. Regarding the second source of (/gm). Consequently, magnesium analyses error, air pollution survey data1* revealed may be performed on membrane filter that the usual atmospheric concentrations of samples which are light enough to count, magnesium are several orders of magnitude and this magnesium may be related to the lower than in-plant concentrations. Other weight of asbestos present on the filter. From magnesium-bearing process materials can be j i die count and weight on the same sample, eliminated as a source in textile plants since a direct count-weight ratio may be made. the other materials do not contain signifi * Several potential sources of error are im cant amounts of magnesium. ? i mediately apparent. Serpentine, the non- Membrane filters from which, a mall fibrous parent mineral from which chryso- wedge had been removed for counting were tile asbestos developed, is usually present tr. tlic'-d in half with a :harp blade against a THIS P'0'011 WENT WAS NOT A RECORD OF PPG INDUSTR.ES, INC. DID i JOT COME FROM IT'S FILES AND CANNOT BE AUThENTICAlED BY PPG INDUSTRIES. INC. __ _ | BB 0020727 J 323.9 ~NG-TE-:4H r NOT CQ?/ih rh'jiVi -I _ rr'u ri; 436 Septembtr^October, 1966 tAMLZ vn CoBHt*Wlht Ratios Tyv* Cm >>ToJ1t0--J1lJffllbitbUooomro* Fl*ri por ppm usS..s.o0oo0oo0 bras die. From analyses of magnesium on these filter halves, the count-weight ratios in term* of weight of asbestos, for each type count, were calculated for each of the 230 samples analyzed. The frequency distribu tions of these sets of ratios are given in Figure 2. Given the source 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 vn.yield the approximate value shown in Table It would appear from Figure 2 mat tb* 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 2J 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 **tim^* of m* An independent estimate of the countweight ratio was obtained {roar size distribudons on electron micrographs. Since the number of large fibers visible on these slides is snail, 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 (>i*) was obtained, which yields a count-weight ratio of 8000 fibers/jigm based on a density of 2Jb for chrysotile asbestos. This result is of the same onier of magni tude as fiber ratios obtained from the mag nesium analyses. By applying the impinger-mexnbrane filter equivalents obtained earlier to the weight count ratios given in Table VII, it was found that 1 mg/in* of asbestos by snagnesi um analysis as approximately equivalent to 1 rnppcf 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 nf membrane filters, one of which was pre ceded by a horizontal elutriator with a 7.1 n 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 605c- Ad ditional samples were taken in each plant using two paint 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 405c 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/m* of the total dust would probably contain 0.5-0.6 mg/m* of dust passing an elutriator and 0.3-0.4 mg/m* pasting 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- TH1S DOCUMENT WAS NOT A RECORD OF PPG INDUSTRIES, INC. DID NOT COME FROM IT'S FILES AND CANNOT BE AUTHENTIQAiED BY PPG INDUSTRIES, INC. |_f_0020728 7 3220 NOTE: TW nQCUJV!^ r 'ifi y NO] C0i\;L -n ` * .o American Industrial Hygiene Association Journal 437 ornate* of relative dustiness from membrane cated and expensive laboratory equipment filter samples ware in good agreement with not generally available in state,, local and those made from impinger samples by either plant hygiene laboratories. However, it is concentration or body burden methods of hoped that the intensive study of total and grouping. respirable mass concentrations, made pos Variance ratio tests of different system* sible by this method, will lead to the develop of measurement indicate that most of the ment of a relevant and convenient mass variance seen is due to the variance of the sampling method. 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 m/mSTM or 5000 fibers > /i/jigm or 11,000 total fibers/figm. None of the sets of ratios exhibited any less dispersion than the others. A mass median fiber volume of Acknowledgments The authors wish to acknowledge the valuable contribution made by Mr. Richard E. Kinscr who developed the method for magnesium analysis and by Mr. Thomas A. Brown, who performed a large part of the dust counting. 50 n*, obtained from electron micrographs, yielded a count-weight ratio of 8000 fibers/ ftgm which was of the same order of magni References tude as the ratio obtained by magnesium 1, Fvuw. W. B.. A. Doour, i. L. Utmavi. md K. L. Havre: AiIomdi. PramytvHia Dimbh ai analysis. Based on these ratios, a gross air Labor tad ladwirr. Sooeiol Eu)l:m 42 (1933). 2. Duun. w. C-. }. M. Dw- Void. T. 1. Eaw. borne mass concentration of asbestos equiva J. M. "" and R. R. Soreac: A Study *( dimiui ia tkt ddolu Tixtdt (adwlry. Public Hld lent to one mppcf would be on the order lulltln No, 241. US. Gownanu Friaua* Q&ca, Waobiaatoa, D. C (193S). 1 of 1 mg/m1, and respirable mass concentra S. Lracn. K- M.. iad W. A. Smith. Carriimai et Lain ia AibaHaMitoa. Amtt. ]. Camttr 24: S6 tions, aS determined by elutriator and cy iI90S). clone, would be in the order of 0.3-0.6 mg/ 4. Doll. R.: Martoiily ins Luo* Coacor ia Aibwoo Woriion. Brit. I. lad. UvC 12: SI (ISIS). m* and 0J3-O.4 mg/m1, respectively. J. Mamcubo. T. F., ad E. J. CovLreo; Mrtbodolw* ia Iadwcrial Hrolth Stadia*. ASIA Art*. Basvoa. Hygienic criteria should be relevant to the Htmltk 6; 36 ,1963). (. Ejmouid. P. .: Mortality Aiaour Atbruso Product* disease-producing mechanisms, capable of ac Worfcon ia ih* L'mtrd Sun. dooaili oI .Vm rk Arad. Sri. 132: 136 (Doc. 31. 1963. curate assessment of the environment, -and convenient to use. The impinger is as con 7. Ctalutt. L. }.: Obitrtitti cod Grorrai Pin lor Or* nWnM1 HrjJi* Slody ri tkt Atbtttl Prodaril ladwtry. Dmowo ol Ocnsctioac1 Hc-iiih. Puhln HeiitN venient as any other method and in view of Smict. Dmtnni M Hraitb, Iducatie*, rod Wri- (are (19*2). .. ,_ iA < i the large variance found in the environment, no lea accurate than other measurements. However, it is difficult to reconcile the baa 5, Lracu, J. R-: Albania Study Piaccdnui aad Find* ia*L iraararlm M tkt 22tk dooad klrrtnt *1 tkt domino CWrrrar* d Caooioiuonred Indmunti Uy limn (19*3). 9. Area, H- E-. C. II. Eawoaaa. J. J. Tuow, Ja- aad J. R. Lr Rrlaiiowhia oo lanonDotor C- auu (a ing of die hygienic criteria on counts dom Filar Caacmuabaao bo Monbraaa filar m Arbattaa inated by grains while the fibers are thought Touila Plant*. Drlircrtd u tka nuul rormon W Ik* Aamn Laduunai Hrnoao Auwabaa, Uwh, to be the causes of disease. In this respect (1^63) ia Eawutaa. C. H.. aad J. R. Lyhcx: Tba U.S. Public *- the counting of fibers on membrane filters appears to have an advantage, especially in u--llh Sirnri Mrtbad (or Monbran* Filin' 1 QUO*111boa ai Abnai Dan. Cauubbthtd (19*6). tl. Raoot, 5. A.; A Man Rarwaai Bow lar Air S--alloc Primalam dour. iad. Hit. dnar. J. 22: I (Jaar i industries where asbestos is only a small frac 1964| It SffYBno** Go W.s Stitistwmi ifrtA*4. 7k Un iott < tion of the airborne dust. The relevance of CcUtv* Pwm. Aintt. lew* (1?56). magnesium analysis far asbestos is unknown IX W Air Poihiooc: jp Mimmrrmtmit */ (fu Ait Ssmpiimt ,N>t3*ri, /AS3-/S57, U-5- and the method involves the use of sophisti tVpvrvMut oi H*itK Uwotw*, ud Wciisrt, Public Hoiib Service* Gtininm. ObM (1$6L | BB 0020729 J t 3221 f