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r is a follow-up on thirty-eight workers exposed from 1 to 24 years to average is Payoncentrationsgenerally between 5 and 50 ppm. but as highas 90 (peak of 140) case of one worker. One individual was diagnosed as suffering from mild benzene in Ihening and a small number of workers in the group had blood changes considered pisormaI, but blood dyscrasia or leukemia has not developed in 13 years after benzene rbnoNas terminated in 1964. The population studied was small, but the data, LI@ heless. would appear to support the former ACGIH TLV of 25 ppm. One case of Plleivlde;eesntzse,nheowepvoeirs,otnhiantgthreepmoarrtgeidn from benzene exposures of the order of 40 of safety may be small. The current ACGlH TLV ppm of 10 J'$seem~ reasonable in the light of our findings. PP ~enzeneexposure in the rubber coating industry - a follow-up L E O ~ ~DR. PDAGNOTTO, M.S.; HERVEY 8 . ELKINS, Ph.D." and HEINRICH G. BRUGSCH. M . D B Department of Labor and Industries,Division of Occupational Hygiene, 39 ~ ~ ~ l s Stt.o. nBoston, MA 02116 degree of exposure ssiveAN exposuresin wed within acceptable I PVC plants. introduction A study of benzene exposure in the rubber coatingindustry was published in 1961."' This is a follow-up on 38 workers in one plant from that study who were exposed for 1 to 24 years to benzene in concentrations that were generally between 5 and 50 ppm, but as high as 90 ppm (peak of 140 ppm) in the case of one worker. The paper contains: (a) Air analysis findings and phenol excretion studies performed between 1961 and 1973. (b) The results of blood tests made in 1961 and 1963during the benzene exposure period. (c) The results of blood tests made in 1973 and 1977, eight and 13 years after the use of benzene-containing solvents was discontinued. (d) The health or employment status of the workers in the original group. materials and methods Air samples for benzene were collected on silica gel contained in U tubes which was subsequently *Present address: Consultant in Toxicology and Industrial Hygiene. 303 Mill St.. Belrnont. MA 02178 Present address: Associate Medical Director, Prudential lnsuranca Medical Department, Boston, MA 02199 extracted with 2-propanol and analyzed by ultraviolet spectrophotometry, as previously d~scribed.'~'' Ultraviolet spectrophotometry was preferred over gas chromatography for our study because most of the components of naphtha do not absorb in the ultraviolet region and therefore do not interfere with the analysis and a positive identification can be made from benzene's ultraviolet absorption spectrum. Toluene was the only other aromatic solvent found in the naphthas and when it interfered with the analysis benzene was separated from the sample by a special technique.'2' Sampling time for individual air tests ranged from 10 to 30 minutes. Urine samples were obtained from workers at the end of their shift and analyzed for total phenol with diazotized p-nitr~aniline.'~T'he test was developed in our laboratory for this study. A paper describing the correlation between benzene exposure and phenol excretion was published in 1961 and can be referred to for details."' Samplingtime for air tests ranged from 10 to 30 minutes, but additional tests were performed under different exposure conditions in order to develop a correlation with urine phenol. Spreader and saturator operators made good subjects because their solvent exposure did not usually fluctuate markedly during a work- Copyright 1979, A m n u n Industrid HygieneAssociation American Industrial Hygiene ArsociationJOURNAL (4U)2/79 137 SATURATOR b0 I 1- I I I I II 1961 1962 1963 1964 1965 1973 -Figure 1 Urine phenol and indicated benzene exposure of saturator. run. It was found that urine phenol was a good Blood smears and differential counts on index of benzene exposure. A phenol glass slides (Wright's stain) concentration of 200 mg per liter in end of shift Platelets by the Fonio Method samples corresponded to an average benzene Spin hematocrits exposure of 25 ppm, the threshold limit value at the time of the 1961 study. Our findings were confirmed by B u c h ~ a l d ' ~ ) Hemoglobin, hematocrit, white blood Celland who reported that environmental benzene concentrations of 25 ppm would result in 195225 mg of phenol per liter of urine. Docter and Zielhuis"' found people exposed to 25 ppm benzene produced 170-195 mg per liter of urinary phenol while those exposed to 10 ppm red blood cell counts were performed in 1973 and 1977 with the Coulter automated system.A phase contrast microscope was used for platelets and differential counts. The findings by the old and new methods are comparable. Significant differences are not usually due to methodology, produced phenol concentrations of 70-80 mg per The male "norms" of the clinical laboratory liter. Sherwood and Carter"' reported that 25 ppm of benzene caused a urinary phenol of 170- performing the blood tests are as follows: 190. On the basis of this data, NIOSH selected a White blood cells X lo3: 7.8f3 level of 75 mg per liter of phenol in urine Red blood cells X lo6: 5.4H.7 collected at the end of a work shift to correlate Hemoglobin (gm): 16.W2 with a time weight average exposure of 10 ppm Hematocrit (%): 47f5 of benzene."' Platelets x io5: 2.59 (1.45 to 3.75) Blood samples were taken in 1961, 1963,1973 and 1977 from workers in the original group. Blood analyses were all performed by the same clinical laboratory. The 1961and 1963tests were conducted by the following procedure^:'^' Red blood cells in Hayem's counting fluid White cell counts in 1% acetic acid Hemoglobin by the Sahli method Differentials Eosinophiles: 2-59 Stab(neutr0philic): 2-54 Polymorphonuclear neutrophiles: 65-78 Lymphocytes: 20-259 Monocytes: 3-89 Basophiles: 0.59 138 Am. Ind. Hyg. Assuc. J (40) February, 1974 a 0L 623h a VI a0 x so w w -w4 :Z7S 5 itial counts iin) thod e blood celland ormed in 1973 iated system. A ied for platelets ings by the old )le. Significant 1 methodology. cal laboratory : follows: 7.8+3 5.43.7 16.0k2 47k5 (1.45 to 3.75) 2-596 2-5% des: 6575% 20-25% 3-895 0.5% February, 1979 r/ TABLE I Summaw of Air Test Findings Location Benzene (ppm) Saturator Spreader J&# l g 6 0 Churnroom Saturator Spreaders 1961 Saturator Spreaders SeP,ember.1961 Churnroom Saturator Spreaders pugust 1962 Saturator . Spreaders ppril, 1963 Churnroom Saturator Spreaders 68 5-27 12-17 57 7-25 90 20-25 14-44 22-23 19-36 10-40 3-20 6 33-43 7-39 gsember. 1963 JU~Y. 1964 September. 1969 Saturator Spreader Saturator Spreaders 38-140 17-35 15-67 0 January, 1972 August, 1973 Spreaders Saturator Spreaders 0 0 rubbercoating process Rubber coated fabric is produced in this process. A rubber formulation is first prepared by churning rubber stock with solvents for several hours. The resulting homogeneous mixture is then added to fabric using saturator and spreader machines. On the saturator, the rubber is allowed to penetrate the fabric as it passes through a light rubber mix. The spreader, however, applies a coat of rubber to the surface of the fabric. The thickness of the coating is adjusted with a doctor blade. The solvents used in the plant during the 1961study were naphthas containing 3% and 7.5% benzene."' Solvents introduced subsequent to 1964 and currently used are methyl ethyl ketone, mineral spirits and toluene which have been found to be benzene free. results and discussion Figure 1 shows urine phenol levels on the saturator operator (only one machine and operator on this job) between 1960 and 1973. He was on this job in 1957 and was still performing his task in 1977. From his urine phenol values, it was determined that his average benzene exposure was frequently above 25 ppm (the former ACGIH TLV) and as high as 90 pprn. The comparison of phenol levels with benzene exposure found on the left and right sides of Figure 1 was taken from the 1961 study.""' Air tests on the saturator taken during 8 plant visits between 1960 and 1964 are reported on Table I. The air samples were of 10 to 30 minutes duration. Although they do not represent time 7m- 600- m- U W 0 300z WI 200 100- CHURNROOM WJRIERS . .0 . I I -815 -75 -625 a 4. W a VI - 50 Wz YNz -375: w 4 25 ; Y -12.5 0I I I 1 I II 0 1960 1961 1962 I953 1364 196s 1373 -Figure 2 Urine phenol and indicated benzene exposure of churnroom workers. American IndustrialHygiene'M i a t i o n JOURNAL. (40)2/79 139 SPREADERS 87.5 75 r l i50 62.2 4 a -Wc .;, z ?jz2- -001 WW22 f WJ- -00 33 ! ~~ . 'e : i * .. .. '# . i - WzY 375; W m a -- 25 E% Iv-) w - 1122 55 i iI I I II lIoO 0 1960 1961 1962 1963 a64 1965 1973 -Figure 3 Urine phenol and indicated benzene exposure of spreader operators. weighted averages, it is clear that the worker experienced extremely high benzene exposures. This is confirmed by the phenol analyses which indicate his average benzene exposures. Because of the variable nature of their work, air tests were not frequently performed on churnroom workers. Personal samplers were not in general use in the early 1960's when this phase of the study was performed. Urine phenols, therefore, were relied upon for measuring benzene exposure. These findings are reported on Figure 2. The spread of phenol values was somewhat less than that observed for the saturator. Benzene exposures as measured by this procedure ranged from 10 to 60 ppm. Air tests taken on spreader operators (8 machines were nearly always in operation) showed that benzene exposures ranged from 5 to 39 ppm. See Table I. These samples were also of short duration, 10-30 minutes, but solvent exposures of spreaders did not vary too much. Air test findings agree with phenol results reported on Figure 3 which indicate average benzene exposures were usually below 40 ppm. Tables 11,111, and IV present complete blood findings on 38 workers in 1961 and 1963. They also include data on 17 workers from this group who were still on thejob in 1973 and 12 of the 15 who were still employed by the plant in 1977. Hemoglobin values of churnmen and spreaders are summarized on Table v. A number of these were below 13.5 grams in 1961 and 1963, but tests repeated in 1973 and 1977 showed no exposure related trend. Two hemoglobin values in the 11.0 to 11,s range on Table V belonged to churnroom worker C-14. All of his blood findings are lisw on Table 11. C-14was employed for 6yearsinthe churnroom, 1958 to 1964. For a year betwan 1963 to 1964, he suffered many episodes of n ~ j e bleeds. An examination by a physician found him free of complaints, but he was described appearing very pale. A diagnosis of mild benzene intoxication was made, and he was transferred to another job. His blood picture improved afterwards, and when seen in 1977 he did not complain of any health problems. From phenol levels reported on Table VI, on tests taken from 1/61 to 12/64, C-14'saverage benzeneexposum ranged from 20 to 60 ppm. In contrast to C-14, the saturator operator (SA) had much higher benzene exposurn reaching 140 ppm as indicated by one airsample taken in 1963. See Table VII. His blood pictun, except for the hemoglobin values, was considered normal. This worker was still on the job in 1977, operating the same saturator, and did not complain of any health problems when interviewed. 140 Am. Ind. Hyg. Assoc. J (40) February, 19fl .--- 8 .' 5 75 W a Y3) 0 io S YWz 1752 W m 0 ' ki! c YY) ?.5 irs. Jhurnmen and n Table V. A .5 grams in 1961 1 1973 and 1977 :nd. :he 11.0 to 11.5 to churnroom ndings are listed for 6 years in the a year between episodes of nose ?hysician found vas described as of mild benzene was transferred m r e improved 1977 he did not s. From phenol :eststaken from lzene exposures irator operator ene exposures I one air sample 5 blood picture, values, was was still on the saturator, and problems when TABLE II Blood Analysis Data on Chummen & Name RBC (lo6) WBC(10') Hb(gram8) HCT(X) Plate. (10') E S P L M E c-1 5.33 10.0 15.0 2 70 25 3 3/81 c-2 4.90 10.1 14.9 6 4 66 21 3 4.17 12.6 13.8 41 5 65 25 5 c-3 4.86 c-3 4.73 9.0 8.4 c-4 4.74 16.2 c-4 5.12 6.9 c-4 4.79 8.4 c-5 5.23 8.3 C-6 4.63 14.6 15.5 15.3 15.3 15.1 13.8 16.8 15.3 44 46 45 41 49 2.38 2.54 3.08 2.20 2 1 48 37 12 58 41 1 2 1 69 23 5 53 42 4 6 3 50 31 9 1 2 51 42 4 57 40 2 1 1 1 c-7 4.65 7.3 13.7 3 2 63 29 3 C-8 4.32 11.2 c-9 4.50 c-9 4.56 c-9 4.59 7.8 8.2 5.3 14.3 13.4 14.0 13.9 44 42 40 41 1.52 1.91 2.11 70 19 6 5 1 74 22 3 2 1 49 44 4 c-10 4.97 9.4 14.9 4 3 75 15 3 c-11 c-11 c-11 c-12 c-12 C-13 C-13 C-13 C-13 C-14 C-14 C-14 C-14 C-14 C-14 C-14 C-14 4.87 5.33 5.52 4.87 4.88 5.15 4.92 4.70 5.00 3.95 3.95 3.88 4.47 4.82 4.76 10.1 9.5 8.0 6.7 7.3 8.5 4.4 3.9 5.1 7.2 8.3 6.5 5.5 6.0 8.3 6.0 6.8 15.5 16.2 16.6 13.8 15.1 14.4 11.9 10.8 14.4 11.3 12.2 11.9 11.5 12.2 13.6 13.5 13.1 47 47 45 45 37 34 41 35 39 41 41 39 3.34 2.30 4.21 2.65 3.35 2.89 4.10 3.65 2 1 64 33 8 49 41 2 5 68 21 6 3 1 46 46 4 3 54 41 1 1 2 56 41 1 6 55 34 5 2 54 36 7 1 50 50 69 29 2 2 40 55 2 1 1 57 38 4 64 35 1 1 2 47 47 3 C-15 C-15 C-15 4.83 4.74 4.70 5.0 11.3 5.1 15.8 16.4 14.7 48 43 1.42 2.36 2 1 85 12 1 66 24 8 1 C-16 4.48 7.9 12.8 67 31 2 C-16 4.56 8.4 13.4 3 8 60 24 5 C-16 4.29 8.1 13.8 42 2.19 4 56 34 5 1 C-16 7.3 41 4.50 3 2 74 19 2 C-17 5.29 8.6 16.2 1 1 53 45 C-18 4.50 8.6 15.5 56 42 2 c-19 c-19 4.46 4.65 7.7 4.7 13.7 13.4 c-20I 4.25 8.2 1_2._2 2.09 3 1 59 35 2 5 45 4 4 6 3 1 5- 4 36 6. E = Eosinophiles S=Steb (neutrophilic) p = Polymorphonuclear neutrophiles L = Lymphocytes M = Monocytes 8 = Basophiles February. 1979 k a n Industrial Hygiene Associaban JOURNAL (40) 2/79 141 I. .. ... .- .-... - -- -- ~- .. ~ TABLE 111 3/61 s- 1 4.93 8.4 12.7 5/61 s-1 5.06 8.5 13.7 10163 s-1 4.64 6.6 15.8 8/73 s-1 4.59 8.1 14.9 3/61 10/63 s-2 s-2 4.93 4.60 8.6 11.6 15.8 15.3 10/63 8/77 s-3 5-3 4.68 4.86 6.8 7.1 15.3 14.9 3/61 s-4 5.30 8.4 15.8 10163 s-5 4.44 9.9 14.8 8/73 s-5 5.12 6.9 16.0 8/77 s-5 4.66 7.0 14.8 3/61 10163 8/73 S-6 S-6 S-6 4.85 4.34 4.96 10.0 8.9 7.6 14.9 14.8 16.6 3/61 s-7 5.1 1 8.3 15.8 10/61 8/73 S-8 S-8 4.80 4.67 5.9 4.0 14.2 15.1 3/61 s-9 5.01 10.2 15.5 3/61 10163 8/73 8/77 s-10 s-10 s-10 5-10 5.09 5.42 5.53 5.07 6.7 7.8 6.1 6.4 15.0 14.8 15.8 14.3 3/61 s-11 4.62 7.1 15.0 10163 s-11 4.53 6.1 14.3 8/73 s-11 4.78 6.1 14.9 8/77 s-11 4.92 7.2 15.1 3/61 s-12 4.43 8.0 14.6 3/61 8/73 8/77 S-13 S-13 S-13 4.35 4.72 4.68 8.0 12.7 10.3 13.8 15.1 14.7 3/61 S-14 4.62 7.2 14.0 3/61 S-15 5.12 7.3 15.5 3/61 S-16 4.66 5.1 12.5 5/61 S-16 4.59 6.4 12.3 10/63 S-16 4.32 6.7 12.6 8/73 S-16 4.76 5.0 13.0 8/77 S-16 4.63 5.2 12.5 3/61 S-17 4.43 8.4 14.3 10163 8/73 8/77 S-17 S-17 S-17 4.59 5.06 5.04 9.6 8.1 10.3 14.3 15.5 14.9 ~ ~~~ ~ E = Eosinophiles L = Lymphocytes S = Stab (neutrophilic) M = Monocytes P = Polymorphonuclear Neutrophtles 8 = Basophiles 48 43 46 47 44 44 47 43 46 48 43 47 43 42 43 43 44 45 42 40 38 45 1 2.19 5.46 3.41 2.44 1.31 3.22 2.76 1.94 2.82 1.99 3.25 2.49 3.20 2.00 2.37 3.13 2.40 74 2.0" 3 1 4 55 37 2 7o 26 75134, ,49 44 4 ' 3 64 31 1 60 39 1m363 4 65 29 z 3155401 216433 69 29 2 4 65 3O 1 68 3O 2 225240~ 1 50 36 13 1 59 36 2 8 1 38 42 10 I 5 2 49 38 6 2 2 56 36 4 3 58 36 3 5 74 20 1 2 4 72 19 3 1 3 57 37 2 2260333 2 53 42 3 1 4 59 28 7 1 55 42 1 1 41 58 1 58 36 4 68 32 62 37 1 1160344 1264321 . .. 142 Am. Ind. Hyg. Assoc. J (4OJ February,1979 Amencan !a1 2 49 3 20 6 1 j 37 4 1 26 2 347 '44 . 31 1 39 ) 36 4 1 3 ' 25 ' 43 I 42 6 1 2 I 29 2 1 1 1 41 33 29 30 30 4 2 1 2 4 36 13 36 2 42 I O 38 6 36 4 36 3 20 1 19 3 37 2 33 3 42 3 28 7 42 1 58 36 4 32 37 1 34 4 32 1 2 1 1 1 1 February, 1979 k u n Industrial Hygiene Association JOURNAL 140) 2/79 .. . . i143 m bt v) m 0 Ow vO )v N) m" $ , o o 144 00v)r. oom- oowom OOUJN OdOO z2 X eX s0 m 0m a U Am. I d . Hyg.As&. J (40) February, 1919 7; cod' 4.10 -r COU' is se two and rept 197 in 8 reti em1 -I and ben are rels -I or I OUI of ber Pla co1 65 we bet dit di: El ex hl lbm -,.. .* .- , .* TABLE X White Blood Count vs Benzene Exposure 1961 or 1963 1973 1977 Workor Benzene (ppm) WBC (10') WEC (10') WBC (10') Employment Status c-2 15-40 12.6 `n.d. n.d. Retired c-4 25 C-6 _-- 16.2 14.6 6.9 8.4 Employed by plant n.d. n.d. Employed elsewhere c-8 c-11 c-15 s-2 30 15-20 35 15.33 11.2 10.2 11.4 11.6 n.d. n.d. Died, heart disease ( 1 972) 9.5 8.0 Employed by plant 5.1 n.d. Retired n.d. n.d. Employed elsewhere s-9 s-13 s-17 20-30 --- _-- 10.2 n.d. 9.6 n.d. 12.7 8.1 n.d. 10.3 10.3 Retired Employed by plant Employed by plant *n.d. = no1 determined. February, 1919 Table VI11 summarizes the red blood cell couafi on the workers. One count was below 4,~oo,ooa0nd it was on C-14 mentioned earlier. Table IX shows that 3 workers had white @UntS less than 5,000. From Table I1 and 111,it that C-19's count was 4,700 in 1963, and noothers, C-13 and S-8. had counts of 4,400 4,800 respectively. C-13's count was s p t e d in 1974 and found to be 3,900, but in 1977 it was 5,100. C-19 retired in 1969reportedly ingood health and was still alive in 1977, S-8 is &d because of poor eyesight, and C-13 is still mployed in the spreader room. Ten workers had white counts above IO,OOO, a d this information along with available benzene exposure data and employment status an summarized on Table X. No exposure nlated trend was noted. Table XI summarizes the benzene and health or employment information on the 38 workers in our study exposed to benzeneup to 1964. Fifteen of the workers who have had up to 21 years of benzene exposure were still employed by the plant in 1977. When interviewed, they did not amplain of any health problems. Five retired at 65 years of age reportedly in good health and were still alive in 1977. Four retired early bccause of disabilities and three others have died: one from lung cancer, another from heart base and the third from coronary sclerosis. 11 Eleven of the group left the company. With the exaption of C-5 who is in his eighties, they are employed elsewhere and were said to be free of any serious health problems. summary and conclusions Rubber coating workers in this study were exposed for 1 to 24 years to benzene at concentrations generally between 5 and 50 ppm, but one worker's average exposure was as high as 90 ppm (peak 140 ppm). One case of mild benzene poisoning was reported, but the worker recovered rapidly when he was transferred to a benzene freejob. Some of the other workers had blood pictures which were considered slightly abnormal. The hemoglobin of one worker(S-16) remained low (12.5 grams) and unchanged from 1961 to 1977. No blood dyscrasia or leukemia, however, has as yet been reported in the group 13 years after benzene exposures were terminated in 1964. While it is true that the population studied was small, the data nevertheless would appear to substantiate the former ACGIH TLV of 25 ppm since no significant abnormal blood findings were associated with lower exposures. The margin of safety of the 25 limit, however, appears to be small because one case of apparent mald benzene poisoning developed in a worker exposed for only six years at concentrations of the order of 40ppm. The recent reduction to the current ACGIH TLV of 10 pprn would seem reasonable in the light of our findings. h n lnd&ial Hygiene Association JOURNAL (4'8 2/79 115 .. .. . . . TABLE XI Years Exposed t o Benzene and Health or Employment Status A W Senrow Worker (1977) Exporum ( y r t . 1 Empkymmt Status (1977) SA s-1 s-3 s-5 s-10 s-11 s-12 S-13 S-16 s-17 c4 c-9 c-11 C-13 C-14 45 41 38 54 41 41 42 49 51 52 45 55 49 51 42 . 2 8 4 3 6 10 9 8 21 9 2 4 6 10 7 Still employed by plant s-9 c-2 c-10 C-15 c-19 78 69 70 65 73 9 18 11 9 19 Retired S-6 S-8 c-12 C-16 65 59 58 46 24 6 4 12 Retired at 63, heart disease Retired at 57, legally blind Retired at 58, emphysema Retired at 44, hip injury 54 S-15 C-8 17 Died, coronary sclerosis in 1972 at 72 14 Died, lung cancer in 1975 at 71 13 Died, heart attack in 1972 at 64 s-2 s-7 S-14 c-1 c-3 c-5 C-6 c-7 C-17 c-18 c-20 44 54 41 43 51 81 58 37 37 37 67 6 2 7 6 6 1 2 2 6 2 9 Left 1963, employed elsewhere Left 1962 Left 1961 Left 1965 Left 1975 Left 1961 retired Left 1965 employed elsewhere Left 1962 Left 1961 Left 1961 Left 1977 references 1. Pagnotto. L. D., H. B. Elkins, H. 0. Brugschand J. E. Walkley: Industrial Benzene Exposure from Petroleum Naphtha: 1. Rubber Coating Industry. Am. Ind. Hyg. Assoc. J. 22:417 (1961). 2. Elkinr, H. 6.. L. D. Pagnotto and E. M. Comproni: The Ultraviolet Spectrophotometric Determination of Benzene in Air Samples Adsorbed on Silica Gel. Anal. Chem. 34:1797 (1962). 3. American Conference of Governmcmtal Industrial Hygienists: Committee on RecommendedAnalytical Methods. Manual of Recommended Methods. 4. Walkley, J. E.. L. D. Pagnotto and H. 6. Elkinr: The Measurement of Phenol in Urine as an Index of Benzene Exposure. Am. Ind. Hyg. Assoc. J. 2 2 3 6 2 (1961). 5. Buchwald. H.: The Expression of Urine Analysis --Results Observationson theUse of SpecificGravity Correction. Ann. Occup. Hyg. 7:125 (1964). 6. Doctor, J. H. and R. L. Zidhuir: PhenolExcretionas a Measure of Benzene Exposure. Ann. Occup. Hyg. 10:317 (1967). 7. Sherwood. R. J. and F. W. 0. Carter: The Measurement of Occupational Exposure to Benzene Vapor. Ann. Occup. Hyg. 13:125 (1970). . . .8. C r i t e r i a f o r a r e c o m m e n d e d standard OccupationalExposure fo Benzene. HEW Publication NO. (NIOSH) 74-137. 9. Grulwohl, R. 6. H.: Clinical L8boratory Methods and Diagnosis. Vol. 1, 3rd Ed., C. V. Mosby Co.. 9. Louis (1943). A W U 3~1, i s m 146 Am. Ind. Hyg. AsaK. J (40) February, 1919