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DATA SHEET D-308 (D-Chem 47) Benzene (Benzol) Published by National Safety Council 425 North Michigan Avenue, Chicago 11 1. Benzol is the common name applied to the technical grade of benzene (C#Ha), an organic liquid generally obtained by fractional distillation of coal tar.* 2. Benzene is supplied commer cially in five grades according to the amount of impurities present. These are reagent, thiophene free, nitration, industrial pure, and in dustrial 90 per cent. Production employees are exposed to benzene in crude material and in the coal tar light oil from which it is dis tilled, as well as to these five grades. Characteristics and Uses 3. Benzene in its purest form is a pure colorless liquid with a rather pleasant aromatic odor somewhat like that of gasoline but readily distinguishable from it. The intensity of the odor varies considerably with the grade or purity of the benzene, but a per son with a normal sense of smell can detect the vapor by odor in a concentration of about 75 to 100 ppm. 4. However, in an atmosphere icontaining benzene vapor the sense of smell quickly ceases to respond to that the odor is a very unre liable indicator of the concentra tion or even of the presence of benzene vapor. 5. Pure benzene boils at 80.1 C (176.2 F) and freezes at 5.5 C (41.9 F). The specific gravity at 20 C compared to that of water at * The name benzene must not be con fused with the name benzine. The latter is still sometimes applied to a mixture of hydrocarbons obtained from petro leum, but it is archaic and misleading and should not be used. This Data Sheet is one of a series published by the National Safety Council, re flecting experience from many sources. Not every acceptable safety procedure in the field is necessarily included. This Data Sheet should not be con fused with American Standard Safety codes, federal laws, insurance requirements, state laws, rules and regulations, or municipal ordinances. 20 C is 0.88 for the liquid, and the vapor density compared to that of air is 2.73. Its flash point is ----11.1 C (12 F), and the explo sive limits are 1.5 to 8 per cent by volume in air. The ignition temperature is 538 C (1000 F). 6. The major impurities in com mercial benzene are toluene and xylene. These substances are toxic but somewhat less volatile than benzene and somewhat less hazard ous. Because the appearance and pleasant smell of benzene give no warning of its toxic effect and be cause a victim may become in curably poisoned before he feels ill, benzene is one of the most in sidious poisons ever to find wide industrial use. 7. Benzene is an excellent sol vent for fats, oils, resins, gums, rubber, and many other organic substances. It is therefore used to extract these materials from aque ous solutions or crude mixtures. It is used also in paints, varnishes, lacquers, adhesive cements, and paint removers. 8. In many of these applications its presence is often not declared upon the label, contrary to the agreement between the manufac turers and the -United States Pub lic Health Service. 9. The manufacturers of ben zene under1 this agreement are obligated to apply labels like that shown below, to warn users against its hazardous character istics. Under the agreement, simi-, lar labels must be applied to products for domestic consumption which contain 15 per cent or more of benzene.1 BENZENE (BENZOL) Danger! Extremely Flammable Vapor Harmful--Poison Keep away from heat, sparks, and open flame. Keep container closed. Use only with adequate ventilation. Avoid prolonged or repeated breathing of vapor. Avoid prolonged or repeated contact with skin. 10. In the past, benzene has been used in large quantity in blends with gasoline for automo tive fuel. The need for benzene as the starting material for an enor mous variety of synthetic chemi cals may largely eliminate its use as motor fuel. 11. A partial listing of indus tries and trades in which the use of benzene may bo expected is given in paragraphs 12 through 21. The trades named are examples of those in which precautions against benzene poisoning should be taken. Each operation which uses benzene must be carefully studied, and precautions must be based on the specific conditions of use. 12. The coke and gas industry is the major source of benzene. Workers who may be exposed are: benzene still men by-product operators laboratory workers tank inspection and maintenance men drum fillers tank car loaders Copyright 1950 by National Safety Council SL 029^29 13. The chemical industry con sumes a large amount of benzene. Workers who may be exposed are: nitrobenzene makers aniline dye workers explosives workers nitrocellulose workers phenol makers pharmaceutical workers denatured alcohol workers maintenance men or others working in the benzol area 14. Because benzene is an excel lent solvent for printer's ink, the printing and lithographing indus try commonly uses benzene as a solvent and cleaner. Those who may be affected are: layout men (from rubber cement) lithographers mimeograph operators multigraph operators photoengravers pressmen and helpers typesetters and cleaners 15. The paint industry uses benzene as a solvent in paints, especially quick-drying enamels and asphalt paints. Those who may be exposed are: rsphalt paint makers enamel makers enamelers lacquer makers varnish makers varnishers stain makers lacquerers paint makers paint remover makers shellac makers Stainers airplane dope makers and users painters 16. The rubber industry, es pecially those branches which irfake rubberized fabrics, has many operations in which there will be serious benzene poisoning unless complete protection is provided. Many rubber companies have sub stituted other solvents for benzene in the applications which are most difficult to safeguard. Workers who may be exposed to benzene are: raincoat makers cementers (rubber) compounders (rubber) dryers (rubber) reclaimers overshoe makers rubber sheet makers oilcloth makers vulcanizers rublkcr cement makers 17. The dry cleaning industry uses benzene to some extent, es pecially for "spotting." Dry clean ing workers and spotters may be exposed. 18. The adhesives industry in corporates benzene into certain glues and cements, usually those stated to be waterproof. Glue workers and adhesive workers may be exposed. Those using glues and cements which contain benzene should get a full description of their toxic hazards and set up suit able precautions for their use. 19. The petroleum industry blends benzene with fuels and uses it to extract certain products from petroleum oils. Those who may be affected are blenders (motor fuel) and paraffin wax makers. 20. Laboratories use benzene in various kinds of analytical work. Chemists and technicians may be exposed. 21. The coatings industries commonly use a great deal of ben zene as a solvent. Workmen en gaged in the production of arti ficial leather, patent leather, oil cloth, linoleum, and shade cloth may be exposed. Benzene Poisoning Acute poisoning 22. Causes and characteristics. Benzene poisoning may be acute or chronic. The acute form is in curred most commonly when men enter enclosed spaces such as tanks which contain very high concen trations of benzene vapor. It may also be caused by benzene stored in closed rooms. This form of poisoning is rare, and its dangers are more widely recognized and more immediately apparent than are those of chronic poisoning. 23. In acute benzene poisoning the symptoms result from the powerful anesthetic action of the vapor. The victim usually has little warning after entering the con taminated atmosphere. Although the odor may be very strong, it is not sufficiently irritating to force withdrawal from the closed space. 24. By the time the victim be gins to feel dizzy and unable to control his movements, he is Usu ally too weak to escape. He stag gers, appears drunk, falls to the ground and rapidly becomes un conscious. If not rescued, he be comes more and more deeply anesthetized. In a short time, de pending on the concentration of the vapor, his breathing becomes gradually slower until the respir atory center is paralyzed and death results. 25. If the victim is rescued be fore death occurs, he may be in any stage of unconsciousness or apparent drunkenness. He should be removed to fresh air, and if breathing has ceased, artificial respiration should be begun at once. A physician should be called immediately and oxygen admin istered as soon as it is available. 26. Recovery will exhibit the stages of acute poisoning in re verse. From a state of coma, the victim will progress to movements, muttering, and struggling. Upon regaining consciousness he will tend to become excited, irrational, and stubborn. He must be re strained at this time from run ning about. Dizziness and head ache are the last symptoms to disappear. Medical supervision of all such cases is imperative to pre vent severe after-effects, 27. Prevention. Workmen should never enter closed spaces where benzene vapor may be pres ent until appropriate tests have shown that the air is safe to breathe. As an additional pre caution, workmen entering such closed spaces should be equipped with hose masks or self-contained oxygen breathing apparatus and should wear safety belts with life lines. 28. The respirator and life line should be used even though the tank or other enclosed place has been thoroughly steamed out and has been shown by the benzene- SL 029430 detecting instrument to be free from vapor. Even after steaming out, there may be a residue of ben zene in sludge, under scale, or in crevices which will make the air iii the tank toxic again when it is stirred up. 29. It is extremely important that the tank be properly cleaned and steamed before anyone enters it The steps which should be taken are these: a. See that there ate no open flames,, sparks, or other sources of igni tion in the vicinity. b. Use non-sparking tools. c. Disconnect and blank off all pipe lines. d. Disconnect agitators. e. Connect a steam line to the tank, and steam until the temperature is above the boiling point of ben zene throughout the tank. f. Connect a water line to the tank and flush with cold water at high pressure. g. Ventilate the tank and test for vapors before entering. 30. When the tank is entered, the air supply equipment and life line of the men going into it should be attended by a man out side who has no other duties. 31. Waste benzene should not be discharged into sewers nor boiled off and released into the open air. It should either be distilled and recovered or burned in the open with precautions against uncontrolled fire or ex plosions. 32. Discharge into sewers may result in the accumulation of toxic or explosive quantities of ben zene vapor in the sewer itself or in tunnels, work rooms, or other closed places drained by it. Dis charge into the open air in the presence of adverse winds may lead to explosive or toxic condi tions in adjacent buildings. 33. The same hazards will be encountered if benzene stills are operated beyond the capacity of their condensers. Vents on such stills, as well as on all storage tanks and other containers, should be equipped with flash arresters and should extend high enough above the nearest building that vapor cannot find its way to an inhabited place. Chronic poisoninc 34. Causes and characteristics. Chronic benzene poisoning occurs as the result of repeated or longcontinued exposure to amounts of vapor smaller than those which produce acute poisoning. Many safety codes set maximum safe limits of benzene vapor as between 50 and 100 ppm. However, some very sensitive individuals can probably be harmed by smaller quantities of vapor after a long exposure. 35. Chronic benzene poisoning affects the blood and blood-form ing organs, producing serious de generation in the bone marrow. When slightly damaged by poison, the marrow can still regenerate after removal from the toxic agent When the damage is severe, no known treatment will restore the marrow to normal or make it able to perform its vital functions of furnishing red blood cells, white blood cells, and blood platelets. 36. There is no way to deter mine the sensitivity of an indi vidual beforehand. Cases of fatal poisoning have been recorded from very light exposures to ben zene. Some individuals, on the other hand, are apparently well after prolonged and heavy expo sure. 37. Some of this variation may be due to the very considerable difficulty in determining benzene concentrations in the air as well as to individual differences in sus ceptibility. In any case, safe prac tice dictates that any. employee who shows signs of benzene poi soning should be transferred to work in which he, will, have no exposure. 38. Chronic benzene poisoning may be detected early by signs of absorption or late by symptoms of poisoning. Early benzene poi soning is indicated by changes in the composition of the blood, as determined by blood counts. 39. Such a blood examination should include a red cell count, a white ceil count, a differential count, a measurement of hemo globin (preferably by the Sahli or other accurate method; the Tallquist method is not accurate enough for this purpose), a hematocrit measurement, and a determination of mean corpuscu lar volume. 40. Medical authorities differ as to what change in the blood count is first found in chronic benzene poisoning. Possibly the change which occurs first varies from one individual to another. Some authorities believe that changes in the size and number of the red cells appear first, followed by a decrease in the number of white cells and a shift in the percentage of granulocytes, and still later by a depression of red and white cells and platelets. 41. In general, signs which should arouse suspicion of early chronic benzene poisoning in ex posed individuals are the follow ing: a. Depression of a previously normal red cell count to below 4.5 million. b. Depression of a previously normal white cell count to below 6,000. c. Depression of a previously normal hemoglobin to below 12 grams per 100 cubic centimeters or 80 per cent by colorimeter. d. The appearance of immature or abnomal red or white cells in the differential count. 42. Prevention.. Pre-placement blood counts should be made, and if there is continuous or repeated exposure to benzene vapor, the counts should be repeated at in tervals of from one to six months. 43. These counts should be so recorded that previous counts for an individual can be readily com pared with the current count. The recording system should also per mit easy calculation of the degree of normal variation among all the individuals in the exposed group and should permit the supervis ing physician to obtain a sum mary of conditions in the entire group for any period. $L 029431 3 44. An experienced industrial physician should examine and evaluate all blood count results because many diseases and condi tions not connected with benzene exposure, or with any industrial exposure, may cause changes in blood counts. 45. Chronic benzene poisoning can be prevented by a combined program of medical supervision, industrial hygiene, and safety or ganization. The industrial hygien ist can furnish methods for test ing the air throughout the plant to detect harmful concentrations of benzene vapor and can suggest the best methods of handling the material to prevent these expo sures. 46. The safety organization can train workmen to avoid exposure and can acquaint them with the dangers of the chemicals so that they will promptly report any hazardous conditions to those charged with controlling them. 47. The medical supervisor can watch over the physical condition of the workmen through periodic examinations. These examinations will detect locations or processes where precautions are inadequate. No method of prevention can suc ceed without cooperation among all departments of the organization and on the part of the workmen, who must understand the reasons for the precautions which they are to observe. 48. The medical supervisor should not only study periodic blood counts but also give pre placement examinations and rou tine re-examinations. The preplacement examination should prevent persons with blood dyscrasias, severe liver or kidney dam age, or lung or heart disease from having contact with benzene. 49. Pregnant women and young persons in general are believed by many to be unusually susceptible to benzene poisoning. At the dis cretion of the examining physi cian, they may be excluded from work requiring contact, with ben zene. 50. New workers who have al ready had contact with benzene or other agents likely to cause dam age to the bone marrow (such as TNT, x-ray, or radium) -should receive special scrutiny since they may already be suffering from mild, damage. 51. New workers who show pre placement blood counts in the sus picious or abnormal range should he excluded from contact with benzene if only for the practical reason that their blood counts could not be reliable indices of bone marrow damage. 52. Some benzene is eliminated unchanged in the urine. Some is oxidized in the body to phenols and diphenols which, in turn, are conjugated in the liver with sul fate ions and excreted in the urine, thus increasing the ratio of ethe real cr organic sulfates to total sul fates in the urine. This is the basis of a dependable control test to determine the severity of the over all daily exposure to benzene. 53. The ratio of inorganic sul fates to total sulfates in the urine is normally 85 per cent or above. Upon exposure to benzene this rat:o is decreased, and the decrease is related quantitatively to the se verity of the exposure to the point where nearly all sulfates are elimi nated as organic sulfates. 54. Ratios of less than 70 per cent inorganic are abnormal and may be indicative of benzene expo sure ; they warrant investigation of exposure sources with a view toward correction. Ratios of 60 per cent or less inorganic indicate dangerous exposures warranting immediate correction and termi nation. Concurrent exposure to carbon tetrachloride, or any other material having an adverse effect upon the liver, may decrease the sulfate response so that ratios on the order of 70 per cent inorganic would indicate exposures of great significance. 55. The sulfate test must be made while the worker is on the job. It is not to be considered a method for diagnosing benzene poisoning, because the changes are merely indicative of exposure, not of poisoning nor of damage. 56. It is therein that the great preventive value of this test lies. The indication occurs in advance of demonstrable harmful effects, hut forewarns of their coming if exposures are not reduced. 57. It seems logical to make urine sulfate tests, perhaps weekly, and periodic hematological studies of all workers coining* in contact with benzene vapors. Air analyses for benzene in suspected atmos pheres should be made frequently. 58. If these three control meas ures are consistently performed, and heeded, no fear need be en tertained regarding use of benzene in industrial processes. Skin Contact 59. Hands and arms should never he immersed in benzene or benzene solutions, nor should ben zene be used to wash or rinse grease or other materials from the skin. Rubber gloves used to pro tect the operator should be fre quently inspected for leaks. Pro tective creams insoluble in benzene hut soluble in water may be used on exposed skin surfaces. 60. Benzene almost instantly dissolves fats and natural oils from the skin. It may cause reddening, dry scaling, and cracking, and pre dispose (he skin to secondary in fections. More prolonged contact may result in injuries resembling first and second degree burns. 61. Wetting of a large area of the skin with benzene is extremely hazardous and has caused fatalities in a very short time. Clothing wet with benzene should be re moved immediately, and all por tions of the body which have been wet with benzene should he thor oughly washed with soap and water, Air Analyses Methods of Detection 62. Of the several methods which may be used to measure the SL 029432 concentration of benzene vapor in air one of the most sensitive de pends upon the color produced when benzene is brought into con tact with a solution of formalde hyde in sulfuric acid. The concen tration of the benzene vapor is measured by the amount of air which must be drawn through the sulfuric acid formaldehyde mix ture to produce a standard color.2 Toluene and xylene also react in this test although it is not as sen sitive to them as to benzene. 63. Another sensitive test and a somewhat more quantitative one is the "butanone test" in which the air sample is passed through nitrating acid. After neutraliza tion, the dinitrobenzene formed is extracted with methyl-ethyl-ketone. When some alkali is added to the methyl-ethyl-ketone solution, a purple color is formed, the in tensity of which is a measure of the amount of benzene in the sample.3 As little as 10 ppm can be detected readily and measured with reasonable accuracy by this test, 64. A more rapid and conveni ent though less sensitive test is that obtained with the benzene detector. This is a very sensitive combustible gas indicator equipped with a special filter to exclude water vapor. It has a special low range to get the concentration of interest from a toxicological stand point and provides instantaneous reading on a dial graduated in ppm. 65. If pure lienzene is not in use, the instrument must of course be calibrated in terms of the par ticular mixture employed in the plant. 66. Another convenient instru ment using an optical measure ment is the organic vapor interfer ometer. This instrument measures the change caused by the gas or vapor in refractive index of the air. Since it will respond to any vapor present, it must be cali brated for the particular materials in use in the plant. It is also quite sensitive to sudden changes in tem perature. Under good conditions it provides an extremely sensitive means of measurement. 67. The industrial hygiene sur vey should include air sampling at various strategic locations in all buildings where benzene is used, especially in the laboratory. The samples should be taken while reg ular operations are being carried on. 68. If the process is said to be entirely enclosed, special attention should be paid to the possibility of leakage at pumps, valves, gaskets, and other points of joining. If the benzene is used in open containers, the vapor content at the breathing level at each work station should be thoroughly investigated. 69. The values found in such surveys should be recorded on a floor plan of the workroom. Sources of contamination will usually become apparent on such a map, and the ventilation can be designed to eliminate the hazards. Maximum Allowable Concentration 70. The American Standards Association has set 100 ppm for the maximum allowable concentra tion of benzene vapor in the air.4 In spite of this standard there has been an almost continuous move ment to lower the maximum allow able concentration, and lower values are used by various state authorities. The safety code of one state now goes as low as 35 ppm. Because of these wide variations, the legal requirements for control of benzene vapor should always be carefully investigated. Exhaust Ventilation 71. Benzene vapor is most effi ciently removed by suction as close as possible to the point of genera tion. Increasing general ventila tion to dilute the vapor is gen erally a far more expensive and much less effective method. In fact, sweeping large amounts of air through workrooms may increase the pollution of the air and is sure to increase the cost. 72. Exhaust hoods should pref erably be located just around the upper rims of tanks or around the edges of work tables. These fix tures should have narrow slots so placed as to assure even distribu tion of the exhaust around the working area. This method utilizes the high specific gravity of ben zene vapor (nearly three times that of air) and tends to prevent the vapor from escaping from the point of operation. 73. Inasmuch as benzene vapor is heavy and has a tendency to settle, low points in areas in which benzene is used should be venti lated by suction. However, it should he noted that as the benzene vapor becomes more dilute, the tendency to settle becomes less and the tendency to mix uniformly with the air in the room becomes greater. 74. It is expensive to attempt to oppose the natural air circulation in the area. If heat in the process or some other influence produces a strong thermal movement, this movement should he utilized in the exhaust system rather than op posed. The air should, however, in every instance be exhausted away from the operator and never drawn past his face. 75. All rooms where benzene is used, even in small quantities, should have sufficient positive ven tilation to keep atmospheric con centration below the safe limit in all occupied areas. Open windows cannot provide efficient positive ventilation because they may he closed during inclement weather and at the will of the operators. 76. The exhaust ventilation sys tems should conform to the regu lations of the National Fire Pro tection Association.'"''6 77. The use of a fountain cnj> or some other type of closed con tainer is suggested where benzene or benzene mixed with other in gredients, such as rubber, bakclite, or artificial leather "dope," is used. Where the operation requires that large vessels containing ben zene remain uncovered, the vapors can readily be removed if the ves sel is placed within a .larger con- SL 029433 laitier, at the bottom of which auction is applied. Fire and Explosion 78. The fire and explosion haz ards of benzene are similar to those of other flammable solvents of approximately the same boiling range. Because of its low flash point (12 F, "closed cup"), ben zene is rated as extremely flam mable. Benzene vapors when mixed with air in proper proportions are explosive if ignited. The Under writers' Laboratories' fire hazard classification for benzene is 95 to 100 on a scale in which ether is the standard for 100. 79. One of the best fire preven tion devices, for this and other flammable solvents is a properly designed and operated exhaust sys tem which will keep the vapor con centration throughout the plant well below the explosive range. Prevention 80. Because of its low flash point (12 F) and low explosive limits (1.5 per cent), every pre caution should be taken to prevent the ignition of benzene vapor. 81. Only non-sparking tools should be used in atmospheres which may contain this vapor. Windows and openings of build ings which house benzene proc esses should be carefully screened with copper or bronze screening to. keep sparks from locomotives or other sources from entering and causing ignition. 1 82. Indirect heating systems are preferable to direct systems. No heating apparatus, whether direct or indirect, should be located close enough to the flammable liquid that splashes might strike the heated surface and be ignited. 83.. The "no smoking" and "no matches" rule should be strictly enforced. Open flames, such as torches, heating and welding ap paratus, stoves, or gas burners, should never be used in the same room with open benzene opera tions. 84. When open flame apparatus must be used in the vicinity or even in other rooms, the possibility of its acting as a source of ignition should be thoroughly investigated and the operation should be car ried out only after proper precau tions have been installed and a permit has been issued by the plant fire prevention department. 85. Electrical installations should be made in accordance with the National Electrical Code.7 86. Static discharges should be prevented by the use of electrical grounding and static brushes on all points where there may be static accumulations.8 Protection 87. Sufficient hand and auto matic fire protection apparatus should be provided where open benzene processes are used. Car bon dioxide, foam type, or dry powder extinguishers furnish ex cellent protection. Steam has also been found satisfactory. 88. The location of the hand apparatus is very important; it should be outside the danger area but readily accessible to the place where the benzene is used. 89. Steam jets may be provided to extinguish vent fires caused by lightning.8 Tanks and vats should have heat-actuated automatic cov ers; and other fire protection, such as sprinklers or'other extinguish ing media, should be automatic in operation. Storing and Handung 90. Benzene, like other flam mable solvents, should be stored and handled in accordance with the regulations of the National Fire Protection Association.10 91. The solvent piping systems in the plant should be painted and stenciled in accordance with Amer ican Standard A13-1928, "Scheme for the Identification of Piping Systems."11 Containers and piping systems should be inspected fre quently for breaks and leaks, and all such defects should be corrected at once. When such repairs arc being made, the workmen should be carefully supervised. 92. Large quantities of benzene should be stored outside buildings, preferably in underground tanks, unless the structures are especially designed for benzene storage. Tanks should be vented through flame arresters. SL 029434 REFERENCES 1. "Warning Labels, Manual L-l," Manufacturing Chemists' Association, Inc., 246 Woodward Building, Washing, ton 5, D. C. 2, "Methods for Determining Toxic Gases in Industry, Leaflet #4, Benzene Vapor," His Majesty's Stationery Office, London. 3. "Micro Determination of Ben* zenc," Schrenk, Pierce and Yam, U. S. Bureau of Mines Reports of Investiga tions 3287 (1935) and 3302 (1936). 4. "American Standard Allowable Concentration of Benzene, 2137.4-1941," American Standards Association, 70 East Forty-fifth Street, New York 17, N. Y. 5, "Standards of the National Board of Fire Underwriters for the Installation of Blowers and Exhaust Systems for Dust, Stock and Vapor Removal or Con veying," NBFU Pamphlet 91, National Board of Fire Underwriters, 85 John Street, New York 7, N. Y., 1947. 6. Safe Practices Pamplets No. 32, "Exhaust Systems," and No. 91, "Spray Coating," National Safety Council, Chicago, III. 7. "Standard of the National Board of Fire Underwriters for Electric Wir ing and Apparatus," NBFU Pamphlet 70, National Board of Fire Underwriters, 1947. 8. Safe Practices Pamphlet No. 52, "Static Electricity," National Safety Council. 9. "Standard of the National Board of Fire Underwriters for the Installa tion, Maintenance and Use of First Aid Fire Appliances," NBFU Pamphlet 10, National Board of Fire Underwriters. 10. "Standard of the National Board of Fire Underwriters for the Installa tion of Containers for Storing and Han dling Flammable Liquids," NBFU Pamphlet 30, National Board of Fire Underwriters, 1941. 11. See also Safe Practices Pamphlet No. 88, "Identification of Piping Sys tems," National Safety Council. ACKNOWLEDGMENT This data sheet was prepared by Robert M. Watrous, M. D., medical director, Abbott Laboratories. The many helpful suggestions for improvement of the first draft which were made by the Safe Practices Conference Committee of the National Safety Council arc grate fully acknowledged. The final draft was edited by F. A. Van Atta, director of industrial hygiene. National Safety Council, and approved by the Council's Industrial Conference. 7 SL 029435