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S P E C I A L
INDUSTRIAL HYGIENE W O R K B O O K
F E A T U R E
by H. H. Schrenk, Industrial Hygiene Foundation
Industrial Hygiene of Metals of Recent Industrial Importance Boron, titanium, and zirconium have been known for a long time, but their widespread applications and new uses have created new industrial health problems
I NTENSIVE
RESEARCH
in
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of jet airliners, rockets, and space vehicles has focused attention on high energy fuels and metallurgical -materials that will meet the high temperature requirements. Unlike organic compounds, which, in m a n y cases, can be placed in groups having similar or related physiological action, metals and their compounds, in general, have distinctive characteristic effects. A metal may vary from being virtually nontoxic to being highly toxic, depending on composition, solubility, and physical properties. T h e large dosage of barium sulfate used for roentgenological studies of the gastrointestinal tract attests to its nontoxicity. However, cases of poisoning by barium carbonate and barium chloride have been reported. T h e difference in toxicity is d u e to the marked insolubility of the sulfate. Similarly, lead sulfide and lead chromate are less toxic than the more soluble compounds of lead. Another example is the low toxicity of mercurous chloride compared to mercuric chloride. Particle size is another property which m a y significantly influence physiological effects. Freshly formed zinc oxide fume produces a transient chills-and-fever response, which is probably the result of the absorption of protein material from the tissues of lungs owing to the action of the oxide. This effect is not produced by zinc oxide which has been collected and redispersed. C a d m i u m oxide fume produces marked irritation of the lungs, resulting in pneumonitis with effects similar to those from oxides of nitrogen or phosgene. A n u m b e r of
fatal cases have been reported. O n the other hand, m u c h greater exposures to cadmium oxide dust have not produced serious effects. Another factor that influences the effect of a metal is the acid radical with which it is combined. Chlorides, fluorides, and sulfates of metals will frequently cause irritation of the respiratory tract, owing to the acid produced by hydrolysis. Hence, metals such as tin, titanium, and zinc, which have a low order of toxicity, have irritating properties when present as the chloride and certain other salts. O n e group of metallic compounds usually associated with a high degree of toxicity is the hydrides. Arsine, stibine, phosphine, hydrogen selenide, and hydrogen telluride are known to be highly toxic. Limited data indicate that the hydrides of germanium and tin are also toxic. T h e physiological action of a hydride is usually characteristic and the effects m a y be different from those produced by inorganic compounds of the same element. Boron Compounds T h e recent interest in boron compounds as high energy fuels, as well as the use of boron in alloys, has directed attention to the potential health hazards associated with these materials. Boric acid and borax have been widely used as antiseptic agents and are commonly considered to have a low degree of toxicity. However, cases of accidental poisoning have occurred and these compounds should be classed as moderately toxic. As late as 1946, the toxicity of
boron hydrides was reported as unknown; however, in 1951, the report of clinical observations on four persons exposed to boron hydrides called attention to the hazardous nature of these compounds. Symptoms observed from exposure to pentaborane were nervous tension and exhaustion, followed the next day by dizziness, drowsiness, and severe muscular contractions. Inability to concentrate and confusion were also noted. Subsequent to these clinical observations a number of papers have been published dealing with the acute and chronic toxicity of diborane, pentaborane, and decaborane by various modes of administration to several species of animals. These studies, also, showed a difference in the type of response to the different hydrides indicated in h u m a n cases. D i b o r a n e . Acute toxicity inhalation experiments showed that the lethal concentration of diborane for 5 0 % of the rats exposed for 4 hours was about 50 p.p.m. T h e main effect was irritation and edema of the lungs and the acute toxicity was comparable to that of phosgene. Exposure of rats to about 5 p.p.m. for 6 hours per day, 5 days per week, resulted in pulmonary d a m a g e in 2 to 3 weeks. Similar repeated exposures to about 2 p.p.m. also produced significant pulmonary damage with increased susceptibility to infections. These results demonstrate the cumulative effects of diborane and show its high degree of toxicity. T h e American Conference of Governmental Industrial Hygienists ( A C G I H ) adopted a threshold limit of 0.1 p.p.m. (0.1 mg. per cu. meter) for repeated 8VOL. 49, NO. 9
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hour daily exposures. T h e median detectable concentration by odor of diborane is estimated to be 2 to 4 p.p.m., hence odor is not a reliable indicator or warning of potentially hazardous concentrations. T h e therapeutic effect of a n u m ber of substances was studied. These included dimercaprol (BAL), antihistamines, and barbiturates. None appeared promising. Pentaborane. Unlike diborane, the main point of attack of pentaborane was the central nervous system. Symptoms included weakness, incoordination, tremors, hyperexcitability, and convulsions. T h e lethal concentration for 5 0 % of rats, exposed for 2 hours and observed for 2 hours was 18 p.p.m. Repeated 5-hour daily exposures for 5 days per week to 3.3 p.p.m. also produced deaths. T h e central nervous system effects were more pronounced and aggravated than in the acute studies indicating an accumulative effect. T h e pentaborane is more toxic than diborane and recovery is slower. T h e A C G I H adopted a tentative threshold limit of 0.01 p.p.m. (0.03 mg. per cu. meter) for repeated 8hour daily exposures. T h e median detectable concentration of odor of pentaborane is about 10 p.p.m.; hence, odor is not a reliable indicator or warning of potentially hazardous concentrations. Decaborane. T h e physiological action of decaborane is similar to that of pentaborane in that the primary effect is on the central nervous system. In addition, there was evidence of damage to the liver and kidneys, and changes in nucleic acid metabolism indicate that it is an active metabolic poison. Repeated 6-hour daily exposures for 5 days per week to 20 p.p.m. showed cumulative effects. T h e recovery of animals receiving repeated doses was markedly delayed, compared to those receiving a single dose. Further, the rate of recovery from decaborane was slower than for pentaborane, indicating that detoxification does not occur readily. This further emphasizes the hazardous nature of these compounds. T h e A C G I H adopted a tentative threshold limit of 0.05 p.p.m. (0.3 mg. per cu. meter). As the median detectable concentration by odor of 88 A
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decaborane is about 0.3 mg. per cu. meter, the odor would be detectable at the threshold level. Owing to olfactory fatigue, this should not be depended upon to prevent hazardous exposure. In summary, it can be stated that diborane, pentaborane, and decaborane are highly toxic compounds and adequate control measures must be instituted to prevent serious hazards to health. In addition to hazard from inhalation, there is also a hazard from skin absorption of pentaborane and decaborane. Toxicity d a t a have been obtained concurrently with industrial development of these materials; otherwise serious injury might have resulted from ignorance of their highly toxic nature. Determination of Boron
Concurrently with the investigations of the toxicity of the boron hydrides, analytical methods were developed for monitoring the atmosphere of work places and analyzing biological specimens. A 5 % silver nitrate solution in amyl amine on filter paper was found to give a yellow-to-brown color spot which was sensitive to about 1 7 of decaborane with a somewhat more sensitive response to diborane and pentaborane. Also, the boranes could be titrated iodometrically, using a Tutweiler buret. A third method is based on the reduction of triphenyltetrazolium chloride by boron hydrides to produce redcolored formazan, using filter paper impregnated with the reagent. A field model instrument, based on this principle, is sensitive to 0.1 p.p.m. of decaborane and 0.5 p.p.m. of pentaborane. An automatic instrument is sensitive to 0.1 p.p.m. of each compound. A method for determination of boron in biological specimens is based on the migration of the borate ion to the anode of an electric field and measurement of the red boron—tumeric complex developed on filter paper.
nium, columbium, a n d vanadium, which are being used in increasing amounts. Reports have also been published on the toxicity of some of the rare earth metals, including cerium, lanthanum, praseodymium, neodymium, yttrium, and cesium. T i t a n i u m is generally accepted as having a low degree of toxicity, particularly the dioxide, for which a threshold limit of 15 mg. per cu'. meter has been adopted (lead 0.15, mercury 0.1, and manganese 6 mg. per cu. meter). Health hazards, therefore, are considered minor, except for compounds such as the tetrachloride, which on hydrolysis yield acid a n d produce irritation of the respiratory tract. Zirconium and molybdenum also have a relatively low degree of toxicity. Threshold limits of 5 mg. for soluble compounds and 15 mg. per cu. meter for insoluble compounds have been adopted for molybdenum a n d 5 mg. per cu. meter (as zirconium) for zirconium compounds. V a n a d i u m , on the other h a n d , has been found to have a much higher degree of toxicity, irritation of the respiratory tract being a major effect. Threshold limits of 0.1 mg. for vanadium pentoxide fume and 0.5 mg. per cu. meter for pentoxide dust have been adopted. Cerium, cesium, lanthanum, p r a seodymium, neodymium, yttrium, germanium, and columbium are considered at present to have a relatively low degree of toxicity. However, sufficient data are not available on which to base definite evaluation of potential health hazards. No threshold limits have been established. T h e fact that many of the metals appear to have a low degree of toxicity is fortunate. However, as pointed out, certain compounds m a y have other physiological action owing to some specific chemical or physical properties.
Other Metals
Considerable attention is being given to the potential health hazards that m a y be associated with a n u m ber of other metals such as titanium, molybdenum, zirconium, germa-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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