hexane 1 Toxic Effects and Modes of Action Classification/MAK value: 180 mg/m 3 MAK value dates from: 1982 Chemical name (CAS):

Transcription

1 n-hexane Classification/MAK value: MAK value dates from: 1982 Chemical name (CAS): 50 ml/m 3 (ppm) 180 mg/m 3 hexane CAS number: Structural formula: Molecular formula: C 6 H 14 Molecular weight: Melting point: 95.3 C Boiling point: 68.8 C Vapour pressure at 20 C: CH 3 (CH 2 ) 4 CH hpa 1 ml/m 3 (ppm) = mg/m 3 1 mg/m 3 = ml/m 3 (ppm) 1 Toxic Effects and Modes of Action Exposure to n-hexane vapour results in vertigo, headache, nausea, vomiting and irritation of the eyes and trachea; the first narcotic symptoms may be expected at concentrations as low as 500 ml/m 3 [1]. The initial effect of intensive contact with liquid n-hexane is defatting of the skin; prolonged contact can lead to erythema [2]. n-hexane irritates the mucous membranes of the nose and throat, and the conjunctiva [3] but the effects are less severe than those of olefins such as, e.g., 2-hexene. It has been suggested that even dermal exposure to liquid n-hexane can induce polyneuropathic syndromes [4, 5] but unequivocal proof of this claim has not been published. The first effects of chronic exposure to n-hexane are unspecific symptoms such as headaches, progressive weakness and a tendency to become tired rapidly [6 9]. The first symptom of nerve damage is usually given as numbness in the toes and fingers; in mild cases this remains the only symptom (distal sensory polyneuropathy) [10]. As the disorder progresses, symmetrical sensory impairment is observed in the distal parts of the extremities and later sensorimotor neuropathy with muscle atrophy. After the end of exposure the symptoms can disappear completely within 10 months. n-hexane is taken into the human organism principally by inhalation. Oral intake is unusual and occurs almost without exception either by mistake or with suicidal intention. Intoxications as a result of dermal absorption are not very likely because liquid aliphatic

2 258 n-hexane Volume 4 hydrocarbons are absorbed relatively slowly. The substance is not absorbed through the skin in noteworthy quantities from vapours containing n-hexane. Because it is lipophilic, n-hexane accumulates in adipose tissue. A steady-state level in the tissues is attained after about 4 5 hours exposure to a constant concentration. The retention of n-hexane is lower than that of other organic solvents such as ethyl acetate, toluene, trichloroethene, acetone and ethanol [11]. Assuming 15% retention, about 25 mg n-hexane is taken in by the human body at rest (respiration rate c 8 l/min) during 1 hour exposure to 100 ml/m 3. After the exposure has ended, more than half of the retained n-hexane (c 56 %) is eliminated by exhalation. The fraction remaining in the body is metabolized and excreted in the urine. 1.1 Biotransformation The biotransformation of n-hexane takes place mostly in the liver where, in several oxidation steps, 2-hexanol and then diols, hydroxyketones, diketones and finally hexanoic acids are produced. This process converts the lipophilic n-hexane to hydrophilic products which can be excreted in the urine either directly or after conjugation with glucuronic or sulfuric acids. A schema of the metabolism of n-hexane [12] is shown in Figure 1. The currently available data indicate that the biotransformation of n-hexane follows qualitatively similar pathways in man and in various experimental animals [10] CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 n-hexane O OH OH OH CH 3 C CH 2 CH 2 CH 2 CH 3 methyl-n-butylketone CH 3 CH CH 2 CH 2 CH 2 CH 3 2-hexanol* CH 3 CH CH 2 CH 2 CH CH 3 2,5-hexanediol O OH 1. α-oxidation 2. decarboxylation 3. oxidation O OH HO C CH 2 CH 2 CH CH 3 4-hydroxypentanoic acid lactonization CH 3 C CH 2 CH 2 CH CH 3 5-hydroxy-2-hexanone O O CH 3 C CH 2 CH 2 C CH 3 2,5-hexanedione* 1. enolization 2. oxidation 3. cyclization H 3 C O CH 3 2,5-dimethylfuran * O O C CH 2 CH 2 CH CH 3 1-valerolactone* Figure 1. Biotransformation of n-hexane in man according to [12] * metabolites of n-hexane which have been found in the urine of occupationally exposed persons

3 Volume 4 n-hexane 259 but that there are differences in the relative amounts of the metabolites formed. The lifetime in the organism of 2,5-hexanedione, the metabolite mainly responsible for the neurotoxicity of n-hexane, is longer than that of the other detectable metabolites (2- hexanol, 2,5-dimethylfuran and γ-valerolactone). In human urine, 20 to 30 times more 2,5-hexanedione is detected than 2-hexanol whereas in animal studies (rats and guinea pigs) urine analysis has shown 2-hexanol to be the main metabolite [13 15], present at 3 times the level of the 2,5-hexanedione. 2,5-Dimethylfuran and γ-valerolactone are probably not "genuine" metabolites of n-hexane but are produced during the gas chromatographic analysis by cyclization of hydroxy compounds [16] (Figure 1). It is still not clear to what extent the neurotoxic effects of n-hexane can be enhanced or depressed by other substances. 2 Effects in Man n-hexane is a component of low-boiling mineral oil fractions from which the usual commercial products are manufactured by specific refining processes. Pure n-hexane (nhexane content about 95 97%) is produced only in very small quantities for special purposes, particularly for laboratory use and analytical applications. Almost all the epidemiological studies and published casuistics which have appeared since 1964 on the subject of the effects on man of chronic exposure to n-hexane have been concerned with occupational exposure to mixtures of substances containing n-hexane. Of industrial significance are technical hexane (50 55 % n-hexane w/w), special boiling point solvents (up to 35 % w/w) and gasoline (petrol, motor spirit) (about 2.5% w/w). A large proportion of the gasoline or petroleum ether intoxications described in the literature are primarily n-hexane intoxications [9]. 2.1 Acute toxicity Exposure to a n-hexane concentration of 2000 ml/m 3 for 10 minutes produces no symptoms, but 5000 ml/m 3 for 10 minutes results in vertigo and stupor [1]. No irritant effects were observed after 3 5 minutes at 500 ml/m 3. The recent report [3] that eye irritation was caused by as little as 20 ml/m 3 n-hexane must probably be considered as a special case as the presence of 1 ml/m 3 NO 2 may have increased the irritative effect. 2.2 Chronic toxicity Since 1964, numerous reports have been published of casuistics and epidemiological studies concerned with the chronic effects of various mixtures containing n-hexane in man after occupational exposure or after "glue sniffing". Most of the exposures involved inhalation of n-hexane vapour. Besides unspecific general symptoms, paresthesia develops in the toes and fingers after several months [8, 9] or several years [17], depending on the exposure concentration. It is followed by symmetrical sensory

4 Table 1. Casuistics of chronic intoxications after inhalation of n-hexane at work Mixture inhaled n-hexane (90 %) toluene (10 %) "n-hexane on sale" (70%) toluene (small quantity) n-hexane (purity 95%) technical n-hexane Benzin C (15 25% n-hexane) "mostly n-hexane" "benzene" (probably total hexane) Workplace sandal production (cottage industry) sandal production (cottage industry) pharmaceutical works furniture factory catalytic reactor sandal production (cottage industry) dial production in clock factory n-hexane concentration in ml/m "n-hexane" (probably total hexane) Exposure time not given "several months or more", 8 h/d months, c 48 h/week Number of persons with symptoms Symptoms 35 sensory polyneuropathy (19 cases), sensorimotor polyneuropathy (16 cases); electromyogram changes, nerve conduction velocity slowed; nerve biopsy: demyelination, axonal changes 93 grade I: sensory polyneuropathy (53 cases), grade II: sensorimotor polyneuropathy (32 cases), grade III: sensorimotor polyneuropathy with muscle atrophy (8 cases); muscle biopsy: degeneration of muscle fibres in 3 cases 11 "intoxication polyneuritis of mixed type" (no other details given) months 3 sensorimotor polyneuritis [20] 400 "long period" 6 polyneuropathy (no other details given) [21] (probably total hexane) months 1 12 months "longer" neuropathy (muscle weakness and atrophy) [5] gait disorders with muscle weakness and atrophy of the lower extremities; electromyograph changes, nerve conduction velocity slowed; nerve biopsy: axonal degeneration Ref. [18] [19] [6] [9] 260 n-hexane Volume 4

5 Table 1 (continued) Mixture inhaled n-hexane technical n-hexane Workplace metal factory (production of tungsten alloys) edible oil and sticking plaster industry n-hexane concentration in ml/m 3 58 ± 41 ("n-hexane") Exposure time Number of persons with symptoms Symptoms 1 16 years 14 no damage to the nervous system; in some tests slight tendency to functional disorders of the peripheral nervous system, e.g., "maximal motor nerve conduction velocity slowed" in comparison with control group (14 persons) years 15 ophthalmoscopic maculopathy in 3 cases; no mention of polyneuropathy Ref. [17] [22] Volume 4 n-hexane 261

6 262 n-hexane Volume 4 impairment, primarily in the distal parts of the extremities, of the "glove and stocking" type with reduced sensitivity to touch, temperature and vibration. This is accompanied by loss of the stretch reflexes, in mild cases mostly only the Achilles tendon reflex. Finally symmetrical muscle weakness leads to flaccid paralysis, mainly in the distal parts of the extremities. The affected persons are then unable to grip objects or to step over obstacles (distal sensorimotor polyneuropathy) [10]. In severe cases, the pronounced muscle atrophy can progress proximally and affect some muscles of the trunk. Involvement of the cranial nerves (II, V, VII and XII) has been observed occasionally. Characteristic is the sudden appearance of the disorder and slow progression, probably as a result of long-term intermittent exposure. The disease generally continues to progress for 1 4 months after exposure has ended [10]. Complete regression of mild and moderate neuropathy may be expected within 10 months of the end of exposure; severe cases may require 3 years. Deaths are not known. Casuistics of chronic intoxications after inhalation of n-hexane at work are shown in Table 1. Electrophysiological abnormalities (reduction in nerve conduction velocity, electromyographic changes) progress in parallel with the severity of the clinical symptoms and also disappear during regression [10]. Biopsy revealed extensive nerve damage with paranodal giant axonal swellings [23] which corresponded to accumulations of 10 nm neurofilaments seen in the electron microscope pictures. Simultaneous loss of the myelin sheath (demyelination) was observed. The giant axonal swellings are considered by some authors to be characteristic of n-hexane-induced degeneration of peripheral nerves [10, 24 26]. Intake of high doses of n-hexane during "glue sniffing" (inhalation of solvents containing n-hexane from glues or varnishes) results in development of the neurotoxic symptoms described above but the symptoms are much more severe and paralysis of the extremities develops much more quickly than after occupational n-hexane exposure. The abuse syndrome is described as symmetric, progressive polyneuropathy ascending from distal parts of the extremities with pronounced muscle atrophy [27]. It is possible to confuse the symptoms with those of polyradiculitis in the atypical form of the Guillain- Barré syndrome. It has been suggested that the effects of n-hexane increase in the presence of methyl ethyl ketone [28]. The once accepted amplifying effects of toluene [29] have not been confirmed; in fact, it is now considered possible that toluene could inhibit the metabolism of n-hexane [30]. Recently, authors from Finland described effects of n- hexane exposure on the optical system (maculopathy, defective colour vision) [22, 31] which have not yet been fully evaluated. Whereas almost all authors agree as to the effects of chronic n-hexane exposure on man with respect to the appearance and progression of the neurotoxic symptoms and the histologically detectable changes in the peripheral nerves, the data for workplace concentrations vary widely in the range between 10 and 3000 ml/m 3. The concentrations are often given for "total hexane" or "technical hexane". It is possible that such mixtures also contained substances which perhaps increased or decreased the effects of the n- hexane. Details of method, duration and frequency of the concentration measurements at the workplace are often not given, nor the details of the instruments used. Where measurements were carried out, the number of analyses was generally too small for

7 Volume 4 n-hexane 263 statistical purposes. Control groups were not included. Only one more recent publication [17] comes close to present day standards. The authors concluded from the results of detailed neurological and neuro-physiological studies of persons exposed to n-hexane and a control group of the same size that manifest polyneuropathy is unlikely to develop at exposure concentrations below 100 ml/m 3 n-hexane. 3 Effects on Animals 3.1 Acute toxicity Inhalation of n-hexane in concentrations up to 8000 ml/m 3 was not narcotic for mice [32]. Concentrations between and ml/m 3 led to lateral position of the mice after minutes. After the end of exposure the mice recovered completely. From about to ml/m 3 loss of reflexes and death occurred after minutes [33]. Exposure of mice to "chemically pure" hexane at a concentration of about ml/m 3 caused initial agitation and then, after 2 hours, lateral position; ml/m 3 caused convulsions and death by respiratory paralysis [34]. Intraperitoneal injection of a single dose of 2 ml n-hexane into guinea pigs resulted in 80% mortality within hours [35]. Dermal application caused no macroscopic skin changes within 15 minutes but microscopic examination revealed in all layers of the epidermis shrinking of the cell nuclei which increased with the exposure time [36]. Morphological changes could not be detected in the liver or kidneys. The effects on both intact and damaged (scarified) skin were studied in rabbits: skin changes found after 22 and 24 hours were considered as mild irritation [37]. In rats given liquid n-hexane by intubation, aspiration led to convulsions and sudden death within a few seconds. Lung weights were increased markedly (pulmonary oedema resulting from "chemical pneumonitis") [38]. 0.1 ml technical hexane (c 45% n-hexane) instilled into the conjunctival sac of rabbits caused reddening of the eyes which was classified as "slight irritation" [37]. 3.2 Chronic toxicity Inhalation studies with rats exposed to n-hexane concentrations of 5, 25 and 125 ml/ m 3 (21 h/day, 7 days/week) for up to 34 weeks revealed neither toxic effects nor morphological or functional signs of neuropathy [10]. In inhalation studies with mice exposed to concentrations of 100, 250, 500, 1000 or 2000 ml/m 3 (24 h/day, 6 days/week) for 1 year, clinical, electrophysiological and histological changes were seen only at 250 ml/m 3 or more [39]. Exposure of rats to a n-hexane concentration of 200 ml/m 3 for 24 weeks produced marked damage in the peripheral nerves [40]. The principal neurological symptom in all animals was footdrop (dropping of the foot caused by insufficiency of the anterior muscles of the leg). The main pathohistological findings were giant axonal swellings with demyelination and degeneration of the nerve fibres in the peripheral nervous system (tibial nerves) and central nervous system (cerebellum,

8 264 n-hexane Volume 4 medulla oblongata and spinal cord) ("axonopathy of central peripheral distal type") [23, 41]. Exposure of rats to toluene on its own had practically no effect [42]. Likewise, an inhalation study in which rats were exposed to the C 6 -isomers contained in technical hexane apart from n-hexane at a concentration of 500 ml/m 3 for 6 months revealed no clinical or histopathological symptoms in the peripheral or central nervous systems [43]. 4 Reproductive and Developmental Toxicity 4.1 Animal studies Pregnant Fischer 344 rats were exposed to 1000 ml/m 3 n-hexane in the air for 6 hours daily from day 8 to 12 (7 animals), 12 to 16 (9 animals) or 8 to 16 (8 animals) of gestation. The foetuses were examined shortly before term for external, organ and skeletal anomalies. Comparison with the appropriate control groups revealed no evidence of prenatal toxicity [44]. In another experiment in this study, 14 pregnant rats were exposed as above from day 8 to day 16 of gestation and the pups then observed until they were 7 weeks old. Birth weights were comparable with those in the control group whereas body weight gain was reduced (p < 0.05) slightly and transiently during the postnatal period [44]. In an embryotoxicity and foetotoxicity study, groups of 30 Sprague-Dawley rats were exposed to n-hexane concentrations of 0, 200, 1000 or 5000 ml/m 3 for 20 hours daily from day 6 to day 20 of gestation. The dams were removed from the inhalation chambers on day 20 and the foetuses examined thoroughly for anomalies. Maternal toxicity (reduced body weights) was observed after exposure to the two high concentrations. Furthermore, the 1000 and 5000 ml/m 3 exposures led, respectively, to slight and moderate decreases in foetal weights. The findings in the highest concentration (5000 ml/m 3 ) group were associated with a significant increase in the incidence of delayed ossification in some individual sternum bones. Prenatal mortality or teratogenic effects were not seen at any concentration including the highest, 5000 ml/m 3 [45]. Effects on postnatal development were studied in groups of 15 Sprague-Dawley rats exposed to 0, 100, 2000 or ml/m 3 n-hexane in air for 7 hours daily from day 15 before mating until day 18 of gestation. The pups Were observed until they were 60 days old. There were no effects on litter size or postnatal growth. Various tests to check the hearing and visual function of the young animals yielded no evidence of significant neurotoxic effects [46, 47]. In contrast to the above studies which were carried out with pure n-hexane, a one generation study with CD rats was carried out with a mixture of solvents which contained not only about 38 % n-hexane but also 3-methylpentane (21 %), 2- methylpentane (21%) and methylcyclopentane (14%) as the other main constituents. Inhalative exposure of groups of 20 animals to 0, 100, 500 or 1500 ml/m 3 of this mixture for 6 hours daily for the whole duration of the study (for 100 days before mating and until 21 days after the birth) caused neither substance-related changes in fertility and

9 Volume 4 n-hexane 265 reproductive performance nor postnatal development disorders. Additional subgroups of 20 control rats and 20 exposed to 1500 ml/m 3 were used for a thorough examination of the foetuses on day 21 of gestation. There was no evidence of prenatal toxic effects [48]. In a test for embryotoxicity, a solution of n-hexane in oil was administered to CD-1 mice by stomach tube, daily from day 6 to day 15 of gestation. The foetuses were examined on day 18 especially for external, organ and skeletal anomalies. In the rangefinding study with 4 12 pregnant mice per group, doses up to 2.2 g/kg/day had no embryotoxic or foetotoxic effects. In the second study with animals per group, total daily doses of 2.17, and 9.90 g/kg were administered in three portions. Whereas maternally toxic effects (increased mortality) were seen at doses of 2.83 g/kg/day or more, slight reductions in foetal weights were first found at doses of 7.92 g/kg/day or more. Embryonic or foetal deaths and teratogenic effects were not seen at any dose including the highest [49]. 4.2 Effects in man There are no published reports of developmental toxic effects in man after n-hexane exposure. 4.3 Evaluation The most important studies for the evaluation of the prenatal toxicity of n-hexane are the detailed embryotoxicity studies in rats exposed to concentrations up to 5000 ml/m 3 (inhaled for 20 hours daily) and mice given doses up to 9.9 g/kg/day orally. The other inhalation studies with rats yielded additional information, particularly about postnatal development. In rats inhaling n-hexane in the concentration range from ml/m 3, a sufficient systemic bioavailability has been demonstrated [50, 51]. In addition, it was demonstrated in pregnant rats exposed to 1000 ml/m 3 n-hexane in air that the concentrations of n-hexane and of its metabolites, 2,5-hexanedione and methyl-nbutylketone, in the foetuses are comparable with those in the maternal blood [52]. Similarly, after oral administration of n-hexane to mice the systemic availability of the substance must have been sufficient because doses of 2.83 g/kg/day or more produced maternal toxicity and systemic effects have been seen in the rat not only after inhalative exposure but also after oral administration [53]. The embryotoxicity studies did not reveal any evidence that exposure to n-hexane concentrations up to the highest tested concentration, 5000 ml/m 3, or oral doses up to 9.9 g/kg body weight caused prenatal mortality or teratogenic effects in the rat or mouse. Neither was the postnatal development of rats significantly affected by exposure to n- hexane concentrations up to ml/m 3. Slight delays in foetal development were seen in rats only after high level exposure (concentrations of 1000 ml/m 3 or more) and in mice only after very high oral doses ( 7.9 g/kg/day). However, these doses caused maternal toxicity in both species. In view of these delays in foetal development, a no effect level of 500 ml/m 3 for the rat and 2.8 g/kg/day for the mouse may be deduced from the available study results. Therefore

10 266 n-hexane Volume 4 prenatal toxic effects are not to be expected in man if the MAK value of 50 ml/m 3 is not exceeded. 5 Mutagenic and Carcinogenic Effects Mutagenicity studies (Ames test and mouse lymphoma test) [54] revealed no in vitro mutagenic activity. In vivo with n-hexane concentrations of 150, 293 and 595 ml/m 3, an increase in the incidence of chromosomal aberrations was seen in the bone marrow cells of male albino rats. On the other hand, significant mutagenic activity was not seen in V79 cell cultures [55]. Studies with n-hexane applied to the skin of mice did not reveal carcinogenic or tumour-promoting effects [56, 57] neither did a single subcutaneous application of 0.5 ml lead to tumour development in mice [57]. 6 Manifesto (MAK value, classification) The MAK value for n-hexane was reduced in 1974 from 500 ml/m 3 to 100 ml/m 3 because long-term occupational exposure to 400 ml/m 3 or more was known at that time to lead to the development of polyneuritis. The value was considered to be provisional. Since then numerous investigations and animal studies have provided a clear picture of the mode of action of n-hexane and particularly of its biotransformation in the organism. The presently available data indicate that the quantitative differences in the biotransformation of n-hexane in man and animals in man the formation of 2,5- hexanedione as main metabolite with proven neurotoxic potential, in animals the conversion mainly to 2-hexanol can well account for the fact that the neurotoxic effects first appear in animals exposed to concentrations above 200 ml/ m 3 whereas the human organism can react with polyneuropathic symptoms at as little as 100 ml/m 3. From these results it may be concluded that the findings of animal experiments may not be applied directly to man and also that the MAK value valid from (100 ml/m 3 ) cannot ensure adequate health protection for employees. In 1982 it was therefore considered justified to reduce the MAK value from 100 ml/m 3 to 50 ml/m 3. This value applies only to n-hexane; other evaluation criteria must be applied for mixtures containing n-hexane because of possible synergistic or antagonistic effects. More field studies in which particular attention is given to neurological and neurophysiological symptoms, representative concentration measurements under standardized analytical conditions and excretion studies to determine the time course of the elimination of the main metabolite, 2,5-hexanedione, are necessary to quantify the dose-response relationships. As a substance with systemic effects, n-hexane is classified in category II,1 for the limitation of exposure peaks.

11 Volume 4 n-hexane 267 The developmental toxicity studies with n-hexane inhaled by rats and administered orally to mice have produced no evidence that prenatal toxicity should be expected in man if the MAK value of 50 ml/m 3 is not exceeded. Teratogenic effects were not detected in either animal species even after very high level exposures. Thus, although an inhalation study with a second species has not been carried out, the available results are considered to justify the classification of n-hexane in pregnancy group C. 7 References 1. Patty, F. A., W. P. Yant: U.S. BUR. Mines Rept. Invest. No (1929); cited in Patty, F. A.: Industrial Hygiene and Toxicology, Vol. II, p 1198, Interscience Publishers, New York, London, Sidney, Oettel, H.: Arch. exp. Path. Pharmakol. 183, 641 (1936) 3. Wayne, L. G., J. A. Orcutt: J. occup. Med. 2, 383 (1960) 4. Nomiyama, K., T. Yoshida, H. Yanagisawa: Percutaneous absorption of n-hexane caused severe polyneuropathy, Proc. 46th Ann. Meeting of Jap. Ass. Industr. Hlth, p 420, Takahashi, M., H. Takeuchi, S. Kyo, S. Yorifugi, S. Sanagi, Y. Seki, I. Hara: Med. J. Osaka Univ. 28, 11 (1977) 6. Yamada, S.: Jap. J. industr. Hlth 6, 192 (1964) 7. Yamada, S.: Jap. J. industr. Hlth 9, 651 (1967) 8. Wada, Y., S. Okamoto, S. Takagi: Clin. Neurol. (Tokyo) 5, 591 (1965) 9. Yoshida, T. H., H. Yanagisawa, T. Muneynki, R. Shigiya: Clin. Neurol. (Tokyo) 14, 454 (1974) 10. Spencer, P. S., H. H., Schaumburg, M. I. Sabri, B. Veronesi: CRC Crit. Rev. Toxicol. 7, 279 (1980) 11. Nomiyama, K., H. Nomiyama: Int. Arch. Arbeitsmed. 32, 85 (1974) 12. Perbellini, L., F. Brugnone, I. Pavan: Toxicol. appl. Pharmacol. 53, 220 (1980) 13. Abdel-Rahaman, M. S., L. B. Hetland, D. Couri: Amer. industr. Hyg. Ass. J. 37, 95 (1976) 14. Perbellini, L., F. Brugnone, G. Pastorello, L. Grigolini: Int. Arch. occup. environm. Hlth 42, 349 (1979) 15. Frontali, N., M. C. Amantini, A. Spagnalo, A. M. Guarcini, M. C. Saltari, F. Brugnone, L. Perbellini: Int. Congr. Neurotoxicol., Varese, Abstr. 113 (1979); cited in [10] p Perbellini, L., F. Brugnone, R. Silvestri, E. Gaffuri: Int. Arch. occup. environm. Hlth 48, 105 (1981) 17. Sanagi, S., Y. Seki, K. Sugimoto, M. Hirata: Int. Arch. occup. environm. Hlth 47, 70 (1980) 18. Sobwe, J., M. Iida, Y. Yamamura, T. Takayanagi: Clin. Neurol. (Tokyo) 8, 393 (1968) 19. Yamamura, Y.: Folia psychiat. neurol. jap. 23, 45 (1969) 20. Herskowitz, A., N. Ishii, H. Schaumburg: New Engl. J. Med. 285, 82 (1971) 21. Cavigneaux, A.: Cah. Not. doc , 199 (1972) 22. Raitta, C., A. M. Seppäläinen, M. S. Huuskonen: Graefes Arch. klin. exp. Ophthalmol. 209, 99 (1978) 23. Spencer, P. S., H. H. Schaumburg: in Mechanism of Toxic and Hazard Evaluation, Elsevier/ North Holland, Rizutto, N., H. Terzian, S. Galliazzo-Rizutto: J. Neurol. Sciences 31, 343 (1977) 25. Towfighi, J., N. K. Gonatas, D. Pleasure, H. S. Cooper, L. McCree: Neurology (Minneap.) 26, 238 (1976) 26. Korobkin, R., A. K. Asbury, A. J. Sumner, S. L. Nielsen: Arch. Neurol. 32, 158 (1975) 27. Altenkirch, H., J. Mager, G. Stoltenburg, J. Helmbrecht: J. Neurol. 214, 137 (1977) 28. Altenkirch, H., H. Schulze: Nervenarzt 50, 21 (1979) 29. Shirabe, T., T. Tsuda, A. Terao, S. Araki: J. Neurol. Sciences 21, 101 (1974)

12 268 n-hexane Volume Suzuki, T., S. Shimbo, H. Nishitani, T. Oga, T. Imamura, M. Ikeda: Int. Arch. Arbeitsmed. 33, 115 (1974) 31. Seppäläinen, A. M., C. Raitta, M. S. Huuskonen: Electroencephalogr. clin. Neurophysiol. 47, 492 (1979) 32. Swann, H. E. jr, B. K. Kwon, G. K. Hogan, W. M. Snellings: Amer. industr. Hyg. Ass. J. 35, 511 (1974) 33. Fühner, H.: Biochem. Z. 115, 235 (1921) 34. Lazarew, N. W.: Arch. exp. Path. Pharmakol. 143, 223 (1929) 35. Wahlberg, J. E., A. Bowman: Scand. J. Work environm. Hlth 5, 345 (1979) 36. Kronevi, T., J. Wahlberg, B. Holmberg: Environm. Res. 19, 56 (1979) 37. Hine, C. H., H. H. Zuidema: lndustr. Med. 39, 39 (1970) 38. Gerarde, H. W.: Arch, environm. Hlth 6, 329 (1963) 39. Miyagaki, H.: Jap. J. industr. Hlth 9, 660 (1967); cited in NIOSH Report, p 39, Ono, Y., Y. Takeuchi, N. Hisanaga: A comparative study of the neurotoxicity of petroleum benzine and n-hexane, Dept. of Hygiene, Nagoya University of Medicine, Nagoya, Japan, Spencer, P. S., H. H. Schaumburg: J. Neuropathol. exp. Neurol. 36, 276 (1977) 42. Takeuchi, Y., Y. Ono, N. Hisanaga: Brit. J. industr. Med. 38, 14 (1981) 43. Egan, G., P. S. Spencer, H. H. Schaumburg, K. J. Murray, M. Bischoff, R. C. Scala: Neurotoxicol. 1, 515 (1980) 44. Bus, J. S., E. L. White, R. W. Tyl, C. S. Barrow: Toxicol. appl. Pharmacol. 51, 295 (1979) 45. Mast, T. J.: Inhalation Developmental Toxicology Studies: Teratology Study of n-hexane in Rats, Pacific Northwest Laboratory, Richmond, Washington, USA, Final Report of December Howell, W. E.: A Neurobehavioral Evaluation of the Prenatal Toxicity of n-hexane in Rats, Thesis, Univ. Cincinnati, OH, USA, Howell, W. E., G. P. Cooper: Toxicologist 1, 152 (1981) 48. Schardein, J. L.: Single-Generation Inhalation Reproductive/Fertility Study on a Commercial Hexane, Final Report of January 1987, IRDC, Mattawan, Michigan; American Petroleum Institute Med. Res. Publ. No , Washington, DC, Marks, T. A., P. W. Fisher, R. E. Staples: Drug Chem. Toxicol. 3, 393 (1980) 50. Baker, T. S., D. E. Rickert: Toxicol. appl. Pharmacol. 61, 412 (1981) 51. Bus, J. S., D. Deyo, M. Cox: Fundam. appl. Toxicol. 2, 226 (1982) 52. Bus, J. S., E. L. White, H. D. Heck, J. E. Gibson: Teratology 17, 42A (1978) 53. Krasavage, W. J, J. L. O'Donoghue, G. D. Di Vincenzo, C. J. Terhaar: Toxicol. appl. Pharmacol. 52, 433 (1980) 54. Hazleton Laboratories America, Inc.: In vivo and in vitro mutagenicity studies: n-hexane (Hexane UV), Project No and , Lankas, G. R., C. S. Baxter, R. T. Christian: J. Toxicol. environm. Hlth 4, 37 (1978) 56. Sice, J.: Toxicol. appl. Pharmacol. 9, 70 (1966) 57. Ranadive, K. J., S. V. Gothoskar, B. V. Tezabwala: Int. J. Cancer 10, 652 (1972) MAK value completed Pregnancy group completed 1991