n-Hexane and its toxicologic effects A review 1

K-H. n-Hexane and its toxicologic effects: A review. Scand j work environ health 7 (1981) 157-168. This review is a critical survey and evaluation of the recent literature relevant as medical background for a discussion of hygienic threshold values for hexane. Polyneuropathy and maculopathy, as well as subclinical effects, eg, functional disturbances (conduction velocity of the motor and sensory nerves), are included.


Physical and chemical characteristics
1a!boratories; and in low temperature therm'OOIleters.
Pure n-hexane is seld'OOIl used. Lt is more often mixed with other aliphatic hydro-carbons and toluene.
'Ilhis repO'l't is only concerned with nhexane and not w~th any of the other hexane isomeI1S.
n-Hexane is used widely in .industry as a solvent and tthdnner, eg, in the rubbeT industry inilhe produ'Otion of :tires and the impregnation of materials; in glue products, eg, in the gluing of soles in the shoe industry 'and rim. the production of tape and bandages; in tihe food industry for the extraction of vegetable oils; intihe phar-maceUlt1oa ' lindustry in the production of tablets; and tiJn the perfume inidJustxy. It is also used as a oleaning ·agent ror, a:mong o1fuer thingiS, textiles, furniture, and lea'tJher products; I'n the chemical indus1Jry for <the production of po'lyethylene and polypropylene; as a component in dJhe ex-traa1li.on of benzene {common -gasoline oontainsapproximately 1.5 Dfo n-hex·ane); in Retention is ,thus aPPI"oximaitely 15 %.
There is a lJi,near re1ationship between the concentration ,~n venous blood in millig.rams per kilogram (y) and the ooncentrotion in ,alveolar air in milligrams per liter (x) after 4.5 h of 'exposure: y = 0.77 x -107 (12). After 4-5 h of exposure to an n-hexane concentration of 170 g/m 3 the n-hexane concentrations reach an equilibrium in the venous blood and expired air of rats. Under such circumstances the concentration in venous blood has been found to be 0.15 g/kg (51).
The 'Solubility of n"'hexane in Walter is limitJed. Therefore ,it must be bound to other oomponents in the blood, one suoh componenrtbeing Lipids. The distribution coeffident between olive oil ,and air is approximately 204 at 37°C (K-H Cohr, unpublished results). In addition n-pentane and n-heptane dalta indicalte <that approximately 40 g of n-hexane can he 'bound to 10 4 g of protein in a 5 0 10 water-based solution of protein (41,42).
In inhalation expedments with rats (170 g/m 3 ) equilibrium was found to occur in the brain, liver, kidneys, and adrenal glands after 4-5 h of exposure. The equilibrium CQncenitrations were 0.39, 0.14, 0.20, and 0.49 g/'kg of tissue, respectively. 'Dhese values Icorrespond ,to /the distribution coef-fid~ents for tissue to blood of 2.60, 0.93, 1.33, and 3.27 (13). Accumulation in these tirssues depends on /the lipid content. Four miil!1:igrams of n.Jhexane accumulates per gram of 'lipid. 'Dhe {blood 15 able to bind approximately 25 mg of n-hexane per gram of lipid (13). Equilibrium does not develop~1Jl connection with the a'ccumwabon of n-iliexane in rat liver. 'I'he n-hexane ,conJtenit continues to rise in tihe liver with the simuilta'lleous accumulation of lipid (12).

Elimination
Only >the elimination of n-'hexane through the lungs -and kidneys is described.

Lungs
The n-hexane concentration in expired air 10 min after the 'cessation of exposure is 2-5 % Of ,the equillibrium concentration during exposure (52). Within the first 4 h after 'the end of exposure 50-60 % of the retained quantity is eliminated through the lungs (44).
In 24-h urine specimens 5 Ofo of ilJhe n-hexane injec'ted intraperitoneally into guinea pigs was recovered as 2-hexanol (2); 1.7 % of the n-hexane (330 mg/kg) injected intraperitoneally into rats was recovered as 2-hexanol in 24-h urine specimens (however, no I-hexanol was recovered) (46, 47). In another experiment 0.7 Ofo the n-hexane (73-363 mg/kg) intraperitoneally injected into rats was recovered in the urine as I-hexanol; 70 0 /0 of the I-hexanol was in the glucuronide form and 30 Ofo in the free form. In addition 27 mg of 2-n-hexanone was found irrespective of the dose (21).

Biological half-time
Ten humans were experimentally exposed to n-hexane {306-429 mg/m 3 ; 87-122 ppm) for 4 h. A two-compal1tment kinetic model descriibed the eljlminaition~,rough the hm:gs for the :flirst 4 h after the cessation of exposure. The half-'times were 13 rn!in and 2.5 .h for the smallest and largest exposure, respectively (44).
Experrnentally, rats were exposed to an n-hexane 'conoentration of 3,520 mg/rn 3 (1,000 ppm) for 6 h a day for 5 d. 'I1he elimination of hexane, 2-hexanone, and 2,5-hexanedione from the blood, liver, kidneys, brain, and sciatic nerve was studied after the cessation of exposure. Hexane and 2-hexanone was not detected in any of the tissues after 4--8 h. Less than 0.05 flg of 2,5-hexanedione/g of tissue was discovered in the blood, liver, kidneys, and brain after 24 h. Approximately 0.6 pg/g and approximately 0.5 pglg was recovered from the sojla'fi,c nerve after 12 and 24 h, respectively (15).
Liver microsomes fmm rats tha,t had received phenobarbiital (80 mg/kg) liIlrt:traperitoneally once a day for 2 d had an increased monooxygenase activilty iJ!l vi.,tro. The biotransformaJtion of n-hexane to 2-and 3-hexanol was increased six to seven times (24).

Toxicologic mechanisms
The exact mechanism of the hexacarboill neUil'opathy sympl;om~compl'ex is not known.
Savolainen (54) is of the opinion that man mUist separate -the shol1t-and longterm effedts. Narcosis, coma, and, eventually, respira'tory arrest are the shortterm effeds. 'I1hey may he cauSied by the inoorporation of Ithe solvent molecule into the nerve cell membranes iJn tihe central nervoUis sysltem (eNS), a phenomenon whidhinfluences the transportation of ions.

Skin, mucous membranes and conjunctivas
The inhalJ.ation of vapor is the mosrt frequent type of <contaot with hexane. Both spi1ls and spray can 'lead ,to eye and skin contact.
The toxicologic reference books report thalt :hexane is irrita'ting for human mucous membr!anes ,in 'the respiratory system and the eye in concentrations of about 5,000 mg/m 3 (1,500 ppm) (9, 10, 11, 56). Yamamura (66) examined 93 shoe and leather workers, 59 % of whom had areas of rough, cold, and erythematous skin on the distal parts of their legs.
It has been shown that heXiane is a mild skin irritant of ra!bbits a:liter primary COilltact (32).
One miUmter of n-hexane dropped on guinea pigs causes pycnotk nuclei iIIl all layers of~he epidermis. Longer exposure leads to pr;ogressi've degeneraltion with karyolysis and pertnuclear edema. The eptdermts can 'be separated from the corium between 1Jhe basal membrane and the basallcell layer. Fseudoeosinophi'l ce11:s infil:br:ate the outer layer of the dermis after 4 h (36).
According to the Darize system, dropping 0.1 ml of n-hexane into the conjunctiva of rabbHls resu1lts iln minimal irrHation, grade one (32).

Respiratory organs
lin spite of the frequent oontact of n-hexane WIth I1Jhe respiratory organs, no references show human damage 'to dJhat area.
'I1heaspiration of 0.2 m1 of hexane by vats results in 'a few seCionds in dea:th due to cardiac arrest, respiratory paralysis and asphyxia. The weight of the lungs increases, 010 the average, by approximately 2 'g, which indicates transudation from the alveolar capHla,ri,es into the al'Veolar spaces. Hemorrhaging has been not demonstrated (27).

Liver
Workers and sniffers who have breathed hexane vapors have significantly lower serum cholinesltJerase lev,els than control groups (3, 45). The authors who studied jjhis -subject concluded 1fua>tthis phenomenon is oaused by a primary toxic a'C\tion on the liver. The w-ork-ers 1n 1ili.is e~periment were exposed to 360-2,160 mg/m 3 (100-600 ppm) ,for two to six months. The exact exposure of the sniffers was unlknown. The ccmtell'tof n-'hexane in l1Jhe handled products was high, between 50-95 0/0. However, 1n addition to hexane, to:luene, other akanes, and ()Ither solvents were present.
Liver examinations have for the most part been normal innhe existialg reports and exarn<i.n-ations. However, in a few individual 'oases high values O'f serum cre-aUne phosphokinase, lactate dehydrogenase, and ornithine carbamoyl transferase, have boon demonstrated, as wen as positive urobtlinogen reactions iIIl the urine (28,66).
It has also been demonstrated that n-heXlWle can lead to a decrease in 1Jhe serum triglyceride level (13).
The distributiOn of n...hexane wa,s measured in 1;1he blood, liver, brain, lungs, kidneys, spleen, and adn~na1s of rats that had been exposed ,to n-hexane vapor in the concenkaltion of 170,000 mg/m 3 (47,200 ppm) for 2-10 h. After 10 h the liver was Sltil'l not fully saturated. A linear accumulation of flalt (triglycerides) was nOited. In this manner the affinity of the liver for hexane was iJncreased. Thus, the complete saturation of the 'liver did IlJot take piace as long as the content in fat was increasing (13).
Other studies have not shown liver damage iIIl guinea pigs after skin contact (36) or in rats after oral intake (30). Hexane potenti,ated the liver toxicity ad' trichloromellhanes when the ,two were given simultaneously to rats (30).

Blood and blood-forming organs
In humans expo'sed to n-hex:ane vapor seV'eflal inv-estigClltors have found mild hypochronic anemia that becomes normal under hospitalization (51,62,66). In addibon pleocytosi,s has been found in a young sniffer (38).
Rabb~ts wer,e g~ven intravenous and subcultaneous dai'ly injections of n-hexane for 6 d. After '1!he fiIlst day, ,a drop in the Leucocyte cou:ntt from 2,800/mm 3 to 600/ mm 3 was noted. Thereafter, 1Jhe count gradually nonmali2Jed over the five remaining days. Mlber the sixth day the bone marrow was microscopi'Cally examined and found to be normal (8).
Mild changes in the reticuloendo!fJhe'liJal system of t!he spleen in '1Jhe rorm of hemosid€lfin piJgmenta-tion and JIlhe occurrence of giant cells has been described for rats -exposed to an n-hexane concentra,tion of 3,060 mg/m 3 (850 ppm) for 143 d (37).

Kidneys and the gastrointestinal and cardiovascular systems
No pafhological Changes in ,the kidneys, stomach, gastrointestinal system, heart, or blood vessels have been described after exposure to n-hexane.

Central nervous system
Shom exposure to high doses of n-'hexane ca'UlSe8 na.rc-oSiis in man. Headache and nausea are caused by 5,400 mg/m 3 (1,500 ppm). ConiLJlSion and dizziness are caused by 'e:lq>OSUre ,t-o 18,000 mg/m 3 (5,000 ppm) for 10 min (43). Headache, nausea, and anorexia are the most oommen eNS symptoms -after the inhal!ation of n-hexane.
The same morphoilogkal changes have been f'OUJnd in boitlh the peI1iphem'l and centra'! nervous system. These histo-pathoiogicaJ changes had been found in the spinal cord, the medulla oblongata, the cerebellum, and the cerebrum (4, 56).
Shaumburg & Spencer (56) postulated 1Jhat dysfunotion aIt tihe peripheral nerve level masks signs of disturban!Ce in the CNS. C-ompli€lte healilIlg of CNS lesions seldom OCCUI1S. This si:1Juation can explain the reflex abnormaliJties and spasitiJcity of the :legs that have been observed 2 a after exposure.

Peripheral nervous system
The outstanding toxic eiffects of n-he)ljane invulve rthe peripheral nervous system (tabl,e 1). The symptom compl'ex, which is known as hexacarbon neuropathy, is comprised of sensory and motor abnormalities, the motor abnormalities playing the larger role.
Folyneuropatlhy has been described following occupational exposure to nhexane, aHer the sni:fJfing of hexane vapor in order to be "high," and for experimental animals, especially rats and md.ce, that harv,e been exposed only to n-hexane.
Under tJhe electron miJcroscope, the swollen axons have been found to contain an increased number of closely packed neurofilaments interspaced with a few glycogen granules. "Dense bodies" and mitochondria were found spread throughout the myelinized fibers. Within the Schwann cells proliferation processes and cytoplasmic increases of both glycogen and Reichert granules have been described (3,28,29,38,51,56,58,59,63).
Hel1skowttz et al examined 1fue intramuscular sensory and motor end plates in man under the electronic microscope (29). 'TIhe neI1ves Icont,ainedan increased amount of neurofi'lamenlts. 11he mitochondria that contained "dense bodies" had a significant "onion-bulb" formation and abnor:mal membrane struotures. The mOltor end plates oontained a swelling of the terminal axons, an iillcreasedamount of degener'ated miJtodhondriJa, 'and glycogen granules. In add~1Ji.on "dense !bodies," laI1ge osmiophil bodies, and an increased number of both synaptilc folds and vesicles were described.
The histograms of myelinized nerve fibers from the sural nerve in sniffers 360-2,160 mg/m 3 (100-600 ppm) with maximum exposure at 9,000 mg/m 3 (2,500 ppm) 70-95 % n-hexane 2-6 months Sensation disturbances; muscle weakness; and distal symmetric pain in the legs Muscle atrophy; hypotonic, decreased muscle strength; deficient reflexes with foot drop and in some cases hand drop; paresthesia and hypoesthesia distally in the arms and legs Decreased motor nerve conduction and decreased sensory nerve conduction with an increased distal latency period; electromyographic indications of neurogenic damage SwelHng of nerves with thinning of the nerve sheath and paranodal retractions; muscle atrophy of fibers Swelling ofaxons with closely packed masses of neurofilaments; other signs of degeneration In certain cases between 2-3 months after the cession of exposure Very long (0.5-1 a); in some cases symptoms and clinical polyneuropathy after 2 a 1,14,18,29,45,48,51,66 1,400-36,000 mg/m 3 (400-10,000 ppm) n-Hexane alone 4--5 months Walking disturbances

Muscle atrophy and foot drop
Decreased motor nerve conduction and sensory nerve conduction; increased refractory period; decreased excitability Local dilatation ofaxons with a thinning of the myelin sheath; ovoid change of the myelin Closely packed masses of long neurofilaments 37,56,58,64