Acetone

Jennifer Butler

SUMMARY

• Acetone is absorbed by all routes; inhalation is the main route of exposure in the workplace

• Vapour exposure leads to irritation of the mucous membranes, eyes, nose and throat

• Systemically, acetone is a central nervous system (CNS) depressant leading to dizziness, confusion and drowsiness which can progress to stupor and coma depending on the concentration and length of exposure

• Toxicity following acetone exposure is non-specific; systemic effects may be confused with other medical conditions

• Acetone has not been reported to increase the risk of cancer in occupationally exposed individuals

• Although animal studies suggest that acetone at high concentrations may be teratogenic, there is very little information to suggest that it has the same effects in humans

DESCRIPTION

Synonyms

Acetone, dimethyl formaldehyde, dimethyl ketal, dimethyl ketone, ketone propane, methyl ketone, propanone, pyroacetic acid, pyroacetic ether, beta ketopropane, 2-propanone, allylic alcohol, ketone propane, acetone oil

Identification numbers

CAS UN

RTECS EINECS

67-64-1 1090

AL 3150000 2006622

PHYSICAL AND CHEMICAL PROPERTIES

chemical formula molecular formula molecular mass physical form relative vapour density (air = 1) flash point (°C)

CHa-

-CHa

58.09

volatile, highly flammable liquid 2.00 -18

boiling point (°C) autoignition temperature (°C) refractive index

56.5

1.356

explosive limits in air (%v/v)

Odour threshold

A wide range of odour thresholds have been documented for acetone ranging from 13 to 680 ppm (Amoore and Hautala, 1983; Ruth, 1986; Morgott, 2001). There may be a degree of tolerance in the perception of acetone odour. Workers with previous occupational exposure to acetone had reduced perception of the intensity of acetone at a concentration of 800 ppm, than non-exposed control subjects (Dalton et al, 1997).

OCCUPATIONAL EXPOSURE

Exposure limits

TWA (UK): 750 ppm (1810 mg/m3) TWA (ACGIH): 500 ppm (1190 mg/m3)

Conversion factors

1 ppm = 2.37 mg/m3 1 mg/l = 421 ppm 1 mg/m3 = 0.421 ppm

Biological monitoring

The ACGIH biological exposure index for acetone is an end of shift urinary acetone concentration of 50 mg/l (ACGIH, 2000). Acetone can occur naturally in humans as a by-product of metabolism (giving a blood concentration of up to 10 mg/l), and may be present in the urine of individuals who have been exposed to other agents; this must be taken into account when interpreting blood and urine results (ACGIH, 2000; Baselt, 2000).

Acetone is ubiquitous; in addition to its extremely wide use as a solvent, it is a naturally occurring metabolite in plants and animals. In humans, it is produced in the breakdown and use of stored fats. Industrially, its role can be divided into three main areas; use as chemical feedstock, as a formulating solvent for commercial products, and as an industrial process solvent. Owing to its many useful properties it is the preferred formulating solvent for a huge range of paints, varnishes, inks, car-care products, coatings and even for use in health and beauty products.

Acetone is absorbed by all routes, but in the industrial setting, toxicity is most likely by the inhalation route, and then only in exceptional circumstances. Acetone toxicity does not have a characteristic syndrome and signs of intoxication may be confused with other medical conditions (Foley, 1985; Morgott, 2001).

Normal blood acetone concentrations can be up to 10 mg/l (Baselt, 2000). The amount of acetone present can be affected by many physiological, clinical and chemical factors such as, pregnancy, lactation, dieting, vigorous physical exercise/exertion, starvation, diabetes mellitus and prolonged vomiting (Morgott, 2001). In relation to the workplace, an important contributing factor is increased physical effort, which may increase acetone production and this should be taken into account when interpreting blood and urine concentrations.

TOXICITY

Following exposure to acetone, blood concentrations may not correlate with clinical effects. In a case reported by Wijngaarden et al. (1995) an acetone concentration of 290 mg/l was associated with decreased levels of consciousness. The patient was reported to have a Glasgow coma scale (GCS) of 8 with a blood acetone concentration of 490 mg/l. The patient was acidotic (pH 7.11), and this may also have been a contributing factor to her altered level of consciousness. An alcoholic patient, who had ingested an unknown quantity of isopropanol, was awake and conversant with an acetone concentration in excess of 1,600 mg/l (Gaudet and Fraser, 1989). A concentration of 4,450 mg/l was recorded in a child who had ingested an estimated 160 ml of nail varnish remover (acetone 60%, isopropanol 10%). The patient was comatose, with areflexia and respiratory depression (Gamis and Wasserman, 1988). Ingestion of 5,440 mg acetone (80 mg/kg) in a male volunteer (an estimated 7 ml of pure acetone) had no reported ill effects. Acetone was detected in the blood soon after exposure at a concentration of 720 mg/l (Haggard et al., 1944).

Fatalities following exposure to acetone are extremely rare and patients can survive massive exposure with good supportive medical management (Strong, 1944; Gitelson et al., 1966; Gamis and Wasserman, 1988; Gaudet and Fraser, 1989; Wijngaarden et al., 1995).

Absorption

Acetone is rapidly absorbed via the lungs. It has a high blood:air partition ratio, suggesting that a large portion of inhaled acetone is absorbed (Haggard et al., 1944). Volunteers exposed to acetone vapours at concentrations of 100 and 500 ppm had peak blood concentrations of 100-700 mg/l at two hours post exposure (Baselt, 2000).

On ingestion acetone is rapidly absorbed. Ingestion of 10,000 mg of acetone, on an empty stomach achieved a peak blood acetone concentration of 327 mg/l, at ten minutes post ingestion (Baselt, 2000).

Distribution

There is very little information on the distribution of acetone. However, it is a non-ionic substance, miscible with water, and these properties allow it to passively diffuse across cell membranes, and distribute throughout body fluids (IPCS, 1998; Morgott, 2001). The volume of distribution for acetone is 0.8 l/kg (Baselt, 2000).

Metabolism

Acetone is metabolised by a variety of routes. The three main routes have been identified as the lactate, methylglycoxal and propanediol pathways. It is metabolised by these pathways via several intermediates. Most of the intermediates and final metabolites are not considered to be toxic (Harbison and Garvey, 1998; IPCS, 1998; Morgott, 2001). Acetone is gluconeogenic, i.e., it can be used as a material source for the biosynthesis of glucose. The body also utilises the metabolites of acetone for the synthesis of other endogenous compounds.

The first step in acetone metabolism is cytochrome P450 oxidation to acetol by the enzyme acetone monooxygenase. The acetol is further metabolised by either of two pathways, an intrahepatic methylglycoxal pathway or an extrahepatic propanediol pathway. The oxidation of acetol to methylglycoxal is also cytochrome P450-dependent. At larger acetone concentrations, above those resulting from normal metabolic processes, the activity of cytochrome P450 CYP2E1 is induced, and consequently functions at a much higher rate. This in turn increases the enzymatic elimination of acetone. This self-induction allows acetone to regulate its own metabolism (Morgott, 2001).

Elimination

Endogenous acetone is eliminated via metabolic pathways (IPCS, 1998; Morgott, 2001). At higher concentrations, these pathways can become saturated, and elimination is mainly via the lungs (Morgott, 2001). This was confirmed by human volunteer studies carried out by Nomiyama and Nomiyama (1974). The authors studied the elimination of seven organic solvents via the respiratory tract. Acetone was found to have a high ratio of eliminated to retained solvent, suggesting that acetone is eliminated unchanged via the lungs with a small portion retained in the blood. The remaining acetone is eliminated via the kidneys and by its metabolic pathways.

The elimination half-life of acetone is variable but usually falls in the range 18-27 hours (Natowicz et al., 1985; Sakata et al., 1989; Jones, 2000). Acetone is removed by haemodialysis at a rate of 7,000 mg per hour (using a 1.0 m2 standard dialyser). This is approximately 40 times the rate of urinary elimination (Rosansky, 1982). Urinary elimination of acetone is linearly related to the amount of acetone absorbed (Pezzagno et al., 1986).

Mode of action

The mechanism of toxicity of acetone is not fully understood. It appears to be moderately toxic to the liver causing haematological effects, but the mechanism of these effects is unknown. In rodent studies, acetone has been reported to be nephrotoxic possibly due to hyaline droplet formation, but this mechanism is not relevant in humans. Liver and kidney weights are increased in experimental animals exposed to acetone. This may be a result of acetone induction of microsomal enzymes (reviewed in IPCS, 1998 and Morgott, 2001).

The CNS effects witnessed in acetone intoxication may be attributed in part to the metabolic acidosis caused by acetone. Accumulation of ketones can occur in certain disease states, e.g., in diabetic and alcoholic ketoacidosis. The resultant acidosis can cause CNS effects such as lassitude, dizziness, delirium, and drowsiness.

Metabolic interactions

Acetone can interact with other agents in two main ways: Enzyme induction

Acetone is an inducer of cytochrome P450 CYP2E1 and other microsomal enzymes (IPCS, 1998; Morgott, 2001). As many other industrial chemicals are metabolised via this enzyme system, increased activity can increase the potential toxicity of other workplace chemicals.

'Co-solvency'

Geller et al. (1979) have suggested a mechanism of 'co-solvency', which may be particularly significant for nervous system tissue, where there are barriers between the circulation and the nerve cells. Membranes with a high fat content such as myelin, preferentially take up hydrophobic molecules, whereas penetration to the cytoplasm of the nerve cell bodies is facilitated by a hydrophilic molecule. In some instances combinations of solvents may have a synergistic effect, acting as a single agent. This synergism is not limited to interactions involving acetone.

• Carbon tetrachloride (CCl4)

Acetone is considered a major potentiator of CCl4 toxicity (Folland et al., 1976; IPCS, 1999). This may be related to acetone-induced enzyme induction, increasing the production of the reactive metabolites of CCl4.

Both hexane and MnBK are metabolised to the neurotoxic metabolite, 2,5-hexanedione. The serum and nerve concentration of 2,5-hexanedione was significantly increased in rats treated with 2,5-hexanedione in combination with acetone compared to 2,5-hexanedione alone. The effect with acetone was weaker than that of methyl ethyl ketone but stronger than that of toluene (Zhao et al., 1998).

• Methylene chloride

One of the routes of metabolism of methylene chloride is a microsomal oxidation process (involving cytochrome P450). Carbon monoxide is formed by this microsomal oxidation pathway (Gargas et al., 1986) and concurrent exposure to acetone may increase the production of carbon monoxide, by the induction of this pathway.

In volunteer studies, exposure to a mixture of MEK and acetone had no effect on neurobehavioural performance. MEK does not potentiate the effect of acetone and there is no pharmacokinetic interaction between the two ketones (Brown et al., 1987; Dick et al., 1988; Dick et al., 1989).

• Trichloroethylene

Trichloroethylene is metabolised principally in the liver by two pathways, one of which is by oxidation via cytochrome P450. Induction of this isoenzyme system by acetone may increase the metabolism of trichloroethylene to its toxic metabolites. The toxic metabolites of trichloroethylene may affect dopaminergic transmission in the brain resulting in CNS dysfunction (Mutti and Franchini, 1987).

CASE REPORTS

Coma and metabolic acidosis following application of a muscle liniment

A 47 year old woman presented to a rural hospital with a 2 to 3 week history of increased weakness, dizziness and fatigue. The main concern at the time of presentation was a 24 hour history of decreasing level of consciousness. At presentation the patient had normal vital signs but a GCS of 8 to 10. Initial blood biochemistry showed an anion gap acidosis, with a normal blood glucose concentration (5.1 mmol/l). The patient required intubation and was transferred to a regional hospital. On arrival at the emergency department, she had a GCS of 10. The only other striking feature was a marked metabolic acidosis (pH 7.09-7.11). Aspartate aminotransferase (AST) and creatinine were slightly elevated. The patient underwent haemodialysis. Further blood results showed an acetone concentration of 8 mmol/l (464 mg/l). After 10 hours of dialysis her mental status improved considerably. Blood acetone concentrations 6 hours post dialysis were 3 mmol/l (174 mg/l). The following morning her level of consciousness deteriorated again. Acetone concentrations were repeated and were 5 mmol/l (290 mg/l). She was dialysed for a further 6 hours, with a post dialysis acetone concentration of 1 mmol/l (58 mg/l). The acetone concentrations remained at this level for a further 24 hours with the patient awake and well. On closer questioning it was discovered that the patient had, for a 1-2 week period, been applying a muscle liniment for leg discomfort. The most recent application had occurred on the day before admission. On examination the liniment in question was found to be 70% v/v acetone. Inhalation, as well as the dermal route, was thought to be significant in this case (Wijngaarden et al., 1995).

Acute inhalation of acetone

Several workers became unwell after occupational exposure to acetone. They had been involved in cleaning out a pit into which water from a burst water main had flowed 10 days previously. Near the pit there were four 10 gallon tanks which were ultrasonically agitated. Two tanks contained acetone and two contained 1,1,1-trichloroethane. The water from the burst pipe had escaped into the pit, joining water which had seeped through the foundations. Four workers began removing the 4 inch depth of water from the bottom of the pit. Two shovelled water into a bucket which was pulled out of the pit on a rope. Two of the workers noticed a sickly sweet smell and one complained of weakness and a headache. The other complained of eye irritation and feeling drunk. After returning from an hour lunch break he collapsed in the pit. One of the workers who had been pulling the bucket out of the pit went to help him. However, he felt faint and sent another worker for help. Four other workers helped the two men in the pit. These workers complained of dizziness, eye irritation, chest tightness and weakness. The two workers who had spent longest in the pit were admitted to hospital. One was unconscious with vomiting and a poor pulse. The other who had been the first to collapse, was drowsy, nauseated, vomiting, confused and ataxic. Both recovered. Measurements in the pit 3 hours, 18 hours and one week later found air concentrations of acetone in excess of 12,000 ppm and 1,1,1-trichloroethane up to 50 ppm. It was thought that the acetone had gradually evaporated from the tanks, moved along the floor to the pit and some had become dissolved in the water. Agitation of this water by shovelling into a bucket had increased the concentration in the pit by releasing dissolved acetone (Ross, 1973).

CLINICAL EFFECTS Acute exposure Inhalation

Human volunteers exposed to acetone vapours of 200 ppm considered it satisfactory for an eight hour exposure period. A concentration of 300 ppm produced slight irritation, and at 500 ppm the subjects complained of irritation to the eyes, nose and throat, but considered the atmosphere still tolerable (Nelson et al., 1943). Workers exposed to acetone concentrations in excess of 12,000 ppm over a period of several hours developed dizziness, confusion, drowsiness, ataxia, and in one case coma (Ross, 1973). Application of a synthetic plaster cast using an acetone based solvent lead to systemic toxicity in the treated patients (Hift and Patel, 1961). See systemic effects below.

Acetone exposed workers may develop a tolerance to the irritant effects of acetone. In a study of occupationally versus non-occupationally exposed individuals, the perceived irritant effects of acetone (800 ppm) were significantly lower in the occupationally exposed group. While the control group complained of a wide range of symptoms including skin irritation, nasal congestion, itching and sweating, the occupationally exposed group had fewer complaints, with nasal irritation the only symptom described as exceeding the rating of 'weak' (Dalton et al., 1997).

Keisswetter et al. (1996) studying the effects of night-shift work and solvent exposure, found that acetone exposed workers (average air concentration, 1,000 ppm) complained more of tiredness, tension and annoyance compared to the control and mixed solvent groups. In a primate study, baboons exposed to 500 ppm acetone for 24 hours per day for seven days, had reduced response times at tasks, but there was no effect on the accuracy of the tasks (Geller et al., 1979).

Systemic effects are unlikely from occupational inhalation exposure except in exceptional circumstances and at concentrations vastly exceeding the current workplace exposure limits.

Dermal

Acetone is a skin irritant. With prolonged contact it may have a defatting action on the skin. Topically applied acetone left on human subjects for 30 to 90 minutes caused considerable damage to the skin, with recovery by 72 hours post exposure (BUA, 1997).

Acetone is absorbed dermally. Systemic effects have been reported in patients exposed to acetone during the application of synthetic casts and from the topical application of a muscle liniment. In these cases however, inhalation may also have played a role in the absorption of the acetone (Strong, 1944; Hift and Patel, 1961; Wijngaarden et al., 1995).

Acetone at concentrations exceeding 500 ppm caused irritation to the eyes of volunteers (Nelson et al., 1943; Dalton et al., 1997).

Direct splash contact to the eye causes an immediate stinging sensation. Serious effects are unlikely if the eye is promptly irrigated. Patchy epithelial injury may occur but usually recovers over 24 to 48 hours (Grant and Schuman, 1993).

Ingestion

Acetone is irritating to mucous membranes. On ingestion it causes pain and redness to the mouth and throat. Ingestion of 200 ml caused swelling of the throat with erosions to the soft palate and entrance to the oesophagus (Gitelson et al., 1966). Acetone is quickly absorbed from the gastrointestinal tract with systemic effects likely via this route.

No toxic effects were reported in adults after oral administration of 40-80 mg/kg (Haggard et al., 1944). Ingestion of 200 ml of pure acetone caused coma and respiratory depression (Gitelson et al., 1966).

Acetone is not an aspiration risk (Panson and Winek, 1980), however, there may be a risk of aspiration of stomach contents in an acetone-poisoned patient, with altered mental status.

Systemic effects

There are many reports describing the clinical effects of acute acetone intoxication (Strong, 1944; Hift and Patel, 1961; Gitelson et al., 1966; Gamis and Wasserman, 1988; IPCS, 1998; Morgott, 2001). Acetone is a CNS depressant leading to dizziness, lethargy, confusion and drowsiness. These effects may progress through stupor to coma, depending on the concentrations and length of exposure. Pupils may be pin-point.

Gastrointestinal effects include nausea, abdominal pain, and late onset vomiting which may contain blood. There is risk of aspiration of stomach contents in the sedated patient. Commonly, the breath has a strong fruity odour. In severe cases Kussmaul breathing and respiratory depression occur. Acidosis is a common feature, and the patient may also be sweating, flushed and tachycardic.

As endogenous acetone is involved in the biosynthesis of glucose, hyperglycaemia may occur following large exposures. In cases where intoxication has been severe, hyperglycaemia may persist after the patient has recovered from the acute phase of toxicity. The persistence of hyperglycaemia has been reported to last from 4 days to 4 months (Strong, 1944; Gitelson et al., 1966).

The clinical picture of acetone intoxication is not specific and can be confused with other medical conditions, e.g., diabetic and alcoholic ketoacidosis (Foley, 1985; Morgott, 2001).

The reports of renal effects following acetone exposure are conflicting. In animal models, acetone has been observed to cause renal toxicity. However, renal effects have not been reported in cases where patients have had high blood acetone concentrations. Hawley and Falko (1982) reported a case of elevated serum creatinine in the absence of renal damage following ingestion of isopropanol. This deranged level was attributed to high acetone concentrations (acetone is a metabolite of isopropanol), possibly interfering with the assay, rather than a direct toxic effect by the parent chemical or its metabolite. The renal effects reported in some cases of acetone poisoning may be secondary to muscle breakdown in the deeply sedated patient.

Chronic exposure

Inhalation

There are very few reports to indicate that prolonged inhalation of low vapour concentrations result in any serious chronic effects in humans. Workers exposed to 1,000 ppm 3 hours per day for 7 to 15 years complained of inflammation of the respiratory tract, stomach and duodenum (Vigliani and Zurlo, 1955).

Immunotoxicity

Acetone has provided conflicting results in sensitisation tests in animals and there are no data available on a sensitising effect in humans. It did not give a positive result in a mouse ear swelling test. This test is performed to judge the allergenic potential by observing the amount of swelling occurring after a topical challenge (BUA, 1997).

Dermal

Acetone is a skin irritant. With prolonged contact it may have a defatting effect. Topically applied acetone left on human subjects for 30 to 90 minutes caused considerable damage to the skin, with recovery by 72 hours post exposure. Acetone has provided conflicting results in sensitisation tests in animals and there are no data available on a sensitising effect in humans. However, as acetone is an endogenous compound present in relatively high concentrations in the normal population, as well as having a role in metabolism, a sensitising effect from acetone is unlikely (BUA, 1997).

Toxicology of Solvents Eye

No information available. Ingestion

No information available.

Carcinogenicity

Acetone has not been evaluated by the IARC as a human carcinogen. A review of the literature by Morgott (2001) found that workers exposed to acetone in various industries did not show any significant increase in the incidence of mortality from non-Hodgkin's lymphoma, multiple myeloma or breast cancer.

Genotoxicity

Various mutation studies, with and without metabolic activation, were negative for various Salmonella typhimurium and Bacillius subtilis strains exposed to 1-10 mg acetone per plate or well. There were also negative findings for a range of sister chromatid exchange assays (reviewed by BUA, 1997 and IPCS, 1998).

Reproductive toxicity

There is very little information on the effects of acetone on reproduction and developmental effects in the human population. The majority of studies have involved workers with co-exposure to multiple solvents.

Agnesi et al. (1997) investigated 108 women exposed to 12 solvents involved in shoe making. The risk of spontaneous abortion was found to be higher in the exposed group compared to controls. However, in a similar study by Taskinen et al. (1989) there was no significant increase in the risk of spontaneous abortion in acetone-exposed workers. This conclusion was reiterated in a later study involving female laboratory workers (Taskinen et al., 1994).

In animal and embryo studies acetone has been found to produce mild reproductive and developmental changes. However, these studies involved the use of acetone at extremely high concentrations (reviewed in Morgott, 2001).

RISK GROUPS

There are no specific risk groups for acetone. However, acetone is an inducer of CYP2E1 and may increase the toxicity of other workplace chemicals metabolised via this enzyme.

HOSPITAL MANAGEMENT

Inhalation

The victim should be removed from exposure and all contaminated clothing removed. The respiratory function should be assessed. Further treatment is symptomatic and supportive. See below for management of systemic effects.

Dermal

Contaminated clothing should be removed and the skin thoroughly irrigated with water or saline. Further treatment is symptomatic and supportive. See below for management of systemic effects.

The eyes should be thoroughly irrigated with water or saline for 15 minutes and then stained with fluorescein. Referral to an ophthalmologist is recommended if there is any uptake of fluorescein.

Ingestion

Gastric lavage is not necessary following ingestion of acetone. Activated charcoal is not of benefit. If there has been any vomiting, coughing or wheezing in a patient with altered mental status, then the patient will need to be assessed to determine if aspiration has occurred. Treatment is supportive. See below for management of systemic effects.

Systemic effects

Treatment for acetone intoxication is essentially supportive. It is important to ensure the patient is adequately hydrated. The electrolytes, blood glucose and blood gases should be monitored. Acidosis should be treated aggressively with bicarbonate. If facilities allow, blood should be taken for the determination of blood acetone concentrations. In patients with decreased mental status the renal function should be monitored. Respiratory depression may require ventilation. Haemodialysis removes acetone at approximately 40 times the rate of urinary elimination, and should be considered in patients with severe CNS, respiratory or metabolic toxicity.

Antidotes

There is no specific antidote for acetone.

Chronic exposure

In most cases of chronic poisoning clinical effects resolve gradually once exposure has ceased. Treatment is symptomatic and supportive care.

REFERENCES

ACGIH. 2000 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Limits. American Conference of Governmental Industrial Hygienists.

Agnesi R, Valentini F, Mastrangelo G. 1997 Risk of spontaneous abortion and maternal exposure to organic solvents in the shoe industry. Int Arch Occup Environ Health 69:311-316.

Amoore JE, Hautala E. 1983 Odor as an aid to chemical safety: Odor thresholds compared with threshold limit values and volatilities for 214 industrial chemicals in air and water dilution. J Appl Toxicol 3 (6): 272-290.

Baselt RC. 2000 Disposition of Toxic Drugs and Chemicals in Man, fifth edition. Chemical Toxicology Institute, California.

Brown WD, Setzer JV, Dick RB, Phipps FC, Lowry LK. 1987 Body burden profiles of single and mixed solvent exposures. J Occup Med 29 (11):877-883.

BUA (Beratergremium für Umweltrelevante Altstoffe). 1997 Acetone. BUA Report 170 (June 1995). S Hirzel, Stuttgart.

Dalton P, Wysocki CJ, Brody MJ, Lawley HJ. 1997 Perceived odor, irritation, and health symptoms following short-term exposure to acetone. Am J Ind Med 31:558-569.

Dick RB, Brown WD, Setzer JV, Taylor BJ, Shukla R. 1988 Effects of short duration exposures to acetone and methyl ethyl ketone. Toxicol Lett 43 (1-3):31-49.

Dick RB, Setzer JV, Taylor BJ, Shukla R. 1989 Neurobehavioural effects of short duration exposures to acetone and methyl ethyl ketone. Br J Ind Med 46:111-121.

Foley RJ. 1985 Inhaled industrial acetylene. A diabetic ketoacidosis mimic. J Am Med Assoc 254 (8):1066-1067.

Folland DS, Schaffner W, Ginn HE, Crofford OB, McMurray DR. 1976 Carbon tetrachloride toxicity potentiated by isopropyl alcohol. Investigation of an industrial outbreak. J Am Med Assoc 236:1853-1856.

Gamis AS, Wasserman GS. 1988 Acute acetone intoxication in a pediatric patient. Pediatr Emerg Care 4 (1):24-26.

Gargas ML, Clewell HJ, Andersen ME. 1986 Metabolism of inhaled dihalomethanes in vivo: Differentiation of kinetic constants for two independent pathways. Toxicol Appl Pharmacol 82:211-223.

Gaudet MP, Fraser GL. 1989 Isopropanol ingestion: case report with pharmacokinetic analysis. Am J Emerg Med 7 (3):297-299.

Geller I, Gause E, Kaplan H, Hartmann RJ. 1979 Effects of acetone, methyl ethyl ketone and methyl isobutyl ketone on a match-to-sample task in the baboon. Pharmcol Biochem Behav 11 (4):401-406

Gitelson S, Werczberger A, Herman JB. 1966 Coma and hyperglycaemia following drinking of acetone. Diabetes 15 (11):810-811.

Grant WM, Schuman JS. 1993 Toxicology of the Eye, fourth edition. Charles C Thomas, Springfield, Illinois.

Haggard HW, Greenberg LA, McCullough Turner J. 1944 Physiological principles governing the action of acetone together with determination of toxicity. J Ind Hyg Toxicol 26 (5):133-151.

Harbison RD, Garvey GJ. 1998 Aldehydes and Ketones. In: Hamilton and Hardy's Industrial Toxicology, fifth edition. Harbison RD (editor). Mosby, St Louis.

Hawley PC, Falko JM. 1982 'Pseudo' renal failure after isopropyl alcohol intoxication. S Med J 75 (5):630-631.

Hift W, Patel PL. 1961 Acute acetone poisoning due to a synthetic plaster cast. S Afr Med J 35:246-250

IPCS. 1998 Environmental Health Criteria 207. Acetone. World Health Organization, International Programme on Chemical Safety, Geneva.

IPCS. 1999 Environmental Health Criteria 208. Carbon Tetrachloride. World Health Organization, International Programme on Chemical Safety, Geneva.

Jones AW. 2000 Elimination half-life of acetone in humans: case reports and review of literature. J Anal Toxicol 24:8-10.

Kiesswetter E, Seeber A, Blaszkewicz M, Sietmann B, Vangala RR. 1996 Neurobehavioural effects of solvents and circadian rhythms. Neurotoxicol 17 (3-4):777-784.

Morgott DA. 2001 Acetone. In: Patty's Toxicology, fifth edition, Volume 6. Bingham E, Cohrssen B, Powell CH (editors). John Wiley & Sons Inc, New York.

Mutti A, Franchini I. 1987 Toxicity of metabolites to dopaminergic systems and the behavioural effects of organic solvents. Br J Ind Med 44:721-723.

Natowicz M, Donahue J, Gorman L, Kane M, McKissick J, Shaw L. 1985 Pharmacokinetic analysis of a case of isopropanol intoxication. Clin Chem 31 (2):326-328.

Nelson KW, Ege JF Jr, Ross M, Woodman LE, Silverman L. 1943 Sensory response to certain industrial solvent vapors. J Ing Hyg Toxicol 25:282-285.

Nomiyama K, Nomiyama H. 1974 Respiratory elimination of organic solvents in man. Benzene, toluene, n-hexane, trichloroethylene, acetone, ethyl acetate and ethyl alcohol. Int Arch Arbeitsmed 32:85-91.

Panson RD, Winek CL. 1980 Aspiration toxicity of ketones. Clin Toxicol 17:271-317.

Pezzagno G, Imbriani M, Ghittori S, Capodaglio E, Huang J. 1986 Urinary elimination of acetone in experimental and occupational exposure. Scand J Work Environ Health 12:603-608.

Rosansky SJ. 1982 Isopropyl alcohol poisoning treated with hemodialysis: kinetics of isopropyl alcohol and acetone removal. Clin Toxicol 19 (3):265-271.

Ross DS. 1973 Acute acetone intoxication involving eight male workers. Ann Occup Hyg 16 (1):73-75.

Ruth JH. 1986 Odor thresholds and irritation levels of several chemical substances: a review. Am Ing Hyg Assoc J 47:142-151.

Sakata M, Kikuchi J, Haga M. 1989 Disposition of acetone, methyl ethyl ketone and cyclohexanone in acute poisoning. Clin Toxicol 27 (1-2):67-77.

Strong GF. 1944 Acute acetone poisoning. Can Med Assoc J 51:359-362.

Taskinen H, Anttila A, Lindbohm M-L, Sallmen M, Hemminki K. 1989 Spontaneous abortions and congenital malformations among the wives of men occupationally exposed to organic solvents. Scand J Work Environ Health 15:345-352.

Taskinen H, Kyyrönen P, Hemminiki K, Hoikkala M, Lajunen K, Lindbohm M-L. 1994 Laboratory work and pregnancy outcome. J Occup Med 36 (3):311-319.

Vigliani EC, Zurlo N. 1955 Arch Gewerbepath Gewerbrhyg 13:528-534. Cited in BUA (Beratergremium für Umweltrelevante Altstoffe). 1997 Acetone. BUA Report 170 (June 1995). S Hirzel, Stuttgart.

Wijngaarden M van, Mock T, Dinwoodie A, LeGatt D, Yatscoff R. 1995 Coma and metabolic acidosis related to the use of muscle liniment. Crit Care Med 23 (6):1143-1145.

Zhao W, Misumi J, Yasui T, Aoki K, Kimura T. 1998 Effects of methyl ethyl ketone, acetone, or toluene coadministration on 2,5-hexanedione concentration in the sciatic nerve, serum, and the urine of rats. Int Arch Occup Environ Health 71 (4):236-244.

Nicola Bates

Benzene

Nicola Bates

SUMMARY

• Acute benzene exposure results in irritation and CNS depression

• The most significant chronic health effects are haematotoxicity, immunotoxicity and carcinogenicity

• Benzene causes bone marrow toxicity varying from mild effects to aplastic anaemia

• Benzene is known to cause cancer, particularly of the lymphatic and haematopoietic systems

• Benzene is a genotoxic carcinogen

• The metabolites of benzene are thought to be responsible for toxicity; the mechanisms of toxicity are complex and multifactorial

• Individual susceptibly to benzene toxicity is variable

DESCRIPTION

Synonyms

Annulene, benzol, benzole, coal naphtha, cyclohexatriene, phenyl hydride, pyrobenzol, pyrobenzole. Note: Benzin and benzine have been used as synonyms for benzene but now refer to a low-boiling petroleum distillate consisting mainly of aliphatic hydrocarbons (Hamilton, 1922; Greenburg, 1926; Barlow and Sullivan, 1982).

Identification numbers

CAS 71-43-2

UN 1114

RTECS CY1400000

EINECS 2007537

PHYSICAL AND CHEMICAL PROPERTIES

chemical formula molecular formula

78.11

molecular mass physical form relative vapour density (air=1) flash point (°C closed cup)

clear, colourless liquid

78.11

autoignition temperature (°C) 498

refractive index 1.5016

Odour threshold

Odour threshold has been measured as 12 ppm (Amoore and Hautala, 1983), 4 ppm (Jex and Wyman, 1996), 1.1 ppm (Gusev, 1965) and 1.4-84 ppm (Ruth, 1986).

OCCUPATIONAL EXPOSURE

Exposure limits

Country TWA

Sweden 0.5 ppm

UK 3 ppm (to be lowered to 1 ppm by 27 June 2003)

USA 1 ppm

Conversion factors

1 ppm = 3.2 mg/m3 1 mg/m3 = 0.31 ppm 1 mg/l = 310 ppm

Biomonitoring

Biomonitoring of benzene exposure usually involves measurement of one or more of the urinary metabolites (outlined below) as this is less invasive than measuring concentrations in blood. However, as the occupational exposure limits have decreased and workers are exposed to lower concentrations of benzene, so the sensitivity of benzene biomarkers has been questioned. This is a particular problem for exposures to concentrations of less than 1 ppm benzene.

Ong et al. (1996) investigated the specificity of various biomarkers for biological monitoring of workers exposed to low benzene concentrations (<1 ppm). They measured environmental benzene concentrations, benzene concentrations in blood and urine and urinary concentrations of trans,trans-muconic acid, catechol, phenol and hydroquinone. Significant correlation was shown between all urinary biomarkers, except catechol, and air benzene concentrations. The strongest correlation was between air and urinary benzene concentrations, followed by blood benzene concentrations. Of the urinary phenol and hydroquinone concentrations, the latter was more strongly correlated to the air benzene concentration. However, when the exposure was low (<0.25 ppm) only urinary benzene concentrations were significantly correlated to air concentrations. No significant differences were found in non-exposed controls compared to subjects occupationally exposed to low benzene concentrations. Consequently, they concluded that the currently used biomarkers are unable to differentiate between subjects exposed to <0.25 ppm benzene and those with only background exposure.

The problem is further complicated by evidence which suggests that at low benzene concentrations the proportion of the different metabolites produced is a function of the exposure dose. Rothman et al. (1998) compared the concentrations of urinary metabolites in workers exposed to benzene at <25 ppm and >25 ppm. In the group with the higher exposure, the ratio of phenol and catechol to total metabolites increased by 6% and 22.2% compared with the lower exposure group, whereas the hydroquinone and trans,trans-muconic acid decreased by 18.8% and 26.7%, respectively.

Measurement of non-metabolised benzene in body fluids following exposure to low concentrations is problematic because there is often interference from exposure to tobacco smoke. The average concentration of benzene in blood or urine is generally 3-5 times higher in smokers (Ong and Lee, 1994). In addition, benzene is a ubiquitous environmental pollutant and smoking, both active and passive, is a significant source of exposure (Wallace, 1989; Wallace, 1996). Consequently, in some cases, occupational exposure to benzene may actually be lower than that from environmental sources.

Adequate monitoring of benzene exposure may depend on the use of more than one biomarker (ATSDR, 1997). For a review of the various biomarkers and the different analytical techniques used to measure them, see Ong and Lee (1994) or Angerer and Horsch (1992).

Phenol is the main urinary metabolite of benzene and has been the most commonly used biological exposure indicator. However, its use as a biomarker for benzene is limited to exposure concentrations above 5 ppm (Angerer and Horsch, 1992; Boogaard and van Sittert, 1995; Ong et al, 1995). This is because phenol may also be present in urine from endogenous production (e.g., from the metabolism of amino acids), and dietary, pharmaceutical and environmental exposure (Lee et al., 1993a; Ong and Lee, 1994; Boogaard and van Sittert, 1996).

• Hydroquinone (quinol)

Good correlation has been reported between hydroquinone concentrations and benzene exposure in the range 1-68 ppm (Ong et al., 1995). However, in some studies this biomarker was unable to distinguish between those exposed at 10 ppm benzene and individuals without exposure (Inoue et al., 1988b). The urinary hydroquinone concentration is higher in smokers (Lee et al., 1993b).

There is poor correlation between catechol concentrations and benzene exposure in the range 1-68 ppm (Inoue et al., 1988b; Ong et al., 1995). The urinary catechol concentration is higher in smokers (Lee et al., 1993b).

• trans,trans-Muconic acid (2,4-hexadienedioic acid)

trans,trans-Muconic acid is a minor urinary metabolite of benzene. Between 2% (Inoue et al., 1989a) and 3.9% (range 1.9-7.3%) (Boogaard and van Sittert, 1995; Boogaard and van Sittert, 1996) of absorbed benzene is excreted as trans,trans-muconic acid and the half-life is less than 6 hours (Johnson and Lucier, 1992; Boogaard and van Sittert, 1995; Boogaard and van Sittert, 1996).

Urinary concentrations of trans,trans-muconic acid in exposed workers correlate with the air benzene concentration (Inoue et al., 1989a; Bechtold et al., 1991; Ducos et al., 1992; Lee et al., 1993a; Ong et al., 1995; Liu et al., 1996), even in the range 0.25-3.5 ppm (Ong et al., 1996). Although Lee et al. (1993a) found correlation with a benzene concentration below 0.1 ppm, a more recent study found that at concentrations below 0.25 ppm this biomarker was unable to distinguish between occupationally exposed and control subjects (Ong et al., 1996). Other studies have also found trans,trans-muconic acid a less reliable biomarker for benzene exposure because of its relatively short half-life (Boogaard and van Sittert, 1996). There is also evidence for preferential formation of trans,trans-muconic acid at low benzene exposure concentrations (Johnson and Lucier, 1992).

High background concentrations of trans,trans-muconic acid may be due to other compounds interfering with the analysis. This is a particular problem with the urine of smokers, and further complicated by the finding that smokers have higher urinary trans,trans-muconic acid concentrations than non-smokers (Lee et al., 1993a; Ruppert et al., 1995). In addition, sorbic acid (trans,trans-2,4-hexadienoic acid) in food is metabolised to trans,trans-muconic acid and this may affect results. However, in a small volunteer study the interference with background trans,trans-muconic acid concentrations after ingestion of 200 mg of sorbic acid was minimal (Ducos et al., 1990).

For benzene exposure concentrations greater than 1 ppm, trans,trans-muconic acid is a suitable biomarker and may be preferred because it is relatively straightforward to measure (Boogaard and van Sittert, 1995).

• S-phenylmercapturic acid (N-acetyl-S-phenyl-L-cysteine)

The minor urinary metabolite S-phenylmercapturic acid has been found to be a sensitive biomarker of benzene exposure at concentrations below 1 ppm and even down to 0.3 ppm (van Sittert et al., 1993; Boogaard and van Sittert, 1995). It is not detected in the urine of individuals without exposure to benzene (Inoue et al., 2000). In some studies smoking did not influence the urinary S-phenylmercapturic acid concentration (van Sittert et al., 1993; Inoue et al., 2000), however it did in others (Boogaard and van Sittert, 1995). The average urinary elimination half-life of S-phenylmercapturic acid is 9 hours and the average proportion of inhaled benzene excreted in the urine as S-phenylmercapturic acid is 0.11% (range 0.05-0.26%) (van Sittert et al., 1993; Boogaard and van Sittert, 1995; Boogaard and van Sittert, 1996). The urinary concentrations of S-phenylmercapturic acid and phenol were strongly correlated (van Sittert et al., 1993). Detection of this compound usually requires sophisticated testing equipment (e.g., gas chromatography-mass spectrometry) and although it is a sensitive method it is not routinely available. However, a simpler high performance liquid chromatography (HPLC) method has recently been developed (Inoue et al., 2000), which may make routine testing of this biomarker more practical.

• 1,2,4-Benzenetriol (hydroxyquinol, 1,2,4-trihydroxybenzene)

This compound is not found in the urine of individuals with no occupational exposure to benzene. There is correlation between exposure and the urinary concentration of 1,2,4-benzenetriol. However, the method of measuring this compound is difficult and the potential use of 1,2,4-benzenetriol as a biomarker is further complicated by the observation that excretion is suppressed by co-exposure to toluene (Inoue et al., 1989b). Consequently 1,2,4-benzenetriol is not recommended as a biomarker for benzene exposure.

• Benzene in blood

In the study by Perbellini et al. (1988) the benzene exposure concentration significantly correlated with the blood benzene concentration but not with the concentration in the breath. This is not the case for smokers. Another study found that, although breath and blood concentrations were correlated, the breath, but not the blood concentration, correlated with the exposure concentration (Brugnone et al., 1989). Brugnone et al. (1998) found that it was not possible to distinguish between occupational and environmental exposure using the blood benzene concentration when the benzene concentration in the workplace was less than 0.03 ppm. However, these workers state that, although it is difficult to measure, the blood benzene concentration is a useful biomarker for benzene exposure (Brugnone et al., 1999). One of the main problems is that smoking complicates interpretation because smokers have higher blood benzene concentrations (Perbellini et al., 1988; Pekari et al., 1992; Brugnone et al., 1998; Brugnone et al., 1999). Ong and Lee (1994) argue that methods of measuring benzene in the blood lack specificity and sensitivity and do not recommend them for routine use.

• Benzene in breath

Benzene breath concentrations are usually higher in smokers (Perbellini et al., 1988). The benzene concentration in breath is correlated with the blood concentration but not the exposure concentration (Perbellini et al., 1988). However, Drummond et al. (1988) found that breath benzene concentrations are more reliable than blood concentrations as a marker of exposure. The method used (gas chromatography-mass spectrometry) was sensitive and sophisticated but is not routinely available.

• Benzene in urine

Ong et al. (1995) found significant correlation between the exposure concentration and the urinary concentration of benzene, but there was insufficient data for evaluating exposure to concentrations of less than 1 ppm.

• Other monitoring

Regular monitoring of the haematological parameters is essential in benzene-exposed workers. All parameters are affected including white blood cell count, red blood cell count, absolute lymphocyte count, haematocrit, mean corpuscular volume (MCV) and platelets. However, the absolute lymphocyte count is the most sensitive indicator of benzene-induced haematotoxicity with a 32% decrease in one study. All other parameters were decreased, except the MCV which was significantly increased, compared to controls (Rothman et al., 1996a).

TOXICITY

Exposure to benzene has long been known to pose a risk to health (Rinsky et al., 1987; Paustenbach et al., 1993; Ross, 1996; Smith, 1996a). The earliest reports of chronic benzene toxicity are in the European literature and date from 1897 (Le Noire and Claude, 1897; Santesson, 1897). Benzene was imported into the USA from Germany until World War I and it was after this, when the USA began producing its own benzene, that the risks of toxicity began to be fully appreciated (Greenburg, 1926) and there were calls to control exposure (Hamilton, 1922). The first case of leukaemia attributed to benzene exposure was reported in 1928 (Delore and Borgomano, 1928). Humans can tolerate relatively high concentrations of benzene and it is this fact that lead to the widespread use of benzene as an industrial solvent (Paustenbach et al., 1993).

Acute inhalation or oral exposure can result in CNS depression, and death can occur within minutes of massive inhalation exposure (Avis and Hutton, 1993). Acute benzene exposure has been reported following chemical spills (Clare et al., 1984), industrial accidents on cargo ships (Midzenski et al., 1992; Avis and Hutton, 1993; Barbera et al., 1998), the use of a benzene-containing product without adequate protection (Drozd and Bockowski, 1967) and solvent abuse (Winek et al., 1967; Winek and Collom, 1971).

The most common route of exposure to benzene is inhalation (Marcus, 1990). Most information on benzene toxicity concerns long-term inhalation exposure. The most significant health effects of benzene are haematotoxicity, immunotoxicity, neurotoxicity and carcinogenicity (International Programme on Chemical Safety (IPCS), 1993). Benzene toxicity is characterised by either early reversible haematotoxicity or irreversible bone marrow damage due to prolonged exposure (Snyder and Hedli, 1996). In early studies of benzene-exposed workers high numbers had evidence of haematotoxicity to varying degrees (reviewed in Smith, 1996a). It is well recognised that exposure to benzene concentrations greater than 30 ppm can cause haematotoxicity (ECETOC, 1984). Benzene is sometimes described as a radiomimetic toxin because the myelotoxic effects resemble that of ionising radiation. Bone marrow effects are of three types: bone marrow depression, chromosome changes and carcinogenicity (IPCS, 1993). It was suggested in the 1950s that benzene could cause leukaemia but clear evidence for this was not available until the 1970s (Paustenbach et al., 1993). With improved industrial hygiene and, consequently, lower concentrations of benzene in the workplace, more recent studies have generally found a much smaller proportion of workers, and in some cases none, with evidence of haematotoxicity. However, occasional outbreaks of benzene-induced haematotoxicity still occur (e.g., Aksoy et al., 1987). There is limited information on haematotoxicity after short-term or chronic oral or dermal exposure (IPCS, 1993).

Benzene toxicity varies between species. Although metabolism in all species studied is qualitatively similar, there are quantitative differences in the proportion of benzene metabolised by different pathways (reviewed in Henderson, 1996). In all species studied it has been found that a greater proportion of absorbed benzene is converted to hydroquinone and ring-opened metabolites following low dose compared to high dose exposures. Mice have a greater capacity to metabolise benzene than rats or primates. Mice and monkeys metabolise a greater proportion of absorbed benzene to hydroquinone metabolites than rats or chimpanzees. Non-human primates metabolise less benzene to trans,trans-muconic acid than rodents or humans. Consequently, there is no suitable animal model for benzene toxicity in humans (Snyder et al., 1993; Harbison, 1998; Golding and Watson, 1999). On the basis of the available evidence, a small increase in leukaemia mortality in workers exposed to low benzene concentrations cannot be distinguished from a no-risk situation (IPCS, 1993). Consequently it is impossible to define a no-effect level for benzene exposure (ECETOC, 1984; Marcus, 1990).

The history of the establishment of occupational exposure limits has been complex and in the USA, controversial (Feitshans, 1989). In 1978, OSHA in the USA reduced the permissible occupational exposure limit from 10 ppm to 1 ppm on the basis of information from case reports and two epidemiological studies (Infante et al., 1977; Ott et al., 1978). The US Supreme Court overturned this in 1980 on the basis that there were insufficient data on quantitative dose-response relationships. However, in the light of further studies, particularly that of Rinsky et al. (1987), OSHA re-imposed the 1 ppm exposure limit in 1987. Nicholson and Landrigan (1989) estimated that this 10 year delay in lowering the exposure limit may ultimately result in between 30 to 490 excess leukaemia deaths in the USA following exposure to benzene at concentrations >1 ppm between 19781987. In addition, deaths from other diseases that may occur following benzene exposure (e.g., aplastic anaemia) will increase this total.

Benzene is no longer used as a general solvent in Europe and North America. However, it is still widely used in some countries. A review of 50,000 workplaces in China found the average concentration of benzene to be 5.6 ppm. Of 508,818 benzene-exposed workers examined, 2,676 cases of benzene poisoning were identified, an incidence of 0.5% (Yin et al., 1987b).

There is a huge amount of literature on benzene toxicity spanning over a century and it has been the subject of several large reviews. For more information see ECETOC (1986), Askoy (1988), Marcus (1990), Paustenbach et al. (1993), IPCS (1993), Snyder et al. (1993), ATSDR (1997) or Snyder (2000). The risk assessments of benzene exposure, in particular, have been extensively reviewed and studied; for further details and criticisms see Brett et al. (1989), Byrd and Barfield (1989), Lamm et al. (1989) and ATSDR (1997).

Absorption

Benzene is readily absorbed by inhalation and ingestion. The proportion of benzene absorbed ranges from approximately 50 to 90% (Srbova et al., 1950; Pekari et al., 1992; Ong et al., 1995).

Benzene can be absorbed dermally but not as well as inhalation or oral exposure (ATSDR, 1997). In an experimental study, the absorption of benzene vapour through the skin was less than 1% of the amount absorbed through the lungs under similar conditions (Hanke et al., 2000). Absorption through the skin of liquid benzene was low at 0.4/mg/cm2/hour. This is unlikely to result in acute poisoning, but there is risk of chronic toxicity from this route (Hanke et al., 2000).

Distribution

Benzene is highly lipid soluble. In three cases of inhalation and dermal exposure where the victims died within minutes the highest concentrations of benzene were found in the body fat, brain and blood (Avis and Hutton, 1993). In a similar case, but where death occurred after approximately 30-45 minutes high concentrations of benzene were found in the liver, heart and brain with lower concentrations in kidney, blood and lungs. The urine contained a very small concentration (2.26 mg/l) of benzene (Barbera et al., 1998). Blood concentrations of benzene in these cases of acute poisoning were 30-120 mg/l (Avis and Hutton, 1993) and 31.7 mg/l (Barbera et al., 1998).

Blood concentrations of benzene in individuals who have died after inhalational abuse range from 0.94-20 mg/l (Winek et al., 1967; Winek and Collom, 1971).

Metabolism

The fraction of benzene metabolised following absorption depends on the route of exposure and the dose (Paustenbach et al., 1993). Metabolism of benzene occurs mainly in the liver involving the enzyme P450 CYP2E1, but other organs including the bone marrow are also involved (Smith et al., 1989; Paustenbach et al., 1993). A reduction in liver metabolism by partial hepatectomy reduces benzene metabolism and toxicity in rats, demonstrating the importance of the liver and benzene metabolism for toxicity (Sammett et al., 1979).

Benzene is oxidised to benzene oxide which is unstable (Smith, 1996b). This compound either binds directly to cellular constituents and ultimately forms phenylmercapturic acid (Cox, 1991), or is hydrated to form dihydrodiol leading to the formation of catechol. Benzene oxide can undergo non-enzymatic rearrangement to form phenol. Phenol is hydroxylated to hydroquinone (quinol) which can then produce p-benzoquinone and 1,2,4-benzenetriol. Phenol can also be hydroxylated to catechol, which can also form o-benzoquinone. A large number of metabolites are present in the urine (see below). The major metabolites are phenol, hydroquinone and catechol which are excreted as sulphates and glucuronides. They account for approximately 25% of the absorbed benzene (Inoue et al., 1988b). The benzene aromatic ring is relatively stable and only small quantities of ring-opened metabolites are formed as a consequence (Snyder, 1987). The urinary metabolites of benzene include (Snyder and Hedli, 1996):

• Hydroxylated metabolites (phenol, hydroquinone and catechol) excreted as glucuronide or sulphate conjugates

• Ring-opened metabolites, e.g., trans,trans-muconic acid and 6-hydroxy-trans,trans-2,4-hexadienoic acid

• Mercapturic acids including L-phenylmercapturic acid, 6-N-acetylcysteinyl-S-2,3-cyclohexadienol and 2,5-diOH-phenylmercapturic acid

• DNA adduct residues including N7-phenylguanine and 8-hydroxy-2-deoxyguanosine.

The dose-dependent metabolism of benzene is probably due to competition between benzene and phenol for the same oxidative enzyme systems. Benzene and its metabolite phenol are both substrates for the enzyme P450 CYP2E1 and because the benzene concentration (as the parent compound) is higher than that of phenol, benzene can inhibit phenol oxidation and decrease the formation of hydroquinone conjugates (Medinsky et al., 1996).

Elimination

The fraction of benzene eliminated unchanged via the lungs varies between 12% and 50% (Srbova et al., 1950; Nomiyama and Nomiyama, 1974; Pekari et al., 1992; Ong and Lee 1994). A very small amount of benzene (0.1-0.2%) is excreted unchanged in the urine (Srbova et al., 1950; Ong and Lee, 1994).

In workers exposed to 0.02-4.1 ppm (85% of samples <1ppm), benzene was detectable in the breath 16 hours after cessation of exposure (before the start of the next shift). There was poor correlation between the concentrations of benzene in the breath and prior exposure. An increase in the benzene concentration in the breath was found during the working week (Money and Gray, 1989).

The elimination of benzene has been reported to be triphasic. The half-lives of these phases are approximately 1 hour, 3-6 hours and >15 hours (Nomiyama and Nomiyama, 1974; Pekari et al., 1992). During exposure to benzene the blood concentrations were higher in males than females, while end-exhalation breath concentrations were equivalent. However, four hours after cessation of exposure the blood and exhaled breath concentrations were higher in females. This may be due to the greater quantity of body fat in females (Sato et al., 1975).

Of the benzene absorbed the proportion of urinary metabolites is as follows:

• Phenol 13.2%, hydroquinone 10.2%, catechol 1.6% (Inoue et al., 1986; 1988b),

• trans,trans-Muconic acid 2-4% (Inoue et al., 1989a; Boogaard and van Sittert, 1995; 1996),

• S-phenylmercapturic acid 0.1% (van Sittert et al., 1993; Boogaard and van Sittert, 1995; 1996). However, it should be noted that these proportions may vary with the exposure concentration.

Mode of action

The mechanism of benzene toxicity is complex and has not been fully elucidated. Benzene itself is not directly toxic and toxicity is believed to involve benzene metabolites and multiple mechanisms (Yardley-Jones et al., 1991). Many benzene metabolites have been identified but it is not known which are ultimately responsible for the bone marrow toxicity of benzene (Schrenk et al., 1996). Myelotoxic metabolites are thought to be formed by hepatic metabolism and then transported to the bone marrow where further transformation may occur (Irons et al., 1980). Since the metabolites are the cause of toxicity the fraction metabolised, rather than the quantity of benzene involved in an exposure, is probably a better measure of risk (Bois et al., 1996).

Some of the mechanisms proposed for benzene toxicity are described below with supporting evidence. It is important to note that the mechanisms that produce chronic haematological effects, genotoxic effects and leukaemia may not be the same (Goldstein, 1989). The phenol metabolites of benzene (phenol, catechol, hydroquinone and 1,2,4-benzenetriol) can all be metabolised by myeloperoxidase which could result in highly toxic semiquinone radicals and quinones. These compounds may be the ultimate metabolites of benzene responsible for its toxicity (Smith, 1996b).

Mechanisms of toxicity

• Covalent binding of a reactive intermediate(s) to DNA or oxidative damage to DNA, followed by one or more proliferative steps resulting in stimulation of genetically altered cells.

The concentration of peripheral lymphocyte 8-hydroxy-2-deoxyguanosine (a sensitive marker of DNA damage due to hydroxyl radical attack) in exposed workers with normal leucocyte counts was found to correlate with the air benzene and urinary trans,trans-muconic acid concentration (Liu et al., 1996). Another study found the urinary concentration of 8-hydroxy-2-deoxyguanosine also correlated with the benzene concentration in exposed workers (Lagorio et al., 1994).

• Alkylation by benzene metabolites of cellular components including DNA.

The benzene metabolites trans,trans-muconaldehyde (a precursor of the urinary metabolite trans,trans-muconic acid) and 6-hydroxy-trans,trans-2,4-hexadienal are multifunctional alkylating agents which have the potential to cross-link cellular DNA and protein. They also react with and deplete glutathione (GSH), which is an important cellular antioxidant (Witz et al., 1996). Alkylation of DNA could result in mutation or chromosome damage.

• Inhibition of topoisomerase II.

Topoisomerases are a group of important chromosomal proteins involved in maintaining the shape and structure of DNA by breaking and resealing strands. They are also involved in chromosome segregation, DNA replication and repair and other cellular processes. Topoisomerase II is believed to play a role in genomic stability and interference with topoisomerase II activity at critical stages of the cell cycle could cause chromosome breakage, aneuploidy or cell death (Chen and Eastmond, 1995b). Phenol and its peroxidation metabolites 2,2'-biphenol and 4,4'-biphenol, have shown inhibitory effects on topoisomerase II but not topoisomerase I. Other benzene metabolites (1,4-benzoquinone and 1,2,4-benzenetriol) also inhibited topoisomerase II, but at higher concentrations (Chen and Eastmond, 1995b).

• Mitotic recombination.

Rothman et al. (1995; 1996b) found an increase of mutations in the glycophorin A (GPA) locus in benzene exposed workers. This assay identifies stem cell or precursor erythroid cell mutations expressed in peripheral erythrocytes. The authors suggested that this finding may be due to mitotic recombination in longer-lived stem cells. The mutation frequency was related to cumulative benzene exposure not to current exposure (Rothman et al., 1995).

• Inhibition of cell division by interfering with microtubules.

Microtubules are polymers of the protein tubulin. They are involved in spindle formation during cell division, maintenance of cell shape, cell growth, intracellular movement of organelles and secretion of cellular products. Hydroquinone (but not phenol or catechol), has been shown in vitro to inhibit microtubule polymerisation. This was an oxidative process where hydroquinone competes for sulphydryl dependent guanosine triphosphate (GTP)-binding sites on the protein (Irons and Neptun, 1980).

• Interference with the differentiation of stem and progenitor cells in the bone marrow.

Haematopoiesis is regulated through a process of proliferation and differentiation in which immature stem cells give rise to a larger quantity of progressively more differentiated cells. These processes are strictly controlled by multiple growth factors or cytokines that work together to control haematopoiesis. An altered response to haematopoietic cytokines is an early feature of the leukaemogenic process. In vitro studies on bone marrow cells suggest that hydroquinone (but not catechol, phenol or trans,trans-muconic acid), alters stem cell differentiation (Irons and Stillman, 1996a; 1996c).

• Increased concentrations of prostaglandins which results in down-regulation of haematopoiesis and inhibition o

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment