Benzene - Aromatic Hydrocarbons

AROMATIC HYDROCARBONS

Benzene

Synonyms

·              Benzol, Benzole, Benzolene, Coal naphtha, Phenyl hydride, Annulene, Carbon oil, Cyclohexatriene, Mineral naphtha, Motor benzol, Phene, Pyrobenzol, Pyrobenzole.

Physical Appearance

·              Colourless, volatile, inflammable liquid, with a strong, pleasant odour. 

Sources

·              Natural sources of benzene include volcanoes and forest fires. Benzene is also a natural constituent of crude oil.

·              Benzene can be recovered from coal tar and produced from the hydrodemethylation of toluene under catalytic or thermal conditions.

·              A chief source of benzene is catalytic reformat, wherein the naphthenes and paraffins contained in naphtha are converted to aromatic hydrocarbons. Solvent extraction is then used to recover the benzene.

·              Most of the benzene produced is generally derived from the petrochemical and petroleum-refining industries. Cigarette smoke also is said to contain benzene.

Uses

·              Benzene is extensively used in industry for the manufacture of drugs, chemicals, insecticides, glues, varnishes, paints, polishes, explosives, batteries, shoes, and rubber tyres. It is also used in printing, photography, and dry cleaning.

·              It is a popular solvent in laboratories.

·              Petrol often has significant concentrations of benzene (as an octane booster).

Clinical Features

Acute Exposure:

·              Benzene can be absorbed through all routes.

·              Most individuals can begin to smell benzene in air at 1.5 to 4.7 parts per million (ppm) and detect the odour of benzene in water at 2 ppm.

·              Brief exposure (5 to 10 minutes) to very high benzene air concentrations (10,000 to 20,000 ppm) can result in death.

·              On inhalation (of lower concentrations), principal mani-festations include vertigo, tinnitus, vomiting, dyspnoea, convulsions, coma, and death. Cardiac arrhythmias are possible.

·              On ingestion, symptoms include burning pain in the mouth and pharynx, epigastric pain, vomiting, vertigo, tachycardia, hypotension, dyspnoea, convulsions and coma.

·              Aspiration produces similar manifestations as in the case of aliphatic hydrocarbons.

·              Locally (on skin), benzene has a strong irritating effect, producing erythema, burning and, in more severe cases, oedema and blistering.

Chronic Exposure:

·              Benzene has been classified as a human carcinogen by various international monitoring agencies. The causal relationship between chronic exposure and a variety of haematologic disorders has been known for the last 50 years or more. These include aplastic anaemia, acute myeloblastic leukaemia, haemolytic anaemia, and pancytopenia. Benzene exposure is associated with translocations between chromosomes 8 and 21, and hyperploidy of 8 and 21 in the circulating lymphocytes of workers exposed to benzene. These aberrations may be involved in benzene-induced leukaemia.

·              Headache, dizziness, irritability, nervousness, fatigue, anorexia and epistaxis may also occur with chronic benzene poisoning.

·              Paroxysmal nocturnal haemoglobinuria (PNH) has been reported in patients occupationally exposed to benzene. PNH is often associated with aplastic anaemia and rarely with acute leukaemia.

·              Insulin-dependant diabetes mellitus has been reported with benzene exposure.

·              An epidemiological study of pregnant women in a large petrochemical industry showed a positive correlation between reduced birth weight and exposure to benzene and work stress.

Diagnosis

·      Benzene is metabolised extensively in the liver and excreted in the urine, with 51 to 87% excreted as phenol, 6% as catechol, and 2% as hydroquinone. Other metabo-lites include phenylmercapturic acid (0.5%), benzene dihydrodiol (0.3%), and trans, trans-muconic acid (1.3%). Monitoring benzene in expired air and urine phenol levels may be useful for observing workers exposed to benzene. Urine phenol levels in unexposed individuals are less than 10 mg/L. Urine phenol levels after chronic exposure to airborne concentrations of 0.5 to 4 ppm are less than 30 mg/L. Urine phenol levels after exposure to 25 ppm average 200 mg/L.

·      Analysis of urinary t, t-muconic acid appears to be a better indicator than phenol for assessment of exposure to low levels of benzene. 

·      Gas chromatography head-space analysis is the preferred method for determining benzene in blood or urine. The lower limit of detection is 0.64 nmol/L for benzene in blood and 0.51 nmol/L in urine.

·      Obtain baseline CBC.

·      Monitor ECG for cardiac arrhythmias.

Treatment

·              Acute exposure is treated on the same lines as in the case of aliphatic hydrocarbons.

·      Ipecac-induced emesis is not recommended because of the potential for CNS depression and seizures.

·      Consider pre-hospital administration of activated charcoal as an aqueous slurry in patients with a potentially toxic ingestion who are awake and able to protect their airway.

·      Consider gastric lavage with a large-bore orogastric tube after a potentially life-threatening ingestion if it can be performed soon after ingestion (generally within 60 minutes).

·      Remove contaminated clothing and wash exposed area extremely thoroughly with soap and water.

·      Administer 100% humidified supplemental oxygen, perform endotracheal intubation and provide assisted ventilation as required. Administer inhaled beta adrenergic agonists if bronchospasm develops. Exposed skin and eyes should be flushed with copious amounts of water.

·      Treat convulsions in the usual manner.

Autopsy Features

·      Marked congestion of brain.

·      Pulmonary oedema. On sectioning the lungs there is exuda-tion of blackish, frothy liquid.

·      Multiorgan congestion.

Forensic Issues

Most cases of exposure (acute and chronic) are occupational in nature.

Toluene and Xylene produce similar (though milder) mani-festations on acute exposure and are managed by supportive measures, with the same precautions in decontamination as for other hydrocarbons.