Benzene gives off very toxic, inflammable vapours and grave risks are thus associated with its industrial use. It exerts the acute narcotic action common to many hydrocarbons and has a local irritant effect on the skin and mucous membranes. The outstanding feature of benzene is its ability to damage blood-forming tissues of chronically exposed persons, with resulting hyporegenerative anaemia of various degree. The onset of chronic benzene poisoning is extremely insidious and its ultimate injury potentially incurable.
Benzene is a clear, colourless, aromatic hydrocarbon which has a characteristic sickly sweet odour. At normal ambient temperatures it is a liquid, but it rapidly evaporates and small amounts are detectable in the atmosphere. During the eighteenth century, benzene was discovered as a component of oil, gas, coal tar and coal gas. Around 1941, commercial production began in the US in the chemical manufacturing industry. An important use was as a solvent and cleaning agent but these uses have been discontinued as a result of benzene's carcinogenic effects.
Benzene is present in petrol and can escape into the air; at filling stations; from the carburettor (evaporation), the fuel tank (vent), the crankcase (past the piston rings) as well as from the engine exhaust. The benzene found in exhaust gases not only derives from the benzene in the fuel but is also produced from other chemicals, such as ethylbenzene, toluene and xylenes, as a result of petrol combustion. It is therefore quite possible for more benzene to be found in the vehicle exhaust, relative to certain other hydrocarbons, than was present in the original fuel.
Benzene is a constituent of coal tar from which benzol is obtained by distillation. The designation benzol 90/100 indicates a substance containing 90% of hydrocarbons distilling below 100 deg C. Benzol is extracted from coal gas and coke oven gas by a stripping or scrubbing operation. In the petroleum industry large quantities of benzene are produced by catalytic reforming, dealkylation and dehydrogenation processes or by cyclization and aromatization of paraffin hydrocarbons. In industry, benzene is used as a fuel, as a chemical reagent and as a solvent. In certain parts of the world a major use of benzene is as an additive of motor fuel and large quantities are used for this purpose. Benzene is chemically reactive and serves as a raw material for a great number of chemical syntheses.
Even though hyporegenerative anaemia resulting from chronic exposure to benzene has been known for over a century, it was not until 1928 that the action of benzene was shown to be a cause of leukaemia. Since then, approximately 200 cases of benzene leukaemia have been reported, either as single cases or as outbreaks. An outbreak of acute leukaemias from benzene exposure has been recently reported among Turkish workers chronically exposed to high concentrations of benzene. Recent epidemiological studies carried out in the USA and in Japan confirm an increased risk of leukaemias in workers with chronic exposure to benzene. Some French and American reports have also attributed cases of chronic leukaemias to benzene.