Benzene C6H6: an odoriferous organic compound. A small constituent of petrol, which is the main source of benzene pollution in the UK. Approximately 70% of total emissions can be attributed to combustion by petrol vehicles; refining, distribution and evaporation of petrol from vehicles accounts for approximately a further 10% of total emissions. Benzene is emitted in vehicle exhaust, not only as unburnt fuel, but also as a product of the decomposition of other odoriferous compounds. Benzene is a proven human carcinogen.