In most urban areas transport or traffic is the largest source of air pollution, particularly for emissions of carbon monoxide, particulates (including black smoke), nitrogen oxides and hydrocarbons. Road transport is an important source of the precursor pollutants that lead to the formation of ground level ozone. This is particularly problematic during periods with long hours of bright sunlight, temperatures above 20°C and light winds. Recently there has been concern over emissions of benzene (a hydrocarbon) as it can cause cancer.

The major source of benzene in the atmosphere is the combustion and distribution of petrol; approximately 70% of benzene emissions are from petrol fuelled cars. In recent years there has been a large increase in the number of vehicles all over the world which has been overshadowed by the even greater increase in vehicle usage. This was reflected in increases in vehicle related pollutant emissions.

Transport related pollution is likely to become increasingly important in the future as vehicle numbers and usage are expected to continue to rise.