The term 'hydrocarbons' is often used when discussing traffic pollution. This refers to a group of chemicals of which volatile organic compounds are a sub-group

Volatile Organic Compounds are a range of chemical compounds all of which contribute, to varying degrees, to the formation of ground level ozone. In addition, certain ones are known to cause cancer. Current attention is focused on 1,3 butadiene, primarily from vehicle exhausts, and benzene. Benzene in the atmosphere either comes from the combustion or evaporation of petrol. Levels are therefore highest close to busy roads or in the vicinity of petrol filling stations.

Long-term exposure to high levels of benzene and 1,3 butadiene has been linked to leukaemia and cancer. Health experts have concluded that the health risks from these pollutants become progressively smaller as cumulative exposure is reduced. Health standards are therefore set based on annual mean levels.

As only very small concentrations of hydrocarbons are typically found in the atmosphere, the monitoring process is very complicated and expensive. As monitoring has only been carried out for the last few years, it is impossible to identify any upward or downward trend in levels. Annual results in the UK suggest the annual mean levels are well below European health standards.