Volatile Organic Compounds
Volatile Organic Compounds or VOCs are organic chemicals that easily vaporize at room temperature. They are called organic because they contain the element carbon in their molecular structures. VOCs have no colour, smell, or taste. VOCs include a very wide range of individual substances, such as hydrocarbons (for example benzene and toluene), halocarbons and oxygenates.
Hydrocarbon VOCs are usually grouped into methane and other non-methane VOCs. Methane is an important component of VOCs, its environmental impact principally related to its contribution to global warming and to the production of ground level or lower atmosphere ozone. Most methane is released to the atmosphere via the leakage of natural gas from distribution systems. Benzene, a non-methane hydrocarbon, is a colourless, clear liquid. It is fairly stable but highly volatile, readily evaporating at room temperature. Since 80% of man-made emissions come from petrol-fuelled vehicles, levels of benzene are higher in urban areas than rural areas. Benzene concentrations are highest along urban roadsides. Oxygenates arise in vehicle exhausts and via atmospheric chemical reactions. Evaporation of solvents, used for example in paints, cause a release of hydrocarbons, oxygenates and halocarbons to the atmosphere.
Some VOCs are quite harmful, including benzene, polycyclic aromatic hydrocarbons (PAHs) and 1,3 butadiene. Benzene may increase susceptibility to leukaemia, if exposure is maintained over a period of time. There are several hundred different forms of PAH, and sources can be both natural and man-made. PAHs can cause cancer. Sources of 1,3 butadiene include the manufacturing of synthetic rubbers, petrol driven vehicles and cigarette smoke. There is an apparent correlation between butadiene exposure and a higher risk of cancer.
In comparison to other pollutants, the monitoring of VOCs in the UK is not yet well developed and there is no long term database of information. The Automatic Hydrocarbon Monitoring Network monitors 25 hydrocarbon species in urban air on a continuous basis, including two known carcinogens, benzene and 1,3-butadiene.