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Polycyclic aromatic hydrocarbons (PAHs)

(see also complex hydrocarbons) Polycyclic aromatic hydrocarbons (PAHs) are the largest group of carcinogenic substances we know of today. The PAH group is made up of several hundred individual chemical substances, more than 500 different PAHs for example having been detected in air samples. PAH is formed when coal or hydrocarbons, e.g. various oils, are heated without there at the same time being sufficient oxygen to provide complete combustion to carbon dioxide. This may happen in industrial processes such as in the cracking of petroleum, or in internal-combustion engines in vehicles. The majority of all PAHs are not used as individual compounds but occur in various mixtures, e.g. in various types of coal and oil products.

From the chemical point of view, PAHs consist of two or more condensed aromatic rings. Benzene is the simplest aromatic hydrocarbon. It consists of six carbon atoms with one hydrogen atom on each carbon atom. Condensed means that the aromatic rings have one side in common. An important property is that the rings are in the same plane.

Much of the biological effect of PAHs is linked to the plane structure of the molecule and its ability to affect DNA in the cell nucleus. Most organisms can convert PAHs. The breakdown products formed can very often be more hazardous than the original substance. In animal studies, many compounds have been found for example to be carcinogenic and cause genetic damage. The compounds are often classified as carcinogenic, here you can search in the Classification list.

PAHs are fat-soluble, generally stable and in some cases bioaccumulative. Being stable means that the compounds are difficult to break down and may be dispersed a long way in the environment before breakdown occurs.

In aquatic environments, PAHs are principally bound to particles which are then transported to sediment, where they can become very persistent. Aquatic ecosystems close to emission sources are therefore most at risk. Many PAH compounds accumulate in invertebrate organisms in the aquatic environment and are built up in the food chain. Mussels, for example, have poor ability to break down PAHs, leading to the compounds accumulating in the mussels.

High-aromatic oils (HA-oils) containing PAH is typically used in car tyres and is a route of exposure of PAH to the environment. Recycled car tyres are then used for manufacture of rubber granulate which can be used in synthetic turf. Here you can read more about PAH in synthetic turf.

The European Parliament and the Council decided in June 2005 to limit the concentration of PAHs in extender oils (e.g. HA oils) used in the manufacturing of new automotive tyres and treads for retreaded tyres. The legislation entered into force 1 January 2010.

Further information:

Regulation EC No. 552/2009

Synthetic turf from a chemical perspective - a status report. PM 3/06, Swedish Chemicals Agency, 2006.

HA oils in automotive tyres – prospects of a national ban. Report on a government commission. Report 5/03, Swedish Chemicals Inspectorate, 2003.

Hazard assessments – Chemical Substances Selected in the Swedish Sunset Project. Report 12/95, Swedish Chemicals Inspectorate, 1995.