Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds with two or more fused aromatic rings. PAHs occur naturally in petroleum and are also produced as by-products of fuel combustion.

PAHs are an important class of environmental contaminants, which are known to accumulate in ecosystems. The US EPA has identified 16 PAH compounds as priority pollutants. Some of these compounds are carcinogenic and/or mutagenic to mammals; in addition, they have both acute toxicity and sub-lethal effects on some aquatic organisms. PAHs may also bio-accumulate in edible shellfish, which gives them a pathway to humans [17].

A source of PAHs is the incomplete combustion of fuel oils and although engines and boilers are designed to optimise the combustion of fuel, exhaust gases will always contain a proportion of incompletely combusted material. This results in gaseous hydrocarbon and particulate emissions that range from methane to very large complex molecules; a proportion of which will include polycyclic aromatic hydrocarbons [3]. Whilst low molecular weight PAHs are mainly found unbound in the gaseous phase of the exhaust stream, heavier molecular weight PAHs constitute a group of the substances that are bound onto soot created during combustion. [4]