Chlorinated and brominated polycyclic aromatic hydrocarbons: Sources, formation mechanisms, and occurrence in the environment

Chlorinated and brominated polycyclic aromatic hydrocarbons (ClPAHs and BrPAHs; XPAHs) are carcinogenic organic pollutants which are mainly produced and emitted from combustion processes. In some environmental matrices, XPAHs display similar toxic properties and even higher toxic equivalent quantities (TEQs) than dioxins. Understanding the sources and formation mechanisms of XPAHs is important for controlling their emissions and human exposure to these ubiquitous pollutants. Nevertheless, comprehensive knowledge on the sources, formation mechanisms, and environmental characteristics of XPAHs, which are considered as emerging persistent organic pollutants (POPs) is lacking. Here, we review and discuss the knowledge on the primary sources (i.e. formation mechanisms, levels, composition pattern, emission factors in combustion and other anthropogenic sources), and secondary formation (i.e. formation from reactions between emitted PAHs and halogens in environmental compartments). Congener profiles of XPAHs released from various anthropogenic sources are evaluated for their possible application as source tracers of XPAHs. Regarding the chlorination of PAHs, we suggest and discuss two possible mechanisms, which result in the production of different congeners under different processes. Finally, knowledge on environmental exposure to XPAHs is reviewed. Research needs with regard to formation, emission, analytical methods, environmental exposure and environmental risk assessment are outlined.