A large number of different products with a wide range of chemical and physical properties can be produced by selecting suitable reaction conditions. Organic chlorine compounds are important synthetic building blocks in the chemical industry, and they are also formed in nature in large quantities. Organochlorine compounds form a sub-group of the organohalogen group. PCBs form a group of compounds which were developed in the 1930s and were mainly used in the electricity supply industry and mining. PCBs are, however, still found in trace concentrations in the sea and in the fatty tissue of marine animals.
Persistence is an important negative criterion in the ecological assessment of chemicals.
These have global effects. Most are already banned in Western Europe but some are still in use in developing countries. POPs and other PTBs are being examined by various national, regional and international bodies with the aim of developing management strategies. PTBs Chemicals which are persistent, toxic and liable to bioaccumulate. These have primarily local effects.