A set of organic substances which have four different properties. They are persistent which means that they degrade slowly. They are bioaccumulative, that is to say they accumulate in living beings and cannot dissolve in water. They are toxic and can travel over long distances. They are found as far away as the Antarctic. Some of them are now banned in most countries of the world. This is the case with PCBs and DDTs. Others have not yet been banned. Examples of these are pesticides, industrial chemical substances or accidental by-products of industrial or other processes. Twelve of them have been recognised by the United Nations as very dangerous chemical substances because their effect on human and animal health is considerable. Les POPs can cause lesions of the central and peripheral nervous system, reproductive and immune system disorders.