POPs are hazardous chemicals which have distinctive and very dangerous properties. POPs persist in the environment for a long time; they can travel long distances through the air or sea; and they are ‘bioaccumulative’. This means that they build up in living organisms, mainly in fatty tissue, and their concentration increases as they rise through each level of a food chain – and so the highest concentrations are normally found in the top predators like humans and polar-bears. POPs are highly toxic and levels found in some people and animals are above those known to cause health and biological effects. Many of these chemicals are ‘endocrine disruptors’ and act like hormones in our bodies; some of them are carcinogenic; the others are mutagenic and affect DNA or are teratogenic and can cause birth defects. A very ‘famous’ POP which is still in use is the pesticide DDT.