Persistent organic pollutants, or POPs, are toxic (poisonous) though nobody realised this when they were first made. They cause cancer, birth defects and other damage to animals' and people's bodies.
The 12 worst of these POPs were banned in May 2004 but they are very stable and take many years to disappear. Because of their stability, they get spread all around the world via the atmosphere and oceans. Many end up in the Arctic, far from their source, and are bad news for the Inuit peoples and polar bears because POPs 'bioaccumulate' (or 'biomagnify') in both plants and animals. This is critical for meat-eating animals at the top of the food chain like people and polar bears. POPs become concentrated in animals' body fat so if a person eats meat from an animal like a fish which has 'bioaccumulated' POPs from its own food, the chemicals become concentrated and can reach up to 70,000 times the levels you would find in the environment.
Before human industry, there were no POPs anywhere. They are not natural chemicals. But some POPs are useful to industry which is why they are made
For more on POPs, including the "dirty dozen" worst offenders, visit the US EPA website.