These chemicals have been linked to cancer, sterility and birth defects and are now banned by many governments. The United Nations is working to reduce their use worldwide, and an international meeting in Johannesburg next week is expected to pave the way for an international convention.
Pops differ from other pollutants because they are extremely difficult to break down.
They can persist in the environment for many years, and can be absorbed by plants and animals. The Johannesburg conference will consider the fate of 12 such chemicals, and draw up measures to eliminate or reduce their use. By far the most controversial is DDT, an organic pesticide that has been used for more than 50 years to control malaria-carrying mosquitoes. Its use has been largely curtailed after studies suggested it was a factor in a wide range of medical conditions. There is some human evidence which gives grounds for concern, there have been studies in North Carolina and in Mexico which suggest that the higher the levels of DDE in a woman's body the shorter the period of time when she's able to breast feed her children.