Persistent organic pollutants are a group of compounds that have either been produced for a specific purpose, examples are PCB and DDT, or have been unintentionally created during production of other compounds or created by industrial processes. The best known example of this is dioxins.
There is a large number of chemicals or compounds classified as POP's. Initially the "dirty dozen" was the focus. The "dirty dozen" is a group of 12 chemicals or compounds that has been identified as key POP's. The Stockholm convention in 2001 saw over 90 countries agreeing to reduce or eliminate the production, use and/or release of these 12 key POP's. The list was later expanded to include 22 and then 28 compounds. However as new, and often replacement, chemicals are being produced so is the expected number of persistent organic pollutants.