Explanation:

 Persistent Organic Pollutants (POPs) are organic chemical substances, that is, they are carbon-based. They possess a particular combination of physical and chemical properties such that, once released into the environment, they:
• Remain intact for exceptionally long periods of time (many years);
• Become widely distributed throughout the environment as a result of natural processes involving soil, water and, most notably, air;
• Accumulate in the fatty tissue of living organisms including humans, and are found at higher concentrations at higher levels in the food chain; and are toxic to both humans and wildlife.

In context of environment, the term “dirty dozen” refers to 12 persistent organic pollutants. These were the 12 initial compounds that were listed under the Stockholm convention.