While inorganic compounds are, generally, simple molecules that remain relatively undisturbed, organic compounds, when reaching the atmosphere, they tend to react and oxidize producing CO2 and water. It is the case of volatile organic compounds.

However, there are other organic compounds that are very stable, this makes their degradation less easy. These are known as persistent contaminants.

Organic compounds with chlorine atoms (that give them great chemical stability) belong to the persistent contaminants group. Some of these compounds have been human synthesized to act as pesticides, as is the case of insecticides DDT, aldrin, endrin, etc. Others, such as polychlorinated biphenyl (PCB) were synthesized to use as dialectrics for transformers.

Other persistent compounds, high in pollutants, are dioxines and dibenzofuranes. Although these compounds are purpose-manufactured, they are also formed unintentionally during the combustion processes of organic matter containing chlorine atoms.

Polycyclic aromatic hydrocarbons (PAH) are hydrocarbons that, even though they lack chlorine atoms, do have merged benzene rings that confer great chemical stability and cause PAHs to also be persistent. Just as dioxines, PAHs form during the combustion processes.