A persistent compound is slow to degrade in the environment, which often results in its accumulation and deleterious effects on human and environmental health if the compound is toxic. Toxic metals such as lead and cadmium, organochlorides such as polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) are persistent compounds.

Persistent molecules are termed recalcitrant if they fail to degrade, metabolize, or mineralize at significant rates. Their compounds can be transported through the environment over long periods of time and over long distances, resulting in long-term exposure and possible changes in organisms and ecosystems. However, organisms and ecosystems may adapt to the compounds, and deleterious effects may weaken or even disappear.

Compounds may be persistent for several reasons. A compound can be persistent due to its chemical structure. For example, in a molecule, the number and arrangement of chlorine ions or hydroxyl groups can make a...