The environmental media for or in which persistence is estimated include air, water, soil, and sediment. The part of the persistence enhancement (or otherwise known as preservation) that bioaccumulation plays an important role in is when the substance is taken up by organisms in the environment at various trophic levels (i.e., in various positions of the food chain).

It is important to distinguish between a chemical’s persistence in a single (environmental) medium and its overall environmental persistence. Although persistence in a single medium is still influenced by the conditions of the immediate environment, environmental persistence is much more dynamic, situational, and complex in that the environment is now viewed as behaving like a set of interconnected media.

Many substances have properties that allow them to partition or dissolve in certain media. Partitioning is the tendency of a substance moving from one medium to another in response to thermodynamic processes (or loosely, some natural forces). If and when partitioning takes place, the chemical may be in a different species not convertible back to its original form.

Thus one other crucial physicochemical property of a substance is its tendency to speciate into a particular form (Mackay et al., 2001). In some cases, speciation is irreversible, such as the conversion of DDT to p,p’-DDE. Another concept related to chemical’s persistence is its availability in the medium or in the overall environment. Persistence of a chemical in the environment hence should be evaluated on the basis of degradation rates, partitioning, speciation, availability, and an appreciation of which media are relevant and irrelevant.