Most of the time for simplicity, chemical persistence is expressed in terms of the substance’s degradation half-life, which is the time for the substance to change one-half of its amount to a different form or compound. The use of half-life implies first order kinetic behavior in chemical concentration, a concept borrowed from radioactive decay. The problem with this usage is that many environmental contaminants are present in more than one place.

The amount of material lost by degradation in a particular medium is determined both by the rate constant specific to the reactivity occurring in that medium, and by the amount present in that medium. It follows that for persistence in the overall environment composing the media of concern, the degradation rate constants for those individual media must be known. At the least, an assessment model for integrated persistence should weigh the persistence in the various media according to where a substance is likely to reside (Wania, 1998).

Again, it is important to note that for chemicals in the environment, their degradation half-lives depend not only on their molecular structure and physicochemical properties, but also greatly on the environmental conditions in which they are present. In any given environment, a chemical’s persistence is susceptible to one or more of the four basic removal processes: photolysis (breakdown by sunlight); chemical decomposition (breakdown through chemical reaction): microbial decomposition (breakdown by microorganisms); and mobility. These are the same four types applied to pesticide persistence by Kerle et al. (1996). In their pesticide terms, mobility can be accomplished through sorption to soil particles or vegetation, uptake by plants, volatilization into the atmosphere, and runoff or leaching to the nearby water table.