When many toxic substances are introduced into the environment, they do not remain in their original form, but are transformed to other products by a variety of biological and non-biological processes. The chemicals and their transformation products are degraded, converted progressively to smaller molecules, and eventually utilized in various natural cycles, such as the carbon cycle. Toxic metals are not degraded but are interconverted between available and nonavailable forms. Some organic compounds, such as polychlorinated biphenyl (PCBs), are degraded over a period of many years to less toxic compounds, while compounds such as the organophosphate insecticides may break down in only a few hours.

The chemical transformations that occur may either increase (referred to as intoxication, or activation if the parent compound was nontoxic) or decrease (referred to as detoxification) the toxicity of the original compound. For example, elemental mercury, which has low toxicity, can be converted to...