The partial breakdown of a (usually) synthetic compound by microbiological action. Synthetic chemicals are widely used in industry, agriculture, and in the home; many resist complete enzymatic degradation and become persistent environmental pollutants. In cometabolism, the exotic molecule is only partly modified by decomposers (bacteria or fungi), since they are unable to utilize it either as a source of energy, as a source of nutrient elements, or because it is toxic. Cometabolism probably accounts for long-term changes in DDT, dieldrin, and related chlorinated hydrocarbon insecticides in the soil. The products of this partial transformation, like the original exotic chemical, usually accumulate in the environment.