Hexachlorocyclohexane (HCH) is a commercial insecticide. It is persistent in the environment, and has been noted in breast milk monitoring studies, but it is not one of the chemicals currently included in the International POPs Elimination Treaty. That is a mistake, in the view of environmentalists and health experts.
HCH is made up of a mixture of eight isomers. Isomers are related but different forms of a chemical. A sample of HCH is usually a mixture of at least four different HCH isomers including the alpha, beta, delta and gamma forms.1 The chart below illustrates what a normal HCH mixture would look like. The composition of HCH is important because different isomer forms have different levels of persistence and bioaccumulate in breast milk differently.