Mixed poly-brominated/chlorinated biphenyls (PXBs): widespread food and environmental contaminants.

Mixed poly-brominated/chlorinated biphenyls (PXBs) are a new class of emerging contaminants. Their environmental occurrence is confirmed by the reported occurrence in foods and human tissues, and the patterns of occurrence suggest that different sources may be contributing to food occurrence in different parts of the world. Important sources of PXBs are thought to include emissions from the combustion of bromine and chlorine containing waste and consumer products and inadvertent contamination in industrial chemicals. Of specific interest are the dioxin-like-PXBs. These are the 62 non-ortho-substituted PXBs with eight congeners of 3,3',4,4'-substitution type (#77A-H), ten of 3,4,4',5-type (#81A-J), twenty four of 3,4,5,3',4'-type (#126A-Z) and twenty of 3,4,5,3',4',5'-type (#169A-V) and many mono-ortho substituted compounds. The toxicological clarification on these contaminants continues, with reports confirming dioxin-like effects at low concentrations, suggesting a greater toxicological significance than PCBs.