The term polybrominated biphenyls or polybromobiphenyls (PBBs) refers to a group of halogenated hydrocarbons, formed by substituting hydrogen by bromine in biphenyl. PBBs are not known to occur as natural products. They have a molecular formula of C12 H(10-x-y)Br(x+y) where both x and y = 1 to 5. Theoretically 209 congeners are possible. Only a few have been synthesized individually and characterized. PBBs, manufactured for commercial use, consist mainly of hexa-, octa-, nona-, and abromobiphenyls, but also contain other homologues.