Many different combinations are possible.
Chlorine atoms can be placed at any or all of ten available sites, with 209 PCB mixtures theoretically possible. Congeners, Isomers and Homologies PCBs belong to a class of organic chemicals known as chlorinated hydrocarbons. PCBs were manufactured and sold as complex mixtures differing in their average chlorination level. The materials now collectively referred to as PCBs are actually several dozen individual PCB homologs clustered around some average degree of chlorination. Congeners may be grouped in terms of the number of chlorine atoms attached to the biphenyl molecule.