The PCBs are chlorinated hydrocarbons that are manufactured commercially by the progressive chlorination of biphenyl in the presence of a suitable catalyst (e.g., iron chloride). Depending on the reaction conditions, the degree of chlorination can vary between 21 and 68% (w/w). The yield is always a mixture of different compounds and congeners. Thus, a total of 209 different chemical components may exist, but only about 130 of these are likely to occur in commercial products or mixtures of these compounds.