 The coplanar PCBs, known as nonortho PCBs because they are not substituted at the ring positions "ortho" to (next to) the other ring, (such as PCBs 77, 126 and 169), tend to have dioxin-like properties, and generally are among the most toxic congeners.Because PCBs are almost invariably found in complex mixtures, the concept of toxic equivalency factors (TEFs) has been developed to facilitate risk assessment and regulation, where more toxic PCB congeners are assigned higher TEF values on a scale from 0 to 1.One of the most toxic compounds known, 2,3,7,8-tetrachlorodibenzo[p]dioxin, a PCDD, is assigned a TEF of 1.