Polychlorinated biphenyls (PCBs) are an environmental contaminant in the news because they have been discovered in schools and other buildings. PCBs are actually a mixture of many different similar chemicals; there are 209 chemically different chlorinated biphenyls that together make up the PCB chemical group. If you spend time learning more about PCBs, a term you may run into is “coplanar PCBs” sometimes also referred to as “dioxin-like PCBs”. Neither of these descriptions are really scientifically accurate, but they have stuck anyway.
Chemically, coplanar PCBs usually refer to 12 of the 209 possible PCB molecules that do not have a chlorine atom stuck in what organic chemists refer to as the “ortho” (or number 2 or 6) position (typically the mono-and di-chloro PCBs are not counted as being coplanar). The absence of an ortho-chlorine atom allows the biphenyl molecule to get closer to being a “flat” molecule; that is one with all 12 carbon atoms lying in a single plane; thus coplanar. Of the 12 chlorinated biphenyls generally considered to make up the coplanar PCB group, 8 are generally absent from commercial PCB mixtures. Based on pioneering analytical work by George Frame at GE’s R&D Laboratory, we know that the total sum of coplanar PCBs in commercial PCB mixtures is well less than 1%.
The reason coplanar PCBs are also referred to as “dioxin-like” is that they have some ability to bind to the same biological receptor protein molecule that dioxin binds to. Other common environmental contaminants, such as the polycyclic aromatic hydrocarbons (PAHs) also bind to this biological receptor. The ability to bind to this receptor does not mean that either PAHs or PCBs have the same type of toxicity or the same potency as dioxin.
The World Health Organization has concluded that certain of the coplanar PCB molecules should be treated as if they were less potent versions of dioxin; the USEPA seems to agree. The relative “toxic potency” of the coplanar PCBs is quantified using Toxicity Equivalence Factors (TEFs). The USEPA and other regulatory agencies routinely require that testing be conducted so that the amount of dioxin equivalent toxicity of wastes and dredge spoils can be calculated.
The TEF approach is not without its critics. There is in fact considerable scientific controversy about the application of TEFs to coplanar PCBs. In a number of cases the use of TEFs has been shown to significantly overstate actual toxic hazards.