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.

Aroclor 1254 was the PCB mixture most commonly used in building materials, based on my personal experience reviewing test results.  Recently, as I was digesting data and researching literature to prepare for my UMass Soils Conference presentation this fall, I discovered something interesting about Aroclor 1254 that I hadn’t known; there are two types of Aroclor 1254, and while they are similar in their physical properties, from the standpoint of environmental toxicology they are very different.

Type 1 Aroclor 1254 was one of the commercial PCB mixtures manufactured and sold by the Monsanto Company prior to 1971; a high percentage of its production was used in electrical equipment like transformers and capacitors, smaller amounts were used in other applications like being formulated into building materials (e.g. paint and caulk).  In its pure form Aroclor 1254 (Type 1 and Type 2) is an highly viscous liquid that is thicker than honey.  While each batch of PCBs differed slightly in its exact chemical composition (based on the percentages of the the individual PCB congeners present), these differences from batch to batch were small. However, around 1971 the formula for making Aroclor 1254 changed in a fundamental way giving rise to what is now referred to as Type 2 Aroclor 1254.

Type 2 Aroclor 1254 was an indirect result of the 1968 Yusho rice oil poisonings. After the Yusho incident, Monsanto (the sole US PCB manufacturer) was seeking alternatives to reduce the toxicity of its PCB mixtures.  They hit on two ideas:

  1. Introducing a new PCB mixture, called Aroclor 1016, that contained lower concentrations of the volatile low molecular weight congeners and the more toxic high molecular weight congeners; and
  2. Stopping the sale of PCBs for use in applications that were not considered to be “totally enclosed”.  The totally enclosed requirement was intended to limit PCB uses that were more likely to result in human or environmental exposures.

Aroclor 1016 was produced by first making Aroclor 1242 (which it resembles), and then through distillation removing the light and heavy ends of the mixture.  As predicted, the toxicity of Aroclor 1016 was significantly less than that of other PCB mixtures; so far so good.   However, at this point in the story you might want to ask: “what happened to the light and heavy ends removed from the Aroclor 1242 to make Aroclor 1016?  Were these simply discarded?”

The answer is no, they were not discarded.  These light and heavy ends were used as feedstock for the manufacture of Type 2 Aroclor 1254.  Type 1 Aroclor 1254 was manufactured as a one step process; biphenyl (derived from coal tar) was chlorinated until the chlorine content in the resulting PCB mixture reached 54%.  In contrast Type 2 Aroclor 1254 was manufactured using a two step process where the by-products of Aroclor 1016 manufacturing (the light and heavy ends) were re-chlorinated to end up with a final product with 54% chlorine.

The physical properties of Type 1 and Type 2 Aroclor 1254 are virtually indistinguishable, but their chemical, toxicological and likely environmental properties have been shown to be different.  Type 2 contains higher concentrations of the so-called “co-planer” congeners (for chemists: that means the ones with no ortho substitutions) including about 5 times more of the three most toxic of these co-planer congeners, PCB 126, PCB 169 and PCB 77.  Type 2 Aroclor 1254 has also been reported to contain significant concentrations of the highly toxic polychlorinated dibenzofurans (PCDFs); these are virtually absent in Type 1 Aroclor 1254.

One interesting question is how much of the total Arochlor 1254 production was Type 2?  The answer from Monsanto’s production records indicates that about 1% of total Aroclor 1254 was Type 2.  Since Monsanto stopped PCB sales for other than totally enclosed uses at about the same time that production of Type 2 Aroclor 1254 began, it is likely that most or all of it went into electrical equipment like transformers and capacitors with little if any going into building materials.

Another interesting observation is that it is likely that Type 2 Aroclor 1254 is the PCB mixture that has been most commonly used in conducting toxicological studies on PCBs.  This is because towards the end of Monsanto’s PCB production (after 1970), most or all of the Aroclor 1254 being produced was Type 2.  As a result, this was the Aroclor 1254 that was available for distribution to researchers when toxicological studies were being undertaken on PCBs.  It took almost 20 years after the end of US PCB production before scientists could detect the chemical differences between Type 1 and Type 2 Aroclor 1254.

For more information please email me at okun@oto-env.com.