Summary - Dioxins and dioxin-like PCBs

Dioxins and dioxin-like PCBs

 

Polychlorinated dibenzo-para-dioxins (dioxins, PCDD) and polychlorinated dibenzofurans (furans, PCDF) are two groups of planar tricyclic compounds that have very similar chemical structures and properties. The basic structure of PCDDs and PCDFs is shown in Figure 1. In both structures the hydrogen atoms at the atoms C1-C4 and C6-C9 can be replaced by chlorine atoms. In this way, dioxins have 75 possible positional isomers and furans have 135 positial isomers, so in total 210 different molecules (the so-called PCDD and PCDF congeners). The nomenclature of these molecules refers to the carbon atoms which are replaced by chlorine atoms.

Dioxins are generally very insoluble in water, are lipophilic and are very persistent. Of the 210 congeners, 17 contribute most significantly to the toxicity of dioxins and furans, the so-called “dirty seventeen”.

 

 

Figure 1. Basic structure of PCDDs and PCDFs. For PCDD, n and m can very between 0 and 4. For PCDFs: 2 ≤n+m≤8.

Polychlorinated biphenyls (PCBs) are a family of commercially produced compounds that have been used extensively since 1930 in a variety of industrial applications, including as dieletrics in transformers and large capacitors, as heat exchange fluids, as paint additives, in carbonless copy paper and in plastics.

The basic structure of PCBs is shown in Figure 2. In this structure, 1 to 10 hydrogen atoms can be replaced by chlorine atoms, ranging from three monochlorinated isomers to the fully chlorinated decachlorobiphenyl isomer. In total, there are 209 different PCBs (congeners). Ballschmiter and Zell have proposed a numbering of the PCB congeners in 1980, which has been adopted by the International Union of Pure and Applied Chemistry (IUPAC).

 

 

Figure 2. Basic structure of PCBs.

The toxicity of PCBs is affected by the number and the position of the chlorine atoms, substitution in the ortho position hinders the rotation of the rings. PCBs without ortho substitution are generally referred to as coplanar and all others as non-coplanar. Coplanar PCBs exhibit dioxin-like effects. The following congeners are classified as dioxin-like: the non-ortho PCBs 77, 81, 126 and 169 and the mono-ortho PCBs 105, 114, 118, 123, 156, 157, 167 and 189. These dioxin-like PCBs are measured together with the PCDDs and PCDFs.

 

For a risk assessment of PCDDs, PCDFs and dioxin-like PCBs, the principle of Toxicity Equivalency Factors (TEF) is applied. These TEF values have been assigned to individual congeners based on a comparison of toxicity of 2,3,7,8-TCDD. In this way, the toxicity of a complex mixture can be indicated by means of one single value, expressed in Toxic Equivalents (TEQ). The TEQ value of a mixture corresponds to the concentration of 2,3,7,8-TCDD having the same toxicity as the mixture of dioxin-like compounds. To calculate the TEQ of a mixture, the concentration of every dioxin-like congener is multiplied with the corresponding TEF value and these weighed concentrations are summed. In Table 1 the WHO-TEF-2005 values of the different PCDD, PCDF and dioxin-like PCBs are listed.

 

Congener	WHO-TEF-2005
PCDD
2,3,7,8-TCDD	1
1,2,3,7,8-PeCDD	1
1,2,4,7,8-HxCDD	0.1
1,2,3,6,7,8-HxCDD	0.1
1,2,3,7,8,9-HxCDD	0.1
1,2,3,4,6,7,8-HpCDD	0.01
OCDD	0.0003
PCDF
2,3,7,8-TCDF	0.1
1,2,3,7,8-PeCDF	0.03
2,3,4,7,8-PeCDF	0.3
1,2,3,4,7,8-HxCDF	0.1
1,2,3,6,7,8-HxCDF	0.1
1,2,3,7,8,9-HxCDF	0.1
2,3,4,6,7,8-HxCDF	0.1
1,2,3,4,6,7,8-HpCDF	0.01
1,2,3,4,7,8,9-HpCDF	0.01
OCDF	0.0003
Non-ortho PCBs
PCB 77	0.0001
PCB 81	0.0003
PCB 126	0.1
PCB 169	0.03
Mono-ortho PCBs
PCB 105	0.00003
PCB 114	0.00003
PCB 118	0.00003
PCB 123	0.00003
PCB 156	0.00003
PCB 157	0.00003
PCB 167	0.00003
PCB 189	0.00003

Table 1. PCDD, PCDF and dioxin-like congeners and their TEF values.