PCDFS

Polychlorinated dibenzofurans (PCDFs) are a group of 135 aromatic congeners with the basic structure shown here. From one to eight chlorine atoms may be substituted on the phenyl rings.* The degree of toxicity of the individual congeners is determined by the position of the chlorines on the rings. The most toxic members of this group are the planar, coplanar, or nearly planar compounds where at least three or four lateral positions are occupied by chlorines,* such as the 2,3,7,8-tetrachlorodibenzofuran.
Their chemical inertness tends to increase with chlorination. PCDFs are usually present in the environment and in tissues together with polychlorinated dibenzo-p-dioxins (PCDDs) and other chlorinated aromatic compounds. When their presence is determined in the environment, the results are often reported as toxic equivalencies (TEQs) based on toxic equivalency factors (TEFs). Toxicity values are assigned to the congeners of PCDD/Fs relative to the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). When a TEF is multiplied by the concentration of a congener in a matrix, TEQ is obtained. The sum of all congener TEQs in a specimen (total TEQ) can be used to compare dioxinlike activity among specimens. TCDD is the most toxic chemical among the PCDD/Fs congeners. The toxicity values are based on their structure–activity relationship for binding and activation of the aryl hydrocarbon receptor (AHR), an initial but insufficient step in the development of TCDD-type toxicity. The variation in the effect on the AHR among congeners with dioxinlike activity is 10 000 fold. Most of the research on structure–activity relationship has been done with TCDD. The substantial species variation is not considered in this approach.

PCDFs have no commercial use and are not deliberately produced by industry. Small amounts of a few congeners have been synthesized in the laboratory for research purposes.

PCDFs are lipid soluble and present in water in extremely low concentrations. They may contaminate surface water and groundwater in areas with point sources. They usually adhere to soil or sediment particles. Their presence has not been reported in municipal drinking water.
PCDD/Fs were determined in samples of archived surface soils collected in the early 1880s, in contemporary surface soils, and in archived subsurface soils collected in 1870/1880 and stored in Rothamsted experimental station in Lancaster, UK PCDFs were detected in most samples. The homolog compositions of PCDFs in earlier soils were similar to patterns in more recent soils, suggesting that similar sources of atmospheric emissions of PCDFs are operating currently as in the past in Europe. The homolog pattern, in samples from Thailand and Australia were different.