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.
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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.
Toxicity evaluation
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.
