Chemistry Reference
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was evidence of reduced reproductive success in 1994, and of high rates of aban-
donment of nests and supernormal clutches in 1995 (McCarty and Secord 1999).
It would appear that tree swallows are particularly insensitive to this type of toxic
action. This finding raises questions about the validity and wider applicability of the
use of TEQ values and brings to mind the large interspecific differences in TCDD
toxicity found in some toxicity tests.
In another study of tree swallows at a Superfund site along Woonasquatucket
River, Rhode Island, during 2000-2001, exceptionally high levels of 2,3,7,8-TCDD
were found in eggs from the most polluted areas (300 to >1000 pg/g wet weight).
Eggs sampled from a reference site contained only 12-29 pg/g. Concentrations in
the food of the birds were more than 6-18 times higher than that regarded as safe
for birds (10-12 pg/g). Hatching success was negatively correlated with TCDD con-
centrations in eggs, and only about 50% of eggs hatched in the most polluted area,
compared to >77% in the reference area. In contrast to the study described in the
area of the Hudson River, 2,3,7,8-TCDD was the dominant residue so far as TEQ
estimations were concerned. PCB levels were below those known to affect avian
reproduction (Custer et al. 2005).
A noteworthy finding of the investigations thus far is the considerable variation
in the relative contributions of PCBs, PCDDs, and PCDFs to TEQ values deter-
mined for wild vertebrates. Thus, the TEQ values for white-tailed sea eagles and
tree swallows on the Hudson River were accounted for very largely by coplanar
PCBs, whereas 2,3,7,8-TCDD was dominant in tree swallows in the Rhode Island
study area. Fish from Saginaw Bay showed relatively low TEQs. PCDDs, PCDFs,
and PCBs all made similar contributions to TEQ. There was considerable variation
between species and age groups of fish, with contributions to total TEQs in the fol-
lowing ranges:
PCDD: 5-38%
PCDF: 13-69%
PCB: 10-50%
7.7 Summary
Both PCDDs and PCDFs are refractory lipophilic pollutants formed by the interac-
tion of chlorophenols. They enter the environment as a consequence of their pres-
ence as impurities in pesticides, following certain industrial accidents, in effluents
from pulp mills, and because of the incomplete combustion of PCB residues in fur-
naces. Although present at very low levels in the environment, some of them (e.g.,
2,3,7,8-TCDD) are highly toxic and undergo biomagnification in food chains.
PCDDs and PCDFs, together with coplanar PCBs, can express Ah-receptor-
mediated toxicity. TCDD (dioxin) is used as a reference compound in the determina-
tion of TEFs, which can be used to estimate TEQs (toxic equivalents) for residues
of PHAHs found in wildlife samples. Biomarker assays for Ah-receptor-mediated
toxicity have been based on the induction of P450 1A1. TEQs measured in field
samples have sometimes been related to toxic effects upon individuals and associ-
ated ecological effects (e.g., reproductive success).
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