Endocrine-Disrupting Chemicals and Their Environmental Impacts - Organic Pollutants: An Ecotoxicological Perspective - page 280

especially bioaccumulative (Guiney et al. 1979; Tyler et al . 1998). PCBs were used
widely in industry as coolants and insulating fluids for transformers and capacitors,
but production was greatly reduced in the 1970s, partly because they were found to
be highly mobile via atmospheric transport and were detected in remote (e.g., arc-
tic) wildlife populations (Muir et al . 1992). PCB residues in the environment have
declined since the 1980s (Fensterheim 1993), and rarely do PCB concentrations in
the aquatic environment now exceed 1 µg/L. Nevertheless, it is estimated that 70%
of the world's production of PCBs is still in use or in stock, and therefore has the
potential to enter the environment. Chapter 6 provides more detailed information on
the chemistry and ecotoxicology of PCBs.
15.6.4 d i of x i n s
Dioxins are important environmental pollutants. Of the 75 congeners, 2,3,7,8-tetra-
chloro-dibenzo-p-dioxin (TCDD) is considered the most reproductively toxic. Uses
included as a herbicide, and exposure of organisms to extremely low concentrations
of TCDD (0.1-1 µg/kg/d) can lead to alterations in the reproductive systems of the
subsequent offspring that are consistent with demasculinization (Gray et al . 1995).
Gray and colleagues (1995) showed that a dose of 1 µg TCDD/kg on a single day
during gestation delayed puberty and testicular descent in rats and hamsters and
caused a 58% reduction in the ejaculated sperm count, all consistent with demascu-
linizing modes of action. Polychlorinated dibenzodioxins (PCDDs) are lipophillic
and in the aquatic environment are generally found at low concentrations, although
some ecosystems are highly contaminated, for example, paper mill effluents, where
concentrations may rise to 40 µg/L (Merriman et al . 1991). In nonaquatic environ-
ments, PCDDs reach their highest concentrations in landfill sites (U.S. EPA 2006),
and leaching from these landfill sites will inevitably have adverse effects for the
local wildlife. Chapter 7 deals with the general chemistry and wider ecotoxicology
of PCDDs.
15.6.5 p o l y b r o m i n a T e d d i p h e n y l e T h e r s
Polybrominated diphenyl ethers (PBDEs) appear to disrupt thyroid function
(Carlsson et al. 2007) via their interactions with thyroid hormone receptors (Marsh
et al. 1998). In rats, exposure to low doses of 2,2′,4,4′,5-pentabromodiphenyl ether
(PBDE-99) has been shown to reduce the concentration of circulating thyroid hor-
mones (Kuriyama et al. 2007). Some PBDE congeners have also been tested for
carcinogenicity and shown significant dose-related increases in liver tumors (Hooper
and McDonald 2000). PBDEs are similar in structure to PCBs. They are used exten-
sively as flame retardants in a wide range of products, including electrical equip-
ment, textiles, plastics, and building materials, and they leach into the environment
from these products (de Wit 2002; McDonald 2002). First detected in the environ-
ment in 1979, PBDEs have now been found very widely in the environment (Allchin
et al. 1999), and levels are generally higher in aquatic species than in terrestrial
species (Sellström et al. 1993; Pijnenburg et al. 1995). PBDEs are highly resistant
to degradation and bioaccumulate in animal tissue (Allchin et al. 1999; Gustafsson