Organic Compounds - Contaminant Geochemistry - page 88

(b) Br

(a)

Br

B r

Br

O

Br

Br

Br

Br

Br

Br O

tetrabromobisphenol A

hexabromocyclododecane

(c)

Br x

O

Br y

polybromodiphenyl ether

Fig. 4.6 Chemical structures of a tetrabromobisphenol A (TBBPA), b hexabromocyclododecane
(HBCD), and c polybrominated diphenyl ethers (PBDEs)

have been associated with a wide variety of toxic effects, affecting (1) thyroid
hormone balance, which can cause hypothyroidism and tumors; (2) the central
nervous system, which may manifest abnormalities in development dysfunction;
(3) hepatic functions, which may cause increased activities of a number of
enzymes, including cytochrome P-450, reduction in vitamin A levels, and tumors;
(4) disturbances to the estrogen balance; and (5) in utero development, which can
cause increased embryo mortality and delayed skeletal formation (e.g., DePierre
2003 ; EPA 2000c ;EU 1997 ; Hooper and McDonald 2000 ; Meerts et al. 2000 ).
Exposure to PBDEs disrupts the thyroid hormone both in humans (Bahn et al.
1980 ) and in animals (Hallgren and Darnerud 1998 , 2002 ; Hallgren et al. 2001 ). It
should be further noted that such effects on the thyroid hormone and its regulatory
functions may cause brain developmental abnormalities, especially in children
exposed in utero or through breast-feeding. PBDEs were found to bind the Ah
receptor in experiments on rats, which in turn regulate several enzymes, including
the cytochrome P-450 system (Hooper and McDonald 2000 ). Due to similarities in
their chemical structure and physical properties with other toxic compounds such
as PCBs, dioxins, and several pesticides such as DDT, PBDEs are suspected of
sharing some toxicological properties as well (DePierre 2003 ).
HBCD distribution in the environment and its effects on humans were discussed
in a review by Covaci et al. ( 2006 ). HBCD was reported to be capable of inducing
cancer by a nonmutagenic mechanism (Helleday et al. 1999 ; Yamada-Okabe et al.
2005 ). Similar to the BPDEs, HBCD is considered capable of disrupting the
thyroid

hormone system (Yamada-Okabe et al. 2005 ). Following neonatal