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elevated in 15-day chicks, but not in adults; reduced nest provisioning behavior and
decreased chick survival were also observed.
6.2.5.3 Population genetics
Genetic resistance to PCBs has been reported in a nonmigratory estuarine fish
(
Fundulus heteroclitus
) from New Bedford Harbor in Massachusetts, a body of water
that is highly polluted by them (Nacci et al. 1999; Bello et al. 2001). Interestingly,
resistant fish from the New Bedford Harbor showed less induction of P4501A1 when
treated with coplanar PCB than did fish from control areas. In a general way, this fits
with the concept of Ah-receptor-mediated toxicity discussed in Chapter 7, Section
7.2.4. Insensitivity to P4501A1 induction suggests that planar PCBs, dioxins, etc.,
are less likely, in resistant fish, to operate the toxic mechanisms normally associ-
ated with this event; in turn, this could mean poor affinity for the Ah receptor in
the resistant fish. It should also be noted that P4501A has an activating function
toward coplanar PCBs such as 3,3′,4,4′-tetrachlorobiphenyl (see Section 6.2.4). In
other words, resistance may be the consequence of failure to induce the activation of
one or more coplanar PCBs—and this again could come back to unresponsiveness
of the Ah receptor to coplanar PCBs.
6.3
PoLyBromInated BIPHenyLS
The principal source of pollution by polybrominated biphenyls (PBBs) has been the
commercial mixture “Firemaster,” which was produced in the United States between
1970 and 1974. Production was discontinued in 1974 following a severe pollution
incident in Michigan, when Firemaster was accidentally mixed with cattle feed on a
farm. In due course, PBBs entered the human food chain via contaminated animal
products. Substantial residues were found in humans from the area, and were subse-
quently found to be highly persistent.
Firemaster is a stable solid, resembling a PCB mixture in its lipophilicity, chemical
and thermal stability, and low vapor pressure. Firemaster contains some 80 out of a
possible 209 PBB congeners, but just two of them—2,2′,4,4′,5,5-hexabromobiphenyl
and 2,2′,3,4,4′,5,5′, heptabromobiphenyl—account for around 85% of the commer-
cial product (Environmental Health Criteria 152). These two compounds were found
to be very slowly eliminated by humans exposed to them during the Michigan inci-
dent. A half-life of about 69 weeks was estimated for 2,4,5,2,4′,5′-HBB.
PBB residues became widespread in terrestrial and aquatic ecosystems of the
United States (Sleight 1979) and Europe. On account of their lipophilicity and slow
elimination by mammals, hexa- and heptabromobiphenyls may be expected to
undergo biomagnification with movement along food chains in an analogous manner
to higher chlorinated PCBs. Although they have been shown to have severe toxic
effects on farm animals receiving high doses, nothing is known of any toxic effects
that they may have had on wild vertebrates.
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