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parent PAH (McCoy et al. 1979; Nikolaou et al.1984; Brack et al. 2003; Lampi et al.
2006).
Photodegradation via photolysis is a major source of elimination of PAHs from various
environmental compartments (Nikolaou et al. 1984), however, this is not the only
pathway that occurs. For instance, anthracene, which is a known photosensitizer, is able
to form the photoproduct 9,10-anthraquinone (ATQ) (Fox and Olive 1979; Nikolaou et
al. 1984; Mallakin et al. 2000). There are a wide variety of other oxyPAHs that are
formed via PAH photomodification. Some of these include phenanthrenequinone
(McConkey et al. 1997), many hydroxylated anthraquinones (Mallakin et al. 2000; Brack
et al. 2003), and a host of other unidentified compounds, resulting from the
photomodification of a variety of PAHs (Huang et al. 1997).
There is little specific data available to evaluate the persistence and bioaccumulation of
specific PAH photodegradation products. However, accompanying photomodification,
which is generally via oxygenation, should lead to an increase in susceptibility to
biotransformation in a similar manner to that observed after phase I metabolism by the
mixed function oxidases (Prince and Walters, 2006) which will minimise the risk of
persistence and bioaccumulation of such compounds.
In evaluating the toxicity of PAH photoproducts or metabolites, one must consider that
they will be less hydrophobic than the parent PAHs, although some hydrophobicity
remains, thereby facilitating partitioning in biological membranes (Krylov et al. 1997;
McConkey et al. 1997). They can then manifest toxicity to a variety of organisms through
specific and non-specific mechanisms (Chesis et al. 1984; Bondy et al. 1994; Zhu et al.
1995; Huang et al. 1997; Li et al. 2000; Shimada et al. 2004). There are few reports of the
presence of photoproducts or metabolites in the aquatic environment (Marvin et al. 2000;
Lampi et al. 2001; Machala et al. 2001; Papadoyannis et al. 2002; Kurihara et al. 2005),
and although in some cases, concentrations may reach that close to the parent (Kurihara
et al. 2005) or interact with co-contaminants (Xie et al. 2006), other work (Lampi et al.
2007) has shown that the contribution of photomodification, although significant, is
outweighed by that of photosensitisation, which is the greatest contributing factor to
photoinduced toxicity, and is already considered in the toxicity assessment of PAHs (e.g.
EU 2008).
Given the increase in polar groups and increase in potential for metabolism, PAH
metabolites should not be considered persistent or bioaccumulative substances. There is a
potential for toxicity of PAH metabolites, but this is less than the potential for effects
from photosensitized generation of reactive oxygen species, and is considered in PAH
hazard assessment. Thus PAH metabolites should not be considered PBT substances.
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