Environmental Engineering Reference
In-Depth Information
to be the most harmful of the PAHs. The effects they exhibit in animal and
human studies are representative of the class as a whole. In addition, these
are the PAHs to which the public is most commonly exposed. Also, they are
found in highest concentration on National Priority List hazardous waste
sites (ATSDR, 1995a and 1995b).
As a class of compounds, PAHs have been classified as carcinogens,
mutagens, and immunosuppressants. Even slight differences in PAH chem-
ical structure and activity result in different toxic potencies and different
health effects from the individual PAHs. The importance of PAH chemical
structure as an indicator of potential carcinogenicity has been reviewed in
Pitot and Dragan (1996). Some PAHs have been classified as carcinogens
only in laboratory animals. Others, including benzo(a)pyrene (BaP) and
benzo(a)anthracene, have been identified as human carcinogens. Still others
are possible carcinogens or not classifiable because the testing is incomplete.
Tumors usually occur at the point of entry into the body (i.e., the skin, lungs,
eyes, intestines). However, metabolism of these compounds can result in an
increase in their toxic potency and tumor formation in secondary organs
(i.e., bladder, colon, liver). Metabolites of these compounds can also be
carried into cells where they form adducts with DNA through covalent
bonding. The best-studied mutation is in the 12th codon of the Hras codon
The PAHs elicit multiple responses from the body's immune system due to
their effects on humoral and cell-mediated immunity as well as host resis-
tance (Burns et al., 1996). The mechanisms of PAH immunosuppression have
been reviewed by White et al. (1994). BaP is often used as an indicator for
risk assessment of human exposure because it is highly carcinogenic, per-
sistent in the environment, and toxicologically well understood. This breadth
of knowledge does not exist for most of the other PAH compounds.
Because PAHs occur as mixtures of different concentrations of different
homologues, toxic equivalency factors (TEFs) were proposed, similar to
those used in the risk assessment of mixtures of polychlorinated biphenyls
(PCBs). The Environmental Protection Agency (EPA, 1984) took the first step
by separating PAHs into carcinogenic and noncarcinogenic compounds. All
of the PAHs were rated, using BaP as a reference and giving it a value of
1.00. However, this method led to an overestimation of exposure risk because
the carcinogenicity of most of the compounds was unknown and the inter-
actions between compounds in mixtures had not been determined. In an
attempt to overcome this liability, Nisbet and LaGoy (1992) developed a new
method based on the compounds' response while testing one or more PAHs
concurrently with BaP in the same assay system (usually lung or skin cell
carcinoma). BaP remained the reference carcinogen, assigned the value of
1.00. Sixteen other PAHs were ranked in comparison to BaP carcinogenicity.
This system was tested by Petry et al. (1996), who assessed the health risk
of PAHs to coke plant workers. There are drawbacks to any system that uses
equivalency factors. The uncertainties in this case arise primarily from deal-
ing with inconsistent mixtures. Carcinogenic potency could be affected by
differences in bioavailability, a competition for binding sites, cocarcinogenic
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