PAHs are generally unreactive and have low acute toxicities yet degradates and biotransformation products of PAHs can be very potent mutagens and carcinogens.23 PAHs may induce cancer of the lungs, bladder, and skin. Several PAHs have been classified by the International Agency for Research on Cancer (IARC) as probable human carcinogens. Exposure to high levels of PAHs has been shown to produce immunosuppressive effects.
PAHs require metabolic activation to produce their mutagenic or carcinogenic effects. The primary mechanism of PAH biotransformation in higher organisms is by cytochrome P450-based monooxygenases leading to detoxification and excretion. However, attack by cytochrome P4501A1 can activate certain PAHs such as B[a]P to form a mutagenic diol epoxide capable of forming DNA adducts. The carcinogenesis of nitro-PAHs involves ring oxidation and nitro-reduction to form Af-hydroxyamino-PAH intermediates that can bind with DNA. The formation of hydroxy-PAH metabolites allows PAHs to be excreted by higher organisms. PAHs can bioconcentrate or bioaccumulate in aquatic invertebrates such as molluscs that do not posses the ability for their biotransformation while fish can effectively biotransform PAHs, preventing biomagnification up the food chain.23
Low molecular weight PAHs are more readily biodegraded than high molecular weight PAHs, which are strongly associated with soil, sediment, and soot particles in the environment.24 Through the action of bacteria, PAHs can become oxidized by incorporating molecular oxygen across an aromatic ring followed by the formation of hydroxy- or dihydroxy-PAHs. Further oxidation can produce ketone, dione and quinone derivatives, and carboxylic acid derivatives as rings cleave.
Atmospheric transformation of PAHs occurs through reactions with hydroxyl radicals (• OH), nitrate radicals (*NO3), and ozone 03 to produce nitro-PAHs (NPAH), oxy-PAHs (OPAH), and hydroxy-PAHs, although these derivatives can also be formed during combustion processes.25-28 In addition to nitrated PAHs, oxygenated derivatives include ketones, diones, quinones, and dicarboxylic acid anhydrides. Oxy- and nitro-PAH concentrations in the atmosphere are about an order of magnitude or less than their parent PAHs, yet they can account for a considerable degree of the toxicity and mutagenicity of urban aerosols.29 Exposure of PAHs to UV radiation in aquatic environments has been shown to produce significant "phototoxicity" to fish and inverte-
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