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EC number: 266-028-2
CAS number: 65996-93-2
The residue from the distillation of high temperature coal tar. A black solid with an approximate softening point from 30°C to 180°C (86°F to 356°F). Composed primarily of a complex mixture of three or more membered condensed ring aromatic hydrocarbons.
PAH are absorbed through the pulmonary tract, the gastrointestinal
tract, and the skin. The rate of absorption from the lungs depends on
the type of PAH, the size of the particles on which they are absorbed,
and the composition of the adsorbent. PAH adsorbed onto particulate
matter are cleared from the lungs more slowly than free hydrocarbons.
Absorption from the gastrointestinal tract occurs rapidly in rodents,
but metabolites return to the intestine via biliary excretion. Studies
with 32P-postlabelling of percutaneous absorption of mixtures of PAH in
rodents showed that components of the mixtures reach the lungs, where
they become bound to DNA. The rate of percutaneous absorption in mice
varies according to the compound.
PAH are widely distributed throughout the organism after
administration by any route and are found in almost all internal organs,
but particularly those rich in lipids. Intravenously injected PAH are
cleared rapidly from the bloodstream of rodents but can cross the
placental barrier and have been detected in foetal tissues.
The metabolism of PAH to more water-soluble derivatives, which is
a prerequisite for their excretion, is complex. In general, parent
compounds are converted into intermediate epoxides (a reaction catalysed
by cytochrome P450-dependent mono-oxygenases), which are further
transformed by rearrangement or hydration to yield phenols or diols and
- following secondary oxidation - to yield tetrols, which can themselves
be conjugated with sulfuric or glucuronic acids or with glutathione.
Most metabolism results in detoxification, but some PAH are activated to
DNA-binding species, principally diol epoxides, which can initiate
PAH metabolites and their conjugates are excreted via the urine
and faeces, but conjugates excreted in the bile can be hydrolysed by
enzymes of the gut flora and reabsorbed. It can be inferred from the
available information on the total human body burden that PAH do not
persist in the body and that turnover is rapid. This inference excludes
those PAH moieties that become covalently bound to tissue constituents,
in particular nucleic acids, and that are not removed by repair.
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