Polycyclic Aromatic Hydrocarbons - Organic Pollutants: An Ecotoxicological Perspective - page 181

9 Polycyclic Aromatic

Hydrocarbons

9.1 Background
Hydrocarbons are compounds composed of carbon and hydrogen alone. They may
be classified into two main groups:

1. Aromatic hydrocarbons that contain ring systems with delocalized elec-
trons, for example, benzene.

2. Nonaromatic hydrocarbons that do not contain such a ring system. Included
here are alkanes, which are fully saturated hydrocarbons; alkenes, which
contain one or more double bonds; and alkynes, which contain one or more
triple bonds.

Some examples of different types of hydrocarbons are given in Figure 9.1. Nonaromatic
compounds without ring structure are termed aliphatic , whereas those with a ring
structure (e.g., cyclohexane) are termed alicyclic . Aromatic hydrocarbons often consist
of several fused rings, as in the case of benzo[ a ]pyrene.
Both classes of hydrocarbon occur naturally, notably in oil and coal deposits.
Aromatic compounds are also products of incomplete combustion of organic com-
pounds, and are released into the environment both by human activities, and by
certain natural events, for example, forest fires and volcanic activity.
Aromatic hydrocarbons, the subject of this chapter, are of particular concern
because of their mutagenic and carcinogenic properties. In the first place, this raises
issues about human health risks. However, there are also questions about possible
harmful effects on ecosystems that are exposed to high levels of aromatic hydrocar-
bons. Nonaromatic hydrocarbons are usually of low toxicity, have not received much
attention in ecotoxicology, and will not be discussed further in the present text. It
should, however, be remembered that crude oil consists mainly of alkanes, and large
releases into the sea due to the wreckage of oil tankers have caused the death of
many seabirds and other marine organisms because of their physical effects of oil-
ing , or smothering (see Clark 1992). Also, released crude oil may act as a vehicle for
other lipophilic pollutants that dissolve in it, for example, organotin compounds and
PCBs, both of which may be present in or on wrecked vessels.

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