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Organic pollutants 169

which suggest they have an oestrogen-like action. Microorganisms can break down PAHs in
soil or water after a period of weeks to months, though some PAHs are resistant to decay.
Microorganisms degrade low molecular weight PAHs more easily than higher molecular
weight PAHs. Non-substituted PAHs will also be degraded faster than alkyl-substituted
PAHs (Lundstedt et al., 2003).
With respect to humans, PAHs are generally associated with chronic risks, which are
often the result of exposure to complex mixtures of aromatic compounds rather than to low
levels of a single compound. Toxic effects include DNA adducts and cancer. In general, the
heavier (4-, 5-, and 6-ring) PAHs have greater carcinogenic potential than the lighter (2-
and 3-ring) PAHs (ATSDR, 2013). Human exposure to PAHs is mainly via the inhalation
of contaminated air in the work environment and outdoors, and the ingestion of grilled or
charred meats or contaminated water, cow’s milk, or other foodstuffs (e.g. cereals, vegetables,
fruits).

9.5 CHLORINATED HYDROCARBONS

Chlorinated hydrocarbons, also known as organochlorins , are hydrocarbons with one or
more chlorine substituents. These compounds can consist of aliphatic as well as aromatic
structures, and include a wide range of compounds. The carbon–chlorine bond is strong and
makes organochlorins very stable. Because of this property, many chlorinated hydrocarbon
compounds have been synthesised and used extensively in agricultural and industrial
applications. In particular, the use of organochlorins as pesticides has resulted in a widespread
contamination of aquatic and soil systems. Chlorinated hydrocarbons that are pesticides
include DDT , aldrin , dieldrin , heptachlor, chlordane, lindane, endrin, and hexachloride.
The chemical structures of these compounds are depicted in Figure 9.1. Other sources of
environmental contamination by chlorinated hydrocarbons include spills from chemical
facilities, improper disposal or leakage from storage containers or waste sites, and the burning
of waste containing plastics, especially polyvinylchloride (PVC ).
Because organochlorins are very stable, these molecules tend to persist in the
environment for a long time and biomagnify in the food chain. Chlorinated hydrocarbon
compounds are very toxic to fish and invertebrates, as well as to any other animals that may
-1
feed on them. The 96-hour LC50 for tested animals ranges from 1–60 μg l , depending
on the species. To account for their great bioaccumulation and biomagnification potential,
the US Environmental Protection Agency has stipulated that the amount of chlorinated
-1
hydrocarbon compounds permitted in the water must not exceed 0.001 μg l . However,
even at low chronic doses, these compounds have the potential to cause problems, including
hormone disruption leading to reduced reproductive success and damage to the central
nervous system. In humans, they may also cause liver and kidney damage. Because of their
persistence, tendency to bioaccumulate, and toxicity to non-target species, most chlorinated
hydrocarbon pesticide uses have been phased out in the developed countries.

Figure 9.1 Examples of some organochlorine pesticides.

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