Organic Compounds - Contaminant Geochemistry - page 79

release, many nontarget organisms can be affected severely. Pesticide residues and
their degradation products are toxic to many components of ecological systems,
either by being lethal to certain living forms or by changing environmental con-
ditions that in turn alter and usually decrease biodiversity in these systems. In
some cases, pesticides can have synergistic effects with other contaminants, thus
increasing overall toxicity. Exposure routes of pesticides include direct intake of
pesticide residues through, for example, digestion of pesticides adsorbed on crops
or dissolved in water and indirect paths through intake of pesticides or their
degradation products that have been concentrated in food chains and intake of
other environmental mediators.
Official lists of pesticides contain over 1,500 substances (Wood 2006 ). Pesti-
cides are divided into several major groups (e.g., herbicides, fungicides, insecti-
cides), which are subdivided into chemical or other classes (e.g., chloroacetanilide
herbicides or auxins). These compounds may be arranged according to their tox-
icity (WHO 2005 ), chemical structure, and/or activity. Here, we briefly discuss
only a few of the more common groups of pesticides; the interested reader should
consult readily available references (e.g., Barbash 2003 ; Matthews 2006 ; Milne
1995 ; Briggs 1992 ) as a primary source of more detailed information.
4.1.1 Organochlorine Insecticides
Organochlorine insecticides (e.g., DDT, aldrin, dieldrin, heptachlor, mirex,
chlordecone, and chlordane) were used commonly in the past, but many have been
removed from the market due to their negative health and environmental effects
and their persistence. However, insecticides of this group are in some cases
still used as active ingredients in various pest control products, such as gamma-
hexachlorocyclohexane (lindane). Lindane also is used as the active agent in the
medicine Kwell , used for human ectoparasitic disease, although it has been
associated with acute neurological toxicity either from ingestion or in persons
treated for scabies or lice. The general chemical structures of some of the orga-
nochlorine insecticides are given in Fig. 4.2 . These compounds are characterized
by cyclic structures, a relatively large number of chlorine atoms on the molecule,
and low volatility. As a result, they usually are resistant to natural degradation
processes and thus stable for very long periods after release to the environment.
Organochlorines are absorbed in the body through ingestion, inhalation, and
across the skin. These substances tend to concentrate in fatty tissues following
exposure. The chief acute toxic action of organochlorine pesticides is on the
nervous system, where these compounds induce a hyperexcitable state in the brain
(Joy 1985 ; Reigart and Roberts 1999 ). This effect is manifested mainly as con-
vulsions, sometimes limited to myoclonic jerking, but often expressed as violent
seizures. Other less severe signs of neurological toxicity, such as paresthesias,
tremor, ataxia, and hyperreflexia, also are characteristic of acute organochlorine
poisoning. Convulsions may cause death by interfering with pulmonary gas