CHLORINATED HYDROCARBONS
J. W. Farrington, Woods Hole Oceanographic
Institution, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
These chemicals are considered in a pollution category because both deliberate and accidental release
to the environment of several of these types of
compounds, for example the industrial chemicals
such as PCBs (polychlorinated biphenyls) and the
chlorinated pesticides p,p
0 DDT (dichlorodiphenyltrichloroethane; formal chemical name 1,1
0 -(2,2,2trichloroethylidene)-bis (4-chlorobenzene)), have
had unintended adverse environmental effects on
diverse plants and animals and on people. Initially,
chemicals such as PCBs and DDT were beneficial to
human civilization: PCBs as industrial chemicals
allowing economical, safe delivery of electricity, and
DDT as a pesticide eradicating vector pests of human
health concern and agricultural crop pests. Only
after these chemicals had entered widespread use did
it become apparent that there were environmental
problems, although in hindsight there was evidence
of potential problems early in the history of their
manufacture and use.
Chlorinated hydrocarbons are chemicals made up
of the elements carbon (C) and hydrogen (H) at the
combine for the ‘hydrocarbon’ part of the molecule,
and chlorine atoms (Cl) substituted for hydrogen
where a hydrogen atom was normally bonded to a
carbon atom. Examples of structures of chlorinated
hydrocarbons are given in Figure 1.
Chlorinated hydrocarbons have a wide range of
molecular weights (related to size), and complexity,
i.e., there are various distinct configurations or arrangements of constituent atoms. For example, there
are 209 individual chlorobiphenyls (known collectively as congeners) making up the family of chemicals known as PCBs. Not all of these are present in
the commercial chemical mixtures of PCBs, but there
are usually 20–50 chlorobiphenyl congeners in a
given commercial mixture.
Smaller molecules among the class of chlorinated
hydrocarbons, such as tetrachloroethylene, trichloroethylene, and carbon tetrachloride are used
for activities such as degreasing of machinery and
dry cleaning. Presently, these compounds are of environmental concern for the oceans only in the nearshore coastal areas where their presence in sewage
effluents and contaminated or polluted ground water
interfacing with coastal sea water results in elevated
concentrations in coastal waters near sources of
input.
Environmental and human health concerns associated with chlorinated pesticides and PCBs have
evolved over the past several decades into wider
concerns with organochlorine compounds of all
types, ranging from those found in plastic trash bags
to the chemicals of Agent Orange defoliant used by
the United States during the war in Vietnam. Among
the chemicals of greatest concern on a per unit
amount basis are tetrachlorodibenzodioxins; often
the name is shortened in general public use to ‘dioxins’. Assessments of risks to human health and
wildlife for the various chlorinated hydrocarbon
pesticides and industrial chemicals are often expressed relative to tetrachlorodibenzodioxin risks.
There are an estimated 10 000 to 11 000 organochlorines in commercial production and many
thousands more may be present, but are as yet unidentified, as by-products of the production. In
addition, processes such as chlorination of sewage
effluent to kill bacteria, result in active forms of
chlorine which react with natural organic chemicals
in the sewage to produce a myriad of organochlorines; perhaps hundreds to thousands depending
on the effluent and the chlorination conditions. Small
amounts of organochlorines are also reported to result from various combustion processes, both natural
fires and volcanic eruptions, and human-controlled
processes such as incineration of wastes.
Analytical chemical, biochemical and molecular
biological methods can detect very low concentrations of these compounds in environmental samples, including marine organisms (ng g
À1
, or about
one unit mass of chlorinated hydrocarbon molecule
per billion unit masses of tissue molecules), sea water
(ng per 1000 liters or kg, ng kg
À1
), and marine
sediments (nanograms per gram of sediment, ng g
À1
).
Given the widespread occurrence of these compounds, and the known or suspected adverse environmental and human health effects at elevated
concentrations, there is a challenge, as with most
chemicals of environmental concern, in establishing
a ‘safe’ concentration in environmental samples. This
has stimulated intense debate among environmental
activists, the chemical industry, researchers, government officials and the public about the adverse
environmental effects and human health risks
163
J. W. Farrington, Woods Hole Oceanographic
Institution, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
These chemicals are considered in a pollution category because both deliberate and accidental release
to the environment of several of these types of
compounds, for example the industrial chemicals
such as PCBs (polychlorinated biphenyls) and the
chlorinated pesticides p,p
0 DDT (dichlorodiphenyltrichloroethane; formal chemical name 1,1
0 -(2,2,2trichloroethylidene)-bis (4-chlorobenzene)), have
had unintended adverse environmental effects on
diverse plants and animals and on people. Initially,
chemicals such as PCBs and DDT were beneficial to
human civilization: PCBs as industrial chemicals
allowing economical, safe delivery of electricity, and
DDT as a pesticide eradicating vector pests of human
health concern and agricultural crop pests. Only
after these chemicals had entered widespread use did
it become apparent that there were environmental
problems, although in hindsight there was evidence
of potential problems early in the history of their
manufacture and use.
Chlorinated hydrocarbons are chemicals made up
of the elements carbon (C) and hydrogen (H) at the
combine for the ‘hydrocarbon’ part of the molecule,
and chlorine atoms (Cl) substituted for hydrogen
where a hydrogen atom was normally bonded to a
carbon atom. Examples of structures of chlorinated
hydrocarbons are given in Figure 1.
Chlorinated hydrocarbons have a wide range of
molecular weights (related to size), and complexity,
i.e., there are various distinct configurations or arrangements of constituent atoms. For example, there
are 209 individual chlorobiphenyls (known collectively as congeners) making up the family of chemicals known as PCBs. Not all of these are present in
the commercial chemical mixtures of PCBs, but there
are usually 20–50 chlorobiphenyl congeners in a
given commercial mixture.
Smaller molecules among the class of chlorinated
hydrocarbons, such as tetrachloroethylene, trichloroethylene, and carbon tetrachloride are used
for activities such as degreasing of machinery and
dry cleaning. Presently, these compounds are of environmental concern for the oceans only in the nearshore coastal areas where their presence in sewage
effluents and contaminated or polluted ground water
interfacing with coastal sea water results in elevated
concentrations in coastal waters near sources of
input.
Environmental and human health concerns associated with chlorinated pesticides and PCBs have
evolved over the past several decades into wider
concerns with organochlorine compounds of all
types, ranging from those found in plastic trash bags
to the chemicals of Agent Orange defoliant used by
the United States during the war in Vietnam. Among
the chemicals of greatest concern on a per unit
amount basis are tetrachlorodibenzodioxins; often
the name is shortened in general public use to ‘dioxins’. Assessments of risks to human health and
wildlife for the various chlorinated hydrocarbon
pesticides and industrial chemicals are often expressed relative to tetrachlorodibenzodioxin risks.
There are an estimated 10 000 to 11 000 organochlorines in commercial production and many
thousands more may be present, but are as yet unidentified, as by-products of the production. In
addition, processes such as chlorination of sewage
effluent to kill bacteria, result in active forms of
chlorine which react with natural organic chemicals
in the sewage to produce a myriad of organochlorines; perhaps hundreds to thousands depending
on the effluent and the chlorination conditions. Small
amounts of organochlorines are also reported to result from various combustion processes, both natural
fires and volcanic eruptions, and human-controlled
processes such as incineration of wastes.
Analytical chemical, biochemical and molecular
biological methods can detect very low concentrations of these compounds in environmental samples, including marine organisms (ng g
À1
, or about
one unit mass of chlorinated hydrocarbon molecule
per billion unit masses of tissue molecules), sea water
(ng per 1000 liters or kg, ng kg
À1
), and marine
sediments (nanograms per gram of sediment, ng g
À1
).
Given the widespread occurrence of these compounds, and the known or suspected adverse environmental and human health effects at elevated
concentrations, there is a challenge, as with most
chemicals of environmental concern, in establishing
a ‘safe’ concentration in environmental samples. This
has stimulated intense debate among environmental
activists, the chemical industry, researchers, government officials and the public about the adverse
environmental effects and human health risks
163