McKay, Donald. "Front matter" Multimedia Environmental Models ...

of other, somewhat similar or homologous chemicals. An example is the series of
chlorinated benzenes that vary systematically in properties from benzene to
hexachlorobenzene.
It is believed that some 50,000 to 80,000 chemicals are used in commerce. The
number of chemicals of environmental concern runs to a few thousand. There are
now numerous lists of “priority” chemicals of concern, but there is considerable
variation between lists. It is not possible, or even useful, to specify an exact number
of chemicals. Some inorganic chemicals ionize in contact with water and thus lose
their initial identity. Some lists name PCBs (polychlorinated biphenyls) as one
chemical and others as six groups of chemicals whereas, in reality, the PCBs consist
of 209 possible individual congeners. Many chemicals, such as surfactants and
solvents, are complex mixtures that are difficult to identify and analyze. One designation,
such as naphtha,
may represent 1000 chemicals. There is a multitude of
pesticides, dyes, pigments, polymeric substances, drugs, and silicones that have
valuable social and commercial applications. These are in addition to the numerous
“natural” chemicals, many of which are very toxic.
For legislative purposes, most jurisdictions have compiled lists of chemicals that
are, or may be, encountered in the environment, and from these “raw” lists of
chemicals of potential concern they have established smaller lists of “priority”
chemicals. These chemicals, which are usually observed in the environment, are
known to have the potential to cause adverse ecological and/or biological effects
and are thus believed to be worthy of control and regulation. In practice, a chemical
that fails to reach the “priority” list is usually ignored and receives no priority rather
than less priority.
These lists should be regarded as dynamic. New chemicals are being added as
enthusiastic analytical chemists detect them in unexpected locations or toxicologists
discover subtle new effects. Examples are brominated flame retardants, chlorinated
alkanes, and certain very stable fluorinated substances (e.g., trifluoroacetic acid) that
have only recently been detected and identified. In recent years, concern has grown
about the presence of endocrine modulating substances such as nonylphenol, which
can disrupt sex ratios and generally interfere with reproductive processes. The
popular book Our Stolen Future, by Colborn et al. (1996) brought this issue to public
attention. Some of these have industrial or domestic sources, but there is increasing
concern about the general contamination by drugs used by humans or in agriculture.
Table 3.2 lists about 200 chemicals by class and contains many of the chemicals of
current concern.
©2001 CRC Press LLC
3.2 IDENTIFYING PRIORITY CHEMICALS
It is a challenging task to identify from “raw lists” of chemicals a smaller, more
manageable number of “priority” chemicals. Such chemicals receive intense scrutiny,
analytical protocols are developed, properties and toxicity are measured, and reviews
are conducted of sources, fate, and effects. This selection contains an element of
judgement and is approached by different groups in different ways. A common thread
among many of the selection processes is the consideration of six factors: quantity,