identify in terms of the types, rates of release, and total amounts of materials
released. In contrast, nonpoint discharges are much more difficult to
characterize. These include materials released from atmospheric emissions,
agricultural runoff, contaminated soils and aquatic sediments, and urban
runoff from such sources as parking lots and residential areas. In most
situations, discharges from nonpoint sources are composed of mixtures of
complex materials. Therefore, the amounts of toxicants released are difficult to
assess, and the rates and timing of discharges are difficult to predict.
Many classes of chemicals can exhibit toxicity. One of the most commonly
studied and discussed is the pesticide. Pesticide can refer to any substance that
exhibits toxicity to an undesirable organism, but its toxicity often constitutes a
broad spectrum. Industrial chemicals also are a major concern because of the
large amounts transported and utilized. Metals, such as cadmium from mining
and manufacturing processes, and as contaminants in oil, are also released to
the environment from various sources. Crude oil and the petroleum products
derived from it are significant sources of environmental toxicity because of
their common usage and persistence in the environment. Importantly, many of
these materials, particularly metal salts and petroleum, can occur in usually
uncontaminated environments. However, the fact that a toxicant is present
does not necessarily mean it will have a toxicological effect. Any chemical
substance can exhibit harmful effects if the amounts introduced into an
organism are sufficiently high. Indeed, the dose, or actual amount of material
that enters an organism, can determine the biological ramifications. At low
doses, it is possible that no apparent harmful effects will be observed. In many
toxicity evaluations, however, an increase in the growth of the exposed
organism may be observed when the dose of the material is very low, while at
high doses mortality may occur. The widely used dose–response relationship
refers to the relationship between dose and biological effect. In some instances,
no effects will be observed until a certain threshold concentration is reached.
1.5.2 THE DOSE–RESPONSE RELATIONSHIP
In environmental toxicology, environmental concentration is often used as a
substitute for knowing the actual amount, or dose, of a chemical entering an
organism.
4 However, in determining the relationship between dose and
response, it is necessary to distinguish between the dose or environmental
concentration and the amount of the material that reaches the target tissue. In
some cases, the concentration of a compound may be high in the environmental medium, particularly if it is water soluble, but if the chemical is not
absorbed and thus does not reach the target organ, no effect will be observed.
5
The fundamental basis of the quantitative relationship between exposure to
an agent and the incidence of an adverse response is the dose–response
assessment. Dose–response usually refers to the relationship between measurable physical, chemical, or biological responses following exposure to a certain
quantity of a chemical. Because, in most experiments, it is almost impossible to
determine the dose of a chemical actually reaching the target tissue, the
Introduction
7
[16:55 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-001.3d]
Ref: 4365 MING-HO YU Chap-001 Page: 7 1-12
released. In contrast, nonpoint discharges are much more difficult to
characterize. These include materials released from atmospheric emissions,
agricultural runoff, contaminated soils and aquatic sediments, and urban
runoff from such sources as parking lots and residential areas. In most
situations, discharges from nonpoint sources are composed of mixtures of
complex materials. Therefore, the amounts of toxicants released are difficult to
assess, and the rates and timing of discharges are difficult to predict.
Many classes of chemicals can exhibit toxicity. One of the most commonly
studied and discussed is the pesticide. Pesticide can refer to any substance that
exhibits toxicity to an undesirable organism, but its toxicity often constitutes a
broad spectrum. Industrial chemicals also are a major concern because of the
large amounts transported and utilized. Metals, such as cadmium from mining
and manufacturing processes, and as contaminants in oil, are also released to
the environment from various sources. Crude oil and the petroleum products
derived from it are significant sources of environmental toxicity because of
their common usage and persistence in the environment. Importantly, many of
these materials, particularly metal salts and petroleum, can occur in usually
uncontaminated environments. However, the fact that a toxicant is present
does not necessarily mean it will have a toxicological effect. Any chemical
substance can exhibit harmful effects if the amounts introduced into an
organism are sufficiently high. Indeed, the dose, or actual amount of material
that enters an organism, can determine the biological ramifications. At low
doses, it is possible that no apparent harmful effects will be observed. In many
toxicity evaluations, however, an increase in the growth of the exposed
organism may be observed when the dose of the material is very low, while at
high doses mortality may occur. The widely used dose–response relationship
refers to the relationship between dose and biological effect. In some instances,
no effects will be observed until a certain threshold concentration is reached.
1.5.2 THE DOSE–RESPONSE RELATIONSHIP
In environmental toxicology, environmental concentration is often used as a
substitute for knowing the actual amount, or dose, of a chemical entering an
organism.
4 However, in determining the relationship between dose and
response, it is necessary to distinguish between the dose or environmental
concentration and the amount of the material that reaches the target tissue. In
some cases, the concentration of a compound may be high in the environmental medium, particularly if it is water soluble, but if the chemical is not
absorbed and thus does not reach the target organ, no effect will be observed.
5
The fundamental basis of the quantitative relationship between exposure to
an agent and the incidence of an adverse response is the dose–response
assessment. Dose–response usually refers to the relationship between measurable physical, chemical, or biological responses following exposure to a certain
quantity of a chemical. Because, in most experiments, it is almost impossible to
determine the dose of a chemical actually reaching the target tissue, the
Introduction
7
[16:55 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-001.3d]
Ref: 4365 MING-HO YU Chap-001 Page: 7 1-12