Encyclopedia of Environmental Science and Engineering, Volume I and II

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TOXICOLOGY

The terms toxicology, toxicity, or toxic substance ( toxicant )

are used daily in the scientific and general literature. Review

of almost any daily newspaper will reveal one or more arti-

cles on the toxic effects of a substance, most of which when

released into the environment are called pollutants. Today

there are scientific journals devoted to the subject of toxicity,

illustrating the importance of this topic. However, many do

not understand the term toxicology or have an understanding

of its concepts. So what is a good definition of toxicity? It

can be best defined as the science of poisons. Of course, this

brings us to the question of what a poison is: any substance

that can result in a detrimental^ effect when the concentration

is increased. An increased response as compared to increasing

concentration has been called a “dose-response curve,” which

will be discussed later.

When using the definition of toxicity provided above,

most will consider poisoning of animals and humans; how-

ever, this definition can be extended to all life forms, includ-

ing microbes (Thomulka et al., 1996) and plants (Haarmann

and Lange, 2000). In the broadest term, toxic insult can be

evaluated from an ecological viewpoint and can encompass

effects to an ecosystem. This is what is commonly consid-

ered when looking at poisoning in an industrial environment.

However, in today’s changing environment, the viewpoint

from an industrial perspective is changing to include the entire

environment. The scope of toxicology is ever-increasing,

and from the point of view of an engineer, especially an envi-

ronmental engineer, should not be limited. Depending on the

focus, toxicity can also be viewed from global impact (e.g.,

mercury release from burning fossil fuels) to that which affects

single-celled organisms in a local pond.

Public awareness has raised the term toxicity to an every-

day usage, although most do not understand how to properly

apply this term. Most consider that when something is listed

as toxic it means an effect from an exposure has occurred.

Certainly in the most general sense this is true. Forgotten

for the term toxicity is that every substance is toxic, at least

in the right dose. So what can be added to the concept of a

poison is that the dose makes the poison.

For engineers, often the terms hazardous substance or

waste are used as substitutes for toxicity. This in the strict

definition is not correct, in that a hazardous waste may not

act as a poison, but rather result in a physical effect (e.g., a

burn). However, even a substance capable of causing a burn

will do so in proportion to the concentration applied. Thus,

even for these types of substances, there is a dose-response

effect. If any effect from a substance is considered a toxic

response, then hazardous waste is another name for toxicity.

In most cases a hazardous waste is a mixture of substances

and
or chemicals at a site, and its release was uncontrolled or

unregulated. Regardless, this mixture will have its own dose-

response, while the individual chemicals or substances will

exhibit separate responses (a differing dose-response curve).

What is of importance to many engineers when examining

toxicity is the use of standard references. Table 1 lists number

of textbooks and governmental sources that contain various

numerical values for toxicity and basic information on chemi-

cals. These sources are a very good staring point to obtain basic

information about a chemical, its regulatory limits, and general

information on the hazards associated with the substance.

AREAS OF TOXICOLOGY

Toxicology can be divided into a variety of subareas. These areas

can be categorized by organ systems, chemicals (substances),

or discipline. Examples of categorization are shown in Table 2,

along with a brief description. For the most part, engineers will

work in the general areas of environmental and occupational

toxicology, although some will venture into others as well. In

special cases, engineers will venture into areas such as forensic

toxicology. What needs to be kept in mind is that toxicology is

an area that borrows from other basic fields of science, such as

chemistry, physics, biology, and mathematics.

ENDPOINTS OF TOXICITY

Historically, toxicology was associated with the response of

animals when exposed to an agent or agents. Mostly this has

been performed using small rodents such as mice and rats.

However, for engineers, animal toxicity data are only one part,

especially for work that relates to the environmental areas. For

example, evaluation of a hazardous-waste site can involve the

toxic effects to plants, invertebrates, microbes, and aquatic

organisms. Commonly, toxicity of a substance or toxicant

is often referred to a single organism. In the environmental

area, as well as in others, there may be many different types of

organisms affected, along with different effects among these

organisms. Use of a single value will not likely represent

toxicity to entire groups or a system. Thus, representation of

toxicity as a single value may be misleading. Toxicity end-

points for a chemical can vary by logarithmic orders, even for

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