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Full Chapter of Toxicology
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EcotoxicologyEnvironmental toxicology is the study of the poisons around us. A general definition of
environmental toxicology would include the hazardous effects that these poisons have on
human health. Specifically, “environment” comes from the French word environ, which means
“around,” and mens is Latin for “mind.” The word “toxicology” comes from the Greek word
toxikon, a poisonous substance into which arrowheads were dipped, and the suffix - logy from
the Greek word logos, which means the study of, or treatise.
Some toxicology terms appear to be similar but should be used with specificity to allow for
accurate communication. For example, Poisons are substances that in relatively small doses act
to destroy life or seriously impair cellular function. There are a variety of poisons, many of
which occur naturally in plants and animals or as minerals. There are also manmade poisons,
which are the direct result of laboratory synthesis.
Toxins are poisonous substances produced by plants (phytotoxins), animals (zootoxins),
or bacteria (bacteriotoxins); a substance is toxic when it acts to destroy or impair cellular
function.
Toxicity is the state of being poisonous. The term venom refers to poisonous substances
secreted by certain animals, such as bees, spiders, and snakes. When substances produce
symptoms that are popularly referred to as intoxication (or poisoning) they are referred to as
toxicants.
Xenobiotics (Greek xenos, a stranger; -biotic, pertaining to life) may include substances,
such as toxicants, that are not naturally produced within an organism.
Modern Toxicology
Modern toxicology is composed of three subdisciplines. The first, descriptive toxicology,
involves toxicity testing of chemicals. Initially, the determination as to whether or not a chemical
is toxic must be made before safety and regulatory issues can be addressed. Toxicity testing
,usually takes place using experimental animals. Second, mechanistic toxicology exam ines the
biochemical processes by which identified toxicants have an impact on the organism. Although
descriptive toxicologists continue to identify agents of toxicity, the exact mechanism by which
many toxicants have their action on the organism awaits continued study. Last, regulatory
toxicology is concerned with assessing the data from descriptive toxicology and mechanistic
toxicology in an attempt to determine the legal uses of specific chemicals, as well as the risk
posed to the ecosystem by the marketing of those chemicals. Many disciplines contribute to an
understanding of toxicology. Of particular interest, clinical toxicology examines the effects of
toxicants on individuals and the efficacy of treatment for symptoms related to intoxication.
Forensic toxicology is concerned with the medical and legal questions relating to the harmful
effects of known or suspected toxicants, and industrial (or occupational) toxicology studies the
disorders found in individuals who have been exposed to harmful materials in their place of
work.
Types of toxic substance
Toxic substances fall into several classes in relation to the way man is exposed to them: drugs,
food additives, pesticides, industrial chemicals, environmental pollutants, natural toxins and
household poisons.
Drugs: Most people in the Western world consume drugs of one sort or another throughout their
lives. Drugs, however, have usually been designed to be highly potent in biological systems and
consequently many are potentially toxic. Drugs vary enormously in chemical structure and
possess a wide variety of biological activities. They are probably the only foreign substances of
known biological activity that man ingests intentionally. Included in this category are alcohol
and the active principles in cigarettes, both of which are used because of their biological activity
and both, of course, have toxic properties. Drugs used in veterinary practice must also be
considered here (and in the next section) as humans may consume meat from or other food
derived from animals treated with these drugs.
Food Additives: This is the second category of foreign substances which are directly ingested.
However, food additives are usually of low biological activity. Many different additives are now
added to food to alter the flavour or colour, prevent spoilage, or in some other way change the
, nature of the foodstuff. There are also many potentially toxic substances which may be regarded
as contaminants occurring naturally in food, resulting from cooking, or from other
contamination, Veterinary drugs and their breakdown products may also be found in foodstuffs.
Most of these substances, both natural and artificial, may be present in food in very small
amounts but for the majority little is known of their longterm toxicity. In many cases they are
ingested daily for perhaps a lifetime and the numbers of people exposed is very large. Public
awareness of this has now begun to influence the preparation and manufacture of food such that
additive free foods are appearing on supermarket shelves.
Industrial Chemicals: Industrial chemicals may contribute to environmental pollution, and they
may be a direct hazard in the workplace where they are used, formulated or manufactured. There
is a huge range of chemical types and many different industries may involve the use or
manufacture of hazardous chemicals. In the broadest sense industrial exposure might include
exposure to the solvents used in photocopiers and typists’ correction fluid. Although in general
exposure is controlled by law, often by the setting of control limits, realistic levels may still
prove to be hazardous in the long term and acute exposure due to accidents will always occur.
The long development time of diseases such as cancer often makes it difficult to determine the
cause until sufficient of the workforce have presented with the disease for the association with
the toxic compound to be made.
Environmental Pollutants: The main sources of pollution are industrial processes and the
deliberate release into the environment of substances such as pesticides. The most visible
pollutant, but perhaps not the most significant, is smoke from power stations and factories.
Factories may also produce and emit more potent substances in smaller quantities although the
level of these is generally controlled. Environmental pollutants may be released into the air, river
or sea water or dumped onto land. Car exhaust fumes with several known toxic constituents
constitute a major source of pollution. Pesticides are deliberately sprayed onto crops or
agricultural land with the potential for exposure either via the crop itself or through
contamination of drinking water or air.
Natural Toxins: Many plants and animals produce toxic substances for both defensive and
offensive purposes. Natural toxins of animal, plant and bacterial origin comprise a wide variety
of chemical types, cause a variety of toxic effects and are a significant cause of human
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