Toxicology is the science of poisons, including not only their physical and chemical effects but also their detection and
antidotes. Toxicity is the ability of a substance to produce injury upon reaching a susceptible site in or on the body.
Substances are ranked according to a system of toxicity ratings used to indicate their relative hazard:unknown, for substances for which insufficient toxicity data are available; no toxicity, for materials that cause no harm under conditions of normal use or that produce toxic effects only because of overwhelming dosages or unusual conditions; slight toxicity, for materials that produce only slight effects on the skin or other organs of the body from either a single (acute) or repeated (chronic) exposure; moderate toxicity, for materials producing moderate effects on the skin or other organs of the body from either acute or chronic exposure; and severe toxicity, for materials that threaten life or cause permanent physical impairment or disfigurement from such exposure.
The dosage level and the period of time over which the dosage occurs determine the effect of a substance on an organism. Normally safe substances, such as salt and water, can cause illness or even death if consumed in sufficient amounts. Recognized poisons can differ from one another in toxicity by factors as great as 10 billion. The study of toxic effects within the body is aided by the study of the biochemicals called porphyrins (see hemoglobin), whose ratios can serve as markers for some kinds of toxic metals and chemicals.
LDQV is a notation describing toxic level: a statistical estimate of the dosage required to kill 50% of an indefinite population of test animals. EDQV, a more general notation, describes the median effective dosage required to produce a specified effect in 50% of the population; such an effect, for example, may be tumor production or inhibition of enzyme production. Other factors in determining toxicity include exposure route, physical nature of the toxicant, temperature, humidity, and condition of the subject. The determination of the toxicity and dosage of a substance involves exposing isolated living tissues, cells, and various animals to the substance. Normally, no testing is performed on human volunteers until animal studies are completed.
A tremendous number of substances can act as poisons, and an organism can be exposed to these substances by various routes. As a result,
toxicology has branched into several specialized areas, including economic toxicology, concerned with chemicals used in drugs, food additives, pesticides, and cosmetics; forensic toxicology, involving the medical and legal aspects of poisonous materials when death or severe injury is the result of their use; industrial toxicology, in which the effects of pollutants in the working environment are evaluated; and environmental toxicology, which is the evaluation of the synergistic effects of chemicals in the environment (see diseases, occupational).
The chemical and pharmaceutical industry has developed a vast number of compounds that are capable of both potential injury and benefit. Many beneficial drugs are poisonous if abused. In general, however, their usefulness outweighs the results of improper use. Government regulations of the pharmaceutical and chemical industry are based on the results of toxicological investigations.