POISON

in nuclear physics, any material that can easily capture neutrons without subsequently undergoing nuclear fission. Examples of poisons are the naturally occurring elements boron and cadmium and the fission products xenon-135 and samarium-149. In nuclear reactors, poisons act as parasitic neutron absorbers and lower the rate of fission. in biochemistry, a substance, natural or synthetic, that causes damage to living tissues and has an injurious or fatal effect on the body, whether it is ingested, inhaled, or absorbed or injected through the skin. Although poisons have been the subject of practical lore since ancient times, their systematic study is often considered to have begun during the 16th century, when the German-Swiss physician and alchemist Paracelsus first stressed the chemical nature of poisons. It was Paracelsus who introduced the concept of dose and studied the actions of poisons through experimentation. It was not until the 19th century, however, that the Spaniard Matthieu Orfila, the attending physician to Louis XVIII, correlated the chemistry of a toxin with the biological effects it produces in a poisoned individual. Both concepts continue to be fundamental to an understanding of modern toxicology. Poisoning involves four elements: the poison, the poisoned organism, the injury to the cells, and the symptoms and signs or death. These four elements represent the cause, subject, effect, and consequence of poisoning. To initiate the poisoning, the organism is exposed to the toxic chemical. When a toxic level of the chemical is accumulated in the cells of the target tissue or organ, the resultant injury to the cells disrupts their normal structure or function. Symptoms and toxic signs then develop, and, if the toxicity is severe enough, death may result. This article considers humans as the primary subjects of poisoning. It first discusses the actions of poisons on the body and then examines principal types of synthetic and natural poisons. in biochemistry, a substance, natural or synthetic, that causes damage to living tissues and has an injurious or fatal effect on the body, whether it is ingested, inhaled, or absorbed or injected through the skin. Stated another way, a poison is a substance that, when consumed by an organism through an appropriate route of exposure at the correct dose, is capable of causing toxic effects or even death by its action on the tissues with which it comes into contact. A poison does not necessarily have an all-or-none effect. Degrees of poisoning are recognized, and some substances are more toxic than others. Potassium cyanide, of which as little as 0.25 gram may be lethal, is rated as highly toxic. Common salt, on the other hand, usually regarded as benign and as an item of the diet, can kill if a large enough single dose is taken. It has a relatively low toxicity. Poisoning may be acute or chronic, defined mainly in reference to time. When a single dose is followed immediately by symptoms that imperil the victim, the poisoning is said to be acute. When the doses or exposures are repetitive or continuous, with either immediate or delayed toxic effects, the poisoning is called chronic. For example, in a delayed reaction, a person may consume extremely low doses of arsenic for weeks or months without symptoms; indigestion, skin rashes, and changes in the nerves of the arms and legs, all symptoms of arsenic poisoning, will appear only gradually and only as the rate of accumulation of the poison in the tissues exceeds the rate of excretion. Thus, although a normal person may harbour a small quantity of a chemical throughout life without any distress, and a somewhat higher level may even confer increased benefit, only when a still higher critical, toxic level is exceeded do adverse complications ensue. Consequently, it is sometimes difficult to classify a substance as a poison without information about such factors as the amount consumed and the duration and pattern of consumption. Chronic poisoning can occur without the causal substance actually becoming stored in the body. The pain-relieving drug phenacetin, taken daily, even in large doses, is completely metabolized and eliminated. Yet a person who indulges in such self-medication excessively over months or years can sustain severe and irreversible kidney damage. A peculiar variant of chronic toxicity is chemical carcinogenicity. Prolonged exposure to certain oils, to benzidine, to beta-naphthylamine, to asbestos, and to cigarette smoke can lead, after an interval, to the development of distinctive and characteristic cancers. No dose-response relationship has been worked out for this process; repeated contact seems to be the essential feature, together, possibly, with a particular chemical configuration. Local effects of poisons include wheals, blisters, and violent inflammation, often followed by necrosis, muscle spasm, and disturbances in sensation. Systemic effects include local or widespread hemorrhages, destruction of blood cells, and abnormal clotting. Irritative effects on the various systems of the body may include excitement, convulsions, vomiting, diarrhea, or tetany; depressive symptoms include clouding of senses, paralysis, and weakening or arrest of respiration and heartbeat. Additional reading Toxic substances Curtis D. Klaassen, Mary O. Amdur, and John Doull (eds.), Casarett and Doull's Toxicology: The Basic Science of Poisons, 3rd ed. (1986), contains in-depth discussions of basic toxicology principles and information on toxins classified according to use and target organs. Analysis of the chemical structure of toxins influencing their effect is provided in Stanley E. Manahan, Toxicological Chemistry: A Guide to Toxic Substances in Chemistry (1989). Michael A. Kamrin, Toxicology: A Primer on Toxicology Principles and Applications (1988), is a concise, nontechnical general introduction. Ernest Hodgson, Richard B. Mailman, and Janice E. Chambers, Dictionary of Toxicology (1988), explains concepts and terminology and covers organizations and authorities in the field. Exposure and response to poisons Robert H. Dreisbach and William O. Robertson, Handbook of Poisoning: Prevention, Diagnosis, & Treatment, 12th ed. (1987), contains concise but essential information on the toxicity and treatment of poisoning by biological toxins and drugs. Avram Goldstein, Lewis Aronow, and Sumner M. Kalman, Principles of Drug Action: The Basis of Pharmacology, 2nd ed. (1973), describes the principles governing chemical absorption, distribution, and excretion of the substances. Matthew J. Ellenhorn and Donald G. Barceloux, Medical Toxicology: Diagnosis and Treatments of Human Poisoning (1988), focuses on the poisons derived from biological sources. Sidney Kaye, Handbook of Emergency Toxicology: A Guide for the Identification, Diagnosis, and Treatment of Poisoning, 5th ed. (1988), surveys almost 200 toxic substances. Sources of poisoning George D. Clayton and Florence E. Clayton (eds.), Patty's Industrial Hygiene and Toxicology, 3rd. rev. ed., vol. 2, Toxicology, parts A, B, and C (198182), is an extensive compendium of information on the toxicology of industrial chemicals. For concise reference on the subject, see Alice Hamilton, Hamilton and Hardy's Industrial Toxicology, 4th ed., rev. by Asher J. Finkel (1983). The toxicology of chemicals found in commercial products is examined in Robert E. Gosselin, Roger P. Smith, and Harold C. Hodge, Clinical Toxicology of Commercial Products, 5th ed. (1984), where information on chemical ingredients of many commercial products is detailed. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 9th ed. by Joel G. Hardman and Lee E. Limbird (1996), is an authoritative source of information on the toxicology of drugs and includes a section on the toxicity of industrial chemicals.The toxicology of food is the subject of Joint FAO/WHO Committee on Food Additives, Toxicological Evaluation of Certain Food Additives (1988), including lists of acceptable consumption of flavourings, preservatives, and colours; Jose M. Concon, Food Toxicology, 2 vol. (1988), a comprehensive survey of food contamination and poisoning; Palle Krogh (ed.), Mycotoxins in Food (1987); and W.F.O. Marasas and Paul E. Nelson, Mycotoxicology: Introduction to the Mycology, Plant Pathology, Chemistry, Toxicology, and Pathology of Naturally Occuring Mycotoxicoses in Animals and Man (1987). Bruce W. Halstead, Poisonous and Venomous Marine Animals of the World, 2nd rev. ed. (1988), is an exhaustive and profusely illustrated compendium on the toxic marine animals of the world, from protozoans to polar bears, including their historical background, names, geographic distribution, biology, mechanism of intoxication, and medical, toxicological, pharmacological, and chemical aspects. Curtis D. Klaassen King Lit Wong