poisons

poisons A poison is any substance that, by chemical action, is capable of killing or injuring living organisms when ingested or absorbed in small amounts. Poison is a quantitative concept, since virtually every known substance is harmful at some dose, but harmless in smaller quantities; the probable lethal dose can vary from a mere taste of those that are supertoxic (cyanide and strychnine, for example) to about a litre of those practically non-toxic (such as chemicals like lanolin and glycerin). All human beings vary in their response to drugs and chemicals, however, and some people can survive a dose generally regarded as fatal. Similarly, individuals may be more susceptible at some life stages than others — primarily infants or the elderly.

The route of administration influences the degree of poisoning that occurs. The major routes by which poisons gain access to the body are through the mouth or lungs, and less commonly by injection or by absorption through the skin. Inhalation is often the most serious type of exposure, as substances are absorbed into the body very effectively through the lungs. Once absorbed, they travel in the circulation and can cause adverse effects at sites remote from that of entry. Most affect one or two organs — the target organs of toxicity of a particular poison.

The central nervous system is most frequently affected by poisoning, and muscle and bone the least. Some substances have a mainly local effect, the damage depending on the route of administration. Corrosives (mineral acids, alkalis, phenols) are in this category. They destroy any tissues with which they come into contact. They burn and erode skin and, if inhaled, damage the bronchi and lungs. When swallowed they produce inflammation and ulceration in the mouth, throat, stomach, and intestines.

Whenever poisoning occurs, death results when the extent and duration of bodily dysfunction has proceeded so far that treatment fails and recovery becomes impossible. A large group of drugs — general anaesthetics (ether, chloroform), barbiturates, opiates (morphine, codeine), and others — cause central nervous system depression, leading to coma and respiratory and circulatory failure. The chlorinated hydrocarbons and certain heavy metals (including arsenic and antimony) cause liver injury and toxic hepatitis. Mercuric salts and ethylene glycol (antifreeze) damage the kidneys. Inhalation of carbon monoxide produces anoxia (lack of oxygen for cell respiration) by binding to haemoglobin and preventing oxygen transport in the blood. Strychnine, atropine, cocaine, certain snake venoms, and other poisons cause convulsions, resulting from altered nerve conduction in the brain and spinal cord. Organophosphorus compounds, the basis of ‘nerve-gas’, now used, with appropriate safeguards, as agricultural and horticultural pesticides, disturb function at nerve endings in the muscles and in the autonomic nervous system.

Poisoning may be acute or chronic. Acute poisonings usually follow a single large dose, and are characterized by a sudden onset of symptoms that run a short course not necessarily leading to death. By contrast, chronic poisonings develop after repeated exposures to small doses. Symptoms emerge gradually, often with remissions and recurrences, as the toxic agent accumulates in the body. Chronic lead and mercury poisoning, for example, follow this course (the term ‘mad hatter’ arose from the deranged behaviour of workers exposed to mercury compounds in the preparation of felt). Cumulative effects are not always associated with repeated exposure, however: carbon monoxide, for example is ‘washed out’ when oxygen or air is breathed, although smokers can have persistently raised levels in their blood. Chronic poisoning is often associated with occupational and environmental exposure, and identification of the specific agent can be difficult.

For many poisons, the effects of acute and chronic poisoning are very different. Acute benzene poisoning, for example, causes depression of the central nervous system, but chronic benzene poisoning can cause leukemia. In fact, chemical carcinogenicity appears to be a variant of chronic toxicity. Prolonged exposure to substances like asbestos and cigarette smoke can lead, often after many years, to the development of characteristic cancers.

Treatment of acutely poisoned patients requires the maintenance of respiratory and circulatory functions, and elimination of the poison from the body. Absorption of swallowed poisons can be interrupted or prevented by gastric lavage (washing out the stomach), induced vomiting, or administration of an adsorbing substance (for example activated charcoal or Fuller's earth). Specific antidotes are infrequently available. In cases of chronic poisoning, removal from the source of exposure and general nursing are most effective. The body may either recover fully, or sustain irreversible damage to some tissues.

During the nineteenth and early twentieth centuries there were many celebrated poison trials, whilst at the same time the science of toxicology (the study of poisons and poisoning) began to develop. For reasons partly connected with availability and partly the result of a continuously developing capacity to detect poisons in the human body, homicidal poisonings are now extremely rare. Figures for 1995 indicated that fewer than 0.5% of fatal poisonings in Britain and the US were proven homicides.

Today, the vast majority of people who suffer acute poisoning are victims of either accidental self-poisoning or suicide attempts. Carbon monoxide (mainly from exhaust fumes or faulty heating systems) accounts for a sizeable proportion of poisoning deaths; children are particularly vulnerable to this and to accidental intake of medicines or household products. Fatal poisonings often follow overdoses of sedatives, tranquillizers, antidepressants, painkillers, and ‘social’ drugs (alcohol, and illegal substances such as heroin and cocaine). It seems likely that acute and chronic accidental poisoning will continue to represent a significant health hazard for as long as vast numbers of toxic substances — including drugs, industrial chemicals, fuels, pesticides, paints, and household cleaning fluids — remain in daily use.

Katherine D. Watson

See also chemical warfare; drug abuse; environmental toxicology; toxicology.