Classification of poisons

A poison is a substance that in definite quantities has properties harmful or fatal to an organism at exposure or absorption, a substance that through its chemical action kills, injures or impairs an organism. A toxin is any of various poisonous substances that are specific products of the metabolic activities of living organisms. Though these two terms are often mixed, they are not absolutely identical. Poisons are of such diverse natures that they are usually classified by origin, physical form, chemical nature or activity, target site, or use.

Poisons are of microbial, plant, animal, or synthetic origin. Microbial poisons are produced by microscopic organisms such as bacteria and fungi. Botulinus toxin, for example, is produced by the bacterium Clostridium botulinum and is capable of inducing weakness and paralysis when present in underprocessed, nonacidic canned foods or in other foods containing the spores. An example of a plant toxin is the belladonna alkaloid hyoscyamine, which is found in belladonna (Atropa belladonna) and jimsonweed (Datura stramonium).

Animal poisons are usually transferred through the bites and stings of venomous terrestrial or marine animals, the former group including poisonous snakes, scorpions, spiders, and ants, and the latter group including sea snakes, stingrays, and jellyfish. Synthetic toxins are responsible for most poisonings. "Synthetic" refers to chemicals manufactured by chemists, such as drugs and pesticides, as well as chemicals purified from natural sources, such as metals from ores and solvents from petroleum. Synthetic toxins include pesticides, household cleaners, cosmetics, pharmaceuticals, and hydrocarbons.

The physical form of a chemical – solid, liquid, gas, vapour, or aerosol – influences the exposure and absorbability.

Because solids are generally not well absorbed into the blood, they must be dissolved in the aqueous liquid lining the intestinal tract if ingested or the respiratory tract if inhaled. Solids dissolve at different rates in fluids, however. For example, compared with lead sulphate granules, granules of lead are practically nontoxic when ingested, because elemental lead is essentially insoluble in water, while lead sulphate is slightly soluble and absorbable. Even different-sized granules of the same chemical can vary in their relative toxicities because of the differences in dissolution rates. For example, arsenic trioxide is more toxic in the form of smaller granules than is the same mass of larger granules because the smaller granules dissolve faster.

A poison in a liquid form can be absorbed by ingestion or by inhalation or through the skin. Poisons that are gases at room temperature (e.g., carbon monoxide) are absorbed mainly by inhalation, as are vapours, which are the gas phase of substances that are liquids at room temperature and atmospheric pressure (e.g., benzene). Because organic liquids are more volatile than inorganic liquids, inhalation of organic vapours is more common. Although vapours are generally absorbed in the lungs, some vapours that are highly soluble in lipids (e.g., furfural) are also absorbed through the skin. Aerosols are solid or liquid particles small enough to remain suspended in air for a few minutes. Fibres and dust are solid aerosols. Aerosol exposures occur when aerosols are deposited on the skin or inhaled. Aerosol toxicity is usually higher in the lungs than on the skin. An example of a toxic fibre is asbestos, which can cause a rare form of lung cancer (mesothelioma).

Poisons can be classified according to whether the chemical is metallic, inorganic, acidic or alkaline. Metallic poisons are often eliminated from the body slowly and accumulate to a greater extent than nonmetallic poisons and thus are more likely to cause toxicity during chronic exposure. Organic chemicals are more soluble in lipids and therefore can usually pass through the lipid-rich cell membranes more readily than can inorganic chemicals. As a result, organic chemicals are generally absorbed more extensively than inorganic chemicals. Classification based on acidity is useful because, while both acids and alkalis are corrosive to the eyes, skin, and intestinal tract, alkalis generally penetrate the tissue more deeply than acids and tend to cause more severe tissue damage.

Electrophilic (electron-loving) chemicals attack the nucleophilic (nucleus-loving) sites of the cells’ macromolecules, such as deoxyribonucleic acid (DNA), producing mutations, cancers, and malformations. Poisons also ay be grouped according to their ability to mimic the structure of certain important molecules in the cell. They substitute for the cells’ molecules in chemical reactions, disrupting important cellular functions. Methotrexate, for example, disrupts the synthesis of DNA and ribonucleic acid (RNA).

Unlike the classifications described above, there is usually no predictive value in classification by target sites or by uses. Such classifications are done, however, to systematically categorize the numerous known poisons. Target sites include the nervous system, the cardiovascular system, the reproductive system, the immune system, and the lungs, liver, and kidneys. Poisons are classified by such uses as pesticides, household products, pharmaceuticals, organic solvents, drugs of abuse, or industrial chemicals. According to the way in which they act on the body, poisons are divided into the following classes: corrosives; irritants; narcotics; delirients, and convulsants.