Toxicity

Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell (cytotoxicity) or an organ such as the liver (hepatotoxicity). By extension, the word may be metaphorically used to describe toxic effects on larger and more complex groups, such as the family unit or society at large. Sometimes the word is more or less synonymous with poisoning in everyday usage. A central concept of toxicology is that the effects of a toxicant are dose-dependent; even water can lead to water intoxication when taken in too high a dose, whereas for even a very toxic substance such as snake venom there is a dose below which there is no detectable toxic effect. Toxicity is species-specific, making cross-species analysis problematic. Newer paradigms and metrics are evolving to bypass animal testing, while maintaining the concept of toxicity endpoints. "Toxic" and similar words derive from Greek τόξον toxon ("bow"), a reference to the use of poisoned arrows as weapons. This root was chosen because the transliteration of ἰός ios, the usual Classical Greek word for "poison", was not distinctive enough from the English word "ion," itself derived from a similar but unrelated Greek root. The literal meaning of the root toxo- is reflected in such biological names as Toxodon ("bow-toothed"). There are generally five types of toxicities; chemical, biological, physical, radioactive and behavioural. Disease-causing microorganisms and parasites are toxic in a broad sense but are generally called pathogens rather than toxicants. The biological toxicity of pathogens can be difficult to measure because the threshold dose may be a single organism. Theoretically one virus, bacterium or worm can reproduce to cause a serious infection. If a host has an intact immune system, the inherent toxicity of the organism is balanced by the host's response; the effective toxicity is then a combination.