toxicity 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. toxicitythe skull and crossbones is a common symbol for toxicity. 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. etymology "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"). types 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. in some cases, e.g. cholera toxin, the disease is chiefly caused by a nonliving substance secreted by the organism, rather than the organism itself. such nonliving biological toxicants are generally called toxins if produced by a microorganism, plant, or fungus, and venoms if produced by an animal. physical toxicants are substances that, due to their physical nature, interfere with biological processes. examples include coal dust, asbestos fibres or finely divided silicon dioxide, all of which can ultimately be fatal if inhaled. corrosive chemicals possess physical toxicity because they destroy tissues, but are not directly poisonous unless they interfere directly with biological activity. water can act as a physical toxicant if taken in extremely high doses because the concentration of vital ions decreases dramatically with too much water in the body. asphyxiant gases can be considered physical toxicants because they act by displacing oxygen in the environment but they are inert, not chemically toxic gases. radiation can have a toxic effect on organisms. behavioral toxicity refers to the undesirable effects of essentially therapeutic levels of medication clinically indicated for a given disorder (dimascio, soltys and shader, 1970). these undesirable effects include anticholinergic effects, alpha-adrenergic blockade, and dopaminergic effects, among others. measuring toxicity can be measured by its effects on the target (organism, organ, tissue or cell). because individuals typically have different levels of response to the same dose of a toxic substance, a population-level measure of toxicity is often used which relates the probabilities of an outcome for a given individual in a population. one such measure is the ld50. when such data does not exist, estimates are made by comparison to known similar toxic things, or to similar exposures in similar organisms. then, "safety factors" are added to account for uncertainties in data and evaluation processes. for example, if a dose of a toxic substance is safe for a laboratory rat, one might assume that one-tenth that dose would be safe for a human, allowing a safety factor of 10 to allow for interspecies differences between two mammals; if the data are from fish, one might use a factor of 100 to account for the greater difference between two chordate classes (fish and mammals). similarly, an extra protection factor may be used for individuals believed to be more susceptible to toxic effects such as in pregnancy or with certain diseases. or, a newly synthesized and previously unstudied chemical that is believed to be very similar in effect to another compound could be assigned an additional protection factor of 10 to account for possible differences in effects that are probably much smaller. this approach is very approximate, but such protection factors are deliberately very conservative, and the method has been found to be useful in a deep variety of applications. assessing all aspects of the toxicity of cancer-causing agents involves additional issues, since it is not certain if there is a minimal effective dose for carcinogens, or whether the risk is just too small to see. in addition, it is possible that a single cell transformed into a cancer cell is all it takes to develop the full effect (the "one hit" theory). it is more difficult to determine the toxicity of chemical mixtures than a pure chemical because each component displays its own toxicity, and components may interact to produce enhanced or diminished effects. common mixtures include gasoline, cigarette smoke, and industrial waste. even more complex are situations with more than one type of toxic entity, such as the discharge from a malfunctioning sewage treatment plant, with both chemical and biological agents. the preclinical toxicity testing on various biological systems reveals the species-, organ- and dose-specific toxic effects of an investigational product. the toxicity of substances can be observed by (a) studying the accidental exposures to a substance (b) in vitro studies using cells/ cell lines (c) in vivo exposure on experimental animals. toxicity tests are mostly used to examine specific adverse events or specific endpoints such as cancer, cardiotoxicity, and skin/eye irritation. toxicity testing also helps calculate the no observed adverse effect level (noael) dose and is helpful for clinical studies. classification the international pictogram for toxic chemicals. for substances to be regulated and handled appropriately they must be properly classified and labelled. classification is determined by approved testing measures or calculations and has determined cut-off levels set by governments and scientists (for example, no-observed-adverse-effect levels, threshold limit values, and tolerable daily intake levels). pesticides provide the example of well-established toxicity class systems and toxicity labels. while currently many countries have different regulations regarding the types of tests, numbers of tests and cut-off levels, the implementation of the globally harmonized system has begun unifying these countries. global classification looks at three areas: physical hazards (explosions and pyrotechnics), health hazards and environmental hazards. health hazards the types of toxicities where substances may cause lethality to the entire body, lethality to specific organs, major/minor damage, or cause cancer. these are globally accepted definitions of what toxicity is. anything falling outside of the definition cannot be classified as that type of toxicant. acute toxicity main article: acute toxicity