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Toxicity is the degree to which something is able to produce illness or damage to an exposed organism. Toxicity can refer to the effect on a whole organism, such as a human or a bacterium or a plant, or to a substructure, such as a cell (cytotoxicity) or an organ (organotoxicity 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".

In the science of toxicology, toxicity is the degree of impact of an external substance or condition and its deleterious effects on living things: organisms, organ systems, individual organs, tissues, cells, subcellular units is the subject of study. A central concept of toxicology is that effects are dose-dependent; even water – generally not considered to be toxic – can lead to water intoxication when taken in large enough doses, whereas for even a very toxic substance such as snake venom there is a dose below which there is no detectable toxic effect.

The skull and crossbones is a common symbol for toxicity.

Toxicity is the ability of a chemical or physical agent to induce detrimental temporary or permanent tissue change or to detrimentally interfere with normal biochemical processing.

There are generally three types of toxic entities; chemical, biological, and physical.

• Chemicals include inorganic substances such as lead, hydrofluoric acid, and chlorine gas, organic compounds such as methyl alcohol, most medications, and poisons from living things.
• Biological toxic entities include those bacteria and viruses that are able to induce disease in living organisms. Biological toxicity can be complicated to measure because the "threshold dose" may be a single organism. Theoretically one virus, bacterium or worm can reproduce to cause a serious infection. However, in a host with an intact immune system the inherent toxicity of the organism is balanced by the host's ability to fight back; the effective toxicity is then a combination of both parts of the relationship. A similar situation is also present with other types of toxic agents.
• Physically toxic entities include things not usually thought of under the heading of "toxic" by many people: direct blows, concussion, sound and vibration, heat and cold, non-ionizing electromagnetic radiation such as infrared and visible light, and ionizing radiation such as X-rays and alpha, beta, and gamma radiation.

Toxicity can be measured by the effects on the target (organism, organ, tissue or cell). Because individuals typically have different levels of response to the same dose of a toxin, a population-level measure of toxicity is often used which relates the probability 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 toxin 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. Obviously, this approach is very approximate; but such protection factors are deliberately very conservative and the method has been found to be useful in a wide 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 assess the toxicity of chemical mixtures than of single, pure chemicals because each component display 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.

Toxicity of a substance can be affected by many different factors, such as the pathway of administration (whether the toxin is applied to the skin, ingested, inhaled, injected), the time of exposure (a brief encounter or long term), the number of exposures (a single dose or multiple doses over time), the physical form of the toxin (solid, liquid, gas), the genetic makeup of an individual, an individual's overall health, and many others. Several of the terms used to describe these factors have been included here.

acute exposure

a single exposure to a toxic substance which may result in severe biological harm or death; acute exposures are usually characterized as lasting no longer than a day.

chronic exposure

continuous exposure to a toxin over an extended period of time, often measured in months or years can cause irreversible side effects.

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"Toxic" and similar words came from Greek τοξον = "bow (weapon)" and via "poisoned arrow," which came to be used for "poison" in scientific language, as the usual Classical Greek word ('ιον) for "poison" would transcribe as "io-", which is not distinctive enough. In some biological names, "toxo-" still means "bow", as in Toxodon = "bow-toothed" from the shape.

• Biological activity
• Biological warfare
• California Proposition 65 (1986)
• Carcinogen
• Drunkeness
• Mutagen
• Toxication
• Nephrotoxicity
• Neurotoxicity
• Ototoxicity
• Paracelsus
• Physiologically-based pharmacokinetic modelling.
• Poison
• RTECS - toxicity database
• Soil contamination
• Teratogen
• Toxin
• Toxicophore

• Agency for Toxic Substances and Disease Registry
• Chemical and Other Safety Information – Oxford University