Toxicity is a measure to the degree to which something is toxic or poisonous. The study of poisons is known as toxicology. 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 the liver. 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 toxicology, however, the subject of such study is the effect of an external substance or condition and its deleterious effects on living things:organisms, organ systems, individual organs, tissues, cells, subcellular units. A central concept of toxicology is that effects are dose-dependent; even water is toxic to a human in large enough doses, whereas for even a very toxic substance such as snake venom there is a dose for which there is no toxic effect detectable.

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

Chemicals include both inorganic substances such as lead, hydrofluoric acid, and chlorine gas, as well as organic compounds such as ethyl alcohol, most medications, and poisons from living things.

Biological toxicity can be more complicated to measure, as the "threshold dose" may be a single organism, as theoretically this 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. In particular, toxicity of cancer-causing agents is problematic, since for many such substances it is not certain if there is a minimal effective dose or whether the risk is just too small to see; here too the possibility exists that a single cell transformed into a cancer cell is all it takes to develop the full effect.

Mixtures of chemicals are more difficult to assess in terms of toxicity, such as gasoline, cigarette smoke, or 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.

Physically toxic entities include things not usually thought of as such by the lay person: direct blows, concussion, sound and vibration, heat and cold, non-ionizing electromagnetic radiation such as infrared and visible light, ionizing non-particulate radiation such as X-rays and gamma rays, and particulate radiation such as alpha rays, beta rays, and cosmic rays.

Toxicity can be measured by the effects on the target (organism, organ, or tissue). But since individuals have different levels of response to the same dose of a toxic exposure, there have also been devised various ways to measure the inherent toxicity of a thing by its measured effects on a whole population, such as LD50 (the dosage at which 50% of the exposed population dies).

When such data do not exist, estimates are made by comparison to known similar toxic things, or to similar exposures in similar organisms. Then "safety factors" must be built in to protect against the uncertainties of such comparisons, in order to improve protection against these unknowns.

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