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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.

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 infectionInfection" is also the title of an episode of the television series Babylon 5; see Infection (Babylon 5). An infection is the detrimental colonization of a host organism by a foreign species. The colonizing organism interferes with the normal functioning. However, in a host with and intact immune systemThe immune system is any system present in an organism to prevent predation by biological agents. All living organisms have these protective measures, although they vary radically in scope and mechanism. In humans and domesticated animals, the immune syst 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 agentAn agent is an autonomous entity with an ontological commitment and agenda of its own. The term originated in philosophy. Each agent possesses the ability to act autonomously; this is an important distinction because a simple act of obedience to a commands. 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 smokeTobacco smoking is the act of smoking tobacco products, especially cigarettes and cigars. Tobacco smoking is considered a significant cause of human health problems, especially cancer and other disorders affecting the lungs. The practice of smoking tobacc, or industrial wasteIndustrial waste is a waste caused by industrial factories or mills. The problem began along with the outset of industrial revolution. Toxic waste and chemical waste are a kind of industrial waste. Sewage treatment can be used to clean water tainted with. 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, heatCommonly, heat is estrus, a period of increased sexual drive in female mammals. For the National Basketball Association team, see Miami Heat. For the movie, see Heat (movie). Heat (abbreviated Q also called heat change is the transfer of thermal energy be 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.