In biology, poisons are substances that cause disturbances in organisms, usually by chemical reaction or other activity on the molecular scale, when an organism absorbs a sufficient quantity.〔("Poison" ) at ''Merriam-Webster''. Retrieved December 26th, 2014.〕
The fields of medicine (particularly veterinary) and zoology often distinguish a poison from a toxin, and from a venom. Toxins are poisons produced by organisms in nature, and venoms are toxins injected by a bite or sting (this is exclusive to animals). The difference between venom and other poisons is the delivery method. Industry, agriculture, and other sectors use poisons for reasons other than their toxicity. Pesticides are one group of substances whose toxicity is their prime purpose.
In 2013, unintentional poisonings cuased 98,000 deaths worldwide, down from 120,000 deaths in 1990.
The term "poison" is often used colloquially to describe any harmful substance—particularly corrosive substances, carcinogens, mutagens, teratogens and harmful pollutants, and to exaggerate the dangers of chemicals. Paracelsus (1493-1541), the father of toxicology, once wrote: "Everything is poison, there is poison in everything. Only the dose makes a thing not a poison"〔
Latin: ''Dosis sola venenum facit.'' Paracelsus: ''Von der Besucht'', Dillingen, 1567
(see median lethal dose). The law defines "poison" more strictly. Substances not legally required to carry the label "poison" can also cause a medical condition of poisoning.
Some poisons are also toxins, which is any poison produced by animals, vegetables or bacterium, such as the bacterial proteins that cause tetanus and botulism. A distinction between the two terms is not always observed, even among scientists. The derivative forms "toxic" and "poisonous" are synonymous.
Animal poisons delivered subcutaneously (e.g., by sting or bite) are also called ''venom''. In normal usage, a poisonous organism is one that is harmful to consume, but a venomous organism uses venom to kill its prey or defend itself while still alive. A single organism can be both poisonous and venomous, but that is rare.
In nuclear physics, a poison is a substance that obstructs or inhibits a nuclear reaction. For an example, see nuclear poison.
Environmentally hazardous substances are not necessarily poisons, and vice versa. For example, food-industry wastewater—which may contain potato juice or milk—can be hazardous to the ecosystems of streams and rivers by consuming oxygen and causing eutrophication, but is nonhazardous to humans and not classified as a poison.
Biologically speaking, any substance, if given in large enough amounts, is poisonous and can cause death. For instance, several kilograms worth of water would constitute a lethal dose. Many substances used as medications - such as fentanyl - have an LD50 only one order of magnitude greater than the ED50. An alternative classification distinguishes between lethal substances that provide a therapeutic value and those that do not.
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