A toxin is a harmful substance produced within living cells or organisms;^[1][2] synthetic toxicants created by artificial processes are thus excluded.^[3] The term was first used by organic chemist Ludwig Brieger (1849–1919)^[4] and is derived from the word toxic.^[5]

Toxins can be small molecules, peptides, or proteins that are capable of causing disease on contact with or absorption by body tissues interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their toxicity, ranging from usually minor (such as a bee sting) to potentially fatal even at extremely low doses (such as botulinum toxin).^[6][7] Toxins are largely secondary metabolites, which are organic compounds that are not directly involved in an organism's growth, development, or reproduction, instead often aiding it in matters of defense.^{[citation needed]}

Terminology

Toxins are often distinguished from other chemical agents due to their biological origin.

According to an International Committee of the Red Cross review of the Biological Weapons Convention, "Toxins are poisonous products of organisms; unlike biological agents, they are inanimate and not capable of reproducing themselves", and "Since the signing of the Constitution, there have been no disputes among the parties regarding the definition of biological agents or toxins".^[8]

According to Title 18 of the United States Code, "... the term "toxin" means the toxic material or product of plants, animals, microorganisms (including, but not limited to, bacteria, viruses, fungi, rickettsiae or protozoa), or infectious substances, or a recombinant or synthesized molecule, whatever their origin and method of production..."^[9]

The word toxin does not specify method of delivery (as opposed to venom, a toxin delivered via a bite, sting, etc.). Poison is a related but broader term that encompasses both toxins and toxicants; poisons may enter the body through any means - typically inhalation, ingestion, or skin absorption. Toxin, toxicant, and poison are often used interchangeably despite these subtle differences in definition. The term toxungen has also been proposed to refer to toxins that are delivered onto the body surface of another organism without an accompanying wound.[2]

A rather informal terminology of individual toxins relates them to the anatomical location where their effects are most notable:

• Hemotoxin, causes destruction of red blood cells (hemolysis)
• Phototoxin, causes dangerous photosensitivity
• Hepatotoxins affect the liver
• Neurotoxins affect the nervous system

On a broader scale, toxins may be classified as either exotoxins, excreted by an organism, or endotoxins, which are released mainly when bacteria are lysed.

Biotoxins

The term "biotoxin" is sometimes used to explicitly confirm the biological origin.^[10][11] Biotoxins can be further classified, for example, as fungal biotoxins, microbial toxins, plant biotoxins, or animal biotoxins.

Toxins produced by microorganisms are important virulence determinants responsible for microbial pathogenicity and/or evasion of the host immune response.^[12]

Biotoxins vary greatly in purpose and mechanism, and can be highly complex (the venom of the cone snail can contain over 100 unique peptides, which target specific nerve channels or receptors).^[13]

Biotoxins in nature have two primary functions:

• Predation, such as in the spider, snake, scorpion, jellyfish, and wasp
• Defense as in the bee, ant, termite, honey bee, wasp, and poison dart frog

Some of the more well known types of biotoxins include:

• Cyanotoxins, produced by cyanobacteria
• Dinotoxins, produced by dinoflagellates
• Necrotoxins cause necrosis (i.e., death) in the cells they encounter.^[14] Necrotoxins spread through the bloodstream.^{[citation needed]} In humans, skin and muscle tissues are most sensitive to necrotoxins.^{[citation needed]} Organisms that possess necrotoxins include:
  • The brown recluse or "fiddle back" spider
  • Most rattlesnakes and vipers produce phospholipase and various trypsin-like serine proteases
  • Puff adder
  • Necrotizing fasciitis (caused by the "flesh eating" bacterium Streptococcus pyogenes) – produces a pore forming toxin
• Neurotoxins primarily affect the nervous systems of animals. The group neurotoxins generally consists of ion channel toxins that disrupt ion channel conductance. Organisms that possess neurotoxins include:
  • The black widow spider.
  • Most scorpions
  • The box jellyfish
  • Elapid snakes
  • The cone snail
  • The Blue-ringed octopus
  • Venomous fish
  • Frogs
  • Palythoa coral
  • Various different types of algae, cyanobacteria and dinoflagellates
• Myotoxins are small, basic peptides found in snake and lizard venoms, They cause muscle tissue damage by a non-enzymatic receptor based mechanism. Organisms that possess myotoxins include:
  • rattlesnakes
  • Mexican beaded lizard
• Cytotoxins are toxic at the level of individual cells, either in a non-specific fashion or only in certain types of living cells:
  • Ricin, from castor beans
  • Apitoxin, from honey bees
  • T-2 mycotoxin, from certain toxic mushrooms
  • Cardiotoxin III, from Chinese cobra
  • Hemotoxin, from vipers

Biotoxins as Bioweapons

Many living organisms employ toxins offensively or defensively. A relatively small number of toxins are known to have the potential to cause widespread sickness or casualties, but these may be appealing to those who would use them nefariously for several reasons. They are often inexpensive and easily available; they may even be able to be refined outside of a laboratory.^[15] They often act quickly and are highly toxic even at low doses, so may be considered more efficient than chemical agents.^[15] Biotoxin used as a weapon of terror is considered to be the most harmful use for such substances.^[16][17] Therefore it is thought that awareness of the potential for biotoxins to be used as weapons, awareness of potential clinical symptoms of biotoxin poisoning, and the development of effective countermeasures including rapid investigation, response, and treatment are vital.^{[16][18] [15]}

Environmental toxins

The term "environmental toxin" can sometimes explicitly include synthetic contaminants^[19] such as industrial pollutants and other artificially made toxic substances. As this contradicts most formal definitions of the term "toxin", it is important to confirm what the researcher means when encountering the term outside of microbiological contexts.

Environmental toxins from food chains that may be dangerous to human health include:

• Paralytic shellfish poisoning (PSP)^[20][21][22]
• Amnesic shellfish poisoning (ASP)^[23][24]
• Diarrheal shellfish poisoning (DSP)^[25][26]
• Neurotoxic shellfish poisoning (NSP)^[27][28][29]

Finding information about toxins

In general, when scientists determine the amount of a substance that may be hazardous for humans, animals and/or the environment they determine the amount of the substance likely to trigger effects and if possible establish a safe level. In Europe, the European Food Safety Authority produced risk assessments for more than 4,000 substances in over 1,600 scientific opinions and they provide open access summaries of human health, animal health and ecological hazard assessments in their: OpenFoodTox^[30] database.^[31][32] The OpenFoodTox database can be used to screen potential new foods for toxicity.^[33]

The Toxicology and Environmental Health Information Program (TEHIP)^[34] at the United States National Library of Medicine (NLM) maintains a comprehensive toxicology and environmental health web site that includes access to toxins-related resources produced by TEHIP and by other government agencies and organizations.^[35] This web site includes links to databases, bibliographies, tutorials, and other scientific and consumer-oriented resources. TEHIP also is responsible for the Toxicology Data Network (TOXNET),^[36] an integrated system of toxicology and environmental health databases that are available free of charge on the web.

TOXMAP is a Geographic Information System (GIS) that is part of TOXNET.^[37] TOXMAP uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency's (EPA) Toxics Release Inventory and Superfund Basic Research Programs.

Misuse of the term

In the context of quackery and alternative medicine, the term "toxin" is used to refer to any substance alleged to cause ill health. This could range from trace amounts of potentially dangerous pesticides, to supposedly harmful substances produced in the body by intestinal fermentation (auto-intoxication), to food ingredients such as table sugar, monosodium glutamate (MSG), and aspartame.^[38]

The use of detoxification or detox as justification for treatments such as infrared saunas, diets,^[39] or chiropractic treatments^[40], is often called the toxin gambit, referring to a marketing technique which can frighten the public into seeking treatments that claim to remove unspecified toxins.^[41] These claims can be harmful financially and physically. Healthy kidneys and liver are all that most people need to remove almost anything potentially toxic that would be ingested.^[42]