Air, water, soil, and food are all unavoidable components of the human environment. Each of those elements influences the quality of human life, and each of them may be contaminated. Food is not only the elementary source of nutrients, but may also contain natural chemical substances with toxic properties, e.g., cyanogenic glycosides (many plants), solanine (green parts of potatoes, sprouted potatoes, and potatoes stored in light), industrial pollutants (heavy metals), biogenic amines (fish), or mycotoxins (moldy foodstuffs).
Poisons have been used since the dawn of civilization, especially at royal courts, to eliminate opponents and to put condemned prisoners to death. They also have been used for hunting (e.g., curare) or for ritual ceremonies in primitive tribes. Hemlock (Conium maculatum L.) is one of the oldest poisons known. In 399 B.C. Socrates was condemned to death by the Athenian court and forced to drink hemlock. Both Caligula and Cleopatra were known to have eliminated their victims by using poisoned food or giving poisoned flower garlands to guests. In the Middle Ages poisoning was the most popular form of convincing a political antagonist of one's rights. The Renaissance Italian family of Borgia was suspected of the most sophisticated murders, committed with arsenic added to foods, candle wicks, or book paper, although this is not confirmed by historians. Historical sources say that the fear of poisoning caused Henry IV of France to eat only eggs that he had cooked himself and to drink only water that he had drawn himself from the Seine river.
A significant historical discovery was that an antidote to arsenic poisoning is the ingestion of gradually increasing doses of the poison: rather than accumulating in the human body, the higher doses of arsenic are excreted in the stool. Similarly, a habit of taking arsenic with foodstuffs developed about 200 years ago among the uplanders in northern Syria and in the Austrian Tyrol. This did not lead to fatal poisonings, but caused an illusive feeling of energy boost that could be used to overcome exhaustion. In the same way, since the 16th century, old horses were fed with small amounts of arsenic to temporarily improve their vitality and energy in order to deceive buyers at horse markets.
These anecdotes show the truth in the statement by Paracelsus, that ''All things are poison and nothing is without poison. Dosage alone determines poisoning.'' This phenomenon is fundamental for homeopathy, a branch of alternative medicine, which suggests treating particular syndromes by using minimal doses of medicines that in higher amounts would trigger the same disease symptoms among healthy individuals ('similia similibus curantur').
As well as being used for intentional poisonings, toxic substances present in plants or spoiled foods have been the causes of accidental poisonings, frequently epidemic and leading to death. Medieval examples include ergot poisonings, known as ignis sacer (holy fire) or ignis Sancti Antonii (St. Anthony's fire) (a French outbreak in 994 A.D. led to over 40000 deaths), while contemporary poisonings may be caused by, for example, preserves containing Clostridium botulinum spores, poisonous mushrooms being mistaken for edible Agaricus campestris, or fish contaminated with mercury compounds (the Minamata disease outbreak in Japan in 1958).
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