The cyanide ion consists of a carbon triple bonded to a nitrogen. It readily reacts with hydrogen to form hydrogen cyanide gas, which has a faint almond-like smell. Most people can smell hydrogen cyanide; however, due to an apparent genetic trait, some individuals cannot. Cyanide gas (HCN) can be generated via combustion, including the exhaust of internal combustion engines, tobacco smoke, and especially some plastics derived from acrylonitrile (because of the latter effect, house fires can result in poisonings of the inhabitants). Cyanides are also produced by certain bacteria, fungi, and algae and are found in a number of foods and plants. Small amounts of cyanide can be found in apple seeds, mangoes and bitter almonds. Hydrocyanic acid (a solution of hydrogen cyanide in water) is present in freshly distilled bitter almond oil (2-4%) prior to its removal by precipitation as calcium ferrocyanide to give food quality oil. Hydrogen cyanide and most cyanide salts readily dissolve in water (or other biofluids) and exists in solution as the cyanide ion. Cyanide ions bind to the iron atom of the enzyme cytochrome c oxidase (also known as aa3) in the fourth complex in the mitochondrial membrane in the mitochondria of cells. The binding of cyanide to this cytochrome prevents transport of electrons from cytochrome c oxidase to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy. Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Because of its respiratory chain toxicity cyanide has been used as a poison many times throughout history. Its most infamous application was the use of hydrogen cyanide by the Nazi regime in Germany for mass murder in some gas chambers during the Holocaust. Hydrogen cyanide (with the historical common name of Prussic acid) is a colorless and highly volatile liquid that boils slightly above room temperature at 26 &deg;C (78.8 &deg;F). Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN-. The salts of hydrogen cyanide are known as cyanides. HCN is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals. Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. It is a weak acid with a pKa of 9.2. A minor tautomer of HCN is HNC, hydrogen isocyanide.