Cyanide refers to chemical compounds containing the CN- ion. They are either salts of hydrocyanic acid (HCN) such as sodium or potassium cyanide (NaCN and KCN) or esters of HCN, such as CH3CN, methyl cyanide, a.k.a. acetonitrile.
Cyanide compounds are used in many industrial processes, most notably in the production of chemicals and in the refining of gold and silver (this is known as the cyanide process). Acetonitrile is used in the production of plastics, synthetic rubber, and acrylic.
The largest source of cyanide pollution is vehicle exhaust, but the chemical and mining industries as well as petroleum refineries, incinerators, landfills, and the use of some pesticides are also major contributors. They are also a component of cigarette smoke. In addition, workers in any of the above industries, as well as people working in pharmaceuticals and photography, face high cyanide exposure. Industrial and house fires may release cyanide as many materials release it when burnt, and it is a fairly common agent of suicide among laboratory workers. Some foods, most notably cassava are also sources of cyanide toxicity over long periods of time.
Cyanide may enter the body through air, contaminants in food, and more rarely, skin contact. It is an extremely lethal poison which acts very quickly. Cyanide gas (HCN) has an odor of bitter almond and exposure at 110 ppm is lethal in 30 minutes. It attacks all body tissues, binding itself to metals found in bodily chemicals. Its primary toxicity is through bonding with, and inactivating, cytochrome oxidase (specifically, its ferric iron), thereby stopping electron transport and thus cellular respiration. This affects tissues with the greatest oxygen requirements most quickly, hence its effects on the brain. Generally, however, inhalation results in respiratory damage, as the lungs are the first tissue to contact the gas.
Lower exposures are rarely dangerous in the long term, as cyanide is quickly detoxified by the body into sulfocyanides, and recovery occurs within a few hours. Thus, medical treatment must happen quickly, with antidotes such as amyl nitrate and sodium nitrate. Long term exposure may, however, lead to vision and hearing problems due to its effects upon the nervous system, as well as thyroid effects, especially if iodine is not present in sufficient quantities. Most long term exposure is due to cassava, a cyanide-containing root that is a staple of some groups in South America. Long term exposure is rarely seen in the U.S. | 
