Nitrites are naturally-occurring chemical compounds that include a nitrogen atom bonded to two oxygen atoms as part of their molecular structure. They are closely related to nitrates, which include a nitrogen atom and three oxygen atoms. Examples include sodium nitrite, potassium nitrite, and ammonium nitrite. Although these compounds are toxic to humans if ingested in large doses, small amounts can be used in medications and as preservatives in the food industry.
Nitrates and nitrites, which can be chemically converted into one another, are found naturally in vegetables and meats and can be metabolized by humans under normal circumstances. Plants require nitrates as a nutrient, so nitrates are also a common component of fertilizers. Problems from excessive nitrate or nitrite consumption most often occur when one of these compounds leaches into a drinking water supply. Very high doses of nitrites can become toxic.
The toxicity of nitrites is due to the ability of these compounds to react with hemoglobin, the substance in blood that transports oxygen through the body. Nitrites cause hemoglobin to become a substance called methemoglobin, which is less able to release oxygen. The lack of oxygen that results from excessive nitrite consumption may cause symptoms such as dizziness, headache, or even unconsciousness in more severe cases.
Pesticides made from nitrites take advantage of the potentially toxic effects of nitrites on living organisms. Ammonium nitrite and sodium nitrite are both used as pesticides, and despite the danger that high doses pose to humans and animals, some nitrite compounds are also used as medicines. For example, the interaction of the methemoglobin formed by nitrites and the poison cyanide results in a non-toxic compound called cyanomethemoglobin. Nitrites can therefore be used as an antidote to cyanide poisoning.
Nitrite compounds can also be vasodilators, substances which cause blood vessels to dilate. This happens when they are converted into nitric oxide, a molecule with one nitrogen and one oxygen atom. Vasodilation can be medically useful in treating heart diseases and lowering blood pressure by allowing more oxygen-carrying blood to pass through dilated veins.
Sodium nitrite is one of the most commonly encountered nitrite compounds due to its widespread use as a meat curing agent. In curing, a tiny amount of the compound is added to table salt, spices, and other miscellaneous flavorings that are applied to the meat. Meat cured in this way is often smoked to enhance the flavor. The nitrite is responsible for the pinkish color of preserved smoked meats and protects the meat from becoming rancid and from bacterial growth.