Nitrites are found in the soil and formed in the human gut by the action of bacteria on nitrate. In nature is is part of the nitrogen cycle and is rapidly converted to nitrate which is absorbed by plants. In humans it can cause problems. Nitrite combines with haemoglobin preventing the blood from carrying oxygen around the body efficiently. A significant degree of the body may be suffocated from within, a condition called methaemoglobinaemia. This is rare in adults but occurs in babies and small children. Nitrites are used in the preservation of some meats and may also present a cancer risk.