nitrite. The subcommittee evaluated this information in the context of the drinking-water standards for those substances and drew conclusions about the adequacy of the current standards to protect human health.
Methemoglobinemia is the primary adverse health effect associated with human exposure to nitrate or nitrite. To cause methemoglobinemia, nitrate must be converted to nitrite. Methemoglobinemia occurs when nitrite oxidizes the Fe2+in hemoglobin to Fe3+, a form that does not allow oxygen transport. Methemoglobinemia can lead to cyanosis (insufficient oxygenation of the blood characterized by bluish skin and lips) and, ultimately, death. Methemoglobinemia in adults is rare; most methemoglobinemia victims are infants who have been fed formula mixed with nitrate-containing well water or food with a high nitrate content or who have diarrhea.
Results of epidemiologic studies are inadequate to support an association between high nitrate or nitrite exposure from drinking water in the United States and increased cancer rates in humans. In laboratory animals, nitrate and nitrite are not carcinogenic unless they are administered concurrently with nitrosatable amines. Studies in humans are also inadequate to support an association between nitrite or nitrate exposure and reproductive or developmental effects. Results of studies in laboratory animals suggest that reproductive and developmental toxicity might occur, primarily at high doses, which also can produce maternal methemoglobinemia. At high doses, inorganic nitrite, but not nitrate, can produce hypotension in humans as a result of its action as a smooth muscle relaxer.