Introduction and Background
Under the Safe Drinking Water Act, the U.S. Environmental Protection Agency (EPA) establishes the concentrations of contaminants that are permitted in public drinking-water supplies. Specifically, Section 1412 of the act, as amended in 1986, requires EPA to publish maximum-contaminant-level goals (MCLGs) and promulgate national primary drinking-water regulations for contaminants in drinking water that might cause any adverse effect on health and that are known or expected to occur in public water systems. The act defines public water systems as systems that provide piped water for human consumption and that have at least 15 connections or regularly serve at least 25 people. MCLGs are to be concentrations at which no known or expected adverse health effects occur and are to allow an adequate margin of safety.
The Safe Drinking Water Act also requires EPA to review its existing standards periodically and incorporate new data if they are available. To that end, the EPA Office of Water has asked the
National Research Council to review the current basis of its MCLGs for nitrate and nitrite in drinking water and to determine whether they are protective of public health. The Research Council, through the Board on Environmental Studies and Toxicology in the Commission on Life Sciences, convened a subcommittee of the Committee on Toxicology and asked it to:
Review the current toxicologic and exposure data on nitrate and nitrite.
Characterize the risk associated with nitrate and nitrite exposure through drinking water.
Determine whether the current MCLGs for nitrate and nitrite are adequate to protect public health.
Identify data needs and make recommendations for future research.
Throughout this report, by the Subcommittee on Nitrate and Nitrite in Drinking Water, concentrations of nitrate and nitrite are expressed in terms of milligrams per liter (mg/L). Another common unit of measurement is milligrams of nitrate nitrogen or nitrite nitrogen per liter. The nitrogen oxides have multiple interconvertible forms, although the deleterious effects of concern arise through a common intermediate. Each of the precursors of that intermediate has a different molecular weight; using a weight measure, such as milligrams, to evaluate usefully the relative contribution of nitrite versus nitrate, for example, requires a conversion factor. The conversion factors are
1 mg of NO3- per liter = 0.226 mg of NO3- nitrogen per liter,
1 mg of NO3- nitrogen per liter = 4.429 mg of NO3-per liter,
1 mg of NO2- per liter = 0.304 mg of NO2- nitrogen per liter,
1 mg of NO2- nitrogen per liter = 3.290 mg of NO2- per liter.
Use of either unit does not permit easy comparisons between nitrate and nitrite or from one medium to another, however, because such concentrations are not comparable on a molar basis. The appropriate unit for comparisons is the mole (or millimole):
1 mmol per liter = 62 mg of NO3- per liter = 46 mg of NO2- per liter, and
1 mmol per liter = 14 mg of NO3- nitrogen per liter = 14 mg of NO2- nitrogen per liter.
Nitrate is a normal component of the human diet. The National Research Council report The Health Effects of Nitrate, Nitrite, and N-Nitroso Compounds (NRC 1981) estimated that a typical daily intake of nitrate by an adult in the United States is about 75 mg. Over 85% of that comes from the natural nitrate content of vegetables, such as beets, celery, lettuce, and spinach. Daily intakes of nitrate by vegetarians can exceed 250 mg/day (NRC 1981). The contribution of drinking water to daily nitrate intake is usually only about 2-3% of the total (NRC 1981). The toxic effects of nitrate are closely related to its conversion to nitrite by bacteria in the alimentary tract, and depends not only on dose, but also on the concentration and type of bacteria present. Therefore, dose-response relationships are highly variable among animal species.
The current MCLG for nitrate in drinking water is 10 mg/L, and that for nitrite is 1 mg/L; both are measured as nitrogen (EPA 1991). The equivalent values are 44 mg of nitrate per liter and 3.3 mg of nitrite per liter. Those values are based on methemoglobinemia, the principal toxic effect observed in humans exposed to nitrate or nitrite. Methemoglobinemia occurs when nitrite oxidizes the Fe2+in hemoglobin to Fe3+, a form that does not allow oxygen transport.
This report reviews the basis of EPA's current MCLGs and evidence of health effects that might result from nitrate or nitrite
exposure in addition to methemoglobinemia, such as cancer and reproductive and developmental effects. It then characterizes the dose-response relationships between exposure and health effects. The human exposure to nitrate and nitrite and the relative contribution of drinking water to that exposure are assessed. Finally, the MCLGs that provide adequate margins of safety for human health are identified.