assumed that the conversion rate of nitrate to nitrite by gastrointestinal tract bacteria in infants is about 10%, from which an RfD of 1 mg of nitrite nitrogen per liter (0.16 mg/kg-day) was calculated. That value is equivalent to nitrite at 3.3 mg/L. The MCLGs for nitrate and nitrite are based on those RfDs. Assuming water consumption of 0.64 L/d by a 4-kg infant, the MCLGs for nitrate nitrogen and nitrite nitrogen are 10 mg/L and 1 mg/L, respectively.
The subcommittee concludes that exposure to the nitrate and nitrite concentrations found in drinking water in the United States is unlikely to contribute to human cancer risk. That conclusion is based on the following observations:
For more than 99% of the U.S. population, about 97% of nitrate intake comes from the diet (99% in the case of vegetarians) and about 99% of nitrite intake comes from the diet. Attempting to limit nitrate or nitrite exposure on the basis of carcinogenicity would implicate the diet, and vegetables in particular, as the primary source of risk for most of the U.S. population. Any theoretical cancer risk should be weighed against the benefits of eating vegetables.
Epidemiologic studies provide inadequate evidence of an association between nitrate exposure from drinking water in the United States and cancer risk.
Studies in laboratory animals do not support an association between nitrate exposure and cancer risk or between nitrite exposure and cancer risk in the absence of concurrent exposure to nitrosatable amines.