Several nationwide surveys of nitrate concentrations in public drinking-water supplies have been conducted and have been reviewed in detail (EPA 1990b). No survey data are available on nitrite concentrations. On the basis of the results of the surveys, EPA (1990b) estimated that of the roughly 219 million people using public drinking-water supplies in the United States, some 92 million (42%) either are not exposed to nitrate or are receiving drinking water with concentrations below 1.3 mg/L. An estimated 127 million (58%) are exposed to water with nitrate concentrations greater than 1.3 mg/L, of whom about 1.7 million, including about 27,000 infants, are exposed to nitrate at greater than 44 mg/L.

ENDOGENOUS SYNTHESIS

Some nitrate and nitrite exposure also originates in the endogenous production of nitric oxide, which can be converted to nitrate, by many types of cells, including macrophages (Iyengar et al. 1987), neutrophils (McCall et al. 1989), endothelial cells (Palmer et al. 1988), neurons (Knowles et al. 1989), and hepatocytes (Billiar et al. 1990). As a result, nitrate excretion in urine exceeds nitrate intake from food and water. In the absence of infection, endogenous nitrate synthesis approximates 62 mg/day (Tannenbaum et al. 1978; Green et al. 1981; Wagner et al. 1983; Lee et al. 1986). Infections and inflammatory reactions can increase endogenous nitrate synthesis in both infants and adults (Hegesh and Shiloah 1982; Wagner and Tannenbaum 1982).

ESTIMATED DAILY INTAKE

Several estimates of daily nitrate intake and its major sources are available. EPA (1990b) concluded that data were insufficient to



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