Nitrate and Nitrite in Drinking Water (1995) / Chapter Skim
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4 EXPOSURE ASSESSMENT
4 EXPOSURE ASSESSMENT
Pages 35-44
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From page 35... ...
nitrite to which humans are exposed is in their diet, as either natural components or intentional additives. Vegetables are the primary source of nitrate and nitrite in food, and cured meat and dairy products can also contribute.
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From page 36... ...
Nitrate concentrations in groundwater are typically less than 10 mg/L but can exceed that in areas of concentrated human sources. Concentrations of nitrate in surface water seldom exceed ~ my except in areas of severe contamination.
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On the basis of the results of the surveys, EPA ~ ~ 990b) estimated that of the roughly 2 ~ 9 million people using public cirinIcing-water supplies in the United States, some 92 million (42%)
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From page 38... ...
(1989) estimated exogenous nitrate intake at ~ ~ 3 midday for males and ~ 84 mg/day for females, on the basis of an integrated approach that used a self TABLE 4-1 Sources of Nitrate Exposure in the United Statesa Sources Nitrate, midday % Vegetables (in omnivores Fruits and juices Cured meat Bread and cereals Other foods Water (usual water supply)
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From page 39... ...
There are losses of nitrate throughout the oral cavity and gastrointestinal tract that are characterized by stepwise reduction of nitrate to ammonia. Thus, urinary nitrate underestimates total exposure.
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From page 40... ...
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From page 41... ...
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From page 42... ...
The amount of nitrite formed depends on the amount of nitrate ingested, the person's nitrate reductase activity, pH, and the number and type of bacteria present and their nitrate reductase activity; but in general, about 5% of ingested nitrate is thought to be converted to nitrite (Stephany and Schuller 1980; Eisenbrand et al.
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From page 43... ...
Nitrite can react with dietary constituents or endogenous compounds, be absorbed by the gastrointestinal tract, or be used as a nitrogen source by bacteria, which recluce it further to ammonia and then to urea and amino acids. Absorbed nitrite reacts with hemoglobin to form methemogIobin; little is transported elsewhere.
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