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Nitrate and Nitrite in Drinking Water 4 Exposure Assessment This chapter identifies the primary sources of human exposure to nitrate and nitrite and assesses the extent of overall exposure. EXPOSURE FROM FOOD AND WATER Dietary Exposure Most nitrate and 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. The highest nitrate concentrations are found in celery, spinach, lettuce, beets, radishes, melon, turnip greens, and rhubarb (over 1000 mg/kg of vegetable) (Walker 1990). Low concentrations of nitrite (less than 10 mg/kg)
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Nitrate and Nitrite in Drinking Water can also be present in those vegetables. Concentrations of nitrate in vegetables depend on agricultural practices, storage conditions, the temperature and light in which they are grown, and the concentrations of nitrate in the soil, fertilizers, and water used to grow the vegetables (NRC 1981; Hwang et al. 1994). The concentrations of nitrate and nitrite in cured-meat products depend on the curing process and on the amounts added as preservatives. Concentrations of nitrite in bacon, for example, can be up to 120 ppm, which is the maximum allowed by law (9CFR 318.7B). Nitrate and nitrite are used as preservatives because of their ability to inhibit the growth of Clostridium botulinum (NRC 1981). Improved manufacturing processes have led to a steady decline in the concentrations of nitrate and nitrite in preserved meats (nitrate is now used only rarely). Dairy products contain low concentrations of nitrate and nitrite in general, rarely exceeding 5 mg/kg in milk (NRC 1981 ). Drinking-Water Exposure Nitrate and nitrite can occur in drinking water as a result of human and other activities. The microbial oxidation of ammonia to nitrate and nitrite is the primary nonhuman source. Inorganic fertilizers and human and animal wastes (from livestock operations and septic tanks) are the primary human sources. Nitrate released to soil can enter groundwater or surface water as a result of leaching or runoff. 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 1 mg/L except in areas of severe contamination. The nitrite in groundwater and surface water is negligible compared with the nitrate; in oxygenated waters, nitrite is rapidly converted to nitrate (EPA 1990b).
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Nitrate and Nitrite in Drinking Water 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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Nitrate and Nitrite in Drinking Water determine the relative contributions of different sources to total intake but that food is the major source for adults. The National Research Council (1981) estimated that food provides a nitrate intake of 40-100 mg/day for males. EPA (1990b) concluded further that water could contribute comparable intake if nitrate were present at 22-44 mg/L. Nitrite intake is thought to be less than 3.3 mg/day for the majority of the United States population. An example of an intake estimate for nitrate is that of Jones (1992), which is shown in Table 4-1. Jones estimated exogenous nitrate intake at 76 mg/day. For most of the population, about 97% of daily intake comes from food and only 3% from drinking water. Endogenous nitrate production contributes about 45% of total exposure. In contrast, Van den Brandt et al. (1989) estimated exogenous nitrate intake at 113 mg/day for males and 184 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, mg/day % Vegetables (in omnivores)b 65 86 Fruits and juices 4 5 Cured meat 2 3 Bread and cereals 2 3 Other foods 1 1 Water (usual water supply) 2 3 Total exogenous 76 100 Endogenous 62 - Total exogenous + endogenous 138 - aBased on Jones (1992). bVegetarians would consume considerably more nitrate (about 260 mg/day).
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Nitrate and Nitrite in Drinking Water administered food-frequency questionnaire and measurements of urinary nitrate. Broccoli and green leafy vegetables were reported to account for 60% of total intake. ECETOC (1988) summarized daily intake estimates from several European reports. Using the duplicate-portion technique, wherein one portion is eaten and another is analyzed, several investigators reported average dietary nitrate intake of 43-131 mg/day. Their results are consistent with those of investigators who analyzed dietary components and calculated nitrate intake on the basis of data on consumption rates and reported estimates of total intake of about 31-130 mg/day for most people. Finally, investigators who measured urinary nitrate concentrations and calculated intake on the basis of observations of the proportion of nitrate excreted in urine have reported intake estimates of 64-297 mg/day for people who consume water with high nitrate concentrations. The latter estimates are unreliable because not all the nitrate that enters the body is excreted as nitrate in urine. 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. The National Research Council report The Health Effects ofNitrate, Nitrite, and N-Nitroso Compounds (NRC 1981) reported estimates of nitrate and nitrite intake based on food-consumption tables. Those estimates are summarized in Tables 4-2 and 4-3. Estimates were made for the average U.S. population, for those whose diets contain greater than average amounts of vegetables or cured meats, and for those who consume nitrate-rich drinking water. For the average population, most nitrate exposure (86%) comes from vegetables, whereas the primary contributors to nitrite intake are cured meats (39%), baked goods and cereals (34%), and vegetables (16%). Those estimates of intake do not necessarily reflect total exposure, however, which also includes endogenous nitrate synthesis and the in vivo conversion of nitrate to nitrite.
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Nitrate and Nitrite in Drinking Water TABLE 4-2 Estimated Daily Nitrate Intake in the United Statesa Daily Nitrate (mg) and Percent Contribution from Each Source U.S. Average High-Cured-Meat Dietb Vegetarian Dietc Nitrate-Rich Waterd Source mg % mg % mg % mg % Diet 73 97 76 97 266 99 73 31 Water 2 3 2 3 2 1 160 69 Total 75 78 268 233 aBased on NRC (1981). bAssumes 4 times daily average consumption of cured meats. cAssumes 4 times daily average consumption of vegetables. dBased on data from central Illinois.
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Nitrate and Nitrite in Drinking Water TABLE 4-3 Estimated Daily Nitrite Intake in the United Statesa Daily Nitrate (mg) and Percent Contribution from Each Source U.S. Average High-Cured-Meat Dietb Vegetarian Dietc Nitrate-Rich Waterd Source mg % mg % mg % mg % Diet 0.76 99 1.69 99 0.76 99 0.76 99 Water 0.01 1 0.01 1 0.01 1 0.01 1 Total 0.77 1.7 0.77 0.77 aBased on NRC (1981). bAssumes 4 times daily average consumption of cured meats. cAssumes 4 times daily average consumption of vegetables. dBased on data from central Illinois,
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Nitrate and Nitrite in Drinking Water Both the measured and calculated estimates of nitrate and nitrite intake discussed above are averages. The primary source of nitrate, for example, is vegetables; both daily vegetable intake and the nitrate content of vegetables vary considerably. Other limitations of the estimates include those inherent in analytic techniques and inaccuracies in food-consumption tables. Nonetheless, the estimates indicate the relative importance of different sources of intake to overall exposure. In particular, they indicate that drinking water is not an important contributor to nitrite exposure and that it contributes substantially to nitrate exposure only in areas of notable contamination. Infants constitute a special case. Breast-fed infants are exposed to very little nitrate or nitrite, but formula-fed infants can be exposed to nitrate from the water used to prepare their formula. Daily fluid intake of newborn infants has been estimated at 850 mL (ICRP 1975) and 150 mL/kg (Hull and Johnstone 1987). BIOLOGIC FATE Ingested nitrate is rapidly absorbed through the small intestine and distributed throughout the body. It is excreted in saliva, sweat, feces, and urine (ECETOC 1988; EPA 1990a). Nitrate excreted in feces is used by fecal microorganisms as a source of nitrogen. Nitrate is reduced to nitrite in areas of the gastrointestinal tract where large numbers of bacteria are found—the mouth, the achlorhydric stomach, the small intestine, and the colon. 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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Nitrate and Nitrite in Drinking Water 1981; Walters and Smith 1981; Stuehr and Marletta 1986). 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 reduce it further to ammonia and then to urea and amino acids. Absorbed nitrite reacts with hemoglobin to form methemoglobin; little is transported elsewhere. Nitrite can also react with secondary and tertiary amines to yield nitrosamines, with secondary and tertiary amides to yield nitrosamides, and with N-substituted ureas and carbamates to yield nitrosourea and nitrosocarbamates. Those reactions are pH-dependent and occur in saliva and in gastric juice at rates that vary considerably among individuals. Ascorbic acid and α-tocopherol (vitamin E) can block the reactions. Human blood concentrations of nitrosamines can reflect gastric and nongastric endogenous synthesis and intake of preformed nitrosamines in food.
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