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Human health effects
Humans are subject to nitrate toxicity, with infants being especially vulnerable to methemoglobinemia. Methemoglobinemia in infants is known as blue baby syndrome. Methemoglobin occurs in normal people in concentrations of 0.5-3.0%. It is when concentrations of methemoglobin exceed 10% do clinical symptoms of methemoglobinemia occur. Any concentration above 50% can result in death. Through the Safe Drinking Water Act, the United States Environmental Protection Agency has set a maximum contaminant level of 10 mg/L or 10 ppm of nitrates in drinking water. This particular standard was set to prevent methemoglobinemia in infants. Infants exposed to water containing nitrates are at highest risk of developing blue baby syndrome during the first 6 months of life. In the United States, it is estimated that 40,000 infants younger than 6 months live in homes with water contaminated with nitrates. This is due to low concentrations of nitrate metabolizing triglycerides during this developmental period. Private water systems, such as well water in agricultural areas, are more likely to have nitrate levels above the maximum contaminant level (MCL). Unlike municipal water, well water is not treated and tested as often. Rural well-water near agricultural fields can become contaminated with nitrates due to manure, fertilizers or septic tanks. Exposure commonly occurs when formula is mixed with well-water containing nitrates or infants under 6 months are fed vegetables washed with the contaminated drinking water. It is important to note that infants who are breastfed by mothers who ingest water with concentrations of nitrates (100 ppm) are not at risk of methemoglobinemia. It is recommended that foods like green beans, carrots, spinach, squash and beets, are not fed to infants under 3 months. These foods have naturally occurring nitrates which can be harmful to the infant. High levels of nitrate in fertilizer may also contribute to elevated levels of nitrate in the harvested plant.

Not only are infants under 6 months a concern, but pregnant women with altered physiological states and compromised immune system can be at risk. Pregnant women show a decrease in methemoglobin levels with increasing gestation. Exposure to nitrate in groundwater during pregnancy at concentrations above the MCL was associated with increased risk for anencephaly. A study done in Texas and Iowa found that mothers of babies with spina bifida were twice as likely to ingest 5 mg or more of nitrate daily from drinking water as mothers of babies without major birth defects. Mothers of babies with limb deficiencies, cleft palate, and cleft lip were, respectively, 1.8, 1.9 and 1.8 times more likely to ingest 5.42 mg or more of nitrate daily than mothers of babies without major birth defects.

While there is evidence that shows nitrates to affect infants and pregnant women, recent evidence shows there are significant scientific doubts as to whether there is a causal link. Blue baby syndrome now is thought to be the product of a number of other factors such as gastric upset, such as diarrheal infection, protein intolerance, heavy metal toxicity etc., with nitrates playing a minor role.

Some adults may be more susceptible to the effects of nitrates than others. In these adults, the methemoglobin reductase enzyme may be under-produced or absent in certain people who have an inherited mutation in the enzyme. Such individuals cannot break down methemoglobin as rapidly as those who do have the enzyme, leading to increased circulating levels of methemoglobin (the implication being that their blood is not so oxygen-rich as that of the others). Diets rich in green, leafy vegetables typically accompany an increased nitrate intake. A wide variety of medical conditions, such as food allergies, asthma, hepatitis, and gallstones, may be linked with low stomach acid; these individuals also may be highly sensitive to the effects of nitrate.

Methemoglobinemia may be treated with methylene blue, which reduces ferric iron(III) back to ferrous iron(II) in affected blood cells.

Another human health effects from the ingestion of nitrate is in the form of processed meat. This form of ingestion can cause pancreatic cancer. Processed meat can be cured with nitrate-based salt to decrease bacterial growth and improve flavor. When ingested, nitrate can become N-nitroso compounds (NOC), a probable human carcinogen. In a study performed by the US Government, there was a positive correlation between nitrate intake of more than 3g/day and pancreatic cancer in men. However, there are few data points which results in only a borderline significance.