The two areas of concerns about the toxicity of nitrate are the following: • nitrate reduced by the microbial activity of
nitrate reducing bacteria is the precursor of
nitrite in water and in the
lower gastrointestinal tract. Nitrite is a precursor to
carcinogenic nitrosamines, and; • via the formation of nitrite, nitrate is implicated in
methemoglobinemia, a disorder of
hemoglobin in
red blood cells, especially affecting infants and toddlers from ingesting nitrates in drinking water.
Methemoglobinemia One of the most common cause of
methemoglobinemia in infants is due to the ingestion of nitrates and nitrites through
well water or foods. In fact, nitrates (), often present at too high
concentration in drinkwater, are only the precursor chemical species of
nitrites (), the real culprits of methemoglobinemia. Nitrites produced by the
microbial reduction of nitrate (directly in the drinkwater, or after ingestion by the infant's digestive system) are more powerful
oxidizers than nitrates and are the chemical agent really responsible for the
oxidation of Fe2+ into Fe3+ in the
tetrapyrrole heme of
hemoglobin. Indeed, nitrate anions are too weak oxidizers in
aqueous solution to be able to directly, or at least sufficiently rapidly, oxidize Fe2+ into Fe3+, because of
kinetics limitations. Infants younger than four months are at greater risk given that they drink more water per body weight, they have a lower
NADH-
cytochrome b5 reductase activity, and they have a higher level of
fetal hemoglobin which converts more easily to
methemoglobin. Additionally, infants are at an increased risk after an episode of
gastroenteritis due to the production of
nitrites by
bacteria. However, other causes than nitrates can also affect infants and pregnant women. Indeed, the
blue baby syndrome can also be caused by a number of other factors such as the
cyanotic heart disease, a
congenital heart defect resulting in low levels of oxygen in the blood, or by gastric upset, such as diarrheal infection, protein intolerance, heavy metal toxicity, etc.
Drinking water standards 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 nitrate in drinking water. An acceptable daily intake (ADI) for nitrate ions was established in the range of 0–3.7 mg (kg body weight)−1 day−1 by the Joint FAO/WHO Expert Committee on Food Additives (JEFCA).
Aquatic toxicity In
freshwater or
estuarine systems close to land, nitrate can reach concentrations that are lethal to fish. While nitrate is much less toxic than ammonia, levels over 30 ppm of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species. Nitrate toxicity remains a subject of debate. In most cases of excess nitrate concentrations in aquatic systems, the primary sources are wastewater discharges, as well as
surface runoff from agricultural or
landscaped areas that have received excess nitrate fertilizer. The resulting
eutrophication and algae blooms result in
anoxia and
dead zones. As a consequence, as nitrate forms a component of
total dissolved solids, they are widely used as an indicator of
water quality. == Human impacts on ecosystems through nitrate deposition ==