A nitrate, salt of nitric acid, is a natural constituent of soil and
vegetation. Nitrate is also a normal metabolite in mammals. Examples of nitrates
include potassium nitrate, ammonium nitrate, calcium nitrate, calcium nitrate
hydrated, and magnesium nitrate.
A nitrite is a salt of nitrous acid; it is also a normal metabolite in
mammals. The average intake of nitrite is considerably less than that of
nitrate. The main source is vegetables, just like that of nitrate. Vegetables
will convert nitrate to nitrite on storage and when contaminated with bacteria
that convert nitrates to nitrites. Occasionally, this can result in very high
concentrations ov nitrites in vegetable juices in particular. The amount of
nitrite ingested from food, however, is only a fraction of that swallowed as a
result of nitrate conversion to nitrite in the mouth (average concentration of
nitrite in saliva is 200 µM). Nitrite is used as a food preservative and
The major acute toxic effect of nitrate and nitrite poisoning is
methemoglobinemia resulting in decrease of the oxygen-carrying capacity of the
blood. The principal concern with exposure to nitrate is its biological
reduction to reactive and toxic nitrite. Nitrate itself is rather harmless.
Symptoms may include bluish discoloration of the skin the may appear within a
few minutes to 45 minutes or more after exposure. Coma and convulsions in severe
poisoning due to severe oxygen deprivation. Abnormally rapid heart rate,
hypotension and collapse may also occur. Nausea, vomiting and abdominal pain may
Nitrite easily transforms into a nitrosating agent in an acidic environment
and can react with a variety of compounds, e.g. ascorbic acid, amines, amides.
Nitrosation can also be mediated by bacteria, e.g. in the stomach. Some reaction
products are carcinogenic (e.g. most nitrosoamines and amides.