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Nitrate
in drinking water: health effects
Nitrate
drinking water standards
How
do we consume nitrate?: nitrate in our diet
Nitrate
and the World Health Organization (WHO)
Methaemoglobinaemia:
a disease caused by nitrate
excess
How
to eliminate nitrates from your water
Nitrate is an inorganic compound composed of
one atom of nitrogen (N) and three atoms of
oxygen (O); the chemical symbol for
nitrate is NO3. Nitrate is not normally dangerous for the health unless it is
reduced to nitrite (NO2).
· Regulatory name: Nitrate
- Molecular formula: NO3-
- Molecular weight:
62 g/mol
Nitrate is one of the most frequent groundwater pollutants in rural
areas. It needs to be regulated in drinking water basically because excess
levels can cause methaemoglobinaemia, or "blue baby" disease.
Although nitrate levels that affect babies are not dangerous for older children
and adults, they do indicate the possible presence of other more serious
residential or agricultural pollutants, such as bacteria or pesticides.
The origin of nitrate in groundwater is primarily from fertilizers,
septic systems, and manure storage or spreading operations. Fertilizer nitrogen
not taken up by plants, volatilized, or carried away by surface runoff ends up
in the groundwater in the form of nitrate. This makes the nitrogen unavailable
to the plants, and can also raise the concentration in groundwater above the
admissible levels for drinking water quality. Nitrogen from manure can be
similarly lost from fields, barnyards, or storage locations. Septic systems
remove only half of the nitrogen in wastewater, leaving the other half to leach
to groundwater, this way raising groundwater nitrate concentrations.
Nitrate in drinking
water is measured either in terms of the amount of nitrogen present or in terms
of both nitrogen and oxygen. The federal standard for nitrate in drinking water
is 10 mg/l nitrate-N, or 50 mg/l nitrate-NO3, when the oxygen is measured as
well as the nitrogen. Unless otherwise specified, nitrate levels usually refer
only to the amount of nitrogen present, and the usual standard, therefore, is
10 mg/l.
Short-term exposure
to drinking water with a nitrate level above the health standard is a potential
health problem especially for babies. Babies drink large quantities of water
considering their body weight, especially if water is used to mix powdered or
concentrated recipes or juices. Also, their digestive systems are inmature, and
thus more likely to allow the reduction of nitrate to nitrite. The nitrite in
the digestive tract of babies can cause methaenoglobinaemia.
Nitrate occurs
naturally in many vegetables, such as lettuce and spinach, and is produced by
microbes in the human gut, with the result that only a small part of the
nitrate in the body comes from drinking water.
The intake of nitrate
from vegetables is unlikely to cause health problems because very little of
this nitrate is converted to nitrite. Meat products account for less than 10
percent of nitrate in the diet, but 60 to 90 percent of the nitrite consumed.
This is basically because sodium nitrite is added to foods such as hot dogs,
bacon, or ham. Fruits, grains, and dairy products contribute almost no nitrate
or nitrite to people's diets.
In the European standards for drinking water, 2nd
edition, published by the WHO after the meeting in Geneva 1970, we find the
following:
Constituents in water which, if present in
excessive amounts, may give rise to trouble:
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Substance
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Nature of trouble which may arise
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Approximate level above which trouble may
arise
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Nitrate (as NO3)
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Danger of infantile methaemoglobinaemia if
the water is consumed by infants.
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- Reccommended: less
than 50 mg/l.
- Acceptable: 50 to 100
mg/l.
- Not reccommended:
more than 100 mg/l
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Definition: Clinical
condition arising from the excessive conversion of haemoglobin to
methaemoglobin, which is incapable of binding and carrying oxygen.
Methaemoglobin is formed when iron in the haemoglobin molecule is oxidised from
Fe2+ to Fe3+.
Toxicology: Methaemoglobin
occurs when haemoglobin is oxidised at a rate exceeding the normal enzymatic
capacity to reduce the haemoglobin. Many agents may be responsible for this
oxidation. The most frequently found are:
- Aniline
- Benzocaine
- Chlorates
- Chloroquine
- Dapsone
- Ground
or surface water contaminated with nitrates
- Nitrates
- Nitrites
- Nitrophenol
- Phenazopyridine
- Primaquine
- Sodium
nitroprusside
- 4-dimethylaminophenol
Methaemoglobinaemia
could also be developed due to non-toxic causes, as congenital enzyme
deficiencies.
Symptoms:
A typically greyish cyanosis can be observed
when the level of methaemoglobin exceeds 1.5 g/dL, which is about 10% of the
total haemoglobin in a normal individual. At this level, the patient may not
notice any symptoms yet.
The symptoms of methaemoglobinaemia
are usually those related to impaired oxygen delivery
(headache, weakness, tachycardia and breathlessness) and develop gradually as
concentrations of methaemoglobin rise above 20%.
Concentrations higher than 50% result in severe
hypoxaemia and central nervous system depression.
Concentrations higher than 70% may cause death.
For individuals that suffer from anaemia,
cardiac failure or pulmonary disease the symptoms of hypoxia may appear at
lower percentage levels of methaemoglobin.
An easy solution to eliminate the nitrites in the water is to oxidize them to
nitrates (as we said before, nitrates are much less toxic than nitrites). This
can be done by injecting ozone in the water. Ozone is a very oxidizing chemical
that would oxidize all the nitrites into nitrates, thus eliminating
the toxicity caused by nitrites.
Click here to know more about our ozone generators.
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