Nitrites in water: origin, risks, standards, and treatment

Nitrites are an intermediate form of the nitrogen cycle. They appear during the transformation of ammonium into nitrates and are normally present at low concentrations in stable water. Their increase can signal a recent contamination, incomplete nitrification, or a malfunction in the distribution network. More reactive than nitrates, they can cause methemoglobinemia, which is particularly dangerous in infants.

Formula: NO₂⁻
Common origin: Nitrification
Critical effect: Methemoglobinemia
EU value: 0.5 mg/L

What are nitrites?

Nitrites are ions with the formula NO₂⁻, composed of one nitrogen atom and two oxygen atoms. They constitute an intermediate form of the nitrogen cycle, situated between ammonium and nitrates.

In well-oxygenated and biologically stable water, nitrites are generally present at low concentrations because they are rapidly transformed into nitrates. An increase may indicate incomplete nitrification, recent contamination by organic matter, or a malfunction in a distribution network.

Nitrites are more reactive than nitrates. Their main recognized health effect is the oxidation of hemoglobin into methemoglobin, a form less capable of transporting oxygen in the body.

Key takeaway: in drinking water, a high concentration of nitrites must be investigated promptly, as it may signal active nitrification or a recent contamination.

Chemical properties of nitrites in water

Characteristic	Information	Importance for drinking water
Formula	NO₂⁻	Dissolved form measured in water.
Nitrogen oxidation state	+3	Intermediate state between ammonium and nitrate.
Solubility	High	Nitrites are not removed by simple mechanical filtration.
Stability	Limited	They can be oxidized to nitrates or reduced to lower forms.
Taste and odor	Generally imperceptible	An exceedance cannot be detected by the senses.
Biological reactivity	High	Nitrogen cycle bacteria produce and consume them.

Nitrous acid, with the formula HNO₂, is the protonated form of nitrite. Its proportion increases as pH decreases. At the typical pH of drinking water, the ionic form NO₂⁻ generally remains dominant.

How are nitrites formed in the nitrogen cycle?

Nitrites mainly appear during nitrification, a two-step biological process carried out by microorganisms.

First step: ammonium to nitrite

Ammonia-oxidizing microorganisms transform ammonium or ammonia into nitrite. This reaction consumes oxygen and alkalinity.

Second step: nitrite to nitrate

Other microorganisms then oxidize nitrite to nitrate. When this second step is slower than the first, nitrites can temporarily accumulate.

Denitrification

In an oxygen-poor environment, certain bacteria use nitrates and nitrites as electron acceptors and progressively transform them into nitrogen gases.

Step	Simplified transformation	Possible consequence
Ammonification	Organic nitrogen → ammonium	Production of a nitrification precursor.
Nitritation	Ammonium → nitrite	Possible accumulation of NO₂⁻.
Nitratation	Nitrite → nitrate	Disappearance of nitrite if the process is complete.
Denitrification	Nitrate / nitrite → nitrogen gas	Biological removal of dissolved nitrogen.

What are the sources of nitrites in water?

Contamination by wastewater

Domestic wastewater and inadequately treated effluents contain ammonia and organic nitrogen. Their infiltration can produce nitrites during microbial transformation stages.

Livestock effluents

Animal waste, slurry, and manure are rich in nitrogen. Recent contamination or incomplete nitrification can lead to the simultaneous presence of ammonium, nitrites, and nitrates.

Nitrogen fertilizers

Ammoniacal fertilizers and urea can be transformed into nitrites before their complete oxidation into nitrates. Nitrites generally persist for less time than nitrates in oxygenated soils.

Industrial discharges

Certain chemical, metallurgical, food processing, or surface treatment industries can produce effluents containing nitrogenous compounds.

Natural decomposition

The degradation of organic vegetable or animal matter can release ammonium, then form nitrites when conditions become favorable for nitrification.

Why can nitrites form in a drinking water network?

Nitrites can be produced directly in the pipes when nitrification develops. This phenomenon is particularly monitored in networks that use chloramines as a residual disinfectant.

Presence of free ammonia

An excess of free ammonia provides a substrate for nitrifying bacteria. Health Canada recommends limiting free ammonia entering the network to reduce this risk.

Long residence time

Tanks, network dead-ends, oversized pipes, and low-consumption zones favor water aging and biofilm growth.

Temperature

A higher temperature generally accelerates biological activity. Episodes of nitrification are therefore often more likely during warm periods.

Low disinfectant residual

A decrease in residual disinfectant can allow nitrifying microorganisms to grow in biofilms and sediments.

Biofilms and deposits

Pipe walls, tanks, and filters can harbor microbial communities capable of transforming ammonia into nitrite and then into nitrate.

Which parameters can signal nitrification?

Parameter	Possible evolution	Interpretation
Free ammonia	Decrease	Consumption by nitrifying microorganisms.
Nitrite	Transient increase	Incomplete or unbalanced nitrification.
Nitrate	Increase	Progressive oxidation of ammonia nitrogen.
Residual chlorine or chloramine	Decrease	Increased demand and biological activity.
pH	Decrease	Nitrification produces acidity.
Alkalinity	Decrease	Consumption during nitrification.
Heterotrophic bacteria	Possible increase	Biological instability of the network.

No single parameter is always enough to confirm nitrification. Interpretation is based on the combined evolution of ammonia, nitrites, nitrates, disinfectants, temperatures, pH, and hydraulic data.

Nitrites and private wells

Nitrites are generally less common than nitrates in wells, because they are rapidly oxidized in water containing oxygen. Their presence can, however, signal a recent contamination or an environment where nitrogen transformations are incomplete.

Shallow well located in an agricultural area;
Proximity to a septic tank or spreading area;
Lack of well-head seal integrity;
Flooding or runoff towards the intake;
Simultaneous presence of indicator bacteria;
High ammonium or nitrate content.

A positive nitrite result in a well generally justifies a broader analysis including at least nitrates, ammonium, and microbiological parameters.

What are the health effects of nitrites?

Hemoglobin oxidation

Nitrites oxidize the ferrous iron of hemoglobin into ferric iron and form methemoglobin. The latter does not effectively transport oxygen.

Methemoglobinemia

When the proportion of methemoglobin increases, symptoms may appear: bluish or grayish discoloration of the lips and skin, fatigue, headaches, shortness of breath, rapid heart rate, confusion, and, in severe forms, convulsions or loss of consciousness.

Acute effect

The main risk of nitrites is linked to acute or short-term exposure, particularly in infants. The severity depends on the dose, the speed of exposure, and the body's ability to reconvert methemoglobin.

Chronic exposure

Research is also examining the formation of N-nitroso compounds and other potential effects. However, drinking water values remain primarily based on the prevention of methemoglobinemia.

Why are infants particularly sensitive?

Infants, especially under six months, have lower activity of certain enzymes that reconvert methemoglobin into functional hemoglobin.

Their fetal hemoglobin is also more easily oxidizable. A less acidic gastric pH can favor the development of bacteria capable of transforming nitrates into nitrites.

Water consumption relative to body weight is high in infants fed with reconstituted formula. Contaminated water can therefore represent a significant dose.

Water exceeding the applicable value must not be used to prepare infant formula. In the presence of bluish lips, respiratory difficulty, or unusual drowsiness, urgent medical care is required.

Which other people might be more vulnerable?

People with a congenital abnormality of methemoglobin reduction;
People suffering from severe anemia;
people with heart or respiratory diseases;
people exposed to certain medications or oxidizing substances;
people suffering from significant gastrointestinal infections;
pregnant women in case of high exposure, upon medical advice.

Individual vulnerability does not change the necessity of complying with the drinking water standard, but it may justify increased vigilance in the event of an exceedance.

Nitrites and N-nitrosated compounds

Nitrites can react with certain amines or amides and participate in the formation of N-nitrosated compounds. Several nitrosamines are carcinogenic in animals.

Formation depends on pH, the presence of precursors, temperature, contact time, and inhibitory substances such as ascorbic acid.

In drinking water networks, some nitrosamines can also appear as disinfection by-products, particularly under specific conditions related to chloramination. However, they constitute a distinct family from the simple nitrite ion and require specific analyses.

A nitrite measurement does not allow for directly inferring the concentration of nitrosamines. The two parameters must be distinguished.

How to convert between nitrite and nitrite nitrogen?

Results can be expressed as the mass of nitrite NO₂⁻ or as the mass of nitrogen contained in the nitrite, denoted as NO₂-N.

Conversion	Factor	Example
Nitrite to nitrite nitrogen	Multiply by 14/46, i.e., 0.304	3 mg/L NO₂ ≈ 0.91 mg/L NO₂-N.
Nitrite nitrogen to nitrite	Multiply by 46/14, i.e., 3.29	1 mg/L NO₂-N ≈ 3.29 mg/L NO₂.

Before making any comparison with a standard, it must be verified whether the laboratory expresses the result in nitrite or nitrite nitrogen.

What are the reference values for nitrites?

Organization or regulation	Value	Note
World Health Organization	3 mg/L	Guideline value for short-term exposure.
World Health Organization	0.2 mg/L	Provisional guideline value for long-term exposure.
European Union – Directive (EU) 2020/2184	0.5 mg/L	Parametric value at the tap.
European Union	0.1 mg/L	Value to be complied with at the exit of treatment plants.
France	0.5 mg/L	Quality limit at the point of compliance.
Canada	3 mg/L	Maximum acceptable concentration, equivalent to approximately 1 mg/L of nitrite nitrogen.
United States – EPA	1 mg/L	Expressed as nitrite nitrogen, i.e., approximately 3.3 mg/L of nitrite.

In the European Union, the following combined condition must also be met: [nitrate]/50 + [nitrite]/3 ≤ 1, with concentrations expressed in mg/L of nitrate and nitrite.

For water distributed in France or the European Union, the practical value to retain at the tap is 0.5 mg/L of nitrites.

How to analyze nitrites in water?

Spectrophotometry

Colorimetric methods based on the Griess reaction are commonly used. Nitrite forms a colored compound whose intensity is measured by spectrophotometry.

Ion chromatography

This allows for the separation and quantification of several anions, notably nitrites, nitrates, chlorides, fluorides, and sulfates.

Online analysis

In some networks, automatic analyzers allow for close monitoring of nitrites and other nitrification indicators.

Sample stability

Nitrites can change after collection due to chemical or biological reactions. The deadlines, temperatures, and any conservation instructions from the laboratory must be strictly followed.

use the provided or recommended vial;
indicate the exact sampling point;
note the presence of any treatment or disinfectant;
transport the sample quickly;
confirm any exceedance with a new sample.

How to interpret a nitrite result?

Result	General interpretation	Recommended action
Not detected or very low	Normal situation in biologically stable water	Continue scheduled monitoring.
Unusual increase	Nitrification or possible contamination	Analyze ammonium, nitrates, disinfectant, and microbiology.
Close to 0.5 mg/L	Near the European limit at the tap	Confirm quickly and investigate the cause.
Greater than 0.5 mg/L	Exceedance in France and the EU	Follow instructions from the health authority.
Presence in a private well	Recent contamination or possible incomplete transformation	Complete water check and inspection of the catchment.

What technologies reduce nitrites?

Reverse osmosis

A suitable reverse osmosis membrane can reduce nitrites. Performance depends on the membrane, pressure, temperature, maintenance, and recovery rate.

Anionic ion exchange

Certain anionic resins can retain nitrites. Their capacity depends on other anions present, notably sulfates, nitrates, and bicarbonates.

Oxidation to nitrates

On a collective scale, nitrites can be oxidized to nitrates through controlled chemical or biological treatment. This transformation does not remove total nitrogen and must be followed by nitrate monitoring.

Biological denitrification

Biological processes can transform nitrites and nitrates into nitrogen gas. They require precise control and a final step to guarantee microbiological quality.

Electrodialysis

Ion-exchange membranes combined with an electric field can separate dissolved anions. This technology is primarily used on a collective or industrial scale.

Distillation

Distillation can retain non-volatile nitrite salts, provided the device is correctly designed and maintained.

Standard activated carbon

Conventional activated carbon is generally not considered a reliable technology for the reduction of dissolved nitrites.

Mechanical and ceramic filtration

Dissolved nitrites pass through ordinary sediment filters and microporous barriers.

Boiling

Boiling is not a reliable method for elimination. Evaporation can concentrate dissolved substances.

Comparison of treatment technologies

Technology	Potential	Main limitations
Reverse osmosis	High with validated membrane	Pressure, maintenance, water discharge, and regular checks.
Ion exchange	Variable to high	Competing anions, saturation, and regeneration.
Controlled oxidation	Transforms nitrite	Produces nitrate and requires monitoring.
Biological denitrification	High	Control, biomass, and microbiological safety.
Electrodialysis	High	Cost and management of concentrate.
Distillation	High	Energy, low flow rate, and maintenance.
Standard activated carbon	Low or unproven	Do not extrapolate from organic compounds.
Mechanical filtration	Ineffective	Nitrite is dissolved.
Boiling	Ineffective	May concentrate dissolved ions.

How to evaluate a filter against nitrites?

Nitrite reduction must be demonstrated by a specific test. Performance on nitrates, chlorine, or sediments does not automatically prove identical performance on nitrites.

Element to check	Importance
Contaminant explicitly named	The test must involve nitrite.
Unit	Distinguish nitrite and nitrite nitrogen.
Inlet concentration	Allows judging the severity of the test.
Outlet concentration	Must remain below the applicable limit.
Total treated volume	Saturation can cause breakthrough.
Water composition	Competing anions alter capacity.
Flow rate and pressure	Influence membranes and contact time.
Result at end of life	Initial performance is not enough.

The performance of a filtration system must be evaluated based on specific tests carried out by its manufacturer. In the absence of published results regarding this contaminant, no quantified reduction can be claimed.

Consult Monderma certifications and analyses

A preventive strategy is generally more effective than a late reaction after the appearance of an exceedance.

How to prevent contamination of a private well?

keep the wellhead above ground level;
ensure the integrity of the casing and cap;
keep the well away from septic tanks, manure storage, and spreading areas;
prevent surface water runoff toward the catchment;
analyze water after flooding or work;
monitor at least nitrates, nitrites, and microbiology;
have a well inspected if it presents repeated abnormal results.

Frequently asked questions about nitrites in water

What is the nitrite limit in France?

The quality limit at the tap is 0.5 mg/L.

What is the European Union value?

The parametric value is 0.5 mg/L at the tap and 0.1 mg/L at the exit of treatment plants.

What is the WHO value?

The WHO indicates 3 mg/L for short-term exposure and 0.2 mg/L as a provisional long-term value.

Why does Canada use 3 mg/L?

This maximum acceptable concentration is based on the prevention of methemoglobinemia, particularly in infants.

What is the American limit?

The EPA sets it at 1 mg/L expressed as nitrogen, which is approximately 3.3 mg/L of nitrite.

Are nitrites and nitrates the same?

No. Nitrite is a more reactive intermediate form, while nitrate is more oxidized and generally more stable.

Do nitrites have a taste?

No. They cannot be reliably detected by taste, smell, or appearance.

Why are nitrites rare in wells?

In oxygenated water, they are usually quickly converted into nitrates.

What does the presence of nitrites in a well mean?

It may indicate recent contamination or incomplete nitrogen transformation.

What are the symptoms of methemoglobinemia?

Bluish skin discoloration, fatigue, shortness of breath, headaches, confusion, and, in severe cases, loss of consciousness.

Why are infants more sensitive?

Their enzymatic system is less efficient at converting methemoglobin back, and their hemoglobin is more easily oxidized.

Can I prepare a baby bottle with water that exceeds the limit?

No. You must use water that complies with standards.

Does boiling the water remove nitrites?

No. Boiling is not a reliable treatment, and evaporation can concentrate the ions.

Does activated carbon remove nitrites?

Standard activated carbon is generally not considered a reliable solution.

Does a ceramic filter remove nitrites?

No. Dissolved nitrites pass through a simple microporous barrier.

Is reverse osmosis effective?

A suitable, validated, and maintained membrane can significantly reduce nitrites.

Can nitrification occur in a public water system?

Yes, especially in the presence of free ammonia, chloramines, biofilms, and long residence times.

What parameters should be monitored alongside nitrites?

Ammonium, nitrates, residual disinfectant, pH, alkalinity, and microbiological parameters.

What analysis should be requested from the laboratory?

Request a separate measurement for nitrites, ideally accompanied by nitrates and ammonium.

What should be done if the limit is exceeded?

Use compliant water, confirm the analysis, investigate the cause, and follow the health authority's guidelines.

Scientific and administrative sources

This fact sheet is based on regulatory texts and recommendations from public organizations.

World Health Organization – Nitrate and nitrite
View WHO chemical fact sheet
World Health Organization – Guidelines for drinking-water quality
View WHO recommendations
European Union – Directive (EU) 2020/2184
View the directive on EUR-Lex
Health Canada – Nitrate and nitrite in drinking water
View the technical document
Environmental Protection Agency – National Primary Drinking Water Regulations
View EPA standards
Monderma – Certifications and laboratory analyses
View Monderma certifications and analyses

Contaminants and associated parameters

Nitrates Ammonium Chlorides Sulfates Iron Manganese Fluorides Arsenic

Health warning: this fact sheet is provided for informational purposes only. It does not replace an analysis carried out by a laboratory, the recommendations of the competent health authority, or the advice of a healthcare professional. Water that exceeds the applicable value must not be used to prepare baby bottles.