Nitrate vs. nitrite in drinking water: What’s the difference?

Nitrate and nitrite can both be found in drinking water, but they are not the same substance. Understanding the difference can help you make sense of water-quality test results, understand why drinking-water standards treat them separately, and choose a water filter based on evidence rather than assumptions.

The distinction matters for health, too. Nitrate can be converted into nitrite in the body, and nitrite can interfere with the blood's ability to carry oxygen. The potential effects depend on factors such as concentration and exposure, so the presence of nitrate in water does not automatically mean that it is unsafe to drink.

The difference is also important when choosing a filter. A product tested to reduce nitrate has not necessarily been shown to reduce nitrite. Each claim needs to be supported by evidence for the specific substance.

What is the difference between nitrate and nitrite?

Nitrate and nitrite are related chemical compounds, but they have different structures and behave differently.

The simplest way to distinguish them is by their chemical formulas:

	Nitrate	Nitrite
Chemical formula	NO₃⁻	NO₂⁻
Number of oxygen atoms	Three	Two
Chemical form	Nitrogen-oxygen ion	Nitrogen-oxygen ion
Relationship	More oxidised form	More reduced form

Both are part of the natural nitrogen cycle. They can occur naturally in soil and water, while activities such as agriculture and wastewater management can contribute additional nitrogen to the environment.

Nitrate is generally more stable in oxygen-rich water. Under certain conditions, microorganisms can convert nitrate into nitrite. This means the two substances are related, but the amount of one does not tell you how much of the other is present.

For more detail on health implications and drinking-water guidance, the World Health Organization's drinking-water guidelines explain how nitrate and nitrite are assessed.

How can nitrate turn into nitrite?

Nitrate and nitrite are different forms of nitrogen, and one can be converted into the other through biological and chemical processes.

For example, certain microorganisms can convert nitrate into nitrite in water or soil under suitable conditions. A similar conversion can also occur in the human body. Some of the nitrate we consume is converted into nitrite by bacteria in the mouth.

This matters because nitrite can affect the way haemoglobin carries oxygen in the blood. In certain circumstances, this can lead to a condition called methaemoglobinaemia, in which the blood's ability to transport oxygen is reduced.

This condition is particularly relevant to infants, who are more vulnerable to its effects. If you'd like to understand why nitrate limits exist and what this condition can mean, our article What “blue baby syndrome” tells us about nitrates in drinking water explains the connection in more detail.

Researchers are also studying whether long-term nitrate exposure may be associated with other health effects, including certain cancers. The evidence is still developing, and a possible association should not be confused with proof that nitrate itself causes cancer. Our article Nitrates and cancer: what research says about long-term exposure explores the research in more detail.

What are the drinking-water limits for nitrate and nitrite?

Nitrate and nitrite have separate drinking-water standards because they are different substances with different effects.

In England and Wales, the drinking-water standards at the consumer's tap are:

• Nitrate: 50 mg/L.

• Nitrite: 0.5 mg/L.

You can check these values in the Drinking Water Inspectorate's guidance on drinking-water standards. The applicable rules can differ between countries, so check the regulations that apply where you live.

There is also an additional requirement when nitrate and nitrite are present together. Under the rules for England and Wales, their combined concentrations must satisfy a calculation set out in the regulations. This means that being below the individual limit for each substance does not, by itself, guarantee that the combined requirement is met.

The Water Supply (Water Quality) Regulations 2016 set out the legal requirements, including this combined condition.

The World Health Organization also publishes health-based guideline values for nitrate and nitrite. These guidelines are not necessarily the same as a country's legal drinking-water standards.

It is important not to interpret a limit as meaning that any detectable amount is automatically unsafe. Concentration matters, and public drinking-water supplies are monitored against the standards that apply to them. If you use a private well or another private water source, you may need to arrange your own testing.

Why does the difference matter when choosing a water filter?

A water filter can reduce one substance without reducing another. This is particularly important for nitrate and nitrite, because both are dissolved in water but are not identical.

For example, a test showing that a filter reduces nitrate does not, by itself, demonstrate that it reduces nitrite. The same applies in reverse. Evidence for one substance should not be treated as proof of performance against the other.

It is also important to distinguish between different filtration technologies. Conventional activated carbon and ordinary particle filters generally do not remove dissolved nitrate effectively. Technologies such as nitrate-selective ion exchange and reverse osmosis can reduce nitrate when the system is properly designed and used under suitable conditions.

If you want to understand the available options, our guide to nitrate in drinking water, including its limits, sources and filtration options explains why some treatment methods can reduce nitrate while others cannot.

The key is to look for evidence relating to the specific filter you are considering. A general statement that a filter removes contaminants is not enough to establish that it reduces nitrate or nitrite.

How can you tell whether a water filter removes nitrate or nitrite?

If you are comparing filters, look for clear information about their performance against the substance you want to reduce. You should not have to rely on a broad marketing claim alone.

1. Look for the manufacturer's test information

Start with the manufacturer's product page and technical information. Look for a performance report, laboratory test results or a clear explanation of which substances the filter has been tested against.

If you cannot find this information, contact the manufacturer and ask directly:

• Has this specific filter been tested for nitrate reduction?

• Has it been tested for nitrite reduction separately?

• Can you provide the results and explain the conditions under which the tests were carried out?

If the manufacturer cannot provide evidence for a particular claim, do not assume the filter removes that substance simply because it reduces other contaminants.

2. Check exactly what was measured

Read the test report carefully to see whether it refers to nitrate, nitrite or both. A report covering nitrate alone does not establish nitrite-reduction performance.

Also check how the result is expressed. Reports may use different terms or units, and these can affect how you interpret the numbers.

3. Compare the concentrations before and after filtration

A useful performance report should show how much of the substance was present in the water before treatment and how much remained afterwards.

For example, the table below uses hypothetical figures for illustration only. It is not a real test result and does not represent the performance of any Weeplow product.

Illustrative nitrate test	Concentration
Before filtration	40 mg/L
After filtration	10 mg/L
Calculated reduction	75%

In this example, the concentration falls from 40 mg/L to 10 mg/L, which represents a 75% reduction. The calculation shows the change under the conditions of this hypothetical test; it does not establish how the filter would perform with different water or after prolonged use.

A percentage alone is not enough to decide whether a system is suitable. You also need to consider the final concentration, the applicable drinking-water standard and how consistently the filter performs.

4. Check how much water the filter can treat

A filter may reduce a substance effectively at first, but its performance can change as more water passes through it.

Look for information about its treatment capacity, maintenance requirements and replacement schedule. If the technology has a limited capacity, ask what happens when that capacity is reached and how you can tell when maintenance or replacement is needed.

5. Understand the conditions behind the results

Filter performance can depend on the starting concentration, water chemistry, flow rate and volume treated. These factors can make a difference when comparing two products or deciding whether a test result is relevant to your own water.

Independent laboratory testing can provide useful evidence, but it is important to understand what was tested and what the results actually demonstrate. A laboratory report is not automatically the same as a product certification, and a test under specific conditions does not guarantee identical performance in every household.

What should you look for when testing your water?

If you want to understand whether nitrate or nitrite is a concern for your household, a reliable water test is a useful starting point. Here is what to check when you receive the results.

Check that nitrate and nitrite are reported separately

Look for separate results for nitrate and nitrite. A report showing only one of them does not tell you the concentration of the other.

If the report does not make this clear, ask the laboratory or water supplier to explain which substances were measured.

Check the units

Results are often expressed in milligrams per litre (mg/L), but you may see different ways of reporting the nitrogen content.

For example:

• Nitrate (NO₃⁻) reports the concentration of the nitrate ion.

• Nitrate-nitrogen (NO₃-N) reports only the nitrogen contained within nitrate.

• Nitrite (NO₂⁻) reports the concentration of the nitrite ion.

• Nitrite-nitrogen (NO₂-N) reports only the nitrogen contained within nitrite.

These figures are not numerically interchangeable. Before comparing a test result with a drinking-water standard, make sure both use the same basis of measurement.

Check the reporting limit

A laboratory report may say that a substance was not detected or was below a stated reporting limit. This does not necessarily mean that absolutely none is present. It means that the result was below the level the laboratory could reliably report under the test conditions.

If this affects how you interpret the result, ask the laboratory to explain its reporting limit.

Check where and when the sample was taken

A result describes the water sampled at a particular place and time. It may not represent every part of the water supply or how its quality changes over time.

Check whether the sample came from a public tap, a private well or another source, and note the date of testing. If you rely on a private water source, repeat testing may be appropriate depending on the source and local guidance.

Compare the result with the right standard

Once you know what was measured and how it was reported, compare the result with the drinking-water standard or guidance that applies to your location.

If you receive water from a public supplier, you may be able to find local water-quality information through your supplier or the relevant authority. In England and Wales, the Drinking Water Inspectorate's guidance on your water supply explains where consumers can find information.

If you use a private well or another private supply, you may need to arrange testing through a suitable laboratory. Local authorities can often advise on the requirements and recommended testing for private water supplies.

Frequently asked questions

Is nitrate the same as nitrite?

No. Nitrate (NO₃⁻) and nitrite (NO₂⁻) are related but distinct substances. They have different chemical structures, and drinking-water standards assess them separately.

Can nitrate turn into nitrite in the body?

Yes. Some of the nitrate we consume can be converted into nitrite by bacteria in the mouth. Nitrite can affect the blood's ability to carry oxygen, which is one reason nitrate and nitrite exposure is considered in drinking-water guidance.

Does a filter that removes nitrate also remove nitrite?

Not necessarily. A nitrate-reduction test does not prove nitrite-reduction performance. Look for separate evidence for each substance you want the filter to reduce.

Does a normal water filter remove nitrate?

Not always. Conventional activated carbon and ordinary particle filters generally do not remove dissolved nitrate effectively. Treatment methods such as nitrate-selective ion exchange and reverse osmosis can reduce nitrate, but performance depends on the particular system and how it is used.

Should I test my tap water for nitrate and nitrite?

If you receive water from a public supplier, you can start by checking the water-quality information available from your supplier or the relevant authority. If you use a private well or another private source, testing can be particularly useful because the quality of the water may vary.

What should I do if my water has elevated nitrate or nitrite?

First, check the test result, units and standard that applies to your water supply. If the result is above the relevant standard or guidance value, contact your water supplier or local public-health or environmental-health authority for advice. If treatment is appropriate, look for a system with evidence of performance against the specific substance concerned.

The bottom line

Nitrate and nitrite are closely related, but they are not the same. Nitrate can be converted into nitrite under certain conditions, and both substances are assessed separately in drinking-water guidance.

If you want to understand what's in your water, start with a reliable test. Check which substances were measured, how the results are expressed and which standard applies to your supply.

If you are considering a filter, look for evidence that the specific system reduces the substance you want to address. A claim about nitrate does not automatically establish performance against nitrite, and a general claim about contaminant reduction is not a substitute for specific test results.

Understanding the difference makes it easier to interpret water-quality information and choose a treatment approach based on your own water and the evidence available.