EWG's Tap Water Database



July 2017


Nitrate is one of the most common contaminants in drinking water. It gets into water from fertilizer runoff, manure from large animal feeding operations and wastewater treatment plant effluent. EWG data show that detectable levels of nitrate are present in the water served to over 218 million Americans. Tap water in agricultural areas frequently has the highest nitrate concentrations. Private drinking water wells in the vicinity of animal farms and intensely fertilized fields, or in the locations where septic tanks are commonly used, can also have unsafe levels of nitrate.

The federal limit of 10 milligrams per liter, or mg/L, equivalent to parts per million, for nitrate in drinking water was set in 1962 and has not been updated. This standard was developed to prevent acute cases of methemoglobinemia, known as blue baby syndrome, in which an infant suffers from oxygen deprivation in the blood due to excessive ingestion of nitrate.

New research suggests that the federal nitrate limit is not sufficiently protective. Scientists at the National Cancer Institute found greater incidence of bladder cancer among people who drank water with nitrate concentrations above half the federal limit. Some studies also report that nitrate contamination of tap water can increase the risk of developmental defects for children born to mothers who drank nitrate-contaminated water during pregnancy.

Click here to see the nationwide testing results for nitrate. Some states test for nitrate and nitrite combined.

What is nitrate and how does it get into water?

Nitrate occurs naturally in water and soil, but nitrate pollution of water supplies has increased dramatically with use of nitrogen-containing fertilizer. Urban areas can also have high levels of nitrate in water due to sewage effluent.

EWG’s Tap Water Database found that more than 218 million people drink water from a public water system with detectable levels of nitrate. Many water utilities report the concentration of combined nitrate plus nitrite. Nitrite is even more toxic than nitrate and water utilities often must install specialized treatments to remove both contaminants.

Private wells, especially in farm country, are at risk of containing excessive nitrate. A 2000 study by the U.S. Geological Survey found almost one-fifth of shallow drinking water wells in agricultural areas and 4 percent of those in urban areas exceeded the legal limit for nitrate.

How can I be exposed to nitrate?

The primary source of nitrate exposure for most people in the U.S. is through food. Foods high in nitrate include green leafy vegetables, such as spinach, and cured meats, which are commonly preserved with nitrates. Although nitrate in cured meat is linked to an increased risk of cancer, cancer-fighting effects have been reported for consumption of green vegetables, even when they contain high amounts of nitrate. Researchers believe antioxidants present in vegetables can inhibit nitrate’s carcinogenic potential.

The contribution of drinking water to overall nitrate intake varies by region and dietary patterns. Tap water can be the dominant source of exposure to nitrate for formula-fed infants. EWG recommends that baby formula should be mixed only with filtered tap water or with bottled water, to avoid exposing the baby to tap water contaminants.

Why is nitrate harmful to health?

At levels exceeding the federal limit, nitrate can cause methemoglobinemia, also known as blue baby syndrome, where a child suffers from oxygen deprivation in the blood. Nitrate-contaminated water has been linked to adverse reproductive effects and changes in thyroid function. A 2010 study by the National Cancer Institute found that women consuming nitrate-contaminated water face a greater risk of thyroid cancer. A 2012 publication from the same group reported a link between nitrate intake and subclinical hypothyroidism in women.

According to the International Agency for Research on Cancer, once ingested, nitrate is converted into N-nitroso compounds such as nitrosamines and nitrosamides by bacteria in our digestive systems. These substances damage DNA and cause cancers in multiple animal species and different organs, including the stomach, bladder, colon, esophagus and blood.

Human epidemiological research linked nitrate intake from water with increased risk of cancer. A 2001 study led by Peter Weyer at the University of Iowa found an association between long-term nitrate exposures from drinking water and increased risks of bladder and ovarian cancers in women.

National Cancer Institute studies consistently find that ingestion of nitrate from drinking water, especially in the range of 5 to 10 mg/L, increases the risk of colon, kidney, ovarian and bladder cancers. These risks are highest for people with low vitamin C intake, high consumption of red meat and for smokers – all conditions that increase the formation of N-nitroso compounds in the body.

How much nitrate is harmful to health?

The federal legal limit for nitrate in drinking water is sufficient to protect infants from the acute effects of methemoglobinemia. But it fails to address the growing concerns of chronic, low-level exposures to nitrate and potential cancer risk from nitrate in drinking water.

Based on the findings from the National Cancer Institute, EWG endorses a nitrate level of 5 mg/L – half of the federal legal limit for nitrate in tap water. As more research becomes available, health risks may be found even below this benchmark. In the meantime, EWG urges public water utilities to minimize nitrate levels as much as possible and to strive to provide water with less than 5 mg/L of nitrate.

What should be done to reduce nitrate pollution of water supplies?

The concentrations of nitrate in drinking water sources will likely continue to rise as fertilizer use increases in agricultural areas without pollution control measures. Once nitrate enters ground and surface water, it lingers for years without breaking down or degrading.

Removing nitrate from water requires expensive and energy-intensive treatment technologies. Therefore, preventing nitrate pollution of ground and surface water is critical, by reducing use of nitrogen-based fertilizers and by using farming techniques that limit nitrate runoff from the fields.

What can I do to reduce my exposure to nitrate?

For many tap water quality issues, consumers can use EWG’s Water Filter Buying Guide to find a filter that can remove specific contaminants. But home treatment options for nitrate are limited because ordinary carbon filters cannot remove the chemical. Reverse osmosis can remove it, but this technology is relatively expensive. Still, if your water has excessive nitrate, investing in a reverse osmosis filter may be appropriate.

People who have private wells should have their water tested, especially if they live in agricultural areas, because nitrate levels can change over time. Private well testing is especially important for households with young children or those planning to have children.

A substantial portion of nitrate and nitrite exposure can come from eating red and processed meats. Cutting back on these foods is a great way to reduce exposure.


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