EWG's Tap Water Database — 2021 UPDATE

picture of test tube and beaker in laboratory


November 2021


Nitrate is one of the most common contaminants in drinking water. It gets into water from fertilizer runoff, manure from animal feeding operations and wastewater treatment plant discharge.

Tap water in agricultural areas frequently has the highest nitrate concentrations. Private drinking water wells in the vicinity of animal farms and intensively fertilized fields, or in 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 1992 and has never been updated. This standard was based on a 1962 U.S. Public Health Service recommendation to prevent acute cases of methemoglobinemia, known as blue baby syndrome, which can occur when an infant’s excessive ingestion of nitrate leads to oxygen deprivation in the blood.

Epidemiological research suggests that the federal nitrate limit does not sufficiently protect public health. Studies conducted in the U.S. and in other countries found greater incidence of colorectal, ovarian, thyroid, kidney and bladder cancers among people exposed to nitrate in drinking water. Researchers in Europe have found elevated risk of colorectal cancer associated with drinking water concentrations more than 10 times lower than the federal limit. Epidemiological studies also report that nitrate contamination of tap water can harm the developing fetus.

Click here to see the results of nationwide test for nitrate. Some states test for nitrate and nitrite combined. Nitrite is even more toxic than nitrate, and water utilities often must install specialized treatments to remove both contaminants.

What is nitrate, and how does it get into water?

Nitrate pollution of water supplies has increased dramatically with the use of nitrogen-containing fertilizer. Analysis by EWG shows that nitrate can be a problem for water systems serving both rural and urban areas. High levels in water systems in large cities can be due to run-off, the release of municipal wastewater, or if the drinking water source has been polluted upstream. EWG’s Tap Water Database data for 2019 show that 237 million people drink water from a public water system with detectable levels of nitrate.

Private wells can contain excessive nitrate, especially in farm country. A 2000 study by the U.S. Geological Survey found that 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?

People are exposed to nitrate in both food and water. Cured meats, which are commonly preserved with nitrates, can be a significant source of dietary nitrate intake. Nitrate also occurs naturally in leafy green vegetables.

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 suggest that 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 be mixed with filtered tap water only, or with bottled water if filtered tap water is not available, to avoid exposing the baby to tap water contaminants.

Why is nitrate harmful to health?

At levels exceeding the federal limit, nitrate can cause blue baby syndrome. 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 who consume 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 by bacteria in the digestive system. These substances damage DNA and cause cancer in multiple animal species, in the blood and in different organs, including the stomach, bladder, colon and esophagus.

Nitrate pollution of U.S. drinking water may be responsible for up to 12,594 cases of cancer a year, at a cost of up to $1.5 billion for health care, according to a peer-reviewed study by EWG.

How much nitrate is harmful to health?

The federal legal limit for nitrate in drinking water fails to address growing concerns about chronic, low-level exposure to nitrate and potential cancer risk.

According to the findings of a peer-reviewed analysis by EWG scientists, a level of nitrate in drinking water that is protective against cancer risk and adverse birth outcomes is 0.14 mg/L – 70 times less than the federal standard. This standard corresponds to a one-in-a-million cancer risk.

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 in the absence of pollution control measures. Once nitrate enters groundwater and surface water, it lingers for years without breaking down or degrading.

Removing nitrate from water requires expensive and energy-intensive treatment technologies. So it is critical to prevent nitrate pollution of groundwater and surface water by reducing the 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, but this technology is relatively expensive. Still, if your water has excessive nitrate, investing in a reverse osmosis filter may be appropriate.

People who use 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 people planning to have children.

In addition, since 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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