Inorganic
Nitrate and nitrite in drinking water: a plain-English guide
Nitrate and nitrite contaminate drinking water mainly from agricultural runoff (synthetic fertilizers, manure) and septic systems. Acutely dangerous to infants under six months (methemoglobinemia, the 'blue-baby syndrome'). EPA's MCL is 10 mg/L for nitrate (measured as nitrogen) and 1 mg/L for nitrite. Boiling water with nitrate concentrates it, it does not remove it.
Where nitrate in drinking water comes from
The dominant sources are agricultural:
- Synthetic nitrogen fertilizer applied to row crops (corn, wheat, sorghum). U.S. agricultural land receives roughly 11 million tons of synthetic N per year. Excess nitrogen runs off into surface water and infiltrates into groundwater.
- Animal waste from concentrated feedlot operations.
- Septic systems, especially older or failing ones in rural areas.
- Industrial discharges (food processing, fertilizer plants), in more localized cases.
The Midwest corn belt, parts of California’s Central Valley, and the southeastern Coastal Plain are the U.S. regions with the highest background nitrate in drinking-water sources.
Why it matters for health
The acute risk is methemoglobinemia in infants under six months, often called blue-baby syndrome. Infant gut bacteria convert nitrate into nitrite; nitrite binds hemoglobin and prevents it from carrying oxygen. Severe cases can be fatal. Healthy adult guts do not make this conversion at a meaningful rate.
For older children and adults, long-term exposure has been linked to:
- Pregnancy complications: pre-term birth, low birth weight, and some neural tube defects.
- Possible colorectal and thyroid cancer: epidemiologic evidence is mixed but several studies show associations at exposures below the current MCL.
- Thyroid dysfunction at high chronic exposure.
EPA regulation
- Nitrate MCL: 10 mg/L measured as nitrogen (mg/L NO3-N).
- Nitrite MCL: 1 mg/L as N.
- MCLG: same values as MCL.
The MCL is set on the infant methemoglobinemia endpoint. Many public-health researchers argue it does not adequately protect against chronic-exposure pregnancy outcomes and that a lower threshold (around 5 mg/L) would be more appropriate; EPA has not moved on this.
What to do
- Never give untreated, high-nitrate water to an infant under six months. Use bottled water or treated water for formula preparation. This is the single most important action.
- Boiling does NOT help. It evaporates the water and concentrates the nitrate.
- If you live on a private well in an agricultural area, test annually. Most state health departments offer low-cost testing.
- For pregnant people or households with infants, point-of-use reverse osmosis or distillation is reliable.
Treatment
Treatment that works:
- Reverse osmosis (very effective)
- Anion exchange with nitrate-selective resins
- Distillation
Treatment that does NOT work: carbon filters, sediment filters, ozone, UV. Boiling makes it worse.
This page is general information and is not medical advice. If you live in an agricultural area and have an infant or are pregnant, talk to your clinician about appropriate water sources for the home.
Sources
- US EPA, “National Primary Drinking Water Regulations”: https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- CDC, “Nitrate and Drinking Water from Private Wells”: https://www.cdc.gov/healthywater/drinking/private/wells/disease/nitrate.html
- US Geological Survey, “Nitrate in Groundwater”: https://www.usgs.gov/mission-areas/water-resources/science/nitrate-groundwater
Editorial review: reviewed 2026-05-11 by RK 2026-05-11. Editorial standards.