What is Nitrate?

Nitrate (NO3 ) is a water-soluble molecule made up of nitrogen and oxygen. It is formed when nitrogen from ammonia or other sources combines with oxygenated water. Nitrate is a natural constituent of plants. It is found in vegetables at varying levels depending on the amount of fertilizer applied and other growing conditions. 

According to the World Health Organization, most adults ingest 20 to 70 milligrams of nitrate-nitrogen per day. Most of this comes from foods, such as lettuce, celery, beets, and spinach. When foods containing nitrate are eaten as part of a balanced diet, nitrate exposure is not thought to be harmful.

Nitrate Levels in Drinking Water

Water naturally contains less than 1 milligram of nitrate-nitrogen per liter. It is not a major source of exposure. 

Higher levels indicate that the water has been contaminated. Common sources of nitrate contamination include fertilizers, animal wastes, septic tanks, municipal sewage treatment systems, and decaying plant debris. 

State and federal laws set the maximum allowable level of nitrate-nitrogen in public drinking water at 10 milligrams per liter (10 parts per million). These laws apply to all city and village water supplies. They are used as an advisory for private wells. 

The ability of nitrate to enter well water depends on the type of soil and bedrock present. It also depends on the depth and construction of the well.

Health Effects

High nitrate levels in drinking water pose a risk to infants. Infants who are fed water or formula made with water that is high in nitrate can develop a condition that doctors call methemoglobinemia. The condition is also called "blue baby syndrome" because the skin appears blue-gray or lavender in color. This color change is caused by a lack of oxygen in the blood.

All infants under six months of age are at risk of nitrate poisoning. Some babies may be more sensitive than others. Infants suffering from "blue baby syndrome" need immediate medical care. The condition can lead to coma and death if it is not treated promptly.

When nursing mothers ingest water that contains nitrate, the amount of nitrate in breast milk may increase. No confirmed cases of "blue baby syndrome" have been associated with nitrate in breast milk. It is still advisable for nursing women to avoid drinking water that contains more than 50 milligrams per liter nitrate-nitrogen.

Some scientific studies have found evidence suggesting that women who drink nitrate-contaminated water during pregnancy are more likely to have babies with birth defects. Nitrate ingested by the mother may also lower the amount of oxygen available to the fetus.

People who have heart or lung disease, certain inherited enzyme defects, or cancer may be more sensitive to the toxic effects of nitrate than others. Some experts believe that long-term ingestion of water high in nitrate may increase the risk of certain types of cancer.


When laboratory tests determine that water contains more than 10 milligrams per liter nitrate-nitrogen, the following actions are recommended:

  • Avoid drinking the water on a daily basis during pregnancy.
  • Do not attempt to remove the nitrate by boiling the water. This will only concentrate the nitrate making levels even higher.
  • Do not give the water to infants less than 6 months of age or use the water to prepare infant formula.
  • Seek medical help immediately if the skin of an infant appears bluish or gray in color. Sometimes the color change is first noticed around the mouth or on the hands and feet.
  • Identify the nitrate source and take action to reduce contamination. Remedial actions may include reducing fertilizer use, improving manure handling methods, pumping septic tanks, or upgrading wells.
  • Limit your daily intake if you have chronic health problems that increase your sensitivity to nitrate, or if you are concerned about scientific uncertainty regarding the health effects of long-term exposure to nitrate-contaminated water.

Testing for Nitrate

The only way to determine the nitrate level in water is to have a water sample tested by a certified laboratory. Public water supplies are tested regularly for the presence of nitrate. 

A nitrate test is recommended for all newly constructed private wells and wells that have not been tested during the past five years. Testing is recommended for wells used by pregnant women. It is essential for wells that serve infants under six months of age. 

Wells with nitrate-nitrogen levels below 5 milligrams per liter should be retested every few years. If the levels are between 5 and 10 milligrams per liter, owners should consider testing more often to check for seasonal changes. Additional testing may be useful if there are any known sources of nitrate or if high nitrate levels are detected in nearby wells.

View Nitrate Testing Instructions.

Additional Information

Contact your city/county health department or the Division of Health office in your region if you would like more information about the health effects of nitrate.

To find out how to have your well tested for nitrate, contact your county health department or regional Department of Natural Resources headquarters.

DHSS Health Offices

  • Central Office
    1414 E. Washington
    Madison, WI 53703-3044
    Phone: (608) 266-0923
  • Northern Region
    1853 N. Stevens Street
    Rhinelander, WI 54501-1165
    Phone: (715) 369-9100
  • Western Region
    312 Barstow
    Suite 2
    Eau Claire, WI 54701-6143
    Phone: (715) 836-5362
  • Northeastern Region
    200 N. Jefferson Street
    Green Bay, WI 53401-5158
    Phone: (414) 448-5223
  • Southern Region
    3518 Memorial Drive
    Building 4
    Madison, WI 53704-1104
    Phone: (608) 243-2351
  • Southeastern Region
    819 North Sixth Street
    Milwaukee, WI 53203-1697
    Phone: (414) 227-4860

DNR Water Supply

  • Northwest District
    Highway 70 W.
    Box 309
    Spooner, WI 54801
    Phone:(715) 635-2101
  • North Central District
    107 Sutliff Avenue
    Box 818
    Rhinelander, WI 54501
    Phone: (715) 365-8900
  • Western District
    404 S. Barstow
    Box 4001
    Eau Claire, WI 54702
    Phone: (715) 839-3700
  • Lake Michigan District
    1789 Shawano Avenue
    Box 10448
    Green Bay, WI 54307
    Phone: (414) 492-5885
  • Southeast District
    4041 N. Richards Dr.
    Box 12436
    Milwaukee, WI 53212
    Phone: (414) 229-0800
  • Southern District
    3911 Fish Hatchery
    Fitchburg, WI 53711
    Phone: (609) 275-3266