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Module 2: The Health Impacts of Nitrate Contamination in Drinking Water

1) An Introduction to the Health Impacts of Nitrate

Nitrate contamination of drinking water is a significant public health concern, particularly in agricultural regions where fertilizers and animal waste can leach into groundwater. High nitrate exposure can reduce the blood's ability to carry oxygen, placing infants, pregnant individuals, and other vulnerable populations at risk for serious health effects, including methemoglobinemia ("blue baby syndrome"). Emerging research also suggests that long-term exposure to nitrate levels below the current federal standard may be associated with increased risks of certain cancers, adverse birth outcomes, and other chronic health conditions. Because many private wells are not routinely monitored, healthcare professionals play an important role in recognizing potential exposure risks, educating patients, and promoting testing and prevention strategies to protect community health.

 

Watch the following Video Introduction:

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2) Specific Health Outcomes Associated with Nitrate Exposure in Drinking Water

Adult Cancer

Long-term nitrate exposure in drinking water is associated with elevated mortality, with effects observed at levels below the current safety limit of 10mg/L. Clinical implications include:

  • 73% higher cancer mortality rate
  • 21% higher overall mortality rate, with effects seen even at nitrate levels below the current safety limit.

Read More:

  • Long-term cancer and overall mortality associated with drinking water nitrate in the United States

Gastric Cancer

Several studies found a positive association between nitrate exposure in drinking water and gastric cancer.

Read More:

  • Drinking Water Nitrate and Human Health: An Updated Review
  • The potential human health hazard of nitrates in drinking water: a media discourse analysis in a high-income country

Colorectal Cancer

Evidence for an association between nitrate exposure and colorectal cancer is not entirely conclusive across studies. However, several studies have found that nitrate in drinking water is associated with a higher risk for colon cancer. One nationwide cohort study found that people drinking water with nitrate levels above 3.87 mg/L had a higher risk of colon cancer.

Read More:

  • Drinking-water nitrate and cancer risk: A systematic review and meta-analysis

Ovarian cancer

  • A long-term study found that higher nitrate levels in drinking water—even below regulatory limits—were associated with a small increased risk of high-grade serous ovarian cancer.
  • A large study found that higher nitrate levels in drinking water and higher dietary nitrate intake were associated with a small increase in ovarian cancer risk. However, the drinking-water nitrate finding was not statistically significant.

Read More:

  • Exposures to Drinking Water Contaminants in Community Water Systems and Risk of Ovarian Cancer in the California Teachers Study Cohort
  • Nitrate and disinfection by-products in drinking water and risk of ovarian cancer

Prostate cancer

  • A recent study found that men exposed to nitrate levels above 10 mg/L in drinking water had a modestly higher risk of prostate cancer, especially aggressive prostate cancer. Lower levels showed weaker or uncertain associations.

Read More:

  • Drinking water nitrate, disinfection byproducts, and prostate cancer incidence in the Agricultural Health Study

Additional Studies of Note:

  • Drinking Water Nitrate and Human Health: An Updated Review
  • Nitrate and nitrite contamination in drinking water and cancer risk: A systematic review with meta-analysis
  • Nitrate in drinking water and colorectal cancer risk: A nationwide population-based cohort study

Pediatric Cancer

A geospatial study in Nebraska found a higher incidence rate of pediatric brain and other central nervous system (CNS) cancers, leukemia, and lymphoma at higher groundwater nitrate concentrations (2.1 to 5 mg/L).10 However, a cohort study found some evidence linking nitrate exposure to childhood central nervous system cancer, but not leukemia or lymphoma.11 More research is needed to address these inconsistencies. 

Read More: 

  • Geospatial Distribution of Age‐Adjusted Incidence of the Three Major Types of Pediatric Cancers and Waterborne Agrichemicals in Nebraska
  • Exploring the Joint Association Between Agrichemical Mixtures and Pediatric Cancer

Maternal/Fetal Health Outcomes

The Environmental Protection Agency (EPA) has set the Maximum Contaminant Level (MCL) and Maximum Contaminant Level Goal (MCLG) of nitrate in drinking water at 10 mg/L in response to methemoglobinemia (commonly known as blue baby syndrome). Infants under 6 months are especially at risk. Multiple studies have also found links between nitrate contamination in drinking water and increased risk of neural tube defects, preterm births, and lower birth weight related to exposure during pregnancy.

Providers should consider discussing potential nitrate exposure with patients who are planning a pregnancy or are currently pregnant, especially in regions with documented nitrate contamination in drinking water sources. Findings include:

  • A particularly increased risk of birth defects affecting the eyes, nervous system, ears, face, and neck was observed.
  • Pregnant women exposed to nitrate concentrations of 5 to <10 mg/L in drinking water had 47% higher odds of giving birth very early (between 20 and 31 weeks) compared to those with lower nitrate exposure. For nitrate concentrations ≥10 mg/L, pregnant women had about 2.5 times the odds of very preterm birth compared with women exposed to <5 mg/L (OR = 2.52). The link was weaker for babies born a little later (32 to 36 weeks).
  • A large Iowa study found that pregnant women exposed to relatively low levels of nitrate in drinking water early in pregnancy had a slightly higher risk of preterm birth and low birth weight.

Read More:

  • Nitrate contamination in drinking water and adverse reproductive and birth outcomes: a systematic review and meta-analysis
  • Nitrate in drinking water and risk of birth defects: Findings from a cohort study of over one million births in Denmark
  • Blue babies and nitrate-contaminated well water
  • Nitrate in Drinking Water during Pregnancy and Spontaneous Preterm Birth: A Retrospective Within-Mother Analysis in California
  • Medical education for obstetricians and gynecologists should incorporate environmental health
  • Maternal Exposure to Nitrate from Drinking Water and Diet and Risk for Neural Tube Defects
  • A Review of Nitrates in Drinking Water: Maternal Exposure and Adverse Reproductive and Developmental Outcomes
  • Drinking water nitrate and risk of pregnancy loss: a nationwide cohort study
    • Note – No consistent associations were found when investigating the exposure as a categorical variable and null findings were also found in trimester specific analyses. In the spline model using the continuous exposure variable, a modestly increased hazard of pregnancy loss was observed for the first trimester at nitrate exposures between 1 and 10 mg/L, with the highest adjusted hazard ratio at 5 mg/L of nitrate of 1.16 (95% CI: 1.01, 1.34). This trend was attenuated in the higher exposure ranges.
  • Nitrate in drinking water and pregnancy outcomes: A narrative review of epidemiological evidence and proposed biological mechanisms

Chronic and acute conditions

  • Drinking Water Nitrate and Human Health: An Updated Review