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

Objectives: Nitrate is a probable carcinogen regulated in drinking water by the US Environmental Protection Agency (EPA) to a maximum contaminant level (MCL) of 10 mg/L nitrate-nitrogen (NO-N; equivalent to 44.3 mg/L NO). We aimed to determine the association of US drinking water nitrate levels with overall as well as cardiovascular, cancer, and other cause mortality.

Study Design: This study used a population-based retrospective cohort design.

Methods: We analyzed data from 2029 participants of the 2005-2006 National Health and Nutrition Examination Survey followed for mortality until 2019 for a median of 13.9 years. We used Cox proportional hazards regression to estimate the hazard ratio (HR) and 95% confidence interval (CI) for mortality associated with drinking water nitrate, adjusting for covariates that included socio-economic factors and pack-years of cigarette smoking.

Results: Drinking water nitrate was detected in 50.8 % of the samples, had a median concentration of 0.77 mg/L NO, and was above US EPA MCL in 0.4 % of participants. In adjusted analysis, drinking water nitrate detection was associated with 73 % higher cancer mortality (HR: 1.73, 95% CI: 1.19-2.51), whereas a 10-fold increase in drinking water nitrate levels was associated with 69 % higher cancer mortality (HR: 1.69, 95% CI: 1.24-2.31) and 21 % higher overall mortality (HR: 1.21, 95% CI: 1.00-1.46). Drinking water nitrate below EPA MCL was still associated with higher cancer mortality (HR: 1.61, 95% CI: 1.07-2.43 per 10-fold increase and HR: 1.61, 95% CI: 1.08-2.42 for detection).

Conclusions: Levels of drinking water nitrate may be an overlooked contributor to cancer mortality in the United States.