Tom Walton is an attorney in Dallas County.

In March 2024, Dr. Peter S. Thorne, Department of Occupational and Environmental Health at the University of Iowa, with his colleague Dr. Angelico Mendy, an epidemiologist at the University of Cincinnati College of Medicine, published an important scientific paper on the association between drinking water nitrate levels with the risk of death from cancer. Thorne is the former Chairman of the U.S. Environmental Protection Agency’s Scientific Advisory Board.

The report has received little, if any, coverage by others, including in Iowa. It deserves more.

The study, “Long-Term Cancer and Overall Mortality Associated with Drinking Water Nitrate in the U.S.” analyzed data from 2,029 participants who were followed for a median period of 13.09 years. After adjusting for variables such as age, Body Mass Index, pack-years of cigarette smoking, and other health-related factors (i.e., “covariants”), the researchers used Cox proportional hazards regression to estimate the hazard ratio (HR) and corresponding 95 percent confidence level for the risk of mortality associated with drinking water nitrate.

Scientists use this method of analysis when they want to understand how an exposure to nitrate, for instance, correlates with a person’s cause of death. This analysis helps answer the question, “Does drinking nitrate-contaminated water make people more likely to die from cancer than other causes?”

The “hazard ratio” compares two groups’ risks. If the risk of the event is the same for both groups, the HR is 1. If the HR is greater than 1, the group being studied has a higher risk, and a HR that is less than 1, means the studied group has a lower risk than the comparison group.  

What did Thorne and Mendy discover? In simple terms, the study found that people in the United States who had nitrate detected in their drinking water had a 73 percent higher risk of dying from cancer compared to those without detectable nitrate, and every tenfold increase in drinking water nitrate level was linked to a 69 percent higher risk of cancer death. Even exposures to nitrate levels below the federal Maximum Contaminant Level “were still associated with higher cancer mortality.”

Evidence that long-term exposure to nitrates in water causes cancer continues to mount. An April 28, 2025 study entitled “The association between increases of nitrates in drinking water and colorectal cancer incidence rates in California, USA”, appeared in the peer-reviewed journal Cancer Causes & Control. The lead scientist is Dr. Ricardo Cisneros of the University of California. The study states:

The findings reveal a consistently positive association across all examined lags between [nitrate] exposure and colorectal cancer incidence, affirming the hypothesis that increasing concentrations of [nitrate] in public drinking water systems correlate with rising rates of colorectal cancer.

This study may be added to the extensive research that indicates chronic exposure to elevated nitrate levels below federal permissible levels is linked to various health risks, including cancers such as colorectal cancer. Leaders must take effective steps to reduce exposure to nitrate-contaminated public drinking water.