In California, an Estimated 15,000 Cancer Cases Linked to Chemicals in Tap Water

A study published in Environmental Health found that drinking California tap water over a lifetime increases the risk of cancer. Specifically, between 2011 and 2015, contaminants found in California public water systems could contribute to approximately 15,500 cancer cases over the course of a lifetime.

Researchers evaluated 2,737 California public water systems by assessing the level of reported contaminants. They then calculated cancer risk by identifying yearly averages of all reported contaminants and adding them together to determine cumulative risk.

Something in the water

The contaminants found in the water included arsenic, hexavalent chromium, and radioactive elements such as uranium and radium.

The authors divided the water systems into four categories of risk. In the highest-risk category, they observed a cancer risk of more than one in 1,000 people being diagnosed over the course of a lifetime from drinking tap water. Approximately 500 community water systems were classified as high risk, although the researchers did not identify which water systems were considered high risk. However, they did note that small communities had the highest exposure to waterborne contaminants.

Most (47%) of the cancer risk was related to arsenic contamination. Water systems that posed the highest risk of cancer from arsenic exposure all served communities with fewer than 1,000 people. Nearly a third of the estimated cancer diagnoses was attributed to disinfection byproducts, and 16% resulted from hexavalent chromium.

The combined levels of those contaminants were estimated to cause an additional 15,449 lifetime cancer cases, or 221 cases per year.

“We need to look at contaminants as a group—not just one at a time. It’s more important to analyze co-occurring contaminants to understand the real-world exposure,” lead author Tasha Stoiber, a senior scientist with the Environmental Working Group, told CNN.

The researchers noted that despite the findings, the majority of California water systems meet the legal standards for water contaminants.