Drinking water byproducts linked to higher uterine cancer risk

Clinical takeaway: Continue counseling patients about established uterine cancer risk factors. This study adds to emerging evidence that some drinking water disinfection byproducts may also contribute to risk.

Uterine cancer is the most common gynecologic cancer in the United States, and incidence continues to rise. While obesity and hormonal factors are well-established contributors, environmental exposures have received less attention. A large prospective cohort study suggests that long-term exposure to some drinking water disinfection byproducts may be associated with a modestly increased risk of uterine cancer.

Researchers followed 53,100 female California educators who had lived at the same home for at least 10 years and were free of cancer and had not undergone hysterectomy at study entry. Over approximately 19 years of follow-up, women with the highest long-term exposure to total trihalomethanes (THMs), byproducts formed during water disinfection, had an 18% higher risk of uterine cancer and a 23% higher risk of endometrioid tumors than those with the lowest exposure.

Among the individual disinfection byproducts, chloroform showed the strongest association with endometrioid tumors. Higher exposure to total haloacetic acids, another class of disinfection byproducts, was associated with an 86% higher risk of nonendometrioid uterine tumors, although these analyses included fewer cases. No associations were observed for nitrate, arsenic, or uranium.

Notably, the associations persisted even among women whose contaminant levels were below current US regulatory limits, although the increases in risk were modest and the observational study cannot establish causality.

The findings add to limited prior evidence suggesting that some drinking water disinfection byproducts may influence uterine cancer risk through endocrine-disrupting and other carcinogenic mechanisms. Additional studies are needed to confirm the findings and determine whether reducing long-term exposure could lower cancer risk.

In an accompanying editorial, Shady Abohashem, MD, MPH, of Massachusetts General Hospital and Harvard Medical School, noted that this study replicates the only previous prospective evidence linking drinking water disinfection byproducts to uterine cancer, despite lower contaminant levels and a different study population. He also highlighted that the observed associations occurred at exposure levels below current US regulatory limits, supporting further research into whether existing standards adequately protect against long-term cancer risk and whether additional reductions in disinfection byproduct exposure could provide public health benefits.

Abohashem concluded, "Clinicians counseling patients with newly diagnosed endometrial cancer rarely think of the tap. This study is a reminder that, for environmental cancers, the most important exposures are often the ones we have stopped noticing."

Source: Spaur M, et al. (2026 Jul 22) JAMA Netw Open. Drinking water contaminant exposures and risk of uterine cancer