Key takeaways:

• Women with the highest exposure to trihalomethanes in drinking water had increased risk for uterine cancer, specifically endometrioid tumors.
• Haloacetic acids exposure was associated with nonendometrioid tumors.

Regulatory limits on water contaminants found in public drinking water may not be strict enough to protect individuals from developing cancer.

An analysis of more than 50,000 women from California showed those exposed to the highest government-approved levels of trihalomethanes in community water systems had significantly greater risk for uterine cancer overall and specifically endometrioid tumors.

Data derived from Spaur M, et al. JAMA Netw Open. 2026;doi:10.1001/jamanetworkopen.2026.24391.

Additionally, women in the highest tertile of haloacetic acids exposure had significantly elevated risk for nonendometrioid tumors.

“Are current limits protective against this cancer? That’s a fair question to ask,” Rena R. Jones, PhD, MS, senior investigator in the division of cancer epidemiology and genetics at NCI, told Healio.

“The data are not without limitations, but the findings point to a signal that I think should be taken seriously.”

Are current levels safe?

Uterine cancer incidence and mortality have steadily increased over the past 20-plus years, according to NCI, and researchers estimate both numbers will grow substantially through 2050, Healio previously reported.

Prior studies have shown regional variation with regard to both endometrioid and nonendometrioid malignancies, indicating an individual’s environment could be a risk factor.

About 90% of people living in the U.S. get their water from community systems.

“In these community and public water systems, we disinfect the water to kill bacteria and viruses that can cause infectious diseases,” Jones said. “That process of disinfecting or treating the water has been a big public health success. We have fewer people getting sick from pathogens in the water, but the consequence of that disinfection, which is often done by chlorination, is we get these byproducts of disinfection, trihalomethanes and haloacetic acids.”

The Environmental Protection Agency regulates certain contaminants in drinking water, including total trihalomethanes, which have been associated with bladder cancer.

“The levels that are set under the Safe Drinking Water Act are thought to be protective against the development of bladder cancer, but one question motivating our research is, ‘Is that level also protective against the risk of other cancers?’” Jones asked.

Jones and colleagues previously published data showing a link between drinking water disinfection byproducts and uterine cancer among women in Iowa.

“That was the first such finding ever,” Jones said. “Anytime there’s a known or suspected carcinogen in your water and see such a relationship, you want to investigate that a little further.”

Researchers used the California Teachers Study, a cohort of women educators without a history of cancer or hysterectomy, enrolled between 1995 and 1996 and followed through 2020, to see if they could replicate their findings.

They included 53,100 women (median age, 50 years; interquartile range, 43-60; 85% white) who had an address served with community water and maintained that residence for at least 10 years.

Incidence of overall uterine cancer, and endometrioid and nonendometrioid tumors, served as the primary endpoint.

‘Important’ consideration

Women in the highest vs. lowest tertile of total trihalomethanes exposure from drinking water had significantly increased risk for uterine cancer overall (HR = 1.18; 95% CI, 1.02-1.38) and endometrioid tumors (HR = 1.23; 95% CI, 1.05-1.46).

An evaluation of individual trihalomethanes showed women in the highest tertile for chloroform exposure also had significantly greater risk for uterine cancer (HR = 1.18; 95% CI, 1.02-1.37) and endometrioid tumors (HR = 1.27; 95% CI, 1.07-1.5), and those with the

highest bromodichloromethane exposure had a significantly elevated risk for endometrioid tumors (HR = 1.22; 95% CI, 1.03-1.44).

Women in the highest tertile for total haloacetic acids exposure had significantly greater risk for nonendometrioid tumors (HR = 1.86; 95% CI, 1.06-3.25), as did those with the highest exposure to monochloroacetic acid (HR = 1.86; 95% CI, 1.01-3.44).

“The association we had previously observed in Iowa was not likely a chance finding,” Jones said. “Those results are replicating, nearly in the same way, what we had seen in a completely different geographic location, in a completely different group of women.”

Researchers acknowledged study limitations, including lack of water quality data prior to 1990.

“Another important caveat is we don’t have estimates of how much women are ingesting, apart from a small subset,” Jones said. “We just know the concentrations in the water that’s serving their home. We don’t have a lifetime assessment of how often they’re drinking water, or if they’re filtering, things like that, which could impact their exposure levels.”

Jones and colleagues do have water ingestion data in another population that they plan to evaluate in the future.

Jones also emphasized these studies do not confirm regulatory limits for water contaminants pose cancer risks.

However, the “evidence is accumulating,” including those showing nitrate concentrations in water may have an association with ovarian cancer.

“The allowed level of nitrate in drinking water is set not based on cancer at all, it’s based on the risk of blue-baby syndrome,” Jones said. “We haven’t had many cases of blue-baby syndrome in the U.S. since that level was set, but we don’t know if it is protective against cancer, and some of our analyses would suggest that it’s not. Denmark recently lowered its nitrate standard in drinking water about 10-fold, in part based on an accumulation of evidence from cancer studies, including some of our work. At a national level, they appear to view this as an important issue to consider.”

“We’ve had several studies that have made this link [with water contaminants and cancer] consistently,” she added. “The number of studies is still fairly small, but it’s worth continued attention.”

For more information:

Rena R. Jones, PhD, MS, can be reached at rena.jones@nih.gov.