Can the chemicals in treated tap water raise a woman’s cancer risk?
Yes, at least for uterine cancer. Women with the highest long-term exposure to chlorination byproducts in their drinking water had an 18 percent higher risk of uterine cancer than women with the lowest exposure.
That finding comes from the California Teachers Study, a prospective cohort of female educators. It included 53,100 women who were cancer-free and had not had a hysterectomy when they enrolled in 1995 and 1996, and who had lived at their enrollment address for at least 10 years. Researchers linked each woman’s address to the monitoring records of the community water system that served it, then followed the group through December 31, 2020. Over that period, 1,038 women were diagnosed with uterine cancer: 864 endometrioid tumors and 154 nonendometrioid tumors. The contaminants that turned up in the results were not the ones most people worry about.
What are trihalomethanes and haloacetic acids?
About 90 percent of the US population gets its drinking water from a community water system, and those systems disinfect. That step has saved an enormous number of lives, and nothing in this study changes that. Disinfection also leaves behind a set of chemicals called disinfection byproducts.
Trihalomethanes and haloacetic acids are two families of these byproducts. They are known or suspected carcinogens, and some have endocrine-disrupting properties, which is the reason anyone would look for a link to a hormone-sensitive cancer of the uterus in the first place. Very few studies have asked the question directly. This one asked whether long-term exposure adds up to something measurable.
What the data show
Women in the highest exposure tertile for total trihalomethanes had an 18 percent higher risk of uterine cancer overall, with a hazard ratio of 1.18 and a confidence interval of 1.02 to 1.38, and a p for trend of .06. Because the low end of that range sits just above 1.0, the result is statistically significant but not far from the line.
The risk was not spread evenly across cancer types. For endometrioid tumors, higher trihalomethane exposure came with a 23 percent higher risk, a hazard ratio of 1.23 with a confidence interval of 1.05 to 1.46 and a p for trend of .01. Chloroform on its own tracked the same way, with a hazard ratio of 1.27 and a confidence interval of 1.07 to 1.50 for endometrioid tumors.
Haloacetic acids showed a different pattern. The sum of 5 haloacetic acids was tied to nearly double the risk of nonendometrioid tumors, with a hazard ratio of 1.86, a confidence interval of 1.06 to 3.25, and a p for trend of .01. Monochloroacetic acid on its own gave a hazard ratio of 1.86 with a confidence interval of 1.01 to 3.44. Those confidence intervals are wide, because nonendometrioid cancers are uncommon and fewer cases means less certainty.
When the researchers modeled the contaminants as a mixture rather than one at a time, each interquartile increase in the mixture of individual trihalomethanes, nitrate, arsenic, and uranium carried a hazard ratio of 1.21 with a confidence interval of 0.96 to 1.53 for uterine cancer, and 1.34 with a confidence interval of 1.04 to 1.74 for endometrioid tumors. Chloroform and dibromochloromethane were the major contributors.
The null results matter too. Nitrate, arsenic, and uranium, three contaminants that get far more public attention, showed no link to uterine cancer here at any tertile.
Dr. Kumar’s Take
I read this one carefully, because the practical question is what a woman should actually do differently on Monday morning. The honest answer is: probably not much, but it is worth knowing.
An 18 percent relative increase sounds alarming until you remember what it is a percentage of. Uterine cancer is the most commonly diagnosed gynecologic cancer in the US, with about 69,120 cases expected in 2025, which is 3.4 percent of all new cancer diagnoses. It is not rare, but a modest bump in relative risk does not translate into a large number of extra cases for any one woman.
The subtype pattern is the part I keep coming back to. Two different chemical families lined up with two different tumor types, and that is not the shape random noise usually takes. It suggests something biological rather than statistical luck.
I would also sit with how these contaminants get regulated. The EPA sets enforceable maximum contaminant levels for community water systems, and those levels weigh economic and technical feasibility alongside public health benefit. A legal limit is a policy decision, not a biological threshold, and this study is a reminder of the gap between the two.
I would not tell anyone to stop drinking tap water over this. Untreated water carries risks that are immediate and severe. But a basic carbon filter is cheap and the downside is close to zero.
How confident should we be in these findings?
This was an observational cohort study, so it can show a link but cannot prove that the chemicals caused the cancers. Exposure was estimated from the water system serving each woman’s enrollment address, not from what she personally drank. Someone who used a filter, drank bottled water, or moved often would be misclassified. The Cox models adjusted for age, body mass index, and smoking status, so other drivers of uterine cancer risk could still be doing some of the work.
The design has real strengths. It followed a very large group prospectively from enrollment in 1995 and 1996 through the end of 2020, which means exposure was recorded before anyone got sick and removes a common source of bias. Restricting the analysis to women who had lived at the same address for at least 10 years makes the exposure estimate more credible. The trihalomethane result also matches the only prior study to look at this question, conducted in the midwestern US, and independent replication carries more weight with me than any single borderline p value.
Practical Takeaways
- If you want to reduce disinfection byproducts at home, an activated carbon filter, either a pitcher or a faucet mount, is the simplest and cheapest option to try.
- Ask your water utility for its annual water quality report and look at the reported levels of trihalomethanes and haloacetic acids for your system.
- Do not switch away from treated municipal water to untreated well or surface water based on this study, since the infection risk from undisinfected water is far larger and far more immediate.
- Report any postmenopausal bleeding or unusual bleeding between periods to your doctor promptly. In my practice that remains the single most useful early warning sign for uterine cancer, regardless of what is in your water.
Does boiling water remove trihalomethanes?
Boiling is not the tool most people assume it is. It is aimed at germs, not at the chemicals left behind by disinfection, and the effect on trihalomethanes depends on how long you boil and whether the pot is covered, since boiling also concentrates whatever remains in a smaller volume of water. If your goal is to reduce these particular compounds, filtration is the more sensible route, and it does not require you to heat anything.
Are levels of these chemicals higher at certain times of year?
Concentrations in a given system move around over time, which is exactly why a single water test is a weak measure of what someone has been drinking. This study did not rely on a snapshot. It used 15-year mean concentrations from 1990 to 2005 for trihalomethanes and inorganics, and mean concentrations from 1990 to 2013 for haloacetic acids, and restricted the analysis to women who had lived at the same address for at least 10 years. That averaging is a large part of why I take the exposure estimates seriously.
Should women with a family history of uterine cancer do anything differently?
Nothing in this study speaks to inherited risk, so anything specific there would be speculation from me. What it does support is that if you are already in a higher risk group, the cheap, low-downside steps get easier to justify. Filtering your drinking water costs very little and carries no real risk. Body weight remains the more powerful lever: in this cohort, 13 percent of all the women had a body mass index of 30 or higher, compared with 26 percent of the women who went on to develop uterine cancer. That, along with diabetes and hormone exposure, is what I would discuss with your doctor first.
Related Studies and Research
- Heavy pesticide exposure linked to 2 to 8 times higher cancer risk
- Long daytime naps linked to higher death risk in older adults
- How vitamin C levels drop during the common cold, and what it means for recovery
- Small changes in sleep, exercise, and diet linked to 9 extra years of life
Bottom Line
In 53,100 California women followed from enrollment in 1995 and 1996 through the end of 2020, higher long-term exposure to chlorination byproducts in drinking water was linked to an 18 percent higher risk of uterine cancer and a 23 percent higher risk of endometrioid tumors, with haloacetic acids tied to nearly double the risk of the nonendometrioid subtype. Nitrate, arsenic, and uranium showed nothing. This is a single observational study and not proof of cause, but it replicates the one prior study on the question, and it points to a risk that a cheap carbon filter can partly address.
