For more than a century, chlorination has helped make public drinking water safer by killing bacteria, viruses, and other disease-causing organisms. It remains one of the most important public health tools in modern water treatment.
But there is a lesser-known trade-off: when chlorine or chloramine reacts with natural organic matter in water, it can form disinfection byproducts. One of the most studied groups of these byproducts is trihalomethanes, commonly called THMs.
A recent global analysis published in Environmental Health Perspectives reviewed the epidemiological evidence on THM exposure in drinking water and cancer risk. The analysis found that people in the highest exposure groups had a 33% higher relative risk of bladder cancer and a 15% higher relative risk of colorectal cancer compared with those in the lowest exposure groups. The authors described the evidence as “limited-suggestive,” meaning the findings point to a concern but should not be interpreted as proof that THMs cause cancer in every individual case. You can read the study summary here: Exposure to Drinking Water Trihalomethanes and Risk of Cancer: A Systematic Review and Dose-Response Meta-Analysis.
Why chlorine is still essential
This research does not mean families should avoid municipal tap water or that chlorination is unsafe overall.
Chlorine and chloramine are used because they kill harmful germs in drinking water and help keep water protected as it travels through pipes to homes and businesses. The CDC explains that disinfectants such as chlorine and chloramine are added to drinking water to kill germs, while also noting that disinfection byproducts can form when those disinfectants react with dirt or other materials naturally present in water. According to the CDC, disinfection byproducts can cause health problems over time at high enough levels. Learn more from the CDC here: About Water Disinfection with Chlorine and Chloramine.
In other words, the issue is not whether water should be disinfected. It absolutely should be. The more practical question is how households can keep the benefits of disinfection while reducing unnecessary exposure to byproducts.
What are THMs?
THMs form when chlorine-based disinfectants react with organic matter such as decaying leaves, algae, or other natural materials in source water. The four regulated THMs are typically grouped together as total trihalomethanes, or TTHMs.
The U.S. Environmental Protection Agency regulates disinfection byproducts through the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules. These rules apply to many public water systems that add disinfectants and are intended to reduce exposure to byproducts such as TTHMs and haloacetic acids. EPA explains the rules here: Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules.
Current U.S. regulations set an enforceable limit for TTHMs, measured as a running annual average. However, the newer research suggests that elevated cancer associations may occur even at levels below some current regulatory limits. That does not mean every home with detectable THMs is unsafe, but it does mean long-term exposure is worth paying attention to.
The health concern: long-term exposure
The cancers most consistently discussed in relation to THMs are bladder cancer and colorectal cancer.
The 2025 meta-analysis reviewed 29 publications assessing 14 different cancers. The summary risk estimates were based on thousands of cancer cases and tens of thousands of participants. For bladder cancer, the analysis found a relative risk of 1.33 for the highest exposure group compared with the lowest exposure group. For colorectal cancer, the relative risk was 1.15. In plain language, that translates to a 33% higher relative risk of bladder cancer and a 15% higher relative risk of colorectal cancer among the higher-exposure groups in the studies reviewed. Source: Environmental Health Perspectives study summary.
A key point for readers: “relative risk” does not mean that one in three people will get bladder cancer from chlorinated water. It means that, in the studies reviewed, the higher-exposure group had a higher risk compared with the lower-exposure group. Individual risk depends on many factors, including age, genetics, smoking history, occupational exposures, diet, and overall water quality.
Still, because drinking water exposure can happen daily over many years, even modest increases in relative risk can matter at the population level.
Can home filtration reduce THMs?
Yes, the right filtration technology can help reduce THMs.
Granular activated carbon, or GAC, is widely used to reduce chlorine, taste and odor issues, and many organic compounds. The Water Quality Association describes granular activated carbon as useful for reducing small organic compounds and disinfection byproducts. You can read more here: Water Quality Association: Granular Activated Carbon.
For homeowners, this is where point-of-use and whole-home filtration can play a meaningful role. A properly selected and maintained carbon-based filter can help reduce THMs after water has already been disinfected by the utility. That means families can benefit from municipal disinfection while adding another layer of protection at the tap.
What to look for in a filtration system
Not every filter is designed for the same contaminants. A basic taste-and-odor filter may reduce chlorine smell, but that does not automatically mean it is certified for health-related contaminant reduction.
When evaluating a filtration system, look for:
Activated carbon or granular activated carbon media
Carbon is commonly used to adsorb organic compounds, including many disinfection byproducts.
Third-party certification
Look for systems that have been independently tested for the contaminants you want to reduce. NSF/ANSI standards are commonly used for drinking water treatment products. You can search certified drinking water treatment units through NSF here: NSF Certified Drinking Water Treatment Units.
Proper sizing and contact time
Carbon needs enough contact time with water to work effectively. Flow rate, filter size, and water chemistry all matter.
Routine filter replacement
Activated carbon filters do not last forever. Once the media is exhausted, performance declines. Following the manufacturer’s replacement schedule is essential.
Water testing or local water quality reports
Your annual Consumer Confidence Report can show whether your utility detected TTHMs and at what levels. The CDC notes that consumers can learn what disinfectant is used and how well a utility follows disinfection rules by reviewing the utility’s Consumer Confidence Report. Source: CDC: About Water Disinfection with Chlorine and Chloramine.
A balanced approach to safer water
At Aquamor LLC, we believe the conversation should not be “chlorine versus no chlorine.” Disinfection is essential. The safer and more practical question is: How do we preserve the benefits of disinfection while reducing the byproducts left behind?
For many homes, the answer is a well-designed filtration system using granular activated carbon or a multi-stage approach. These systems can help reduce THMs and other unwanted compounds while allowing municipal treatment to continue doing its primary job: protecting the public from waterborne disease.
What you can do next
Start by reviewing your local water quality report and looking for TTHM or total trihalomethanes. If you are unsure what the numbers mean, or whether your current filter is designed to reduce these byproducts, ask a water filtration professional.
Clean water is not only about what you can see, taste, or smell. It is also about understanding what may be present in small amounts over many years.
At Aquamor LLC, we help families make informed decisions about filtration, water quality, and peace of mind. If you have questions about THMs, chlorinated water, or which filtration system is right for your home, we would be happy to help.
