Across the nation, municipal water utilities treat and filter their water supplies to make them safe for consumption. This disinfection is essential, eliminating pathogens that would cause cholera, typhoid, dysentery and other dangerous illnesses. The downside of this disinfection is, however, the production of small quantities of toxic by-products called trihalomethanes (THMs). These compounds are so common that they show up in city water systems across the United States and beyond. Growing scientific evidence points to health concerns that can result from drinking water with THMs.
What Are Trihalomethanes and How Do They Form?
Trihalomethanes are toxic substances formed when chlorine or similar disinfectants react with natural organic matter like decaying leaves, algae, or soil in the water supply. The four most common THMs regulated by the Environmental Protection Agency (EPA) are:
- Chloroform
- Bromodichloromethane
- Dibromochloromethane
- Bromoform
With the current methods of disinfection available to municipalities, the production of THMs is unavoidable. Anyone who drinks this water deserves to know the THM levels in their local water supply. It is also essential to understand what the health effects of THMs are.
Health Risks Linked to Trihalomethanes
The Environmental Protection Agency has set a limit of Total THMs (TTHMs—the sum of all four THMs) at 80 parts per billion. This limit was set because long-term exposure to high levels of THMs has been linked to cancers of the bladder, colon, and rectum. Studies have shown that people consuming water with higher THM levels face a measurable increase in cancer incidence compared to populations with lower exposure.
But cancer is not the only concern. Consumption of THMs has also been associated with:
- Reproductive issues: Some research has linked exposure to higher rates of miscarriage and developmental problems during pregnancy.
- Liver, kidney, and central nervous system damage: Animal studies show that chronic exposure to certain THMs can damage vital organs and impair nervous system function.
- Gut microbiome disruption: Recent science has revealed how vital the microbiome is to immune function, digestion, and overall health. THMs, because they are chemical disinfectants by nature, can alter the delicate balance of gut bacteria. Over time, this disruption may weaken the immune system and increase susceptibility to disease.
When a person is exposed to THMs in their water, the exposure typically goes on for years. While these chemicals do not accumulate in the body, the damage comes from this chronic, low-level exposure.
Other Methods of Exposure to THMs
One of the less obvious ways people are exposed to THMs is through inhalation and skin absorption. These chemicals are volatile, meaning they can escape into the air. When hot water is used in the shower, during dishwashing, or even while boiling water for cooking, THMs vaporize and are breathed in. This route of exposure can sometimes deliver as much or more of the chemical into the body as drinking the water itself.
That means your morning shower or nightly bath could be a significant source of exposure—an important point to consider when thinking about how to reduce risks.
How Common Are THMs in Municipal Water Supplies?
THMs are among the most widespread contaminants found in public water supplies. While the EPA sets a maximum contaminant level for total trihalomethanes or TTHMs (80 parts per billion), compliance does not mean that levels are always low. A 2025 report from the Environmental Working Group noted that more than 700 water systems had TTHM levels above EPA limits. These cities include Bethlehem, New York; Seminole County, Florida; and Boerne, Texas.
Even when water utilities maintain TTHM levels below 80 parts per billion, the long-term cumulative exposure is still a concern, especially for children, pregnant women, and the elderly.
Reducing Your Exposure to TTHMs
There are methods available to you to reduce the trihalomethanes your family is exposed to. These filtration systems can reduce the TTHMs present in your household water:
- Activated carbon filters: These are highly effective at removing THMs and other volatile organic compounds. Whole-house systems can protect not only your drinking water but also your shower and bath water.
- Aeration systems: Because THMs are volatile, a treatment system that exposes your water to air before it reaches your tap can remove these substances.
- Point-of-use filters: Faucet or pitcher filters using granular activated carbon can significantly reduce THMs in drinking water.
Before investing in filtration, it is essential to know what you are dealing with. That’s why you should get a professional water test done before installing any kind of filtration system.
The Importance of Professional Water Testing
Professional water testing laboratories like ETR Laboratories can tell you how much of each chemical is present. Then, if the levels exceed EPA standards or you want to reduce them to extremely low levels, you’ll know and can install the right system to meet your goal.
ETR Laboratories provides fast, accurate results from our sophisticated laboratory. We provide thousands of water tests every month for homeowners, municipalities and businesses. The Premium Water Test from ETR Laboratories provides levels on all four THMs and much, much more. Order the Premium water test today!
