Disinfection byproducts: TTHMs

Total trihalomethanes (TTHMs) in Minnesota community water systems:

Disinfecting water helps prevent disease

Public water systems play an essential role in protecting public health through treatment and disinfection processes. The most common method of disinfection is through the addition of chlorine to drinking water supplies. Chlorine effectively kills waterborne bacteria and viruses and continues to keep the water safe as it travels from the treatment plant to the consumer's tap.

Disinfection makes our water safer to drink, and we do not have to worry about the waterborne diseases of the past. Both the World Health Organization and the Centers for Disease Control and Prevention regard disinfection of drinking water as one of the most important advances in public health.

What are disinfection byproducts (DBPs)?

Although chlorine has been a literal lifesaver with regard to drinking water, it also has the potential to form byproducts that can cause harmful health effects. Chlorine can react with organic materials in water to form disinfection byproducts (DBPs).
The formation of DBPs is usually a greater concern for water systems that use surface water, such as rivers, lakes, and streams, as their source. Surface water sources are more likely to contain the organic materials that combine with chlorine to form DBPs.

Health effects of TTHMs

Some people who drink water containing total trihalomethanes in excess of the MCL over many years could experience liver, kidney, or central nervous system problems and increased risk of cancer.

The risk of not disinfecting drinking water—and exposing people to microorganisms that can cause illnesses—outweighs the long-term, low level risk of DBPs, particularly at the low levels typically found in U.S. water supplies. Water systems review their operations to minimize TTHM formation without compromising public health protection from disinfection.

Standard and guidance for TTHMs in community water systems

All community water systems that disinfect test for TTHMs and ensure levels meet the Environmental Protection Agency drinking water standard, or Maximum Contaminant Level (MCL). The MCL for TTHMs is 80 µg/L (one µg/L equals one part per billion). This MCL is for the four TTHMs added together.

The Minnesota Department of Health sets health-based guidance values for some DBPs, including three of the four TTHMs: bromodichloromethane, bromoform, and chloroform. These values are protective for the most sensitive and/or highly exposed populations. Using limited resources to treat drinking water to meet health-based guidance values may not always provide the greatest public health benefit for a community. Guidance values are used to make decisions about managing the health risks of contaminants found in groundwater used as a source of drinking water. To learn more, visit Guidance Values and Standards for Contaminants in Drinking Water.

Most community water systems provide drinking water with TTHMs below the standard

Mean TTHM levels of Minnesota community water systems

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The MCL is 80 µg/L. Concentrations are the mean annual levels for community water systems in Minnesota.
The MCL is 80 µg/L. Concentrations are the mean annual levels for community water systems in Minnesota.

The graph above shows the percent of community water systems by TTHMs annual mean concentration. 

Maximum TTHM levels of Minnesota Community Water Systems

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The MCL is 80 µg/L. Concentrations are the maximum annual levels for community water systems in Minnesota.
The MCL is 80 µg/L. Concentrations are the maximum annual levels for community water systems in Minnesota.

The chart above shows the percent of systems by annual maximum concentration category. A system may have a number of different locations in the distribution system with different levels of TTHMs. In this chart, the maximum value reflects the sample with the highest concentration at a single location. The level of TTHMs in the water that ultimately reaches a consumer's tap might be different.


Population served by community water system mean TTHM concentration, 2022

TTHM Systems Population served Percent
0 - 20 118 936,490 99.5%
20+ - 40 60 4,841 0.50%
40+ - 60 0 0 0.0%
60+ - 80 0 0 0.0%
80+ - 100 0 0 0.0%
100+ 0 0 0.0%


What can be done about TTHMs in drinking water?

All public water systems that disinfect must regularly test their treated water to determine if regulated DBPs are present and at what levels. If they are above the limits set by EPA, the water system must take action to reduce the DBPs. Actions could include adjustments to organics removal processes, disinfection dose and location, and distribution system management. The water system must also notify all of their customers of the DBP levels.

You can find the level of TTHMs in your community water system detected by reading their Water Quality Report (also known as a Consumer Confidence Report [CCR]). Call your community water system to get a copy of your CCR, or find it online at Search for Your Consumer Confidence Report (CCR).

If you want to take additional steps to reduce your exposure to DBPs in drinking water, you can use a home water treatment system. For more information on home water treatment, visit Home Water Treatment.

For more information on DBPs, see Disinfection and Disinfection Byproducts

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