Disinfection Byproducts: TTHMs

Total Trihalomethanes (TTHMs) in MN Community Water Systems:

Disinfecting water helps prevent disease

Many community water systems (CWS's) use disinfectants in drinking water. Microbes such as giardia and cryptosporidium can cause gastrointestinal illness. When these disinfectants react with organic material (like decomposing plants), they form disinfection byproducts (DBPs). People may be exposed to DBPs when using tap water for drinking, cooking, bathing, or swimming. DBPs are not normally found in private well water.

High levels of DBPs may be harmful to health

Some studies suggest that they might increase the risk of cancer and adverse pregnancy outcomes. However, the risk of not disinfecting drinking water (and exposing people to microbes that can cause gastrointestinal illnesses) far outweighs the risk of DBPs - particularly at the low levels typically found in US water supplies. 

All community water systems that disinfect their water (generally all surface water systems and some groundwater systems) are required to monitor for disinfection byproducts. The DBPs that are tracked are trihalomethanes (TTHMs) and haloacetic acid (HAA5). The Maximum Contaminant Level (MCL) for total trihalomethanes (TTHMs) is 80 µg/L.


Mean TTHM levels of Minnesota Community Water Systems

Percent of tested systems by mean TTHMs concentration
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

Tested systems by maximum TTHMs concentration
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.


Most drink water with TTHMs below levels of concern

The table below shows the number of people served by community water systems by mean concentration of TTHMs. Note that the estimates of number of people served by each system are rough calculations periodically updated by water system managers.

Population Served

Number of people served, by mean annual TTHM concentration for Minnesota Community Water Systems, 2015
TTHM Systems Population served Percent
0 - 20 526 2,600,350 70.2%
20+ - 40 172 1,367,871 23.0%
40+ - 60 39 262,614 5.2%
60+ - 80 8 5,509 1.1%
80+ - 100 2 1,985 0.3%
100+ 2 1,497 0.3%

How do you find out about DBPs in your water?

Customers of community water systems can find out the DBP levels, if any, in their drinking water by reading the Consumer Confidence Report issued each year by their water utility. For those wishing to take extra precautions, granulated activated carbon (GAC) filters are effective in lowering DBP levels in drinking water, and several types of GAC filters are available for home use.

For more information on DBP regulations and health effects, see Disinfection Byproducts.

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