TTHM in Drinking Water: Information for Consumers

Information about Total Trihalomethanes in Drinking Water (Rev. Jan. 2018)

What are TTHM?

Total trihalomethanes (TTHM) are a group of disinfection byproducts that form when chlorine compounds that are used to disinfect water react with other naturally occurring chemicals in the water.  They are colorless, and will evaporate out of the water into the air.  There are four significant TTHM potentially found in disinfected drinking water and their combined concentration is referred to as total TTHM.

Levels of TTHM generally increase in the summer months due to the warmer temperatures, but can also be affected by seasonal changes in source water quality or by changing amounts of disinfection added.  Water systems often can experience temporary increases in TTHM due to short-term increases in chlorine disinfection.  Chlorine disinfection increases can occur when there is a water main break, when water systems are under repair, or when there is a potential microbial (example:  bacteria) problem or threat.

All water systems that use chlorine to disinfect the water are required by federal and state law to sample for TTHM on a regular basis (quarterly, or once every three months).

Why is chlorine added?

Chlorine is used to disinfect drinking water.  Disinfection of water supplies is necessary to prevent illness from waterborne disease causing bacteria; it is a federal and state requirement.  The practice of disinfection has nearly eliminated most acute waterborne diseases in the United States.

Disinfection of the water first kills any microorganisms that it may contain.  Then, a small amount of disinfectant is needed in the water as it travels through the pipes in the distribution system.  This prevents regrowth of microorganisms, or contamination from an outside source, such as during a water main break. 

What is the Drinking Water Standard for TTHM and how is compliance determined?

Drinking water standards are called maximum contaminant level (MCLs).  MCLs are set to limit risks to people from chemicals in drinking water.  Some MCLs limit the daily amount consumed (for chemicals that pose an immediate risk), and some limit the amount averaged over a long period of time (for chemicals that pose a long-term risk).  The TTHM MCL is set at a level that balances the immediate risk of bacterial contamination should the water not be adequately disinfected and the long-term risk of health effects, such as cancer, potentially associated with long term exposures to TTHM.  The USEPA and MassDEP have set an MCL for TTHM of 80 parts per billion (ppb) or micrograms per liter (ug/L) as an annual average.  Federal and state compliance with the MCL requires that the running annual average of four samples (i.e., quarterly, or once every three months over a year) not exceed the MCL at each sampling location.

How can consumers be exposed to TTHM in drinking water?

People may be exposed to TTHM in drinking water from ingestion (i.e., drinking the water and ingesting it in foods and/or ice prepared with the water).  In addition, TTHM vaporize readily into the air so inhalation exposure to TTHM can be significant, especially when showering and bathing, as can exposure from absorption through the skin.

What are the health risks associated with using water containing TTHM?

The MCL for TTHM is based on potential cancer risks following a lifetime of drinking the water.  TTHM are considered to be possibly carcinogenic to humans by USEPA because of evidence of carcinogenicity in experimental laboratory animals and limited evidence in people.  Some of the individual chemicals that comprise TTHM have also caused other effects in experimental laboratory animals following high levels of exposure, including toxicity to the liver, kidneys, neurological and reproductive systems.  Various adverse reproductive and developmental effects have been observed in experimental laboratory animals following exposure to disinfection byproducts (which include TTHM).  In some, but not all, studies in people, similar effects have also been reported.  In general, young children may be more susceptible to the effects from any chemical exposure, such as TTHM, because their ability to metabolize chemicals is not mature and because their exposures may be greater for their size than in adults.  More research is being conducted to better understand the potential risks from using water containing TTHM.

Based on the available information, long term consumption of TTHM in drinking water above the MCL may increase the risk of certain types of cancer (e.g., bladder, colon, and rectal) and other adverse effects in some people.  The degree of risk for these effects will depend on the TTHM level and the duration of exposure.  Consumption of water with TTHM levels somewhat above the MCL for limited durations, for example, while corrective actions are being taken to lower the levels, is not likely to significantly increase risks of adverse health effects for most people.  Because some data indicate that disinfection byproducts may increase the risk of developmental effects, women who are pregnant or may become pregnant may wish to avoid consuming water containing TTHM and other disinfection byproducts exceeding the drinking water standard.

If you are concerned and would like to reduce your exposure to TTHM, what can you do?

If you are concerned about TTHM and want to reduce your exposure, you can do the following:

  • Use bottled water.  Bottle water sold in Massachusetts must meet all federal drinking water quality standards and, if originating in Massachusetts, must also meet state drinking water quality requirements.  or
  • Use water filters (e.g., a pitcher style or a point of use treatment filter that can be mounted to the faucet, under the sink or on the counter top) or install a point-of-entry whole-house filtration system.  Any filter that is purchased should be certified by National Sanitation Foundation (NSF), Underwriters Laboratories (UL) or the Water Quality Association (WQA) to remove TTHM (look for the seals on the box.  For information on selecting a water treatment system that’s right for you, visit NSF international at or call their hotline at 1-800-673-8010.
  • To reduce overall TTHM exposure risk:
    • Ventilate the bathroom when bathing or showering;
    • Operate room exhaust fans or ventilate room (open window) when boiling water, washing with hot water or running the dishwasher;
    • Reduce the length of showers and baths;
    • Reduce the temperature on hot water heaters; and
    • Limit time spent in or around chlorinated pools or hot tubs.

Where can I get additional information?

If you have concerns about your exposure, particularly pregnant women and women of child-bearing age who may be at increased risk, you may wish to seek the advice of your health care provider.