Total Trihalomethanes (TTHMs) in tap water

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States Affected
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People Affected
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Toxicity Rating
Health Effects

Associated with various types of cancer as well as reproductive and developmental issues

The legal limit is
Health Guideline
0.15 ppb
what is considered unhealthy
Legal Limit
80 ppb
What are trihalomethanes?
Trihalomethanes are dangerous chemicals, part of a larger family of chemicals known as disinfection byproducts (DBPs), that form when chlorine and other disinfectants interact with water. TTHMs refers to the combined concentration of the following four trihalomethanes:
  • Chloroform (Trichloromethane).
  • Bromodichloromethane.
  • Bromoform (Tribromomethane).
  • Dibromochloromethane.

Trihalomethanes can be ingested, inhaled, or absorbed through the skin. TTHMs are associated with various types of cancer as well as reproductive and developmental issues. The greatest exposure to TTHMs tends to come from drinking water.
What are disinfection byproducts (DBPs)?
DBPs are the result of chemical reactions that occur when water interacts with disinfection agents such as chlorine. Hundreds of DBPs exist, but trihalomethanes (THMMs), haloacetic acids (HAAs), chlorite, and bromate are common DBPs associated with tap water.
DBPs vary based on a variety of factors including water quality as well as the disinfectants used to treat water. The U.S. EPA has set Maximum Contaminant Levels (MCLs) for TTHMs, HAA5, chlorite, and bromate to regulate the volume of these DBPs found in U.S. drinking water supplies. The U.S. Food and Drug Administration (FDA) set allowable levels of TTHMs, HAA5, chlorite, and bromate to regulate the volume of these DBPs found in bottled water.
How do trihalomethanes get into drinking water?
Trihalomethanes are produced when chlorine and other disinfectant agents interact with water supplies being treated for public consumption. 98% of tap water supplies in the U.S. are treated with chlorine. The amount of chlorine used constantly changes in response to new and emerging contaminants, damaged and aging systems, and natural disasters like flooding and fires. This change in chlorine levels can increase TTHMs. Plus, as other disinfectants are added, removed, and adjusted, TTHMs can also increase.
Are trihalomethanes dangerous in water?
Yes. Long-term exposure to elevated levels of TTHMs is associated with an increased risk of cancer—including bladder cancer, colon cancer, and rectal cancer—as well as reproductive and developmental issues. TTHMs has also been linked to pregnancy-related issues such as miscarriage, low birth weight, and specific birth defects known as neural tube defects (NTDs).
Are trihalomethanes carcinogenic?
Two trihalomethanes—chloroform (trichloromethane) and bromodichloromethane—are classified as “possibly carcinogenic to humans” by the International Agency for Research on Cancer (IARC). Plus, the latest ‘Report on Carcinogens’ from the U.S. Department of Health and Human Services labeled both chemicals as “reasonably anticipated to be human carcinogens.” In other words, multiple organizations believe these chemicals could have the potential to cause cancer in humans.
How much TTHMs is safe?
The U.S. EPA set a Maximum Contaminant Level (MCL) of .080 mg/L, or 80 parts-per-billion (ppb), for Total Trihalomethanes (TTHMs). MCL is measured as an annual average. Therefore, TTHMs can exceed the MCL at any given time.
Are trihalomethanes in bottled water?
Yes, trihalomethanes can be found in bottled water. While the EPA regulates tap water, the FDA regulates bottled water because it is classified as food. The FDA allows .080 mg/L of TTHMs in bottled water, matching the regulations set for tap water by the EPA.
Does boiling water remove trihalomethanes?
It’s unclear if boiling water eliminates the four trihalomethanes that make up TTHMs. To protect yourself, we recommend getting a water filter certified to remove TTHMs.
Can water filters remove trihalomethanes from tap water?
Clearly Filtered’s breakthrough Affinity® Filtration Technology removes up to 99.9% of TTHMs from your tap water. Upgrade to our superior water filters so you never have to worry about TTHMs lurking in your water again. Remember, few carbon and charcoal filters remove TTHMs.
How does Clearly Filtered do at Removing Total Trihalomethanes (TTHMs) in tap water?
Clearly Filtered with
Affinity Filtration Technology Product
Water Pitcher Filter
Under-the-Sink FIlter
Water Bottle Filter
Refrigerator Filter
Total Trihalomethanes (TTHMs) in tap water Removal

1.EWG Tap Water Database. Total trihalomethanes (TTHMs). Accessed April 1, 2021.

2. CDC. Biomonitoring Summary: Disinfection By-Products (Trihalomethanes). April 1, 2021.

3. U.S. EPA. National Primary Drinking Water Regulations. April 1, 2021.

4. Massachusetts Department of Environmental Protection. TTHM in Drinking Water: Information for Consumers.,the%20water%20into%20the%20airAccessed April 1, 2021.

5. WHO. Trihalomethanes in Drinking-water. April 1, 2021.

6. IARC. Monographs On The Identification of Carcinogenic Hazards to Humans: List of Classifications. April 1, 2021.

7. NTP. Report on Carcinogens (ROC), Fourteenth Edition: Chloroform. Accessed April 1, 2021.

8. NTP. Report on Carcinogens (ROC), Fourteenth Edition: Bromodichloromethane. April 1, 2021.

9. U.S. FDA. Small Entity Compliance Guide: Bottled Water and Residual Disinfectants and Disinfection Byproducts. April 1, 2021.