Trihalomethanes (TTHM’s) in Drinking Water | What Washington & Idaho Homeowners Should Know

Total trihalomethanes (TTHM’s) come from disinfectant byproducts (DBPs). The Mountain Empire Community College (MECC) found in a study that “THMs [trihalomethanes] and HAAs [haloacetic acids] are most often found in chlorinated drinking water.”

There are four types of trihalomethanes:

• Chloroform
• Dibromochloromethane
• Bromoform
• Bromodichloromethane

Disinfectant Byproducts (DBPs) in Drinking Water

When the city disinfects municipal water, the intention is to prevent microbial diseases. According to the Environmental Working Group (EWG), “when chlorine and other disinfectants react with animal waste, plant matter, and other organic materials in drinking water supplies, they can form harmful contaminants, known collectively as disinfection byproducts.”

Potential Health Effects of Consuming Disinfectant Byproducts

The MECC found that:

• Those who drink chlorinated surface water have a higher risk of developing cancer of the bladder, rectum, and colon.
  
• Chlorinated drinking water may contribute to the risk of birth defects of the brain and spinal cord, low birthweight babies, premature births, and miscarriages.
  
• Exposure to very high levels of certain DBPs resulted in kidney and liver damage, reproductive effects, and cancer in animal studies.

Testing for Total Trihalomethanes

If you’re concerned you may have total trihalomethanes in your water, have your water tested. We offer complimentary in-home water testing for homeowners in Eastern Washington and Northern Idaho. You can also order certified mail-in water tests here.

How to Filter Dysinfectant Byproducts

When looking for a way to filter disinfectant byproducts, look for a point-of-entry or a point-of-use filter (MECC):

• Point-of-entry filter: This filter starts at the entrance of your home and filters the water throughout the whole house. This means your plumbing, appliances, showerheads, sinks, and more. They’re the most effective at filtering DBPs from your home’s water. See point-of-entry filters.

• Point-of-use filter: An under-sink drinking water system. These specifically filter the water you consume. Point-of-use systems filter more contaminants than point-of-entry filters because they’re designed for drinking water. See point-of-use filters.

SOURCES

Environmental Working Group (EWG) “EWG’s Tap Water Database: Contaminants in Your Water.” Ewg.org, 2021, www.ewg.org/tapwater/reviewed-disinfection-byproducts.php.

Mountain Empire Community College (MECC) “Disinfectant Byproducts (THM) in Drinking Water.” Mecc.edu, 2026, water.mecc.edu/exam_prep/THM.html.

Published on: 2/4/2026

Last updated on: 2/4/2026