What Total Trihalomethanes (TTHM) are, where they come from, and how they compare to Haloacetic Acids (HAA5) and chloroform.
Data current as of July 2026
🔍Straight from EPA
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Total Trihalomethanes (TTHM) are a group of disinfection byproducts formed when chlorine used to disinfect drinking water reacts with naturally occurring organic matter. They show up on almost every water quality report that uses chlorine disinfection, which is most public water systems in the US.
What's in the TTHM group
Total Trihalomethanes (TTHM)
A group of disinfection byproducts formed when chlorine reacts with organic matter. Long-term exposure above the legal limit has been linked to increased cancer risk in some studies.
Haloacetic Acids (HAA5)
A group of disinfection byproducts formed similarly to TTHM, regulated together as a combined measurement.
Chloroform
One of the individual trihalomethane disinfection byproducts, formed when chlorine reacts with organic matter.
Filtration
Disinfection byproducts are one of the categories a standard activated carbon filter handles well — whole-house carbon, under-sink reverse osmosis, and under-sink carbon systems are all generally effective against them, provided the specific filter is certified for the substance. See the comparison table on any water quality report for how a given area's actual results line up.
Live in this area? Simple Water Heater & Filtration can size a filtration system to whatever's actually on file for your utility.