Total Trihalomethanes (TTHM) in Drinking Water

EPA Limit: 0.080 mg/L (annual average)

TTHMs are regulated under the Stage 2 Disinfectants and Disinfection Byproducts Rule. The MCL of 0.080 mg/L is based on a locational running annual average, meaning each monitoring location must individually comply.

Where Does Total Trihalomethanes (TTHM) Come From?

Trihalomethanes form when chlorine used to disinfect drinking water reacts with natural organic matter (decaying leaves, algae) in the source water. They are disinfection byproducts, not contaminants from the source itself. Systems using surface water with high organic content tend to produce more TTHMs.

Health Risks

Long-term exposure to elevated TTHM levels is associated with increased risk of bladder cancer and may affect liver, kidney, and central nervous system function. Some studies suggest possible reproductive effects.

What Should I Do?

Activated carbon filters (including pitcher filters) can reduce TTHM levels. Letting water sit uncovered or aerating it allows some THMs to dissipate. However, do not stop drinking tap water — the risks from disinfection byproducts are much lower than the risks from waterborne diseases if disinfection were stopped.