Total Trihalomethanes (TTHMs)
TTHM is the sum of four trihalomethanes — chloroform, bromodichloromethane, dibromochloromethane and bromoform — formed when chlorine reacts with organic matter. Unlike the haloacetic acids they are volatile, so they are an inhalation concern in the shower as well as an ingestion concern. The EPA limits TTHM to 80 ppb (0.080 mg/L), measured as a running annual average for the water system rather than as a single-sample ceiling, so an individual reading above 80 ppb is not by itself a violation.
EPA MCL
0.08 mg/L
EPA has not set a health goal (MCLG) for this contaminant. This is the enforceable limit water systems must meet.
Health Effects
EPA lists the long-term effects of exposure above the limit as liver, kidney or central nervous system problems and an increased risk of cancer. The cancer signal in the epidemiology is strongest for bladder cancer at the population level. The risk is chronic and lifetime rather than acute.
Source: EPA — National Primary Drinking Water Regulations — Disinfection Byproducts (retrieved 2026-07-30)
Common Sources
- Byproduct of disinfecting drinking water with chlorine
- Formed when chlorine reacts with natural organic matter such as decaying leaves, runoff, and algae
- Levels are highest in warm-season surface water
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The EPA MCL for Total Trihalomethanes (TTHMs) is 0.08 mg/L. Upload your water test (PDF or a photo) and we'll email a plain-English reading of where your result lands — and every other number — free. Educational only, no medical claims.
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Water filters certified under the following NSF standards are effective at removing Total Trihalomethanes (TTHMs):
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About the EPA MCL
This is the enforceable federal limit water systems must meet. EPA has not set a separate non-enforceable health goal (MCLG) for this contaminant.
Contaminant Type
Total Trihalomethanes (TTHMs) is classified as a Disinfection Byproducts contaminant.
Related Disinfection Byproducts Contaminants
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