Total THMs
Total trihalomethanes (TTHM) are the sum of four byproducts — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — formed when chlorine reacts with organic matter. The EPA limits TTHM to 80 ppb (0.080 mg/L).
EPA MCLG Level
0 mg/L
Maximum level that poses minimal health risk based on the latest science
Health Effects
Long-term exposure to trihalomethanes is associated with an increased risk of bladder cancer at the population level (about 8,000 of 79,000 annual US cases, EPA estimate), with possible liver, kidney, and central-nervous-system effects at high doses. The risk is chronic and lifetime, not acute, and chlorination prevents far more illness than its byproducts are estimated to cause.
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
Is your Total THMs result safe? Get a free reading
The EPA MCLG for Total THMs is 0 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.
Your upload also helps us keep local water data accurate — we only ever share anonymized, area-level summaries.
Upload my testHow to Remove It
Water filters certified under the following NSF standards are effective at removing Total THMs:
Get Total THMs Updates
Research updates, regulation changes, and filter recommendations — in your inbox.
Free forever. Unsubscribe anytime. We never share your email.
Where is Total THMs Found?
Total THMs has been detected above EPA health guidelines in 1,367 cities serving 30.5M people across the US.
and 1,362 more cities
EPA MCLG Level
The EPA MCLG represents the maximum level that poses minimal health risk based on the latest scientific research. It's often more protective than federal legal limits.
Contaminant Type
Total THMs is classified as a Disinfection Byproducts contaminant.
Related Disinfection Byproducts Contaminants
Check Your Water
Find out if Total THMs is in your tap water.
Search cities for Total THMs dataTest Your Water for This Contaminant
Public water reports may not test frequently enough or at your specific tap. Professional home testing provides current, location-specific results.
SimpleLab
Standard Home Water Test
$232
Comprehensive water analysis testing over 200 contaminants including bacteria, heavy metals, and chemical compounds.
SimpleLab
Advanced Home Water Test
$369
Most comprehensive home water test including all standard tests plus additional parameters for ultimate peace of mind.
Tap Score
Advanced Microplastics Test
$636
Cutting-edge testing for microplastics particles in drinking water using advanced laboratory techniques.
Stay Informed About Your Water Quality
Get EPA reports, filter recommendations, and safety alerts for your area.
Join 10,000+ people protecting their families. Unsubscribe anytime.