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 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
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.
Source: EPA — Stage 1 & Stage 2 Disinfectants and Disinfection Byproducts Rules (retrieved 2026-07-27)
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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Upload my testHow to Remove Total THMs
How do I get rid of Total THMs in drinking water?
For Total THMs, the verified treatment is activated carbon. Activated carbon is a conditional treatment. Requires an NSF/ANSI 53 TTHM-certified carbon and adequate contact time; carbon adsorbs TTHM but breaks through over time. EPA's municipal BAT is GAC10/GAC20 with enhanced coagulation or softening.
Source: NSF/ANSI 53 (TTHM reduction) / EPA Stage 1 & 2 D/DBP Rule, 40 CFR 141.64
What filter certification covers Total THMs?
Activated carbon is certified under NSF/ANSI 53. Check that the certification listing names Total THMs or its group: a certification to a standard is not a certification for every contaminant in it.
Source: NSF/ANSI 53 (TTHM reduction) / EPA Stage 1 & 2 D/DBP Rule, 40 CFR 141.64
Does a basic water filter pitcher remove Total THMs?
Only if it is certified for it. A basic pitcher is an activated-carbon device and carbon does adsorb Total THMs, but only a pitcher certified under NSF/ANSI 53 for this reduction has been tested to do it, and the cartridge breaks through as it loads — so replacement on schedule is part of the answer. An NSF/ANSI 42 certification, which most pitchers carry, covers taste and odor only and says nothing about Total THMs.
Source: NSF/ANSI 53 (TTHM reduction) / EPA Stage 1 & 2 D/DBP Rule, 40 CFR 141.64
Does boiling water remove Total THMs?
Mostly, from the water you drink — and this is the half of the disinfection-byproduct group where that is true. Trihalomethanes are volatile, so heating drives Total THMs out of the water rather than destroying it: EPA measured volatilization losses approaching 75% when water was boiled even for brief periods, and 90% when the boiled water was poured and served. It leaves as vapour into the room, which is why the cited EPA work is about ingestion exposure — it lowers what you swallow, not what the household breathes. Open containers lost more than closed ones, because a lid traps the vapour. None of this is advice to boil your water: boiling does nothing for the non-volatile half of this group and can raise haloacetic acids where residual chlorine is present, so it is no treatment for disinfection byproducts as a group.
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Is Total THMs in your water?
Total THMs has been recorded above its legal limit in 57 cities, serving 0.1M people. Each system below was judged against a limit of 80 PPB.
Most affected systems, by population served
- Lemoore, CA26,093 people served110 PPBlimit 80 PPB
- Duncan, OK23,000 people served86 PPBlimit 80 PPB
- Huron, CA9,306 people served100 PPBlimit 80 PPB
- Liberty, OK8,370 people served109 PPBlimit 80 PPB
- Mounds, OK8,370 people served109 PPBlimit 80 PPB
- Winchester, OK8,370 people served109 PPBlimit 80 PPB
- White Cottage, OH6,540 people served80.2 PPBlimit 80 PPB
- Apalachicola, FL4,500 people served85 PPBlimit 80 PPB
- Alderson, OK4,306 people served83.8 PPBlimit 80 PPB
- Aubrey, AR4,240 people served89.2 PPBlimit 80 PPB
and 47 more cities with a recorded result above the guideline
These are the largest systems that reported a result above the guideline — not the highest results. A city missing from this list has not been shown to be clean: it may have no Total THMs result on file at all. We publish a level only where a system reported one.
Check your city or ZIP code for Total THMs
Part of a group: disinfection byproducts
This contaminant is rarely measured on its own. Disinfection byproducts in drinking water covers the whole group — what they are, which limits are enforceable, and where they come from.
Others in this group
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 THMs is classified as a Disinfection Byproducts contaminant.
Related Disinfection Byproducts Contaminants
Check Your Water
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