Contaminant

Total trihalomethanes in drinking water

Also listed as TTHM · TTHMs · Trihalomethanes · Total Trihalomethanes

We haven’t published a full explainer for Total trihalomethanes yet, so this page only shows what we can stand behind and where to go next.

This page can’t tell you what’s in your tap water.

Only a test of water from your own tap can. Public-system results describe the system, not your home.

3 sources

01Check your water systemWater Check · system results, not your tap 02Test your tapHow to get a reliable result 03Treatment approachesWhat can reduce it at the tap

On this page · 9 sections

01Reference values
02Is it in my water?
03Where it comes from
04Testing
05Water filters
06Evidence
07Next steps
08Ask about total trihalomethanes
09References

Reference values

An enforceable limit for public water systems.

Each value says what kind of reference it is, and they mean different things. A lower or missing number never means water is safe.

Enforceable limit · MCLFederal · public water systems

80 ppb

The highest level allowed in water a public system delivers. A legal limit, not a health goal.

Applies to: Water delivered by public water systems, not private wells
Status: In force
Source: EPA NPDWR MCL (0.080 mg/L) · 40 CFR §141.64

Your stateNot held yetStates may set their own references. We don’t hold a state reference for total trihalomethanes yet.

Is it in my water?

Only a test of your own tap can tell.

Choose what you have. We’ll show what it can and can’t tell you.

If you haven’t tested

Nobody can tell without a measurement, including us.

Three things you can do, in this order.

Test water from your tapThis is the only way to know your level. Testing guidance ↓See what your system reportsSystem results aren’t a tap measurement. Open Water Check →On a private well?Public-system reports don’t cover wells. Open Well Water →

Where it comes from

Form when water disinfectants such as chlorine or ozone react with naturally occurring chemicals in the water
Levels can increase temporarily due to short-term increases in disinfectants during water main breaks or system repairs
Seasonal changes in source water quality can affect disinfectant byproduct levels

Why it matters

Possibly carcinogenic to humans based on evidence of carcinogenicity in laboratory animals and limited evidence in people

Inhalation and skin absorption can be significant, especially during showering and bathing, because these chemicals readily vaporize into the air · Effects on the liver, kidneys, and reproductive system have been reported in experimental animals exposed to high levels · Children and infants are generally more sensitive to effects than others and typically consume more water for their size than adults

Testing

How to test your tap water for total trihalomethanes

Water systems that disinfect with chlorine or ozone are required to sample for total trihalomethanes quarterly and report a running annual average of four samples

01Use a certified labA state-certified lab, or a kit that sends samples to one.
02Follow the sampling stepsHow and when you collect the water changes the result.
03Check the reportIt should state units and the detection limit.

Certified labNot sponsored

Use a state-certified labYour state keeps a list of labs certified for drinking water.

Choosing a filter for total trihalomethanes

None qualify

No products currently qualify for a total trihalomethanes pick here.

Nothing in our database has a total trihalomethanes claim for an exact configuration we can verify.

Find a fit in Finder Read treatment approaches

Evidence

What the evidence means

We keep evidence types apart. A result for one configuration never stands in for another, and we never average across types.

No product in our database holds a reduction record for total trihalomethanes yet.

How we read evidence →

Next steps

Take the next step

Match a system to your homeFinder asks about your home first. It suggests total trihalomethanes as a starting point, and nothing counts until you confirm it.

Ask about total trihalomethanes

Still have a question?

Answers come only from WaterDecoder’s reviewed sources and show citations. Ask can’t interpret your test result or choose a product. Don’t include personal details.

AI assistant

Ask about Total trihalomethanes

Answers draw on cited public-health sources and the filters certified to reduce Total trihalomethanes. Every source is linked, and if the sources don’t cover a question, the answer says so.

Questions to start with

Educational information from cited sources. Not medical or professional advice.

References

Where these facts come from

1Regulatory reference40 CFR §141.64EPA NPDWR MCL (0.080 mg/L)
2Knowledge BaseFactsheet: Haloacetic Acids and Total Trihalomethane in Drinking Water — Information for ConsumersMassachusetts Department of Environmental Protection
3Knowledge BaseReport on Carcinogens, 15th Edition — Haloacetic Acids Found as Water Disinfection Byproducts (Selected)National Toxicology Program (NTP/NIEHS/HHS)

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