Trihalomethanes (THMs)
The federal limit is 80. The recent human evidence does not wait that long.
Trihalomethanes (THMs) are four disinfection byproducts in tap water, chloroform plus three brominated relatives, formed when chlorine reacts with natural organic matter in the source. A 2025 meta-analysis found bladder and colorectal cancer risk climbs above 41 µg/L total THM, below the 80 µg/L federal limit. Over a lifetime the shower can deliver as much of your dose as the glass, and both routes are filterable.
Trihalomethanes (THMs): what is it, and where does it come from?
Trihalomethanes are four single-carbon compounds, chloroform plus three brominated relatives, that form when the chlorine used to disinfect drinking water reacts with natural organic matter in the source. Nobody adds them on purpose. They appear after chlorination and keep forming in the distribution pipes as the water travels and warms. Chloroform is usually the largest share. When the source water also carries bromide, the brominated species rise, and those are the ones that matter most. Levels run higher in summer and higher the farther a home sits from the treatment plant.
Trihalomethanes (THMs): why does it matter?
Chlorination is one of the great public health wins of the last century, and it still earns its place. By the time the water reaches you the disinfectant has done its job, so the byproduct is worth removing at that point. Two of the four THMs, bromodichloromethane and bromoform, are genotoxic enough that EPA's own health goal for each is zero, yet the enforceable limit sums all four together and lets those two hide inside a permissive number. The exposure surprise is the route. A single shower can raise the chloroform in your indoor air more than forty-fold, while drinking a liter of the same water barely moves your blood level. Over a lifetime, the dose you inhale and absorb in the bathroom can equal or beat the dose you drink. A kitchen filter alone, the right move for lead, only covers part of the THM picture.
- A 2025 systematic review and dose-response meta-analysis found bladder and colorectal cancer risk rising above 41 µg/L total THM, below the 80 µg/L US limit, with bladder cancer risk about a third higher at the highest exposures versus the lowest.Helte et al., 2025 (Environ Health Perspect). THM exposure and cancer, dose-response meta-analysis
- A 2025 time-updated meta-analysis of 16 studies and over 8,000 bladder cancer cases found long-term consumption of chlorinated drinking water was associated with bladder cancer in men, with the association concentrated in males rather than females.Xie et al., 2025 (J Hazard Mater). Chlorinated water and bladder cancer, time-updated meta-analysis
- In a controlled study of household water use, showering raised indoor air chloroform more than forty-fold, while drinking a liter of tap water produced only a small rise in blood THM, which is why the bathing route carries much of the lifetime dose.Gordon et al., 2005 (Environ Health Perspect). Breath THM levels from household water use
- Estimates from household-water modeling put lifetime inhalation dose at roughly 0.6 to 1.5 times the dose from drinking, and as high as 5.7, with dermal absorption adding further, so the non-drinking routes can match or exceed ingestion.Maxwell et al., 1991 (Regul Toxicol Pharmacol). Inhalation and dermal chloroform exposure from household water
Trihalomethanes (THMs): what do we grade it against?
Trihalomethanes (THMs) is held to two different numbers, and they do not agree. The health-based level for Total THM, cancer-risk inflection (µg/L) is 41 (risk rises above this in pooled human data), from Helte et al., 2025 (Environ Health Perspect). The enforceable legal limit is 80. The table below lists every level on this page next to the source that set it.
| Contaminant | Health-based level | Legal limit | Source (health-based) |
|---|---|---|---|
| Total THM, cancer-risk inflection (µg/L) | 41 risk rises above this in pooled human data | 80 EPA TTHM MCL, sum of all four | Helte et al., 2025 (Environ Health Perspect) |
| Chloroform (µg/L) | 0.4 1-in-a-million lifetime cancer risk | 80 no separate MCL; counts toward TTHM 80 | CA OEHHA Public Health Goal, 2020 |
| Bromodichloromethane / BDCM (µg/L) | 0 genotoxic; EPA health goal is zero | 80 no separate MCL; counts toward TTHM 80 | US EPA Stage 2 D/DBPR (MCLG) |
| Bromoform (µg/L) | 0 EPA health goal is zero | 80 no separate MCL; counts toward TTHM 80 | US EPA Stage 2 D/DBPR (MCLG) |
Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints, while legal limits are enforceable compromises. Your report grades to the health column.
Trihalomethanes across US water systems: what we measured
We pulled trihalomethane levels for 17,538 US community water systems out of state testing records, mostly sampled between October 2024 and July 2026, and graded each one against the same two lines: the 41 ppb level where pooled human data shows cancer risk rising, and the 80 ppb federal limit. Parts per billion is the same unit as µg/L.
3,049 of those systems, about one in 6, average at or above 41 ppb. Only 221 are at or above the legal limit, because the legal limit is set almost twice as high. That gap runs roughly 14 to one: nearly every system in the higher-exposure group is obeying the law perfectly.
Half the systems that detect any trihalomethanes sit at or below 19.1 ppb. One in four is above 39.09 ppb and one in ten is above 56.87 ppb. The highest single system average in the set is 248.75 ppb.
2,828 systems sit in the space between the health line and the legal limit, serving 104 million people who would read their utility's annual report as unambiguously good news. The 80 ppb limit dates from the 1970s and was carried forward; the 41 ppb figure comes from the pooled human data published in 2025 and cited on this page.
16,031 of the same systems report haloacetic acids (HAA5), the second regulated group of disinfection byproducts. 159 average at or above the 60 ppb federal limit, and the typical detecting system reads 12.4 ppb. No health-based line below that limit exists in the research we use, so we grade HAA5 against the limit. The two groups also behave differently in a house: trihalomethanes evaporate into shower air while haloacetic acids stay in the glass, so a filter certified for one does not automatically handle the other.
The yearly average hides the summer spike
Compliance is judged on a running annual average, and trihalomethanes swing hard with the season and with distance from the treatment plant. Where our records hold both the annual average and the highest single reading behind it, 2,107 systems, the peak runs a median 8.7 ppb above the average, and the top tenth of that group runs 36.4 ppb or more above it.
The consequence is countable: 276 systems hold an annual average under the 80 ppb limit while their highest recorded reading sits at or above it. Those systems are not in violation and the reading is not an error. It is what averaging is for, and it is also what averaging hides from anyone reading a single published number. If you ask your utility for data, ask for the range as well as the average.
Which states run highest on trihalomethanes?
Arkansas leads the trihalomethane ranking: 59.3% of its 123 reporting systems average at or above 41 ppb, at a state median of 46 ppb. Pennsylvania (42.9%), Louisiana (36.6%), New Jersey (33.6%) and Oklahoma (32.2%) follow.
| State | Typical system (ppb) | Above 41 ppb | Above 80 ppb |
|---|---|---|---|
| Arkansas 123 systems reporting, 66% of the state | 46 | 59% 73 systems | 2 |
| Pennsylvania 252 systems reporting, 57% of the state | 36.29 | 43% 108 systems | 6 |
| Louisiana 202 systems reporting, 30% of the state | 26 | 37% 74 systems | 8 |
| New Jersey 143 systems reporting, 26% of the state | 28 | 34% 48 systems | 3 |
| Oklahoma 791 systems reporting, 96% of the state | 25.5 | 32% 255 systems | 61 |
| Massachusetts 245 systems reporting, 62% of the state | 26 | 32% 78 systems | 5 |
| New Hampshire 48 systems reporting, 45% of the state | 22.1 | 31% 15 systems | 0 |
| Illinois 1,001 systems reporting, 63% of the state | 23 | 26% 257 systems | 5 |
| Florida 1,496 systems reporting, 96% of the state | 17.18 | 23% 338 systems | 26 |
| California 1,828 systems reporting, 67% of the state | 12.48 | 20% 367 systems | 21 |
| Iowa 867 systems reporting, 89% of the state | 14 | 14% 121 systems | 2 |
| Kentucky 365 systems reporting, 100% of the state | 22.5 | 14% 50 systems | 0 |
| Alaska 308 systems reporting, 97% of the state | 1.77 | 11% 33 systems | 8 |
| New Mexico 245 systems reporting, 51% of the state | 4.34 | 10% 24 systems | 3 |
| South Carolina 374 systems reporting, 80% of the state | 9.2 | 10% 36 systems | 0 |
27 states qualify. A state is ranked only with at least 30 reporting systems and at least 25% of its systems covered, because ranking a state on a small slice of it would publish our own collection coverage as that state's water quality. The 20 states held back average 2.27 times higher above the line than the ranked ones, the pattern the rule is there to catch: a state can look clean only because we hold too few of its systems.
A single system's number can rest on one sampling point, and we cannot check that against the utility's own published figure. A state median over dozens of systems survives that; one system's number would not.
The pattern follows the source water. Trihalomethanes form when chlorine meets natural organic matter, so a system drawing warm, organic-rich river water produces more of them than a system pumping cold groundwater, whatever either utility does about it. The states at the top of this table are surface-water states.
How this was built, and what it cannot tell you
Every trihalomethane figure here is drawn from one dataset of 38,332 community water systems. Below is how it was built, including the things this data cannot tell you.
- Records
- 17,538 community water systems with a trihalomethane result on record, drawn from 38,332 systems that passed every filter listed here. The full panel set holds 42,081 systems.
- Filters applied
- Community water systems only (316 non-community systems removed). Wholesalers removed (2,164), because their population is already counted under the retail systems that buy from them. Systems whose most recent report predates 2023 removed (929). Systems EPA’s inventory does not classify removed (340).
- Sample dates
- July 2018 to July 2026. Nine in ten records are dated October 2024 or later. The median is October 2025. Some older samples are valid: several analytes are on a three-year monitoring cycle, so a sample taken years ago can still appear in a current report.
- Sources
- Contaminant levels: state drinking-water monitoring records and utility reports, assembled into one panel set dated 2026-06-16. System type, population served and purchased-water status: EPA SDWIS, vintage 2026Q1 (file dated 2026-04-08). Health and legal lines: the Stasis threshold registry, version 2026-08-08, which is the same registry that grades a customer report, so a page and a report can never disagree.
- Provenance
- 13,111 of 17,538 results are reconstructions we computed from raw state monitoring records, rather than figures taken straight from a utility’s published water quality report. Full breakdown: 13,111 reconstructed running annual average, computed from the state's raw quarterly samples; 4,406 the utility tested and reported no detection; 21 read from the utility's published water quality report. We compile and interpret these figures; we do not take the samples ourselves.
- Scoring rule
- A system is counted against the health-based level. Both lines are printed on every figure so the gap between them is visible. Non-detects are counted as zero and included in the denominator, so a share above a line is never inflated by dropping clean systems.
- Automation
- Software assembled the dataset. It reads state monitoring records, normalises them and combines them, and every figure on this page comes out of one program rather than being typed by hand. This is how 42,081 systems are held to one standard. A person chooses which comparisons to draw and which limits to name, and reviews what the numbers mean. The health thresholds come from the cited primary literature, not a model.
- Raw data
- The row-level extract is not published. It was assembled out of several hundred separate state and federal sources. The findings, the method, the denominators and the limits are all on this page so the work can be checked, and every threshold cited comes from a public primary source linked in place, but the compilation itself is not a free download. Any figure can still be checked at retail: against a utility’s own published report, or by putting one address into the lookup and comparing what it returns. Researchers and reporters who need specific rows can write to data@stasishome.com and say what they are working on.
- Known limitation: how states qualify to be ranked
- A state is ranked only if at least 30 of its systems have a trihalomethane result AND at least 25% of its community systems do. 27 states clear both. 20 more clear the first and fail the second, so we hold them back from the ranking. The gate exists because coverage in this dataset runs from under 3% of a state’s systems to over 99%, and ranking a state on a 3% subsample would publish our own collection coverage as that state’s water. The excluded states average 38.9% above the line, against 17.1% for the ranked ones, 2.27x higher. The direction differs by analyte, and we recompute this figure from the data on every build.
- Known limitation: purchased water
- 7,416 of the 38,332 systems here buy finished water from a wholesaler instead of treating their own. For those systems, plant-level analytes are often sampled under the seller’s permit number, so our record shows “not reported” where the water was in fact tested upstream. A claim that some share of utilities does not test for trihalomethane would therefore be false, and nothing on this page makes one. Reporting rates are stated against systems that treat their own water: 45.6% of those 30,916 systems have a trihalomethane result, against 46.3% of the 7,416 that purchase. The two rates are within 0.7 points here. That means this analyte is measured in the distribution system by whoever operates it, so purchased water does not leave a hole for it.
- Known limitation: what a running average is
- The reported figure is the highest locational running annual average in the system, which is how federal compliance is calculated. It is an average of quarterly samples at one location, so it is not what came out of any tap on any day, and reconstructing it exactly from raw quarterly records is the hardest case in this dataset. Where a utility's own published running average exists, that is the better number, and it is the one the Orange County benchmark uses.
- Known limitation: no lead
- No figure built on this dataset will ever be a national lead number. Lead enters at the household plumbing between the street and the tap, so utility data structurally understates it.
- Corrections
- If a figure here disagrees with your utility’s published report, the published report is more likely to be right and we want to know. Write to data@stasishome.com with the system name and we will check it against the source and correct the record.
These figures describe water systems as a whole. They cannot tell you what is at your own tap, and a system with no result on record is one nobody has tested rather than one known to be clean.
What helps against Trihalomethanes (THMs)?
5 approaches below reduce Trihalomethanes (THMs) in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Point-of-use carbon block at the kitchen tap. Each entry also names what it cannot do, because no single fix covers every exposure route.
Direct fixes
- Point-of-use carbon block at the kitchen tap
An NSF/ANSI 53 carbon block certified for VOC and THM reduction, with chloroform as the test surrogate, lowers THMs in the water you drink and cook with. A dense carbon block holds up better than loose granular carbon.
It covers the drinking route only, which for THMs is the smaller share of your exposure. THM capacity runs out before chlorine taste fades, so it cannot be judged by taste and must be replaced on schedule.
- Shower carbon filter
A shower-stream carbon filter, certified to the NSF/ANSI 177 standard, cuts free chlorine and volatile byproducts in the spray. That spray is where much of the THM dose enters, through the air you breathe and your skin. It installs in minutes.
Per-product performance varies by cartridge and source water, and the filter needs replacement on a 6-to-12-month cycle to keep working.
Bigger retrofits
- Reverse osmosis at the kitchen tap
An NSF/ANSI 58 RO system removes THMs along with lead and PFAS, which makes it the right choice when several of those are present at once in a home's water.
It is more than you need if THMs are your only concern, and it covers the drinking water it treats, not the shower or bath.
- Whole-house carbon at the point of entry
Point-of-entry activated carbon is the most complete answer for THMs, since it treats every tap, shower, and bath in the home rather than one fixture.
Where PFAS is present this is not the first move, because a whole-house carbon bed can saturate and release captured PFAS, and it strips the chlorine residual that keeps home plumbing clean, so decide it per home from the water test.
Free and behavioral
- Heating water for coffee, tea, and cooking
Boiling drives off about 96% of chloroform in five minutes, and 50 to 90% comes off at 70 to 90 degrees C, so anything you heat is already lower in THMs at no cost.
It helps the drinking route, and only for THMs. A hot shower does the opposite, sending more THM into the air you breathe. It also does nothing for haloacetic acids, the other regulated byproduct group, which are not volatile and can go the wrong way when chlorinated water is boiled.
See how well each fix works
Chlorine keeps the water pathogen-free from the plant to your house, and by the time it arrives that work is done. The natural complement is removing the leftover byproducts at the point of use, on both the drinking and the bathing routes. Keep the disinfection, then remove what it leaves behind.
Common questions about Trihalomethanes (THMs)
What are trihalomethanes in water?
Trihalomethanes are a group of four disinfection byproducts, chloroform, bromodichloromethane, dibromochloromethane, and bromoform, that form in tap water when the chlorine used to disinfect it reacts with natural organic matter from the source. They were not in the raw water and are not added on purpose. Chloroform is usually the largest share, and the brominated forms rise when the source water carries bromide. Every home on a chlorinated public system carries some level.
What is a safe level of TTHM?
No level has been proven safe. EPA sets the enforceable total THM limit at 80 µg/L, the sum of the four compounds. But the 2025 human evidence finds cancer risk rising above 41 µg/L, and California's health-based goals for the individual compounds sum to about 1 µg/L, with the two genotoxic ones set at zero. Treat 80 as compliance, not safety, and aim lower.
Are THMs in shower water dangerous?
The risk comes from lifetime dose. THMs are volatile, so a hot shower sends chloroform into the air you breathe, more than forty-fold in one controlled study, and your skin absorbs a comparable share during the wash. Across a lifetime the shower and bath can deliver as much THM as everything you drink, the route a kitchen filter misses. A shower carbon filter covers it.
Should I stop drinking tap water?
No. Filtering handles it. A certified carbon block at the kitchen tap lowers THMs in what you drink and cook, and heating water for coffee, tea, or cooking removes most of the chloroform on its own. Pair that with a shower filter to cover the route most people miss.
Are all four THMs equally concerning?
No. Chloroform usually dominates the total, but the brominated compounds, bromodichloromethane and bromoform, carry an EPA health goal of zero and are the ones worth watching when bromide is in the source water. A test that breaks out the four, rather than reporting one lumped total, tells you which story your home is in.
Does boiling water remove THMs?
Mostly, for the drinking route, and only for THMs. About 96% of chloroform comes off after five minutes of boiling. That covers anything you heat, but it does nothing for the shower and bath, where heating the water actually pushes more THM into the air. It also does nothing for haloacetic acids, the other regulated byproduct group. Because they are not volatile, boiling leaves them behind, and in chlorinated water it can raise one of them.
Does a Brita filter remove trihalomethanes?
Only if that specific cartridge carries an NSF/ANSI 53 certification for VOC reduction, and most basic pour-through cartridges are certified to NSF/ANSI 42 for chlorine taste and odor only. Chloroform, the dominant trihalomethane in US chlorinated supplies, is the surrogate compound the NSF 53 VOC test is run on. Format matters as much as the mark: a carbon block outperforms the loose granular carbon in a pitcher, because higher density and longer contact time do the removing.
Sources cited for Trihalomethanes (THMs)
Every number on this Trihalomethanes (THMs) page traces to one of the 7 sources below: 4 peer-reviewed, 2 government or regulatory.
Peer-reviewed
- Helte et al., 2025 (Environ Health Perspect). THM exposure and cancer, dose-response meta-analysis
- Xie et al., 2025 (J Hazard Mater). Chlorinated water and bladder cancer, time-updated meta-analysis
- Gordon et al., 2005 (Environ Health Perspect). Breath THM levels from household water use
- Maxwell et al., 1991 (Regul Toxicol Pharmacol). Inhalation and dermal chloroform exposure from household water
Government & regulatory
Institutional & standards
Related water science
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