At a glance
| Chemical family | Volatile halogenated organic compounds — the largest class of disinfection by-products (DBPs). The regulated group ("total trihalomethanes," TTHM) is four compounds: chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3). The brominated members form where source water contains bromide. |
| CAS number | No single CAS (a regulated group). Members: chloroform 67-66-3; bromodichloromethane 75-27-4; dibromochloromethane 124-48-1; bromoform 75-25-2 |
| Classification | Chloroform and bromodichloromethane are IARC Group 2B (possibly carcinogenic to humans). EPA Stage 2 D/DBP Rule sets a total-trihalomethanes (TTHM) Maximum Contaminant Level of 0.080 mg/L (80 µg/L), as a locational running annual average. WHO sets individual guideline values per THM. Not acutely toxic at drinking-water levels; the concern is long-term. |
| Where you encounter it | Chlorinated (and chloraminated) municipal drinking water — anywhere disinfection reacts with natural organic matter. Exposure occurs by drinking, but also by inhalation and dermal absorption during hot showers, baths, and swimming (THMs are volatile). Highest in systems with high source-water organic content, long distribution times, and warm water. |
| Sleep micro environment relevance | Direct-adjacent — because THMs volatilize from hot water, a chlorinated shower or bath before bed releases them into bathroom and bedroom air. This is the compound that most clearly bridges the tap-water Atlas and the indoor-air Atlas: chloroform in particular is measured in indoor air after showering. |
| Activated carbon capture | Effective — and unusually so for a water contaminant. THMs are organic and volatile, so activated carbon adsorbs them well; a certified carbon filter (NSF/ANSI 53 for VOC/THM reduction) is the standard, low-cost fix. Inferred from established DBP water-treatment engineering; carbon adsorption is the recognized point-of-use method for THMs |
Regulatory & certification status
Where trihalomethanes stand across the major drinking-water systems. THMs are regulated as a treatment by-product, so the standards balance disinfection benefit against by-product risk rather than aiming for zero.
| United States | EPA regulates total trihalomethanes (TTHM) at a Maximum Contaminant Level of 0.080 mg/L (80 µg/L) under the Stage 2 Disinfectants and Disinfection Byproducts Rule (2006), compliance based on a locational running annual average so that no single point in the distribution system persistently exceeds it. A parallel limit covers five haloacetic acids (HAA5) at 60 µg/L. Regulatory — US EPA |
| European Union | The recast EU Drinking Water Directive (2020/2184) sets a total-THM parameter of 100 µg/L (with a long-term aim of 60 µg/L where feasible without compromising disinfection). The Directive explicitly notes disinfection must not be compromised in pursuit of lower DBPs. Regulatory — European Commission |
| Canada | Health Canada sets a maximum acceptable concentration of 0.100 mg/L (100 µg/L) for total THMs, as a running annual average, and 0.060 mg/L for bromodichloromethane specifically — with the standing instruction that efforts to reduce THMs must not compromise disinfection. Regulatory — Government of Canada |
| World Health Organization | WHO sets individual guideline values — chloroform 300 µg/L, bromodichloromethane 60 µg/L, dibromochloromethane 100 µg/L, bromoform 100 µg/L — and a rule that the sum of each THM's ratio to its guideline should not exceed 1. WHO stresses that disinfection should never be compromised to meet THM guidelines. Regulatory — WHO |
| International | IARC classifies chloroform and bromodichloromethane as Group 2B (possibly carcinogenic to humans). The classification reflects animal carcinogenicity plus the epidemiological bladder-cancer signal; it is a "possible," not "known," designation — appropriately calibrated to the strength of evidence. Regulatory — IARC Monographs |
| Certifications | For point-of-use reduction, NSF/ANSI 53 certifies filters for VOC reduction (a surrogate that includes chloroform/THMs) and NSF/ANSI 58 covers reverse osmosis — verify the specific VOC or THM reduction claim. This is a case where an inexpensive certified carbon filter is genuinely effective. Industry — NSF |
| The 72-hour test window | Not applicable to products, but notable for air. THMs are not a mattress emission; however, because chloroform volatilizes from hot chlorinated water, it is one of the DBPs actually measured in bathroom and bedroom air after showering — an air exposure created by water, not by any furnishing. Inferred — from THM volatility and indoor-air monitoring studies of post-shower chloroform |
What it is
Trihalomethanes are single-carbon molecules with three halogen atoms — the "tri-halo-methane" name. They form when a disinfectant (chlorine, or chloramine) reacts with natural organic matter: the dissolved remains of leaves, algae, and soil organics that are present in almost every surface-water source. The chlorine that kills pathogens also attacks this organic matter, and trihalomethanes are among the products. Where the source water also contains bromide (common in coastal and some groundwater sources), the brominated THMs form alongside chloroform. Chloroform is usually the dominant member.
This origin is the entire point. THMs are not pollution that leaked in; they are the chemical signature of the disinfection process that made drinking water safe. Before widespread chlorination, waterborne cholera and typhoid killed enormous numbers of people; chlorination is one of the reasons life expectancy rose in the 20th century. So the regulatory posture worldwide is unusually explicit and consistent: reduce THMs where you can, but never compromise disinfection to do it. That instruction appears verbatim in EPA, WHO, EU, and Health Canada guidance, and it is the correct frame for a reader deciding what to worry about.
THM levels are not fixed. They rise with more organic matter in the source (seasonal runoff, algal blooms), with higher water temperature, with higher chlorine dose, and with longer residence time in the distribution pipes — which is why the far end of a distribution system, or a home at the end of a long main, can see higher THMs than the treatment plant's output. Utilities manage THMs by removing organic matter before disinfection, optimizing chlorine dose, and sometimes switching to chloramine, which forms fewer THMs.
Where you encounter it
From drinking chlorinated water
The baseline route. Any chlorinated municipal supply carries some THMs; the level is reported in every utility's annual Consumer Confidence Report as the TTHM running annual average. Systems drawing on high-organic surface water, or with long distribution networks, tend to run higher. This is the exposure the 80 µg/L limit is written around.
From showering, bathing, and swimming
The underappreciated route. Because THMs are volatile, hot water releases them into the air, and you take them up by inhalation and across warm, wet skin. Blood-THM studies show measurable increases after a shower or bath, and a chlorinated indoor pool is a notably high-exposure environment (which is why competitive swimmers and pool workers have been studied). For a home, a hot shower in a poorly-ventilated bathroom is a real, if modest, inhalation exposure — and the reason THMs sit at the seam between the water and air Atlases.
From the seasonal and end-of-system peaks
THMs are not constant. After spring runoff or a warm-weather algal bloom, source-water organic content rises and THM formation climbs; homes at the end of long distribution mains see the highest levels because the water (and its residual chlorine) has had longer to react. A "swimming-pool" smell that intensifies seasonally is often this in action.
What the research says
The bladder-cancer signal — the pooled analyses
Villanueva and colleagues (2004) pooled six case-control studies of bladder cancer from the US, Canada, France, Italy, and Finland using THMs as the DBP marker, and reported rising bladder-cancer risk with increasing long-term exposure — an odds ratio of roughly 1.4 for exposure above ~50 µg/L in men. Peer-reviewed This pooled analysis is the anchor of the THM–bladder-cancer association and a major reason the by-product is regulated as tightly as it is.
Accounting for showering and bathing, not just drinking
Villanueva and colleagues (2007) in the American Journal of Epidemiology examined bladder-cancer risk across multiple THM exposure routes — ingestion, showering, bathing, and swimming-pool use — and found elevated risk associated with the combined exposure, supporting the idea that the inhalation/dermal routes matter, not only drinking. Peer-reviewed This is the evidentiary basis for treating a chlorinated shower as part of the exposure.
The European pooled/meta-analysis
Costet and colleagues (2011) in Occupational and Environmental Medicine pooled European case-control studies and found a consistent, if modest, association between long-term THM exposure and bladder cancer. Peer-reviewed The consistency across independent populations is what elevates this above a single-study finding — while the effect sizes remain moderate and the IARC classification stays at "possible" rather than "known."
Regulatory reference values
The EPA TTHM MCL is 80 µg/L (0.080 mg/L), a locational running annual average, under the Stage 2 D/DBP Rule. Regulatory The WHO background document sets individual THM guideline values with an additive-ratio rule. Regulatory The ATSDR Toxicological Profile for Chloroform details the toxicology of the dominant THM. Regulatory
What helps reduce exposure
Check your utility's TTHM number first. The annual Consumer Confidence Report lists the running-annual-average TTHM for your system. If it is comfortably below 80 µg/L, this is a low priority; if it runs high, the steps below matter more.
Use a certified activated-carbon filter for drinking water. This is the rare water contaminant where the cheap, common filter is the right tool — THMs are organic and adsorb well onto carbon. A pitcher, faucet, or under-sink filter certified to NSF/ANSI 53 for VOC/THM reduction meaningfully lowers the ingestion dose.
Ventilate the bathroom and don't over-heat showers. Because THMs volatilize from hot water, running the exhaust fan and using less scalding water reduces the inhalation route. This is a genuine, low-effort mitigation for the shower exposure.
Let cold-brew or refrigerated water off-gas, and keep drinking water cold. A small, real effect: THMs are volatile, so water left uncovered loses some; but do not confuse this with a primary control — the carbon filter is the reliable step.
Keep the disinfection. The most important instruction on this page: do not switch to untreated or under-treated water to avoid THMs. The microbial risk of undisinfected water is far larger than the THM risk.
What does NOT help
- Abandoning chlorinated tap water. The single worst response. Undisinfected water risks cholera, typhoid, and other pathogens that historically killed far more people than THMs. Reduce THMs; keep the disinfection.
- Boiling (as a primary fix). Vigorous boiling does drive off some THMs, but it also concentrates any non-volatile contaminants and is impractical for all household water — a carbon filter is the correct tool.
- Assuming bottled water is THM-free. Bottled water is not necessarily lower in DBPs and is not required to be; a certified carbon filter on tap water is usually the better and cheaper route.
- Ignoring the shower. A drinking-water filter does nothing for the inhalation/dermal route; ventilation and water temperature are the levers there.
- Treating THMs as an acute poison. At drinking-water levels the concern is long-term, statistical, and modest — not an immediate hazard. Proportion matters.
Open research questions
- The relative contribution of the inhalation/dermal (shower, bath, pool) route versus ingestion to the bladder-cancer association — the 2007 multi-route study implicates it, but partitioning the risk precisely is unresolved. Speculation re: route apportionment; the overall association is peer-reviewed
- The role of the unregulated and emerging DBPs (brominated, iodinated, and nitrogenous by-products) that THMs merely index — THMs are a marker, and some of the toxicity may track co-occurring DBPs not captured by the TTHM number. Speculation
- Reproductive and developmental endpoints (low birth weight, small-for-gestational-age) show inconsistent associations across studies; whether these are causal at typical exposures is unsettled. Speculation
- Optimal utility-side trade-offs between disinfection strength and DBP formation (chlorine vs chloramine vs alternative disinfectants) at the system scale remain an active engineering and public-health balance. Inferred from the drinking-water treatment literature
Where you meet Trihalomethanes across your home
The same compound turns up in more than one place you live. Here's where it shows up in Embr — each links to the full breakdown for that part of your home.
Citations
- US Environmental Protection Agency. Stage 2 Disinfectants and Disinfection Byproducts Rule — TTHM MCL 0.080 mg/L (80 µg/L). epa.gov Regulatory
- International Agency for Research on Cancer. Chloroform (Group 2B) and bromodichloromethane (Group 2B) — Monographs classifications. monographs.iarc.who.int Regulatory
- World Health Organization (2005). Trihalomethanes in Drinking-water — Background document for the WHO Guidelines for Drinking-water Quality. who.int Regulatory
- Agency for Toxic Substances and Disease Registry. Toxicological Profile for Chloroform. atsdr.cdc.gov Regulatory
- Villanueva CM, Cantor KP, Cordier S, Jaakkola JJK, King WD, Lynch CF, Porru S, Kogevinas M (2004). Disinfection byproducts and bladder cancer: a pooled analysis. Epidemiology, 15(3):357-367. DOI 10.1097/01.ede.0000121380.02594.fc Peer-reviewed
- Villanueva CM, Cantor KP, Grimalt JO, Malats N, Silverman D, Tardon A, et al (2007). Bladder cancer and exposure to water disinfection by-products through ingestion, bathing, showering, and swimming in pools. American Journal of Epidemiology, 165(2):148-156. DOI 10.1093/aje/kwj364 Peer-reviewed
- Costet N, Villanueva CM, Jaakkola JJK, Kogevinas M, Cantor KP, King WD, Lynch CF, Nieuwenhuijsen MJ, Cordier S (2011). Water disinfection by-products and bladder cancer: is there a European specificity? A pooled and meta-analysis of European case-control studies. Occupational and Environmental Medicine, 68(5):379-385. DOI 10.1136/oem.2010.062703 Peer-reviewed
Frequently asked questions
What are trihalomethanes and where do they come from?
Trihalomethanes (THMs) are a family of four chemicals — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — formed when the chlorine used to disinfect drinking water reacts with natural organic matter (decaying leaves, soil organics) already in the source water. They are the single most common class of disinfection by-products. In other words, they are not a contaminant that got into the water; they are created by the very process that makes water microbiologically safe. That trade-off is the heart of the story.
Are trihalomethanes dangerous?
The honest answer is proportionate, not alarmist. At the levels typical of well-run water systems (the global average is around 27 µg/L, below the 80 µg/L US limit), the individual risk is small. But long-term exposure to higher levels has been associated with bladder cancer across multiple independent studies, and chloroform and bromodichloromethane are classified as possible human carcinogens. The disinfection that produces THMs prevents cholera, typhoid, and other waterborne diseases that killed far more people than THMs ever will — so the goal is to minimize THMs without abandoning disinfection, not to fear the water.
Should I stop drinking chlorinated tap water because of THMs?
No. Disinfected water is one of the great public-health achievements, and the disease risk of undisinfected water dwarfs the THM risk. The reasonable response if your water runs high in THMs is to reduce them, not to stop drinking treated water: an activated-carbon filter (THMs are one of the few water contaminants carbon does remove well) reduces the ingestion route, and ventilating the bathroom addresses the inhalation route during showering. The framing "disinfection is the enemy" gets this exactly backwards.
Do I absorb THMs from showering, not just drinking?
Yes, and this is an underappreciated route. THMs are volatile, so a hot shower releases them into the air, and you inhale them and absorb some across warm, wet skin. Studies of blood THM levels find measurable increases after a shower or a bath, and some of the bladder-cancer research specifically accounts for showering, bathing, and swimming-pool exposure alongside drinking. This is why THMs bridge the water Atlas and the air Atlas — a chlorinated shower is partly an indoor-air exposure. Ventilating the bathroom and not over-heating the water both help.
Does a carbon filter remove trihalomethanes?
Yes — and this is one of the cases where a standard activated-carbon filter genuinely works. Unlike nitrate or chromium-6 (dissolved inorganic ions that carbon does not capture), THMs are organic compounds that activated carbon adsorbs well. A certified carbon pitcher, faucet, or under-sink filter (look for NSF/ANSI 53 certification for VOC or THM reduction) meaningfully lowers THMs in drinking water. This is the rare instance where the cheap, common filter is the right tool.
Why does my tap water taste and smell like a swimming pool sometimes?
That chlorine-pool smell is the disinfectant and its by-products, and it often intensifies seasonally — warmer water and higher organic matter in the source (for example after spring runoff) increase both chlorine demand and THM formation. Utilities also sometimes switch disinfection methods temporarily. The smell is not proof of danger, but if it is strong and persistent it is a reasonable prompt to check your utility's Consumer Confidence Report for the reported TTHM level and, if you want, to add a carbon filter.
Related compounds
Embr researches the chemistry of where you live — including the water you drink and shower in. See the methodology page for how this Atlas tags claims by evidence strength, the tap-water source hub for how THMs fit alongside the other things in your water, and the indoor-air hub for the shower-air route.
Last reviewed 2026-07-12. If you find a factual error, contact us.