Trihalomethanes are volatile disinfection byproducts that can form when chlorine reacts with naturally occurring organic matter in source water. If your goal is to reduce TTHMs at home, activated-carbon filtration and certain other treatment systems can help—but the right approach depends on whether you want to treat only drinking water or water throughout the house.
How Can You Reduce TTHMs in Drinking Water?
For household treatment, activated carbon is one of the most established approaches for reducing trihalomethanes and other volatile organic compounds. Carbon may be used in countertop, faucet, under-sink, refrigerator, or whole-house systems, but performance depends on the exact filter, carbon volume, contact time, flow rate, and service life.
Some reverse-osmosis systems can also reduce TTHMs, often with an important contribution from their carbon prefilters and postfilters. If showering and bathing exposure is a concern, point-of-use treatment at the kitchen sink will not address those fixtures; a properly designed whole-house system is the more relevant approach.
What Are Trihalomethanes (THMs)?
Trihalomethanes are a family of disinfection byproducts. In U.S. drinking-water regulation, total trihalomethanes (TTHMs) means the sum of four compounds:
- Chloroform
- Bromodichloromethane
- Dibromochloromethane
- Bromoform
They can form when chlorine used for drinking-water disinfection reacts with natural organic matter from sources such as soil, leaves, vegetation and other material in rivers, lakes and reservoirs. Bromide in the source water can influence how much brominated THM forms.
Formation is affected by several variables, including organic precursor levels, disinfectant dose and type, pH, temperature and contact time. TTHMs can continue to form as treated water moves through storage tanks and distribution pipes, which is one reason concentrations may differ by season and location within the same water system.
For a deeper explanation of formation, health research, testing, and utility monitoring, see our complete TTHMs in Drinking Water guide.
What Is EPA's Limit for TTHMs?
EPA's enforceable Maximum Contaminant Level for total trihalomethanes is 0.080 mg/L, equivalent to 80 µg/L or approximately 80 parts per billion (ppb).
The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules apply to covered community and non-transient non-community public water systems that use or deliver chemically disinfected water. Stage 2 strengthened monitoring requirements so that compliance reflects conditions at individual monitoring locations rather than allowing high locations to be obscured by a system-wide average.
| Regulated DBP | Federal MCL | What it represents |
|---|---|---|
| TTHMs | 80 µg/L (80 ppb) | Sum of chloroform, bromodichloromethane, dibromochloromethane and bromoform |
| HAA5 | 60 µg/L (60 ppb) | Sum of five regulated haloacetic acids |
| Bromate | 10 µg/L (10 ppb) | DBP associated particularly with ozonation where bromide is present |
| Chlorite | 1.0 mg/L | DBP associated with chlorine-dioxide treatment |
EPA continues to evaluate potential revisions to its microbial and disinfection-byproduct regulations. As of 2026, EPA anticipates proposing revisions in summer 2027; the current TTHM MCL remains 80 µg/L.
Before Buying a Filter: Check Your Water
If you receive municipal water, start with your utility's annual Consumer Confidence Report (CCR). Look for “Total Trihalomethanes,” “TTHM,” or “TTHMs.”
Pay attention to the units and whether the report provides a range, highest locational running annual average (LRAA), or violation status. A result of 0.050 mg/L is the same as 50 µg/L or approximately 50 ppb.
If the annual report is outdated or you want information closer to your address, ask the utility for recent TTHM results from the monitoring location that best represents your neighborhood.
A certified laboratory can also test household water. TTHMs are volatile, so sampling procedures matter; laboratories typically provide prepared vials and instructions designed to prevent the compounds from escaping before analysis.
1. Activated Carbon Filtration
Activated carbon is widely used to adsorb organic chemicals, including many volatile organic compounds. TTHM reduction depends on the carbon type, amount of media, contact time, water chemistry, flow rate, and how much water has already passed through the filter.
Carbon appears in many treatment formats:
- Countertop gravity filters
- Carbon-block faucet and under-sink filters
- Refrigerator filters
- Water-filter pitchers
- Granular activated carbon (GAC) whole-house systems
However, the presence of activated carbon alone does not prove a specific TTHM reduction level. Check the manufacturer's performance sheet.
What certification should you look for?
NSF maintains listings for drinking-water treatment products certified for TTHM reduction under NSF/ANSI 53 and, for applicable reverse-osmosis products, NSF/ANSI 58. NSF's contaminant-reduction guide also groups TTHMs with VOC reduction claims.
Certification is product-specific. A filter certified only to NSF/ANSI 42 for chlorine taste and odor should not automatically be assumed to have a TTHM reduction claim.
2. Reverse Osmosis Systems
Reverse osmosis can be useful when a household wants to reduce multiple dissolved contaminants in addition to TTHMs. Most household RO systems combine a membrane with carbon prefilters and/or postfilters.
That distinction matters for volatile organic compounds: the carbon stages can play an important role in TTHM reduction. Therefore, do not assume that every RO membrane or every RO system has the same TTHM performance.
When comparing systems, check for a specific TTHM, chloroform, or VOC reduction claim and note the rated capacity, wastewater ratio, storage-tank requirements, and filter-replacement schedule.
3. Whole-House Activated Carbon
TTHMs are volatile. Household exposure can therefore occur not only from drinking and cooking but also when compounds move from water into indoor air during showering, bathing and other hot-water uses.
A countertop or under-sink filter treats only water delivered through that outlet. If the goal is to reduce TTHMs throughout the home, a properly sized point-of-entry or whole-house granular activated carbon system is more relevant.
Whole-house carbon systems require adequate media volume and contact time, and they must be maintained. CDC also notes an important tradeoff: if a whole-home filter removes chlorine or another disinfectant residual, microorganisms may have more opportunity to grow in household plumbing.
Can Aeration or Boiling Reduce TTHMs?
Because TTHMs are volatile, some can move from water into air when water is aerated, heated, or boiled. Chloroform is particularly volatile.
That does not make boiling an ideal household TTHM treatment. Volatilization transfers compounds into the surrounding air rather than capturing them, results vary among the four THMs, and boiling can concentrate contaminants that do not evaporate.
If you are under a boil-water advisory, follow the public-health instructions exactly. The purpose of boiling in that situation is to control microorganisms, not to optimize TTHM levels.
What Does Not Reliably Remove TTHMs?
| Treatment | TTHM role | Why |
|---|---|---|
| Activated carbon | Often effective when appropriately designed | Adsorbs many organic compounds; verify product-specific TTHM/VOC performance. |
| Reverse osmosis system | Can be effective | Often relies in part on the accompanying carbon stages; verify the complete system's claim. |
| Whole-house GAC | Can address all fixtures | Requires professional sizing, adequate contact time, and maintenance. |
| Water softener | Not a standard TTHM treatment | Conventional softeners are designed primarily to remove hardness minerals such as calcium and magnesium. |
| UV disinfection | Not a TTHM-removal method | CDC notes that UV treatment addresses microorganisms and does not remove chemicals. |
| Ordinary sediment filter | Not sufficient by itself | TTHMs are dissolved organic compounds, not simply suspended particles. |
This is why matching the treatment technology to the contaminant matters. A system can be excellent at one task and have little relevance to another.
Do Berkey® Water Filters Reduce Trihalomethanes?
Berkey systems are countertop gravity-fed systems, so performance depends on the filter elements installed.
Published laboratory information for Black Berkey® Elements reports TTHM reduction greater than 99.8% under the referenced test conditions. Black Berkey Elements use a proprietary combination of filtration and adsorption media and have been independently tested across a broad range of contaminants.
A laboratory percentage should always be read in context. It describes performance under the conditions of the referenced test; actual household performance can vary with source concentration, water chemistry, flow, installation, filter condition, and gallons processed.
Berkey systems are point-of-use. They can reduce TTHMs in water processed for drinking and cooking, but they do not treat shower or bath water.
You can compare current options in our Berkey Replacement Filters Guide.
How to Choose a TTHM Filter
- Confirm that TTHMs are actually a concern. Read the current CCR or obtain a certified laboratory test.
- Decide which route of exposure you want to address. Drinking/cooking generally calls for point-of-use treatment; all fixtures require point-of-entry treatment.
- Look for a specific reduction claim. Search for TTHM, chloroform, or appropriate VOC performance—not simply “carbon filter.”
- Check the rated capacity. Adsorption media eventually reaches capacity.
- Consider flow and contact time. These can materially affect carbon performance.
- Replace media on schedule. A filter can continue producing normal-looking water after its contaminant-reduction capacity has declined.
- Verify when it matters. If you are making a treatment decision based on an elevated laboratory result, consider testing the treated water as well.
CDC recommends testing water and selecting treatment based on the specific chemicals or other substances you want to reduce. That contaminant-first approach is more reliable than choosing equipment based on broad marketing categories.
Frequently Asked Questions About Removing TTHMs
What is the best type of filter for TTHMs?
Activated carbon is one of the most established household approaches. Look for product-specific TTHM, chloroform, or VOC reduction data and follow the rated service life.
Does activated carbon remove trihalomethanes?
Activated carbon can adsorb TTHMs, but performance varies with filter design, carbon media, flow rate, contact time, source water, and filter age. Not every carbon filter carries a TTHM reduction claim.
Does reverse osmosis remove TTHMs?
Some complete RO systems can reduce TTHMs, with important contributions from carbon prefilters and postfilters. Check the exact system's contaminant-reduction documentation.
Does a water softener remove TTHMs?
A conventional ion-exchange water softener is primarily intended to remove hardness minerals such as calcium and magnesium. It should not be treated as a standard TTHM-removal technology unless the exact product has supporting performance data.
Does UV remove TTHMs?
No. UV systems are designed primarily for microbial treatment. CDC states that UV treatment does not remove chemicals from water.
Does boiling remove TTHMs?
Heating and boiling can drive volatile THMs from water into the air, but this is not a controlled filtration method and may increase inhalation during the process. It can also concentrate nonvolatile contaminants as water evaporates.
Does a refrigerator filter remove TTHMs?
Some do, while others are certified only for chlorine taste and odor. Check the exact model for TTHM, chloroform, or appropriate VOC reduction claims.
Does a countertop filter reduce TTHMs in shower water?
No. A countertop system treats only the water passed through it. Reducing TTHMs at showers and baths requires treatment upstream of those fixtures.
Does Berkey remove TTHMs?
Published Black Berkey Element laboratory information reports TTHM reduction greater than 99.8% under the referenced test conditions. That result applies to the tested element and conditions and should not automatically be attributed to every filter that can be installed in a Berkey system.
The Bottom Line
Trihalomethanes are a normal regulatory concern associated with drinking-water disinfection, not a reason to avoid disinfected public water. EPA limits TTHMs to a locational running annual average of 80 µg/L in covered public water systems.
If you want to reduce TTHMs at home, start by checking your utility's results or testing your water. Activated carbon is one of the most practical household treatment technologies, and certain reverse-osmosis and whole-house carbon systems can also be appropriate when they have relevant performance evidence.
Choose a treatment based on the specific contaminant and the exposure route you want to address. A countertop system can treat drinking and cooking water; whole-house treatment is needed if the goal includes showers and baths.
For the science, monitoring rules, exposure routes, and health evidence behind these compounds, continue with our Trihalomethanes (TTHMs) in Drinking Water: A Complete Homeowner's Guide.