Trihalomethanes (TTHMs) in Southern California Water: What Chlorination Byproducts Mean for Your Family

The EPA Maximum Contaminant Level for total trihalomethanes in public drinking water is 80 micrograms per liter (80 ppb), set under the Stage 2 Disinfectants and Disinfection Byproducts Rule. TTHMs, which include chloroform, bromodichloromethane, dibromochloromethane, and bromoform, form when the chlorine utilities use to disinfect water reacts with naturally occurring organic matter. Southern California water presents conditions that promote TTHM formation: imported Colorado River and State Water Project supplies carry higher organic loads than many inland sources, summer temperatures accelerate the reaction, and long distribution distances give the chemical process more time to run. Consumer Confidence Reports from Los Angeles, Orange County, and San Diego utilities show TTHM levels that comply with the federal 80 ppb MCL but consistently exceed California public health goals that are far lower. This guide explains the SoCal TTHM picture by county, the documented health effects of long-term exposure, and the certified treatment options that reduce or eliminate TTHM exposure at the point of use.

Published August 20, 2026  ·  Data sourced from EPA DBPR records, utility CCRs, and EWG Tap Water Database  ·  Southern California Water Treatment

What are trihalomethanes and how do they form in tap water?

Trihalomethanes are a family of four compounds: chloroform (trichloromethane), bromodichloromethane, dibromochloromethane, and bromoform. They are not added to water intentionally. They form as an unintended consequence of the disinfection process that makes public water safe to drink.

When water utilities apply chlorine or chloramine to kill pathogens, those disinfectants react with naturally occurring organic matter in the water, principally dissolved humic and fulvic acids that come from decayed plant material in source reservoirs and rivers. The reaction produces a mix of halogenated byproducts, the most regulated group of which is total trihalomethanes.

According to the EPA Stage 2 Disinfectants and Disinfection Byproducts Rule (epa.gov), four conditions accelerate TTHM formation in distribution systems:

• Higher organic carbon in source water: Surface water sources like the Colorado River and California Aqueduct carry more dissolved organic matter than deep groundwater. SoCal imports the majority of its water from these surface sources.
• Water temperature: Warmer water accelerates the chlorine-organic reaction. Southern California's climate means that water in distribution pipes reaches temperatures that drive faster TTHM formation compared to cooler-climate systems.
• Distribution system residence time: The longer water sits in pipes before reaching the tap, the more time the reaction has to produce TTHMs. Large, spread-out systems like those serving Los Angeles and San Diego have longer average residence times.
• Bromide in source water: The Colorado River contains naturally elevated bromide, which reacts with chlorine to form brominated TTHM compounds including bromodichloromethane, which carries the lowest OEHHA public health goal of the four compounds at 0.06 ppb.

SoCal water checks every one of these boxes: surface-water origin, warm climate, large distribution systems, and Colorado River bromide. That is why Los Angeles County, Orange County, and San Diego County all report consistent TTHM detections in their annual water quality reports, even when meeting the federal MCL.

What is the EPA limit for trihalomethanes in drinking water?

The EPA Maximum Contaminant Level for total trihalomethanes is 80 ppb, set under the Stage 2 Disinfectants and Disinfection Byproducts Rule, which became effective for most large water systems in 2012 and 2013. Compliance is evaluated as a locational running annual average, meaning each monitoring point in the distribution system must average 80 ppb or below across four quarters. The Stage 2 rule was stricter than its predecessor (Stage 1) precisely because it addressed high-TTHM locations within distribution systems rather than system-wide averages that could mask hot spots.

California adopted the federal 80 ppb MCL under Title 22 of the California Code of Regulations. California utilities must meet the same standard and report TTHM results in their annual Consumer Confidence Report. Violations trigger notification requirements and corrective actions.

The California OEHHA public health goals are a separate, non-enforceable benchmark. They represent the level at which no significant cancer or health risk is calculated over a 70-year lifetime. The current OEHHA public health goals for individual TTHM compounds are:

• Chloroform: 0.4 ppb (source: oehha.ca.gov)
• Bromodichloromethane: 0.06 ppb (oehha.ca.gov)
• Dibromochloromethane: 0.4 ppb (oehha.ca.gov)
• Bromoform: 0.4 ppb (oehha.ca.gov)

The gap between the enforceable 80 ppb MCL and the OEHHA public health goals for individual compounds is substantial. A SoCal utility reporting 20 ppb total TTHM complies with federal and state law. That same utility almost certainly reports bromodichloromethane at levels well above 0.06 ppb. Understanding this gap is why many SoCal homeowners who know their water is "compliant" still choose treatment options like reverse osmosis or whole-home carbon filtration.

How much TTHM is in Southern California tap water?

The table below summarizes TTHM ranges from utility Consumer Confidence Reports and EWG Tap Water Database analysis across Southern California counties. All figures are below the federal 80 ppb MCL. Range variation within a county reflects differences between utilities and between monitoring sites within a single utility. Verify your specific address at the EWG database or your utility's most recent CCR.

Sources: utility Consumer Confidence Reports 2023 to 2024; EWG Tap Water Database (ewg.org/tapwater); California SWRCB Drinking Water Watch. All readings are below the federal 80 ppb TTHM MCL.

If you are concerned about your specific address, our zip code water checker provides contaminant profiles for your service area, including TTHM data where available. You can also see how other Southern California counties compare on our chromium-6 county-by-county data page.

What health effects are linked to long-term TTHM exposure?

The health case for paying attention to TTHMs comes primarily from three bodies of evidence: animal carcinogenicity studies, epidemiological studies of populations served by chlorinated water, and regulatory risk assessments by the EPA and OEHHA.

The International Agency for Research on Cancer (IARC) classifies chloroform as a Group 2B possible human carcinogen and bromodichloromethane as a Group 2A probable human carcinogen (source: IARC Monographs, monographs.iarc.who.int). The EPA uses bromodichloromethane and chloroform as the basis for TTHM risk calculations in its regulatory analysis.

Long-term TTHM exposure at concentrations above public health goals has been associated in epidemiological research with:

• Bladder cancer risk: Multiple case-control and cohort studies have found associations between long-term chlorinated water consumption and elevated bladder cancer risk. The EPA's own Stage 2 DBPR preamble cites bladder cancer as the primary cancer endpoint driving the TTHM regulation. The association is most consistent for exposure above 50 ppb total TTHM over many years.
• Adverse birth outcomes: Several peer-reviewed studies have linked elevated TTHM exposure during pregnancy to increased risk of miscarriage and low birth weight. The evidence is more consistent for bromodichloromethane than for chloroform. California OEHHA cites reproductive endpoints in its public health goal documentation.
• Dermal and inhalation exposure: Showering and bathing in chlorinated water can result in dermal absorption and inhalation of volatile TTHMs, particularly chloroform. Studies suggest that for some compounds, shower exposure may contribute as much to daily dose as drinking water consumption. A whole-home filtration system addresses both drinking and bathing exposure. A kitchen-only reverse osmosis system addresses drinking and cooking exposure only.

These associations are dose-dependent and concern chronic exposure over years, not short-term consumption. Healthy adults drinking SoCal tap water within the federal MCL face no documented acute health risk. The concern is lifetime accumulation, particularly for pregnant individuals, infants, and those with extended daily exposure to chlorinated water at home.

Which Southern California counties have the highest TTHM levels?

TTHM levels in Southern California are driven primarily by source water origin and distribution system characteristics. Counties and utilities that import more surface water from the Colorado River or State Water Project generally report higher TTHM concentrations than those with significant local groundwater supplies.

Los Angeles County shows the widest TTHM range among SoCal counties, from the mid-teens in groundwater-dominant areas of the San Fernando Valley to above 40 ppb in eastern distribution zones served by State Water Project imports. The LADWP 2023 to 2024 Consumer Confidence Report documents TTHM as a detected contaminant at levels below the 80 ppb MCL across all monitoring locations.

San Diego County imports virtually all of its water and has some of the most consistent TTHM detections in the region. The warm distribution temperatures and long transport distances from Northern California and the Colorado River give TTHMs more time to form. San Diego County Water Authority reports document TTHM as a routine contaminant of concern.

Orange County shows lower TTHM levels in districts that draw more water from the Orange County Water District's Groundwater Replenishment System, which uses advanced purification (reverse osmosis and UV) before rechargingthe aquifer. Districts that rely more heavily on imported MWD surface water show higher TTHM formation.

Riverside County and San Bernardino County show TTHM levels that vary significantly by utility, with groundwater-served areas generally lower and MWD-import-served areas similar to coastal counties. The Inland Empire also carries perchlorate and arsenic concerns in private well water that compound the overall water treatment picture.

To find your specific utility's TTHM data, use the California SWRCB's Drinking Water Watch portal (waterboards.ca.gov) or look up your utility's most recent Consumer Confidence Report. Your utility is required by law to mail or make available a CCR every year by July 1.

SoCal homeowners concerned about TTHMs alongside other contaminants can use our zip code water checker or book a free in-home water test to get a complete picture of what is in their tap water. TTHMs are one of several documented concerns in SoCal water, alongside chromium-6, PFAS, and elevated water hardness in most inland counties.

What filter removes trihalomethanes from drinking water?

Three categories of residential water treatment are effective against TTHMs, with differing scope and certification levels:

One approach that does not work: boiling water. Boiling does not remove volatile TTHMs and may concentrate non-volatile compounds as water evaporates. Pitcher filters with activated carbon reduce TTHMs to some degree but are not independently tested and certified to NSF/ANSI 58 for chloroform removal unless the label specifically states it. Verify any filter's actual certification at the NSF certified product registry (nsf.org).

How do I know if my water has high TTHM levels?

You have three practical ways to find out what is in your water:

Frequently asked questions about trihalomethanes in SoCal water