Disinfection Byproducts in Phoenix Metro Drinking Water: What TTHM and HAA5 Data Show

Phoenix-area tap water is treated with chloramine, a disinfectant blend of chlorine and ammonia that utilities adopted in part because it produces fewer total trihalomethanes (TTHMs) than free chlorine. TTHMs and haloacetic acids (HAA5s) are the two main classes of disinfection byproducts the EPA regulates under the Stage 2 Disinfectants and Disinfection Byproducts Rule. The EPA sets a Maximum Contaminant Level of 80 parts per billion (ppb) for TTHMs and 60 ppb for HAA5s (epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations). Phoenix Water's 2023 Consumer Confidence Report shows the utility measured TTHMs at approximately 18 ppb and HAA5s at approximately 12 ppb, both well below federal limits. Levels that low are consistent with a chloramine system: chloramine generates fewer TTHMs than free chlorine but can produce higher concentrations of some HAA5 compounds and different byproducts altogether. Understanding what your utility's data actually shows, and where real exposure gaps exist, is the starting point for any household decision about water filtration.

What are disinfection byproducts and why does the EPA regulate them?

When disinfectants react with naturally occurring organic matter in source water, they form chemical compounds called disinfection byproducts. TTHMs are a family of four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. HAA5s are five haloacetic acid compounds. The EPA established MCLs for both under the 1998 Stage 1 and 2006 Stage 2 Disinfectants and Disinfection Byproducts Rules (epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations). The TTHM limit is 80 ppb and the HAA5 limit is 60 ppb. These limits reflect a balance between the measurable cancer and reproductive risks from long-term elevated exposure and the non-negotiable public health benefit of disinfecting water supplies. The EPA states clearly that the risk of disease from waterborne pathogens is far greater than the risk from disinfection byproducts at levels found in compliant water systems. Disinfection must occur, and byproducts are an unavoidable result. The regulatory question is whether concentrations stay below thresholds linked to long-term harm.

What Phoenix metro water actually shows for TTHMs and HAA5s

The City of Phoenix publishes an annual Consumer Confidence Report at phoenix.gov/waterservices that includes monitored TTHM and HAA5 concentrations across the distribution system. The 2023 CCR data shows Phoenix measured TTHMs at approximately 18 ppb and HAA5s at approximately 12 ppb. Both figures are substantially below the EPA MCLs of 80 ppb and 60 ppb respectively. Phoenix's relatively low TTHM numbers are consistent with its use of chloramine as the residual disinfectant. The EPA's chloramines fact sheet documents that chloramine produces significantly fewer TTHMs than free chlorine when applied to the same source water (epa.gov/sites/default/files/2015-09/documents/chloramines_factsheet.pdf). Phoenix draws source water from the Salt River Project and the Central Arizona Project, both surface-water sources that carry organic matter typical of desert river systems. The EWG Tap Water Database (ewg.org/tapwater/) applies more stringent health guidelines than EPA MCLs and shows TTHM and HAA5 levels for metro utilities compared to EWG's lower health-based thresholds. EWG's guidelines reflect precautionary limits rather than regulatory standards.

• Total Trihalomethanes (TTHMs): Phoenix 2023 CCR reported approximately 18 ppb. EPA MCL: 80 ppb.
• Haloacetic Acids (HAA5s): Phoenix 2023 CCR reported approximately 12 ppb. EPA MCL: 60 ppb.
• Both figures are well below regulatory limits. Download your utility's current Consumer Confidence Report at phoenix.gov/waterservices for the most recent measured values.

Why chloramine reduces TTHMs but shifts the byproduct profile

Phoenix switched to chloramine as its residual disinfectant in part to reduce TTHM formation, a strategy documented in the EPA chloramines fact sheet (epa.gov/sites/default/files/2015-09/documents/chloramines_factsheet.pdf). Chloramine is formed by combining chlorine and ammonia at a controlled ratio. It is a weaker disinfectant than free chlorine but maintains residual protection through long distribution systems while generating substantially fewer TTHMs. The tradeoff is that chloramine produces different byproducts, including some HAA5 compounds, nitrosamines, and iodoacids, some of which are not yet fully characterized. Oversight of disinfection byproduct monitoring in Arizona falls to the Arizona Department of Environmental Quality (azdeq.gov/drinking-water), which enforces EPA MCLs and requires utilities to submit regular monitoring results. Homeowners can also use the Arizona Department of Environmental Quality site to access utility-specific compliance records beyond what appears in the annual CCR.

How are Phoenix residents exposed to TTHMs beyond drinking water?

Drinking is not the only exposure pathway for TTHMs. Chloroform, the most common TTHM compound, is volatile and releases readily from hot water. Public health researchers have documented that showering, bathing, and running dishwashers in homes with chlorinated or chloraminated tap water creates inhalation and dermal absorption pathways that add to ingestion exposure. This is not unique to Phoenix: it applies to any chlorinated water supply where TTHMs are present. A whole-home carbon filtration system installed on the main water line reduces chloramine and the organic precursors from which TTHMs form before water reaches any fixture, addressing shower and bath exposure along with kitchen and drinking water. A point-of-use reverse osmosis system at the kitchen tap reduces TTHMs and HAA5s specifically in drinking and cooking water. Both approaches are described in detail on the whole-home filtration and reverse osmosis pages of this site.

What the EPA says about long-term TTHM health effects

The EPA's National Primary Drinking Water Regulations page (epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations) documents that long-term exposure to TTHMs above the MCL is associated with increased bladder cancer risk and adverse reproductive effects including low birth weight. HAA5s at levels above the MCL are associated with increased cancer risk. These health designations apply to exposure at or above the MCL over many years, not to one-time or short-term consumption. Phoenix's reported levels of approximately 18 ppb TTHM and 12 ppb HAA5 are well below the thresholds where regulatory action is required. Homeowners with documented concerns, particularly those who are pregnant, immunocompromised, or have a family history of bladder cancer, may choose to apply additional point-of-use treatment as a precautionary measure independent of whether the utility is in violation of any standard.

What Phoenix homeowners can do to reduce disinfection byproduct exposure

• Read your utility's Consumer Confidence Report. Phoenix Water publishes it at phoenix.gov/waterservices. Scottsdale, Mesa, Chandler, Gilbert, and other metro utilities publish their own CCRs on city websites.
• Check EWG's Tap Water Database at ewg.org/tapwater/ to compare your utility's measured levels against EWG's independent health-based thresholds alongside the EPA's regulatory MCLs.
• A whole-home carbon filtration system removes chloramine before water reaches any fixture, reducing TTHM precursors site-wide including at showers and bathtubs.
• A reverse osmosis system at the kitchen tap reduces TTHMs and HAA5s in drinking and cooking water to very low concentrations.
• Schedule a free professional water test to confirm current contaminant levels at your specific tap. Utility-wide averages can vary by zone and season.
• Call Aqua Wellness at (928) 451-1591 to schedule a free water test and discuss which combination of treatment matches your household's priorities.

Do TTHM and HAA5 levels vary by season or neighborhood in Phoenix?

TTHM formation rates depend on water temperature, the organic carbon load in source water, and the time water spends in the distribution system. Higher summer temperatures accelerate the chemical reactions between chloramine and naturally occurring organic matter, so Phoenix utilities typically see higher TTHM readings during summer months than in cooler periods. Neighborhoods at the edges of a long distribution system receive water that has had more contact time with pipe surfaces and more opportunity for byproduct formation than water measured directly at the treatment plant output. Phoenix Water's Consumer Confidence Report includes locational running annual average data that lets homeowners compare results across different monitoring zones rather than relying solely on the system-wide average. Downloading the current CCR from phoenix.gov/waterservices and reviewing the reported range across all monitoring locations gives the most accurate picture of what is reaching your specific tap. The Phoenix and Mesa CCR explainer on this site walks through how to find and interpret these locational data tables, including what the running annual average actually measures and how to compare it to the EPA MCL.

How Scottsdale, Mesa, Tempe, and Chandler TTHM levels compare to Phoenix

Phoenix metro water quality varies across utilities because each city operates its own treatment system drawing from a different blend of sources. Scottsdale Water treats a combination of Colorado River water, Central Arizona Project water, and locally recovered water, and publishes its own Consumer Confidence Report each year at scottsdale.gov/water. Mesa, Tempe, Chandler, and Gilbert draw primarily from Salt River Project and Central Arizona Project surface-water supplies treated at their respective facilities. Because these utilities draw from the same general watershed as Phoenix, their TTHM and HAA5 baseline profiles tend to resemble Phoenix's, with variations driven by seasonal source blending ratios, treatment decisions, and distribution system characteristics. Each household's most accurate reference is its own utility's CCR rather than a neighboring city's published data. EWG's Tap Water Database at ewg.org/tapwater/ lets you enter a zip code to pull utility-specific measured concentrations alongside EWG's independent health-based threshold comparisons, whether you are in Phoenix, Scottsdale, Mesa, Tempe, or another metro municipality. Our Scottsdale water quality page covers what Scottsdale's CCR reports for key contaminants including disinfection byproducts.

What other disinfection byproducts exist beyond TTHMs and HAA5s?

The EPA's Stage 2 Disinfection Byproducts Rule sets enforceable Maximum Contaminant Levels only for TTHMs and HAA5s because these are the most extensively studied and widespread disinfection byproduct groups in municipal water. Chloramine systems produce additional byproducts that are less comprehensively regulated. Nitrosamines are one such group that the EPA monitors through its Unregulated Contaminant Monitoring Rule program and has included on the Contaminant Candidate List at epa.gov/ccl as candidates for potential future regulatory consideration. Iodoacids and haloacetonitriles are other byproducts documented in chloramine-disinfected systems in the peer-reviewed literature. The fact that these compounds are currently unregulated does not mean they are known to present health risks at levels found in compliant systems; establishing a Maximum Contaminant Level requires years of epidemiological and toxicological research. For homeowners seeking broad coverage across both regulated and emerging byproduct classes, a combination of whole-home catalytic carbon filtration and a point of use reverse osmosis unit addresses the most relevant exposure pathways: volatile compounds at showers and baths as well as dissolved compounds in drinking and cooking water. A free water test from Aqua Wellness at (928) 451-1591 includes a review of your specific tap water results and which treatment approach matches your water quality data.