Disinfection byproducts — TTHM, HAA5, and the number that is an average

By Andrew Bates, FounderPublished 2026-09-10Updated 2026-09-11

Byproducts formed when the disinfectant that makes your water safe reacts with organic material already in it. EPA caps total trihalomethanes at 0.080 mg/L and the five haloacetic acids at 0.060 mg/L. Both figures in your report are annual averages at a sampling point, and this is the one contaminant here you inhale as well as drink.

Why they exist at all

Disinfection is not optional. The byproducts are the cost of it.

EPA puts the mechanism plainly: disinfectants “can react with naturally-occurring materials in the water to form byproducts including: Trihalomethanes (THM), Haloacetic acids (HAA), Chlorite, and Bromate.”

So this is not contamination that got past the utility. It is produced by the step that made the water safe to drink, and the rules limit it rather than forbidding it.

That framing matters, because it rules out the obvious wrong reaction — wanting the disinfectant gone.

→ Chlorine and chloramine, and why the residual is there

The two limits, and what each covers

Group	Limit	Covers	EPA lists against it
TTHM	0.080 mg/L	Chloroform, bromodichloromethane, dibromochloromethane, bromoform	Liver, kidney or central nervous system problems; increased risk of cancer
HAA5	0.060 mg/L	Monochloroacetic, dichloroacetic, trichloroacetic, bromoacetic, dibromoacetic acid	Increased risk of cancer

Both are group totals. A report line reading “TTHM” is the sum of four compounds, not one.

On the evidence, EPA grades it carefully, and the grading is worth keeping: byproducts have “been shown to cause cancer and reproductive effects in lab animals and suggested bladder cancer and reproductive effects in humans.”

Shown in animals. Suggested in humans. That is a regulatory rationale, not a statement that a compliant supply is harming you.

The decision framework

Four things decide whether this is your problem, and all four are answerable before spending anything.

1. Which disinfectant your system uses

If your report names chloramine, weight this lower. EPA states chloramines are “not as reactive as chlorine with organic material in water” and so “produce substantially lower concentrations of disinfection byproducts.”

If it names chlorine, weight this higher. Same organic material, more reactive disinfectant.

If it names ultraviolet light, stop here. EPA states the rules “do not apply to water systems that use ultraviolet (UV) light.” No residual, no byproducts.

2. What your figures actually say

Find TTHM and HAA5 in the annual report your utility must send you by July 1. Compare against 0.080 and 0.060 mg/L.

→ How to read the report

If both sit well below, the remaining question is exposure route, not concentration.

If either sits close to the limit, go to criterion 3 before deciding anything, because a number near the limit is the one most affected by how it was calculated.

3. How the number was calculated — the part that changes its meaning

This is the criterion most guides skip, and it is the one that decides how much weight the figure deserves.

Under the Stage 1 rule, compliance was “the running annual average (RAA) of all samples from all monitoring locations across the system.” One system-wide average, so a high location could be offset by low ones.

The Stage 2 rule changed it to “the locational RAA (LRAA) for each monitoring location” — each point has to comply on its own.

That is a real tightening. It is still an annual average of four quarterly results at one sampling point.

If your figure is near the limit, treat it as a central tendency rather than a ceiling. EPA built a separate trigger precisely because averages hide peaks, which is criterion 4.

4. Where you sit in the distribution system

Byproducts keep forming while water sits in pipes, and EPA’s sampling design says so. Surface water systems serving more than 10,000 people must take “at least 25 percent of samples at the location of maximum residence time”, with the rest representative of average residence time.

If you are at the end of a long run, the system-representative figure understates you.

If you are close to the plant, it overstates you.

Neither is knowable from the report alone. The utility knows where its sampling points are, and it can tell you.

The trigger nobody mentions, and the report you can ask for

EPA sets an Operational Evaluation Level alongside the limit, for the stated purpose “to reduce peaks in DBP levels and exposure to high DBP levels.”

It is calculated at each location as:

OEL = (Q1 + Q2 + 2Q3) / 4

The two previous quarters plus twice the current one. Weighting the current quarter double is what makes it catch a rising trend that an annual average would smooth away.

Where the OEL exceeds the limit in any quarter, the system must conduct an operational evaluation, “submit a written report of the evaluation to the state no later than 90 days”, and “keep a copy of the operational evaluation report and make it publically available upon request.”

Publicly available upon request. Not published — requested. It is a specific document, it names the operational practices contributing to byproduct formation, and a household can ask the utility for it.

That single request is worth more than any product on this page.

The decision path

Your situation	What decides it	Where that lands you
Report names UV, no chlorine residual	The rules do not apply	No byproduct purchase
Chloramine, figures well below the limits	Lower formation, low result	Exposure route only, if anything
Chlorine, figures well below the limits	Result, not chemistry	Treat if the shower route concerns you
Either figure close to the limit	How it was averaged	Ask about the OEL and the sampling location
OEL exceeded in any quarter	A report exists	Request it before buying anything
Concerned about showering, any figure	Inhalation route	Whole-house, per EPA’s own framing
Concerned only about drinking	Ingestion route	Point-of-use, and it is the cheaper half
Private well	Outside these rules entirely	Different cluster

Choose whole-house if

The route that concerns you is the shower. EPA names disinfection byproducts among contaminants that “easily turn into gases and may pose a risk when inhaled, such as when showering”, and says a point-of-entry device can reduce them.
You want every fixture covered rather than one tap.
You accept the running cost, because every gallon gets treated.

Choose point-of-use if

Your concern is what you swallow, and the shower does not worry you.
You want the treated volume to be the volume you drink. EPA notes reverse osmosis uses approximately three times as much water as it treats, which is why it belongs at one faucet.

Choose to buy nothing if

Your system disinfects with UV, which EPA exempts from these rules.
Both figures sit well below the limits and the inhalation route does not concern you. There is no third thing this purchase would fix.

Mistakes that cost the most money

Treating the disinfectant as the problem. The residual is capped at 4.0 mg/L and it is protecting water all the way to your tap. The byproducts are the regulated health endpoint; the disinfectant is the reason the water is drinkable.

Buying a drinking filter for a shower exposure. This is the most common mismatch in the category, and it is backwards. EPA places byproducts in the inhalation group.

→ Why exposure route decides placement

Reading the reported figure as your tap water. It is a locational running annual average. Four quarters, one sampling point, and EPA weights a quarter of samples toward maximum residence time for a reason.

Assuming compliant means unchanging. EPA built the OEL because averages hide peaks. A compliant annual figure and a bad quarter are compatible.

Forgetting the cartridge. EPA states carbon filters have a specified shelf life and should be replaced on schedule. Whole-house cartridges cost more and are easier to forget than a pitcher.

Switching to bottled water over it. That changes what you drink and nothing about what you shower in, which is the route this contaminant is on.

Three situations, and what changes

Surface water supply, chlorine, figures in the upper half. The one where all four criteria point the same way. Surface water carries more organic precursor, chlorine is the more reactive disinfectant, and EPA requires conventional filtration systems to “remove specific percentages of organic material that may react to form DBPs.” Ask about the OEL, then treat by route.

Chloramine system, low figures, chlorine taste complaint. Two separate problems being conflated. The taste is the disinfectant and is addressable at the tap with the right certified claim. The byproducts are lower by chemistry.

→ Matching a filter claim to your disinfectant

New build at the end of a long main. Residence time is working against you and the system-wide picture will not show it. This is the case for asking the utility which sampling point represents your address, rather than buying on the headline figure.

The objections worth answering

“My water is compliant, so why would I treat it?” Compliance is a legal status. EPA’s own limits sit above a health goal, its OEL exists because averages hide peaks, and the inhalation route is unaffected by any of it. Whether that gap matters to you is a judgement, and a defensible answer is no.

“Surely a shower filter is enough.” It addresses one fixture. EPA’s mapping for byproducts is point-of-entry adsorptive media, distillation, aeration, carbon filtration and reverse osmosis — treatment at the main, not at the head.

“This sounds like an argument against chlorination.” It is the opposite. The alternative to a disinfectant residual is microbial risk, which is the thing the residual exists to prevent. The rules limit a cost; they do not question the trade.

“My state says something different.” It may, legitimately. EPA notes its own summary requires consulting “the federal regulations at 40 CFR 141 and any approved state requirements”, and a state can be stricter than the federal floor.

Who this is for

Households on a public water system who have found TTHM or HAA5 on their annual report and want to know whether the figure warrants action — and anyone who has been sold a drinking-water filter for a contaminant they mostly inhale.

It is not for private well owners. Nobody disinfects a well but its owner, and these rules do not reach it.

→ Well water treatment

What to do this week

Find TTHM and HAA5 on your report, then ask the utility two questions: which sampling location represents my address, and has an Operational Evaluation Level been exceeded there?

Both are free. The second one has a written report attached to it, and you are entitled to ask for it.

→ How to read your annual water quality report → Point-of-use or whole-house, by exposure route → What each NSF standard covers, and why claims vary by product → City water treatment, and the three gaps a utility cannot close → The four whole-house models with a VOC claim, which covers TTHMs → Why no certified shower filter addresses byproducts → The 153 filters certified for trihalomethanes, and the four at the main → What a VOC claim covers, and why it reaches the byproduct group → Coloured water, and why it points at byproduct precursors → Water age: why byproducts rise in summer and at the end of the line

The bottom line

Find TTHM and HAA5 in your annual report and compare them against 0.080 and 0.060 mg/L. Read both as annual averages at a sampling point rather than as your tap, because compliance is calculated that way and EPA's own sampling design targets the far end of the system. If the figures sit close to the limit, ask the utility whether an Operational Evaluation Level was exceeded and for the report that follows. Then treat by exposure route: EPA names these as contaminants inhaled while showering, which makes this the case where whole-house is the correct answer rather than the expensive one.

Who this is not for

This covers regulated byproducts on a public water system. It does not apply to private wells, which are not disinfected by anyone but their owner and fall outside these rules entirely, and it does not apply to systems disinfecting with ultraviolet light, which EPA exempts because UV forms no byproducts. It also cannot tell you your own figures — those come from your annual report, or from asking your utility for the sampling location nearest you.

Questions

What are disinfection byproducts in drinking water?

Compounds formed when a disinfectant reacts with organic material already present in the water. EPA names trihalomethanes, haloacetic acids, chlorite and bromate. They are a consequence of making water microbially safe, which is why the rules limit them rather than banning disinfection.

What do TTHM and HAA5 mean on my water report?

Two regulated groups. Total trihalomethanes covers chloroform, bromodichloromethane, dibromochloromethane and bromoform, capped at 0.080 mg/L. The five haloacetic acids cover monochloroacetic, dichloroacetic, trichloroacetic, bromoacetic and dibromoacetic acid, capped at 0.060 mg/L.

Are disinfection byproducts dangerous?

EPA states some have been shown to cause cancer and reproductive effects in lab animals, and suggested bladder cancer and reproductive effects in humans. It lists liver, kidney or central nervous system problems and increased cancer risk against the trihalomethane limit. The evidence grades differ between animals and humans, and EPA marks that difference.

Is the number in my report what comes out of my tap?

No. Under the Stage 2 rule, compliance is a locational running annual average at each monitoring point, so the figure averages four quarters at one place in the distribution system. EPA also requires larger surface water systems to take at least a quarter of samples at maximum residence time.

What is an Operational Evaluation Level?

A trigger EPA set to reduce peaks. It is calculated at each location as the two previous quarters plus twice the current quarter, divided by four. Where it exceeds the limit, the system must evaluate its practices, report to the state within 90 days, and make that report publicly available on request.

Does chloramine produce fewer byproducts than chlorine?

EPA states that because chloramines are not as reactive as chlorine with organic material, they produce substantially lower concentrations of disinfection byproducts in the distribution system, and that systems using chloramine tend to see fewer taste and odour complaints.

Should I filter byproducts at the tap or the whole house?

Whole house, and this is the case that inverts the usual advice. EPA names disinfection byproducts among contaminants that easily turn into gases and may pose a risk when inhaled, such as when showering. A drinking filter does not address that route.

What removes disinfection byproducts?

EPA maps them to point-of-entry adsorptive media systems, distillation, aeration, carbon filtration and reverse osmosis. That is a technology list, not a product recommendation — whether a given model reduces them is a certified claim on that model.

Sources — 18 claims

EPA — Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules
EPA states that disinfectants can react with naturally-occurring materials in water to form byproducts including trihalomethanes, haloacetic acids, chlorite and bromate. (captured 2026-09-10)
Limits: Names the formation route and the four byproduct groups. It does not say how much forms, which varies with the disinfectant, the amount of organic material, temperature and time in the pipes.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA states the purpose of the Disinfectants and Disinfection Byproducts Rules is to reduce exposure to disinfection byproducts, some of which have been shown to cause cancer and reproductive effects in lab animals and suggested bladder cancer and reproductive effects in humans. (captured 2026-09-10)
Limits: EPA separates the strength of the evidence deliberately: shown in lab animals, suggested in humans. It is a regulatory rationale and not a statement that a compliant supply causes harm.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA changed byproduct compliance from a system-wide running annual average under the Stage 1 rule to a locational running annual average at each monitoring location under the Stage 2 rule. (captured 2026-09-10)
Limits: Both figures are annual averages. Neither describes what came out of a particular tap on a particular day, and the LRAA still averages four quarterly results at one location.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA sets an Operational Evaluation Level for TTHM and HAA5, calculated at each monitoring location as the sum of the two previous quarters plus twice the current quarter divided by four, for the stated purpose of reducing peaks in byproduct levels. (captured 2026-09-10)
Limits: The OEL is a trigger for the system to evaluate its own practices. It is not a second enforceable limit, and exceeding it is not an MCL violation.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA requires a system that exceeds an Operational Evaluation Level to conduct an operational evaluation, submit a written report to the state within 90 days, and keep a copy and make it publicly available upon request. (captured 2026-09-10)
Limits: The report is available on request rather than published. A household has to ask the utility for it, and the requirement applies only where an OEL was exceeded.
EPA — Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules
EPA states the Disinfectants and Disinfection Byproducts Rules apply to systems that add a disinfectant other than ultraviolet light, and do not apply to water systems that use ultraviolet light. (captured 2026-09-10)
Limits: UV is exempt because it forms no byproducts, not because it is superior overall. UV leaves no residual disinfectant in the distribution system, which is a separate trade-off.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA requires public water systems using conventional filtration to remove specific percentages of the organic material that may react to form disinfection byproducts, as a treatment technique. (captured 2026-09-10)
Limits: A requirement on the utility to remove the precursor, not a limit on the byproduct. It applies to conventional filtration systems and does not state the percentages.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA requires surface water systems serving more than 10,000 people to take at least 25 percent of byproduct samples at the location of maximum residence time, with the remaining samples representative of average residence time. (captured 2026-09-10)
Limits: A sampling design requirement. It establishes that the regulator targets the far end of the distribution system, and says nothing about where any individual household sits within it.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA lists the total trihalomethanes group as chloroform, bromodichloromethane, dibromochloromethane and bromoform, and the five haloacetic acids as monochloroacetic, dichloroacetic, trichloroacetic, bromoacetic and dibromoacetic acid. (captured 2026-09-10)
Limits: Composition only. The per-compound MCLG column of EPA’s Table 2 did not survive text extraction from the PDF and is deliberately not recorded here; the enforceable limits are the group totals in contaminant-epadis-0002.
EPA — Comprehensive Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2): Quick Reference Guide, EPA 816-F-10-080, August 2010
EPA states that its quick reference guide summarises federal drinking water requirements and that full compliance requires consulting 40 CFR 141 and any approved state requirements. (captured 2026-09-10)
Limits: EPA is bounding its own summary. State requirements can be stricter than the federal floor, so a state figure may differ from the one quoted here.
EPA — National Primary Drinking Water Regulations
EPA sets a Maximum Residual Disinfectant Level of 4.0 mg/L as Cl2 for both chlorine and chloramines, with noticeable effects listed as eye and nose irritation and stomach discomfort, and anemia additionally for chloramines. (captured 2026-09-10)
Limits: An MRDL is a limit on a deliberately added disinfectant, not a contaminant to be eliminated. EPA Region 9 states the concern behind these limits is control of disinfection byproducts rather than the toxicity of the disinfectants at these levels.
EPA — National Primary Drinking Water Regulations
EPA sets a Maximum Contaminant Level of 0.080 mg/L for total trihalomethanes and 0.060 mg/L for the five haloacetic acids, both byproducts of drinking water disinfection. (captured 2026-09-10)
Limits: These are enforceable health-based limits for public water systems, measured as system-wide running averages rather than at an individual tap.
EPA Region 9 — Chloramine (page as archived in EPA’s 19 January 2017 web snapshot)
EPA states that because chloramines are less reactive with organic material than chlorine, they produce substantially lower concentrations of disinfection byproducts, and that systems using chloramine tend to see fewer taste and odour complaints. (captured 2026-09-10)
Limits: A statement about aggregate system experience, not about how any individual perceives their own water. Some people report the opposite, and this claim does not contradict that.
EPA Office of Water — Home Water Treatment Facts / Filtration Facts (published November 2005)
EPA states that for most contaminants a point-of-use device treats only the water consumed, but that contaminants such as radon, disinfection byproducts and some organic chemicals easily turn into gases and may pose a risk when inhaled, such as when showering, which a point-of-entry device can reduce. (captured 2026-09-10)
Limits: States when whole-house treatment is warranted by exposure route. It does not establish that any particular home has volatile contaminants, which requires a test.
EPA — Filtration Facts (Water Health Series), EPA 816-K-05-002
EPA maps disinfection byproducts to point-of-entry adsorptive media systems, distillation, aeration, carbon filtration and reverse osmosis. (captured 2026-09-10)
Limits: A technology list, not a product recommendation. Whether a given model reduces byproducts is a certified claim on that model, and EPA names no capacity, contact time or replacement interval here.
EPA Office of Water — Home Water Treatment Facts / Filtration Facts (published November 2005)
EPA states that reverse osmosis units use approximately three times as much water as they treat, and are effective in eliminating all disease-causing organisms and most chemical contaminants. (captured 2026-09-10)
Limits: The 2005 source describes the technology class. Certification under NSF/ANSI 58 still grants claims per model, where TDS reduction is the only required one, so this does not establish what any specific unit is certified to reduce.
EPA Office of Water — Home Water Treatment Facts / Filtration Facts (published November 2005)
EPA states that an activated carbon filter by itself is not designed to remove all disease-causing organisms, and that carbon filters have a specified shelf life and should be replaced regularly per the manufacturer instructions. (captured 2026-09-10)
Limits: Concerns activated carbon as a class. Some carbon blocks carry certified cyst reduction claims, which is a model-specific capability rather than a property of carbon.
EPA — CCR Information for Consumers (page last updated 2025-11-21)
EPA requires community water systems to deliver a Consumer Confidence Report to their customers each year by July 1. (captured 2026-09-10)
Limits: The requirement falls on community water systems. It does not reach private wells, and it does not oblige a landlord to pass a report to a tenant.