Disinfection byproducts

Total trihalomethanes (TTHM) in water reports

TTHM is the defined sum of four trihalomethanes formed as disinfection byproducts. It is distinct from HAA5 and from the chlorine disinfectant.

View report examplesBrowse explainers →
EPA dataSource review:

This is a report-reading explainer. It is not a live advisory, a household result, or a personal health assessment.

Examples on this site:
3 entries from annual water reports on this site link to this explainer.
What these results show:
Annual-report data is evidence for the named report period, not live monitoring.
Does this mean a violation?:
Not necessarily. A listed result is not automatically a violation. Check the reported value, applicable limit, report period, and status together.
Check current notices:
Current instructions must come from the water provider or another appropriate official source.

Term Identity

What the term means in an annual report

TTHM is a regulated group measurement for four specified trihalomethanes rather than one individual compound.

Why it may appear

Systems report TTHM because disinfectants can react with material in source water and produce regulated byproducts monitored at distribution-system locations.

How it may be printed

Reports may use TTHM, TTHMs, or Total Trihalomethanes and show a highest locational running annual average, individual-sample range, MCL field, and printed status.

Other names you may see
TTHM; TTHMs; Total Trihalomethanes
Common report labels
TTHM; TTHMs; Total Trihalomethanes

Evidence

Official sources for this explainer

Use official federal, state, utility, or report sources before treating a page as final.

Regulatory Context

How to read the applicable rule language

Use this Total trihalomethanes (TTHM) context to read a report's printed fields; it does not replace the report or the current official rule.

Regulatory context

Framework

Stage 2 compliance uses the locational running annual average at each monitoring location. A range high is not independently compared with the MCL to infer a violation when the report identifies a different compliance statistic.

Limit or program terms

  • MCL
  • Locational running annual average
  • Individual-sample range

Units and source context

Read the reported statistic, location or averaging context, range, unit, and report period together.

What this page does not duplicate

There is no collective TTHM MCLG. Gallup's printed parenthetical HAA5 anomaly is not an alias and does not make the two groups equivalent.

No standalone numeric limit table is rendered here. Read the source report's printed criterion and the current official source together.

Interpretation Boundaries

What a listed result does not establish

These Total trihalomethanes (TTHM) boundaries apply to every report row connected to this explainer.

Household-level boundary

A report's TTHM statistic does not establish a current result at a particular tap or monitoring location outside the report period.

What to verify next

  1. Confirm that the row is TTHM rather than HAA5 or chlorine.
  2. Identify the locational running annual average and individual-sample range.
  3. Use the report's printed status and official provider sources rather than calculating a new conclusion from one range endpoint.

Annual report examples

Examples of total trihalomethanes (tthm) in water reports

Read each result with its report year, public water system, original label, units, and any printed criteria or status.

Utility report2025 report row

Total Trihalomethanes (HAA5)

From Gallup Water System's 2025 report (PWSID NM3508317). Labels, values, units, criteria, and status wording are shown as printed.

Source table
Regulated Substances
Testing or report period
2025
Reported value
Amount Detected: 42
Reported range
Range: 18-41.6
Unit printed in row
ppb
Criterion printed in row
MCL: 80; MCLG column: N/A
Status printed in row
Violation: No

The source prints Total Trihalomethanes (HAA5), Amount Detected 42, and Range 18-41.6.

Table note: The source labels this row Total Trihalomethanes (HAA5). The anomalous HAA5 label is preserved rather than silently corrected.

View row in report context →Open original report
Utility report2025 report row

(TTHM’s) Total Trihalomethanes

From Silver City Water System's 2025 report (PWSID NM3522609). Labels, values, units, criteria, and status wording are shown as printed.

Source table
Disinfectants and Disinfection By-Products
Testing or report period
2025
Reported value
RAA: 22
Reported range
Range Low-High: 5.42-22.4
Unit printed in row
ppb
Criterion printed in row
MCL: 80; MCLG column: N/A
Status printed in row
MEETS EPA STANDARDS: YES

The MCLG column prints N/A and the printed YES remains row- and sample-period-scoped.

View row in report context →Open original report
Utility report2025 report row

Total Trihalomethanes [TTHMs]

From Ruidoso Water System's 2025 report (PWSID NM3513114). Labels, values, units, criteria, and status wording are shown as printed.

Source table
Regulated Substances
Testing or report period
2025
Reported value
Amount Detected: 49
Reported range
Range Low-High: 5.79-80.8
Unit printed in row
ppb
Criterion printed in row
MCL: 80; MCLG column: NA
Status printed in row
VIOLATION: No

The amount 49, range 5.79-80.8, MCL 80, and printed No are preserved without calculating or inferring violation status from the range high.

View row in report context →Open original report

Continue Reading

Related explainers and report-reading guides

Read about topics related to Total trihalomethanes (TTHM), or use a guide to understand the report as a whole.

How to read a water quality report

A plain-English walkthrough of Consumer Confidence Reports, including report years, detected contaminants, legal limits, violations, and source notes.

View How to read a water quality report