TTHMs Explained: Facts About Your Drinking Water
Simple, accurate information about TTHMs, water testing, and EPA rules
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We want our customers to have accurate, understandable information about drinking water quality.
Recently, statements shared online have suggested that our water system is automatically in violation of federal drinking water standards because one TTHM sample result was above the maximum contaminant level. That is not how the federal compliance rule is applied.
Under EPA’s Stage 2 Disinfectants and Disinfection Byproducts Rule, compliance for TTHMs is determined using the Locational Running Annual Average (LRAA) at each monitoring location, based on the most recent four quarters of sampling results at that location.
What TTHM Means
Total trihalomethanes, a group of disinfection byproducts that can form during water treatment.
Federal Standard
80 µg/L, which is the same as 0.080 mg/L or 80 ppb.
How Compliance Is Measured
By the LRAA at each monitoring site, not by one routine sample alone.
Home Filters
Some activated-carbon filters can reduce TTHMs when they are specifically certified and maintained properly.
What Are TTHMs?
TTHM stands for total trihalomethanes. These are disinfection byproducts that can form when disinfectants such as chlorine react with naturally occurring organic material in source water.
TTHMs are regulated by the U.S. Environmental Protection Agency because long-term exposure above the federal standard may increase health risk.
Why Do Water Systems Disinfect Water?
Disinfection is one of the most important parts of producing safe drinking water. It helps control bacteria, viruses, and other microbes that can cause immediate illness.
Water treatment is a balance. Utilities must maintain strong microbial protection while also managing and minimizing disinfection byproducts such as TTHMs.
What Is the Federal Standard for TTHMs?
80 µg/L = 0.080 mg/L = 80 ppb
This is the EPA maximum contaminant level, or MCL, for total trihalomethanes in drinking water.
Does One Sample Above 80 Mean a Violation?
No – not by itself in the normal quarterly compliance framework.
For Stage 2 TTHM compliance, EPA evaluates the Locational Running Annual Average (LRAA) for each sampling location. That means results from the most recent four consecutive quarters at a specific site are used to determine compliance.
A system is in violation of the TTHM MCL if the LRAA at any monitoring location exceeds the federal standard.
A Simple Example
One higher quarterly result is important and must be evaluated, but one result alone does not automatically mean a violation.
Quarter 1: 25 ppb
Quarter 2: 30 ppb
Quarter 3: 40 ppb
Quarter 4: 109 ppb
Average (LRAA) = (25 + 30 + 40 + 109) / 4 = 51 ppb
Because the running annual average in this example is 51 ppb, which is below the 80 ppb standard, this example would not be a violation.
Why Can TTHM Levels Change During the Year?
TTHM levels can vary based on conditions in the water and the distribution system. Warmer temperatures, longer water age, and natural organic matter can all affect disinfection byproduct formation.
That is one reason the rule uses a running annual average rather than relying only on a single routine result.
If There Were a Violation, Would Customers Be Told?
Yes. Drinking water systems are required to report violations and provide public notice when required by the regulatory framework.
Can Home Water Filters Reduce TTHMs?
Some can.
Activated carbon is a recognized treatment approach for reducing trihalomethanes, and some home filters are certified for Trihalomethanes (TTHM) Reduction.
Not every filter is certified for the same contaminants. Customers should check the product label or manufacturer information and look for language such as “Trihalomethanes (TTHM) Reduction”.
The filter also needs to be used and replaced according to the manufacturer’s instructions.
Myth vs. Fact
Myth: “One sample over 80 means the water system is automatically in violation.”
Fact: EPA’s Stage 2 rule bases compliance on the LRAA at each monitoring location using four quarters of results, not on one routine sample standing alone.
Myth: “TTHMs are not regulated.”
Fact: TTHMs are regulated by EPA under the Stage 2 Disinfectants and Disinfection Byproducts Rule.
Myth: “Carbon filters do not remove TTHMs.”
Fact: Some activated-carbon drinking water filters are certified for TTHM reduction and can reduce TTHMs when properly selected and maintained.
Frequently Asked Questions
What does TTHM mean?
Total trihalomethanes, a group of disinfection byproducts that can form when disinfectant reacts with natural material in water.
What is the legal limit?
80 µg/L, also written as 0.080 mg/L or 80 ppb.
Is one high sample automatically a violation?
No. Compliance is determined by the LRAA over four quarters at each monitoring site.
Can home filters help?
Some can. Look for filters specifically certified for Trihalomethanes (TTHM) Reduction.
Our Commitment
We understand that drinking water information can be technical.
Our goal is to provide clear, accurate, and easy-to-understand information. If you have questions about TTHMs, water quality, or home filtration options, please contact us directly.