If you live in O'Fallon and have read the annual water report, you may have noticed a line for total trihalomethanes, often shortened to TTHM. These are disinfection byproducts that can form when chlorine reacts with natural organic matter in source water. Because local supplies draw on Missouri River water, which carries that organic matter, it is a reasonable thing to look up. This guide explains what the term means, where to find it, and what a homeowner can do next without guessing.
What TTHM Is
Trihalomethanes form when a disinfectant added to water reacts with organic material already present in the source. Chlorine is the usual disinfectant, and the reaction can happen during treatment or while water moves through distribution pipes. TTHM is not a single substance. It is a group of related compounds, and the report usually shows their combined total.
This guide does not make health claims about TTHM. A reported value is a tracking and regulatory figure, so the useful step is to read the report and your provider's notes to understand what was measured and how it was compared with the standards that apply to public water systems.
Where to Find It in Your Report
- Open the most recent annual water quality report from your provider. Many providers post it online and mail a copy to customers.
- Look for the table of detected contaminants. Disinfection byproducts are often listed apart from microbial results.
- Note the reported value, any range given, the standard it is compared against, and the sampling year.
- Check whether the report describes where samples were taken. Water near the far end of a distribution line can differ from water measured at the treatment plant.
- Look for any notes about corrective action or planned changes to treatment.
Questions Worth Asking Your Provider
- Where in the system is the sample taken, and how often is it collected?
- How has the reported value changed over recent years, and does it vary by season?
- Is there a pattern tied to temperature or changes in source water?
- What steps are being taken to limit byproduct formation?
Testing at Home
Consumer test strips and simple kits can show whether chlorine is present, but they generally do not measure trihalomethanes. For a specific TTHM result at your own tap, a certified laboratory analysis is the way to get a number you can compare with the provider's report. Ask the lab which method it uses, what sample container it requires, and how long a sample can wait before analysis. Let the tap run normally before collecting, and label the sample with the date and the exact location.
The Missouri State Public Health Laboratory offers low-cost testing for coliform bacteria and E. coli, which is a different question from TTHM. Confirm what each lab analyzes before you send a sample, so the result answers the question you actually have.
Options Homeowners Consider
Some homeowners put a point-of-use filter on one faucet, such as the one used for drinking or cooking. Activated carbon is the material most often used for this purpose. Look for the certification listing on the product, and read which compounds it covers. Performance depends on the specific compound, water temperature, contact time, and how often the cartridge is replaced, so the listing matters more than the marketing.
A whole-house carbon system is a larger decision. It treats all water entering the home, which also means showers and laundry. Before considering one, confirm that the system is sized for your flow rate, and plan replacement intervals in advance so the media does not run out without you noticing.
What to Do Next
- Pull the latest annual report and find the byproduct line.
- Write down the year, the reported value, and the standard it is compared against.
- Call your provider with the questions above.
- If you want a result for your own tap, arrange a certified laboratory test.
- Only then decide whether a filter at one faucet or a whole-house system fits your household.
Treat the report as your starting point and your own measurements as the second step. For homeowners in O'Fallon, that order keeps the decision tied to information you can verify rather than to whatever a seller suggests.