Total trihalomethanes, usually shortened to TTHM, is a group of disinfection byproducts. They form when chlorine, which cities add to kill germs in water, reacts with natural organic matter already present in the source water. O'Fallon draws on a mix of river water and groundwater. River water generally carries more organic material than groundwater, so the chlorine step is where TTHM tends to come from in local supply.
Public water reports for the area list chlorine and Missouri River organics as the sources of disinfection byproducts. The levels you see are not fixed. They can shift with the season, with water temperature, and with how long water sits in pipes and storage before it reaches your tap.
Why Your Tap May Differ From the Plant
TTHM is measured across the distribution system, so the amount leaving the treatment plant can differ from what reaches a house at the far end of a long line. Homes served by water that moves through storage slowly may see different results than homes closer to the plant. A sample taken at your own kitchen faucet tells you what actually reaches the cold tap you drink from, which is the question that matters for your household.
How to Test for TTHM at Home
Standard test strips and most consumer kits do not measure TTHM. You need a laboratory certified for drinking water analysis that can run a total trihalomethanes test. When you call, ask for TTHM by name rather than general chlorine or water quality. Ask how the sample should be collected, whether it must be kept cold, which container to use, and how it should be shipped.
Collection matters. Follow the lab's instructions exactly, use the container it supplies, and label the sample with the date and time. Ask whether the lab wants you to run the tap first and for how long. Write down what you did, because a result without a collection record is hard to interpret later.
The Missouri State Public Health Lab offers low-cost screening for coliform, E. coli, and iron bacteria, which matters most for private wells. For TTHM, a certified commercial laboratory is usually the practical route, so check with any lab before you assume it can run this test.
Filter Choices if TTHM Is a Concern
Activated carbon is a common approach for reducing TTHM. Point-of-use units under a kitchen sink treat only the drinking and cooking tap. Whole-house carbon treats all water entering the home but needs a larger unit and a more deliberate replacement plan. Either way, the filter should carry third-party certification that names TTHM as a contaminant it reduces. A general statement that a filter improves taste or removes chlorine is not the same claim, because chlorine removal and TTHM reduction are different things.
Replacement is part of the decision. A carbon cartridge that is overdue for change can perform poorly, and a spent unit can hold material that you do not want to keep in your water. Follow the manufacturer's replacement interval and keep a dated record.
Questions to Settle Before You Buy
- Do you want treatment at only the drinking tap, or for the whole house?
- Does the filter's certification name TTHM specifically?
- Who will replace the cartridge, and how often does the manufacturer specify?
- Does the unit's flow rate suit how many fixtures you run at the same time?
- Have you tested your own tap for TTHM, not just the plant or a neighbor's report?
What to Do Next
Start with the question, not the equipment. Get a TTHM result from your own tap, record the date and collection method, and compare it with what you already know about your supply. If the result or your concerns point toward a filter, match the certification to the contaminant before you choose a model.
If your home runs on a private well, TTHM depends on whether that well is disinfected with chlorine, so start by confirming that. If it is not chlorinated, focus your testing on the problems that fit a well, such as bacteria, nitrates, iron, and manganese.