Understanding Radon Test Results for West St. Louis County Homes

When you buy a home in Chesterfield, Ballwin, Wildwood, or Town and Country, the radon test results are one of the first things a savvy buyer looks at. That small number in picocuries per liter tells you whether the invisible radon gas seeping from the soil is a minor issue or a serious health risk. But reading those radon test results correctly means understanding what the test actually measured, how long it ran, and whether conditions during the test might have skewed the number.

I have spent years working with Air Sense Environmental on radon testing and mitigation across West St. Louis County. In that time I have seen plenty of homes where a single test gave a borderline reading, and the homeowner did not realize that a mitigation system could bring that number down to near outdoor levels. The key is knowing what to ask after you open that lab report.

What a Radon Test Actually Measures

A typical radon test kit uses an alpha track detector, a small plastic chip inside a canister that records alpha particles emitted by decaying radon. You leave it in the lowest livable level of the home for a set period, usually three to twelve months for the most accurate picture, then mail it to a lab. The lab counts the tracks and reports an average radon level in picocuries per liter (pCi/L). The EPA recommends action at 4 pCi/L or higher, but many experts, including me, think the safe zone is well below that.

Short-term tests, often using charcoal canisters or a continuous radon monitor, run for two to seven days. A continuous radon monitor takes hourly readings and can show spikes that a passive device would miss. That can be useful if you suspect intermittent sources, like a sump pump that pulls soil gas only when it runs. But short-term results are more sensitive to weather and ventilation. A heavy rain or a sealed-up house during a cold snap can push numbers artificially high. If you are buying a home, a short-term test is a reasonable first step, but a long-term test gives you the real annual average.

Reading the Report: What the Numbers Mean

When you get your radon test results, look first at the average concentration. The lab will also give you the test duration and the start and end dates. A reading of 2.7 pCi/L might seem low, but if the test ran for only forty-eight hours during a wet spring, that number could double in a dry winter when the house is closed up tight. Conversely, a reading of 6.8 pCi/L during a drought might drop in a normal year. The radon test results are a snapshot, not a life sentence.

I once worked with a family in Wildwood who had a short-term test showing 5.2 pCi/L. They were ready to walk away from the deal. I suggested a follow-up test with a continuous radon monitor for a week. The monitor showed that readings peaked at night when the furnace ran and dropped during the day when windows were open. The long-term average was actually 3.1 pCi/L, well below the EPA action level. They bought the house and later installed a radon mitigation system as a precaution.

Factors That Affect Test Accuracy

Several things can throw off a radon test. The device must be placed in the lowest occupied area, at least twenty inches above the floor, away from drafts, heat sources, and exterior walls. If you set it in a finished basement that you use as a family room, that is fine. If you put it in a crawl space that you never enter, the reading will not reflect your actual exposure. The house should be kept in its normal living mode: windows closed, doors closed except for normal entry and exit, and the HVAC running as usual. If you open windows for a day during a short-term test, the radon test results will be artificially low.

Radon entry paths matter too. A house with a sump pump crock, a floor drain, or a gap around the radon vent pipe can pull more soil gas than a house with a sealed slab. In older homes in Ballwin and Town and Country, I often see cracks in the foundation slab that were never sealed. Those cracks can let radon gas enter directly into the living space. A test that runs during a period of high barometric pressure might show lower numbers because the soil gas is pushed down. A low-pressure system pulls it up. That is why long-term tests are more reliable.

What to Do if Your Numbers Are High

If your radon test results come back above 4 pCi/L, do not panic. The first step is to confirm with a second test, preferably a long-term alpha track detector or a continuous radon monitor. If the number holds, you need a radon mitigation system. The most common approach in West St. Louis County is sub-slab depressurization. A contractor drills a hole through the basement slab, digs a small pit beneath it, and installs a radon vent pipe that runs up through the house to the roof. A radon fan, mounted in the attic or outside, creates a vacuum under the slab that pulls soil gas away before it can enter the home.

That system also includes a manometer, often a U-tube manometer, that shows you whether the fan is working. The manometer has colored fluid that sits level when the fan is off and shifts when the fan pulls a vacuum. You should check it once a month. If the fluid is level, the fan may have failed or lost power. A simple visual check saves you from living with high radon levels without knowing it.

Active Soil Depressurization vs. Passive Systems

Newer homes in Chesterfield and Wildwood sometimes come with radon-resistant construction built in. That might include a layer of gravel under the slab, a vapor barrier, and a stub of radon vent pipe that goes up to the roof but has no fan. That is a passive system. It works some of the time but not reliably. To make it active, you add a radon fan. That turns it into an active soil depressurization system, which is the gold standard. If your home already has the pipe stub, retrofitting a fan is a straightforward job. If not, you will need a full sub-slab depressurization installation.

Common Misconceptions About Radon Test Results

One myth I hear often is that radon levels are constant year-round. They are not. Levels tend to be highest in the winter when the house is sealed and the stack effect pulls soil gas upward. Summer readings can be lower, especially if you open windows. That is why a test taken in July might give you a false sense of safety. Another myth is that a radon test kit from the hardware store is just as good as a professional test. In my experience, the difference is mostly about placement and handling. A kit that sits in direct sunlight or gets knocked over will give a bad reading. A professional installer knows where to put it and how to seal the house beforehand if needed.

Some homeowners in Ballwin have told me they think radon is only a problem in basements. That is mostly true because radon enters from the soil and accumulates in the lowest space. But if you have a slab-on-grade home with no basement, radon can still enter through cracks in the slab. I have tested slab-on-grade homes in Town and Country that showed levels above 4 pCi/L. The mitigation approach is the same: sub-slab depressurization with a radon fan and radon vent pipe to the outside.

When to Retest After Mitigation

After a radon mitigation system is installed, you should retest within thirty days to confirm the system is working. Use a long-term test if possible, but a short-term test can give you an early indication. The goal is to bring the radon level below 2 pCi/L, ideally under 1 pCi/L. If the retest shows a number above 2 pCi/L, the system may need adjustment. The contractor can check the manometer reading, increase the fan speed, or look for additional entry points.

I recommend retesting every two years after that, or whenever you make major changes to the home, like finishing a basement or replacing the HVAC system. A radon test kit is cheap compared to the peace of mind it provides. For homes in West St. Louis County, where the geology includes limestone and shale that can produce moderate radon levels, regular testing is a simple habit.

The bottom line is that radon test results are a tool, not a verdict. They tell you whether your home has a problem that needs fixing. With a properly designed radon mitigation system, you can reduce your exposure to a tiny fraction of what it was. And that is a result worth testing for.