If you have already tested your home for radon, you might think the job is done. But radon levels are not static. They shift with seasons, weather, changes in your home's foundation, and even the way you run your HVAC system. That is why a radon retest is not just a box to check off. It is a practical step that tells you whether your family's exposure has changed and whether your mitigation system is still working as it should.
Living in Jefferson County, Missouri, means dealing with geology that can produce pockets of radon gas. Homes in Arnold, Festus, High Ridge, and Hillsboro all sit on ground that can release radon into basements and crawl spaces. I have seen homes where the first test came back low, only for a later retest to show levels well above the EPA action guideline. That is the kind of surprise nobody wants, but it happens more often than people expect.
How Radon Levels Change Over Time
Radon enters a home through cracks in the foundation, gaps around pipes, and openings in the slab. The rate of entry depends on the pressure difference between the soil and the indoor air. When the ground is wet or frozen, that pressure difference can increase, drawing more gas inside. A test done in July might show a moderate reading. A retest in January could be double that number.
I once worked with a homeowner in Festus who had tested in late spring and got a reading of 3.2 pCi/L. That is below the EPA action level of 4.0 pCi/L, so they felt fine. But the next winter, they noticed a musty smell in the basement and decided to retest. The result came back at 5.8 pCi/L. That jump was enough to push them into the range where mitigation is strongly recommended. A radon retest caught the problem before it became a long-term health issue.
What a Radon Retest Tells You
A retest does more than give you a new number. It confirms whether your previous test was an accurate snapshot or a fluke. For homes that already have a radon mitigation system installed, the retest is the only way to know if that system is still performing. Sub-slab depressurization systems, for example, rely on a fan that pulls radon from below the foundation and vents it outside. If that fan loses efficiency or if a crack develops in the suction pit, the indoor level can rise again without any obvious sign.
I have also seen cases where homeowners sealed basement cracks and improved basement ventilation only to find that radon levels actually went up after the changes. That counterintuitive result happens because altering airflow patterns can change how radon moves through the house. A retest after any significant renovation or change to the heating and cooling system is smart practice.
Testing Methods and What to Look For
There are several ways to test for radon, and each has strengths and weaknesses. The most common short-term method uses a charcoal canister that sits in the lowest livable area for two to seven days. These are inexpensive and widely available, but they only capture a brief window of time. For a retest, I usually recommend a longer-term approach if you want a more reliable average.
Alpha-track detectors are a good choice for a 90-day to one-year test. They record cumulative exposure and smooth out daily fluctuations. Continuous radon monitors, made by companies like Airthings and Sun Nuclear, give you real-time readings and can show daily patterns. Some homeowners in High Ridge and Hillsboro have started using these monitors because they can see how levels change with weather fronts or when they run the furnace.
When you order a radon test kit, make sure it comes from a reputable source and that it is analyzed by a certified lab. The EPA recommends that tests be conducted by a qualified professional if you are making decisions about mitigation. That is where companies like Air Sense Environmental come in. They understand the local geology of Jefferson County and can help interpret results in context.
When to Retest
There is no single rule for when to retest, but some situations clearly call for it. If you have recently installed a radon mitigation system, you should retest within 30 days to verify the system is working. After that, every two to five years is a reasonable interval, especially if you live in an area with known radon potential.
You should also retest after any major structural work, such as adding a new addition, replacing windows, changing the foundation drainage, or finishing a basement. Even something as simple as sealing a crawl space or adding insulation to the attic can alter the stack effect that pulls radon indoors.
If you move into a home that was previously tested, do not rely on the old test report. The previous owners might have tested during a mild season or might have tampered with the test. A fresh radon retest gives you your own baseline.
The Role of the Health Department
The local Health Department in Jefferson County provides information on radon risk and can sometimes offer test kits at reduced cost. They keep records of test results from the area and can tell you if your neighborhood has shown elevated readings. I have found that checking with them before buying a home or before a retest adds a helpful layer of local data.
For homes in Arnold and Festus, the Health Department data often shows a pattern of moderate to high radon readings, especially in homes built on limestone bedrock. Knowing that ahead of time helps you decide whether to invest in a continuous radon monitor or to schedule a retest every year.
Interpreting the Results
When you get your retest results, compare them to the EPA action level of 4.0 pCi/L. But do not treat that number as a magic cutoff. The EPA itself says there is no safe level of radon. It is a radioactive gas, and any exposure carries some risk. If your retest shows a level between 2.0 and 4.0 pCi/L, you may want to consider mitigation even though it is not required. If it is above 4.0, you should take action.
I have seen homeowners in Hillsboro who tested at 3.9 pCi/L and decided to wait. A year later, a retest showed 6.2 pCi/L. By then, they had already spent months breathing elevated levels. The small cost of a retest is trivial compared to the health risk of prolonged exposure.
Choosing a Mitigation Professional
If your retest indicates a problem, the next step is to hire a qualified radon mitigation contractor. Look for someone certified by the NRD LLC or the Radalink network, and ask if they have experience with homes in Jefferson County. The type of mitigation that works best depends on your home's construction. Sub-slab depressurization is the most common for houses with concrete slabs, but homes with crawl spaces may need a different approach.
Air Sense Environmental, for example, serves the entire Jefferson County area including Arnold, Festus, High Ridge, and Hillsboro. They can assess your home and design a system that fits. But do not take my word for it. Ask for references and check that they follow EPA guidelines.
Final Thoughts on Radon Retesting
A radon retest is not an expensive or complicated process. It takes a few days of your time and the cost of a test kit. But the information it gives you is invaluable. Radon is the second leading cause of lung cancer after smoking, and it is completely preventable. Putting off a retest because you think your home is fine is a gamble that does not pay off.
If you have never tested, start with a simple charcoal canister test. If you have already tested and the result was low, do not assume it will stay low. Set a reminder to retest in two years, or sooner if you make changes to your home. And if you have a radon mitigation system, test it annually. The system is only as good as its last performance check.
Living in Jefferson County means living with a real risk of radon exposure. But it also means you have access to knowledgeable professionals, affordable testing options, and a community that understands the issue. A radon retest is the easiest way to stay ahead of the problem and keep your home safe for the long term.