Why a Radon Retest Matters for Your Home in Jefferson County

If you live in Jefferson County, Missouri, and you have already tested your home for radon, you might think the job is done. But radon levels are not static. They shift with the seasons, with changes to your home, and even with the weather. A single test gives you a snapshot, not a guarantee. That is why a radon retest is something every homeowner should plan for, especially here in parts of Arnold, Festus, High Ridge, and Hillsboro where the geology can produce significant year-to-year variation.

I have been working in radon measurement and mitigation for years, and I have seen too many families assume that because their first test result was low, they are in the clear. The truth is more complicated. Soil conditions, barometric pressure, and even how you use your basement all affect how radon moves into a house. A radon retest catches those changes before they become a health problem.

How Radon Levels Change Over Time

Radon comes from the natural decay of uranium in soil and rock. In Jefferson County, the underlying limestone and granite formations can produce pockets of higher radon potential. But even on a single property, the amount of radon entering a home can vary by a factor of two or three between summer and winter. During colder months, homes tend to be sealed tight, and the stack effect pulls more soil gas inside. A test done in July might show 2.0 pCi/L, while a retest in January could read 5.5 pCi/L.

That is not a flaw in the first test. It is simply the reality of how radon behaves. The EPA recommends action at 4.0 pCi/L, but many health experts suggest mitigation above 2.0 pCi/L because levels fluctuate. A single test is a starting point, not a permanent answer. That is why a radon retest, ideally done in a different season from the first one, gives you a much clearer picture of your long-term exposure.

What to Use for a Retest

When you decide to do a radon retest, you have several options. The most common is a charcoal canister test, which you leave in place for two to seven days and then mail to a lab. These are inexpensive and reliable for short-term screening. Another option is an alpha-track detector, which you leave in place for three to twelve months. Alpha-track detectors give an average reading over a longer period, smoothing out daily spikes. I often recommend them for a retest because they capture the seasonal variation that a short-term test might miss.

For homeowners who want real-time data, a continuous radon monitor from companies like Airthings or Sun Nuclear can be useful. These devices plug in and show hourly readings. They are not cheap, but they are excellent for tracking changes after mitigation or for ongoing peace of mind. I have used continuous radon monitors in homes where the family wanted to see the immediate effect of basement ventilation changes. The data is immediate and clear.

When to Retest

The EPA suggests testing every two years, but I think that is a minimum. If you have made any structural changes to your home, such as finishing a basement, adding a sump pump, or sealing foundation cracks, you should do a radon retest right away. Even changes like installing a new HVAC system or replacing windows can alter air pressure and airflow, which affects how radon enters.

If you already have a radon mitigation system, such as sub-slab depressurization, you should still retest every couple of years. Fans wear out. Pipes can get blocked. A friend of mine in High Ridge had a mitigation system installed five years ago, and when I checked it last winter, the fan was running but the manometer showed zero pressure. The system had failed silently. A radon retest revealed levels had climbed back to 6.2 pCi/L. A simple retest caught a problem that could have gone unnoticed for years.

The Role of a Professional

You can buy a radon test kit at a hardware store or online, and many homeowners do their own testing. That is fine for a basic check. But for a retest that you really trust, consider hiring a certified professional. Air Sense Environmental, for example, serves Jefferson County and the surrounding areas, including Arnold, Festus, High Ridge, and Hillsboro. A professional brings calibrated equipment, knowledge of placement, and the ability to interpret results in context.

One advantage of working with a company that does both testing and mitigation is that they can spot issues that a test kit cannot. For instance, if your initial test was done in a finished basement and the retest shows a higher reading, a professional can check whether the change is due to a new crack in the slab, a failing mitigation system, or simply the season. They can also measure other factors like soil moisture and air pressure that affect radon entry.

What a Retest Tells You That the First Test Did Not

A first test tells you whether you have a problem now. A radon retest tells you whether the problem is getting worse, staying the same, or responding to mitigation. It also helps you verify that a radon mitigation system is working as designed. Without a retest, you are guessing.

I recall a house in Festus where the initial test showed 3.8 pCi/L. The homeowner decided not to mitigate because it was just below the EPA action level. A year later, a retest with an alpha-track detector showed 7.1 pCi/L. The difference was partly seasonal and partly because the homeowner had sealed some crawlspace vents to save energy, which actually increased the vacuum drawing radon inside. The retest caught the change before the family had been exposed to high levels for another year.

Practical Steps for Your Retest

Before you start, close all windows and doors for at least twelve hours before the test and keep them closed during the test period. Place the detector in the lowest livable area of the house, such as a basement or a first-floor room if you have no basement. Do not put it in a kitchen, bathroom, or laundry room where humidity or drafts can skew the reading. Follow the instructions that come with the test kit to the letter. A charcoal canister left out for an extra day will give a falsely high result, while one opened late will give a falsely low result.

If you choose a continuous radon monitor, place it at breathing height, away from windows and exterior walls. Let it run for at least 48 hours before you trust the average reading. Some monitors, like those from Airthings or Sun Nuclear, have smartphone apps that let you see the data in real time. That can be reassuring, but remember that short-term spikes are normal. Look at the long-term average.

Radon and Real Estate Transactions

If you are selling your home in Jefferson County, a recent radon retest can be a strong selling point. Buyers are more aware of radon risks than they used to be, especially in areas like Hillsboro and Arnold where radon is a known issue. A retest that shows levels below 2.0 pCi/L, ideally with a long-term alpha-track detector, can remove a potential objection. If you have a mitigation system, include the retest results in your disclosure. It shows you have maintained the system and that it is working.

On the buying side, never rely solely on a test the seller did a year ago. Request a new test, or better yet, do your own radon retest after you move in. The conditions of an empty house are different from a lived-in one, and the previous test may have been done under ideal conditions that do not reflect real occupancy.

Keeping Track of Results

I recommend keeping a simple log of every radon test you do, including the date, location, type of detector, and result. Over time, that log becomes a history that helps you spot trends. If you see a slow increase over three or four tests, you can investigate before levels become high. Some companies, like NRD LLC and Radalink, offer cloud-based platforms where you can store your test history, but a notebook works just as well. The important thing is to have a record.

Your local Health Department may also keep records of radon tests in the area. While they cannot give you individual results, they can tell you whether your neighborhood tends to have higher readings. That information can help you decide how often to retest.

Final Thoughts

Radon is a serious health risk, but it is also a manageable one. Testing is cheap and easy. Mitigation is effective when done right. The missing piece for many homeowners is the follow-up. A radon retest is not an admission that the first test was wrong. It is a recognition that conditions change and that your health deserves more than a single data point. Whether you use a charcoal canister, an alpha-track detector, or a continuous radon monitor, make retesting part of your home maintenance routine. It is one of the simplest things you can do to protect your family.