How to Read Your Radon Test Results and Take Action

When you buy a home in St. Charles County, Missouri, or anywhere in the state, the radon test is one of those things that can feel both urgent and confusing. You get a packet back from the lab, see numbers like 4.2 pCi/L or 8.7 pCi/L, and maybe a note about the EPA Action Level. But what does that actually mean for your family? And what do you do next? I have been through this myself, both as a homeowner and as someone who has worked alongside mitigation pros at Air Sense Environmental. The short answer is that your radon test results are not just a pass-fail grade. They tell a story about your home’s soil, foundation, and air pressure that you can use to decide on next steps.

What the Numbers Mean

Radon is measured in picocuries per liter of air, or pCi/L. The EPA recommends fixing any home that has a level at or above 4 pCi/L. That is the Action Level. But here is the nuance: there is no truly safe level of radon. The EPA also says that any exposure carries some risk, and the risk increases with the level and with the time you spend breathing that air. So a reading of 3.9 pCi/L is technically below the Action Level, but it is still higher than outdoor air, which averages around 0.4 pCi/L. Some experts suggest that if you are in the 2 to 4 range and you have the budget, you should still consider mitigation.

Your radon test results also depend on the type of test you used. A short-term test, often a charcoal canister or a continuous radon monitor sitting in your lowest living area for two to seven days, gives you a snapshot. That snapshot can be influenced by weather, by whether the windows were closed, and by how much the furnace ran. A long-term test, which runs for three months to a year, gives you an average that smooths out those swings. If you are selling a home, you usually get a short-term test. If you are testing your own home for peace of mind, a long-term test is more reliable.

The Shape of the Results

When I first got my radon test results back for a house in St. Peters, the number was 5.8 pCi/L. That is above the Action Level, so I knew I had to do something. But I also noticed that the report noted the test was a short-term test done in winter. Cold weather can create a stack effect in a house, pulling more soil gas upward and giving a higher reading. So the question was: would a summertime test give a lower number? Possibly, but the EPA still says that if your short-term test is above 4, you should take action. You cannot just wait for better weather.

Another thing that matters is where you placed the test. The EPA says to test in the lowest livable area of the home. That could be a basement, a crawl space that you use as a family room, or even a slab-on-grade living room. If you put the test in an unfinished basement that you never use, the result might be higher than what you actually breathe. But if you spend time in that basement, even occasionally, then that reading is directly relevant to your exposure.

Comparing to Neighbors

One common question I hear from people in O'Fallon and Wentzville is whether they can guess their radon test results based on their neighbor’s numbers. The answer is no. Radon levels can vary dramatically from house to house, even on the same street. I have seen a house in St. Charles read 2.1 pCi/L while the house next door read 14.3 pCi/L. The difference came down to the soil type directly under each foundation, the condition of the slab or crawl space, and the direction of the prevailing wind. So do not skip testing just because your neighbor had low numbers.

That said, there are some broad patterns. In St. Charles County, Missouri, much of the area lies over limestone and glacial till, which can contain uranium-bearing minerals. The EPA maps show Zone 1 counties (highest potential) covering most of the state, including our area. That means that on average, homes here are more likely to have elevated radon than homes in, say, coastal Texas. But average is not individual. The only way to know is to test.

What to Do with High Results

If your radon test results come back at or above 4 pCi/L, the next step is radon mitigation. The most common and effective method is sub-slab depressurization, sometimes called soil suction. This involves a contractor drilling a small hole in your basement floor or your slab, pulling soil gas from beneath the house with a radon fan, and venting it safely above the roof line. The fan runs continuously, creating a vacuum under the slab that keeps radon from getting inside. I have seen systems that drop a home from 12 pCi/L down to 1.2 pCi/L after installation.

Other methods exist too, like sealing cracks and joints or pressurizing the basement, but those are less reliable on their own. Sealing alone rarely works because radon can find new pathways. Sub-slab depressurization is the gold standard. A good contractor, like Air Sense Environmental, will also test after installation to make sure the system is working. You should always get a post-mitigation test, preferably a long-term one, to confirm that your radon test results now fall below the Action Level.

Short-Term vs. Long-Term: Which One to Trust

I have had homeowners tell me they took a short-term test, got a 3.8, and decided they were fine. But then they did a long-term test and got a 5.1. Which set of radon test results should they believe? Both are valid, but they answer different questions. The short-term test tells you what the level was during that particular week. The long-term test gives you a more accurate average of what you actually breathe over months. If you have the time and the patience, the long-term test is better for making a mitigation decision. If you need to sell a house quickly, the short-term test is the industry standard.

One practical tip: if you do a short-term test and the number is borderline, like 3.5 or 3.8, do not just shrug it off. Run a long-term test next. The cost is minimal, maybe thirty dollars for a kit, and the peace of mind is worth it. Or if you want a digital readout over several days, you can rent a continuous radon monitor from a company like Air Sense Environmental. That gives you hourly data and shows you how the level changes with weather and with your daily activities, like running the dryer or opening windows.

The Human Side of the Results

I understand that getting a high number can feel alarming. But it is also fixable. Radon is a natural radioactive gas that comes from the ground. It is not a sign that your home is dirty or built wrong. It is just a sign that the soil under your house happens to release more of it. Once you know, you can act. And acting is usually straightforward: call a mitigation professional, get a system installed, and retest. The whole process, from test to fix, can take a few weeks.

One thing I always tell people: do not wait. Radon exposure is the second leading cause of lung cancer in the United States, after smoking. The risk is cumulative. Every year you live with a level of 8 pCi/L, you are exposed to roughly the same radiation dose as having over 100 chest X-rays. That is not a reason to panic, but it is a reason to take your radon test results seriously and move forward with a plan.

Final Thoughts on Testing

If you have not tested yet, do it. You can buy a simple charcoal canister test at a hardware store or order one online. Place it in the lowest lived-in area of your home, close windows and doors for 12 hours before the test starts, and keep them closed as much as possible during the test. Mail it to the lab and wait for the report. When you get your radon test results back, read them with the context I have described here. And if the number is 4 or higher, call a licensed radon mitigation contractor in St. Charles County, Missouri. They will know exactly what to do.