Understanding Your Radon Test Results: What St. Louis Homeowners Need to Know

When I bought my first home in St. Louis, the radon test results came back at 6.2 picocuries per liter. I remember staring at the number and having no idea what it meant. Was that high? Low? Dangerous? The real estate agent handed me the report and said something like "you might want to look into mitigation," but I was left with more questions than answers. Over the years, working as a home inspector and eventually with Air Sense Environmental, I have learned that radon test results are one of the most misunderstood pieces of a home inspection report. Let me walk you through what those numbers actually mean, how they are generated, and what you should do once you have them in your hands.

The Numbers Behind the Gas

Radon is a radioactive gas that comes from the natural decay of uranium in soil and rock. It seeps into homes through cracks in the foundation, gaps around pipes, and other openings, especially in basements. The EPA has set an action level of 4 picocuries per liter (pCi/L). That does not mean anything below 4 is safe, and anything above is a crisis. It is a threshold where the EPA recommends you take steps to reduce the radon level. The higher the number, the greater the health risk, which includes an increased risk of lung cancer over many years of exposure.

Radon test results come in two basic flavors: short-term and long-term. A short-term radon test usually lasts 2 to 7 days. It gives you a snapshot of the radon level during that specific window. A long-term radon test runs for 90 days or more and provides a more accurate average of the radon level over different seasons and weather conditions. For a real estate transaction, a short-term test is standard because time is tight. But if you are curious about your home's actual year-round exposure, a long-term test is better.

Why Continuous Monitors Matter

There is a big difference between a charcoal canister you leave in the basement for a few days and a continuous radon monitor. The charcoal canister is a passive device. You set it out, wait, mail it to a lab, and get your radon test results days or weeks later. A continuous radon monitor, like the ones Air Sense Environmental uses, is an electronic device that sits in the home and takes readings every hour or even more frequently. It can detect tampering, power outages, and drastic swings in radon concentration. In a real estate transaction, that kind of transparency is invaluable. It protects the buyer and the seller because the data is hard to dispute.

What Influences Radon Test Results

Many things can affect your radon test results. Weather is a big one. A heavy rain or a rapid drop in barometric pressure can cause radon to surge into the home for a day or two. That can push a short-term test result higher than the home's true average. On the flip side, if you run the test during a warm, calm week with all the windows open, the radon level may read artificially low. The EPA and most testing protocols require windows and doors to be closed as much as possible during a short-term test, except for normal entry and exit. That is why a home inspector will often ask homeowners to keep the house closed up during the testing period. It is not about inconvenience; it is about getting a result that reflects the worst-case living conditions.

Another factor is the location of the test. In St. Louis, many homes have basements, and that is where radon tends to accumulate. If you place the testing device in a finished basement versus an unfinished one, the results may differ because of airflow and humidity. A good testing company will place the monitor in the lowest livable area of the home, typically the basement if someone spends at least four hours a day there, or the first floor if the basement is unfinished and rarely used.

When Radon Test Results Come Back High

If your radon test results come back above 4 pCi/L, do not panic. It is a common problem in the St. Louis area because of the region's geology. The first step is to confirm the result with a second test, preferably a long-term test if time allows. If you are in the middle of a real estate transaction, you may not have that luxury. In that case, the next step is radon mitigation. A qualified radon mitigation contractor can install a system that reduces the radon level in the home. The most common approach is sub-slab depressurization. This involves drilling a small hole in the basement floor, inserting a pipe, and attaching a radon fan that pulls radon gas from beneath the slab and vents it safely outside. It is usually effective and not as disruptive as you might think.

Radon remediation is not a DIY job. It requires knowledge of pressure differences, soil composition, and proper venting. A poorly installed system can actually make the problem worse by drawing more radon into the home. I have seen homes where a well-meaning homeowner sealed cracks in the basement floor but did nothing to address the pressure difference, and the radon level stayed high. Professional radon mitigation is the safe way to go.

The Role of Radon Test Results in Real Estate Transactions

In St. Louis, radon is part of the seller disclosure. Sellers are required to disclose any known radon test results. If you are buying a home, you have the right to request a radon test as part of your home inspection. Many buyers waive it to save time or money, but that is a mistake. Knowing the radon level before you close gives you leverage. You can ask the seller to pay for mitigation or adjust the price accordingly. In my experience, most sellers are willing to negotiate because they do not want the deal to fall apart over something fixable.

If the radon test results are low, say below 2 pCi/L, you are in good shape. It does not mean radon will never be an issue, but it reduces the immediate concern. If the results are between 2 and 4, the EPA recommends considering mitigation, especially if you plan to finish the basement or if children will spend time down there. Every 1 pCi/L increase in radon level raises the lifetime risk of lung cancer by about 16 percent. That is not a scare tactic. It is a statistical fact backed by decades of research.

New Construction and Radon-Resistant Construction

If you are building a new home in St. Louis, consider radon-resistant construction from the start. It is relatively inexpensive to install a passive sub-slab depressurization system during the build. It involves a layer of gravel, a vapor barrier, a vent pipe that runs from the gravel layer up through the roof, and sealing all foundation openings. If radon test results later show a high level, you can easily add a radon fan to the passive system to turn it into an active one. It is much cheaper than retrofitting a system after the home is built.

Final Thoughts from the Field

I have seen too many homeowners ignore radon test results because they seem abstract. The numbers do not smell or taste or cause immediate symptoms. But radon is the second leading cause of lung cancer in the United States, after smoking. It is responsible for an estimated 21,000 lung cancer deaths per year, according to the EPA. That is a real health risk, but it is also a manageable one. Testing is cheap and easy. Mitigation is straightforward and often costs less than a new roof or furnace.

If you get your radon test results and they are high, call a professional. Air Sense Environmental has been helping St. Louis homeowners and real estate agents navigate this process for years. They use continuous radon monitors that give you reliable, tamper-proof data. Whether you are buying, selling, or just want to know what is in your basement air, the first step is getting a test. The second step is understanding what the numbers mean. And the third step is acting on them. It is that simple.