How to Read Your Radon Test Results and What to Do Next

If you have just finished a radon test in your St. Louis home, you might be staring at the numbers wondering what they actually mean. I have been in that spot myself, both as a homeowner and as a professional performing radon measurements for other families. The short answer is that your radon test results tell you the average concentration of radon gas in the air you breathe over the test period. But the real value comes from understanding the context, the guidelines, and the practical steps that follow.

Radon gas is a natural radioactive decay product of uranium in the soil. It seeps into homes through cracks in the foundation, gaps around pipes, and other openings. Because it is invisible and odorless, the only way to know your risk is through proper radon measurement. In the St. Louis area, the geology includes a fair amount of shale and limestone that can produce elevated radon levels. The EPA radon guidelines recommend action when the average annual concentration reaches 4.0 picocuries per liter (pCi/L) or higher. But even levels between 2.0 and 4.0 pCi/L carry some risk, and the EPA suggests considering mitigation in that range.

When you receive your radon test results from a professional radon testing service like Air Sense Environmental, the report typically shows the average level, the test duration, and any notes about conditions during the test. A continuous radon monitor provides a time-stamped record of readings, so you can see whether levels spiked at certain hours or stayed steady. That kind of detail is far more informative than a single number from a radon test kit that you mail away. For example, I once saw a test where the average was below 4.0 but the monitor showed nighttime spikes above 6.0. That told us there was a soil gas entry pathway that sealed up during the day when the house was more ventilated. The average number alone would have missed that story.

Short-Term vs. Long-Term Tests

One of the first choices you make is between a short-term radon test and a long-term radon test. Short-term tests, usually lasting two to seven days, give you a quick snapshot. They are useful for real estate transactions or for initial screening. But radon levels can vary significantly with weather, season, and how you use your home. A short-term test taken during a windy cold snap might show higher numbers than a test taken in calm summer weather. That variability is why many experts, including the EPA, recommend a long-term radon test lasting at least 90 days. Long-term tests average out those fluctuations and give a much more reliable picture of your year-round radon exposure.

In the St. Louis region, long-term tests are especially valuable because the Missouri radon map shows that many counties, including St. Louis County and St. Charles County, have a moderate to high potential for elevated radon in homes. A single short-term test might leave you guessing. A long-term test, especially one done with a continuous radon monitor, provides the kind of data you can trust to make decisions about radon mitigation.

Interpreting the Number

Let us talk about the actual number. If your radon test results show an average below 2.0 pCi/L, your radon risk is relatively low. You might still want to retest every few years, especially if you make changes to your home that could affect air flow or foundation integrity. If the result is between 2.0 and 4.0, you are in the gray zone. The EPA radon guidelines say you should consider fixing your home, but they do not require action. My advice to homeowners in St. Louis has always been: if you can afford mitigation, do it. The cost is typically between eight hundred and fifteen hundred dollars for a radon reduction system, and it can reduce levels by 99%. Compared to the health risks of long-term radon exposure, which is the second leading cause of lung cancer after smoking, that investment is small.

If your radon test results come back at 4.0 pCi/L or higher, the EPA says you should take action to reduce the level. That means hiring a qualified radon mitigation contractor to install a radon reduction system. In most homes, this involves a vent pipe running from the sub-slab gravel layer up through the roof, with a fan that creates suction to pull radon out from under the house before it can enter your living space. The system is sometimes called a sub-slab depressurization system. It works well for most slab-on-grade and basement homes, which are common in St. Louis.

What a Continuous Monitor Adds

When I work with Air Sense Environmental, I always use a continuous radon monitor for the tests I perform. These devices take a reading every hour or even more frequently, storing the data so we can see the pattern. They are more expensive than a simple radon test kit, but the extra information is worth it. For instance, if your radon levels are high only during certain times of day, a continuous monitor will reveal that. A charcoal canister that sits for three days and then gets mailed to a lab will only give you one lump number. The continuous monitor also has the advantage of detecting tampering or unusual conditions, which adds credibility when the results are used in a real estate transaction.

One thing I have learned over the years is that radon in homes often follows a seasonal pattern. In St. Louis, levels tend to be highest in the winter when homes are sealed up and the stack effect pulls soil gas inside. Summer levels can be lower because windows are open and the pressure difference is smaller. A continuous monitor used over several months will capture that seasonal swing, giving you the data you need to decide whether to install a radon mitigation system or to adjust your ventilation habits.

After You Get the Results

Once you have your radon test results, the next step depends on the number. If the average is 4.0 or above, schedule a radon remediation consultation as soon as possible. Many radon testing services, including Air Sense Environmental, can recommend qualified mitigation contractors. If the level is between 2.0 and 4.0, consider a long-term follow-up test to confirm the reading before spending money on mitigation. If the level is below 2.0, you can relax a bit, but do not ignore radon entirely. Retest every two years or after any major renovation that affects the foundation or HVAC system.

Another practical detail: if you are buying a home in St. Louis and the seller provides radon test results, check whether the test was done by a professional using a continuous radon monitor or by a do-it-yourself radon test kit. The quality of the test matters. A kit placed in the wrong location or exposed to drafts can give a false low reading. A professional radon testing service follows strict protocols to ensure the results are accurate and defensible. That is especially important when the sale of a home hangs on the outcome.

Common Mistakes to Avoid

Testing in a closed room during a heat wave or cold snap without normal ventilation. The EPA guidelines require closed-house conditions for short-term tests, but many homeowners forget to keep windows and doors closed for the full duration.
Placing the radon detector in a basement that is unfinished and rarely used. The test should be in the lowest livable area, but if you never spend time there, you might want to test the first floor instead.
Ignoring radon test results because they are just below 4.0. The risk is continuous, not a cliff. A level of 3.9 is still higher than most people would accept if they understood the cumulative effect.
Assuming one test is enough for the life of the home. Radon entry patterns can change with soil settling, new construction nearby, or changes to your foundation.

One more thing I want to emphasize: radon test results are not just numbers on a page. They represent your family's potential radon exposure over many years. In the St. Louis area, where the geology and housing stock combine to create real risk, taking those results seriously can make a meaningful difference in your health. I have seen homes with initial levels above 20 pCi/L drop to below 1 pCi/L after a properly designed radon reduction system. That is a huge win, and it starts with the test.

If you have not tested yet, consider hiring a radon testing service that uses a continuous radon monitor and follows the EPA radon guidelines. If you have already tested and have your radon test results, use them as the basis for a conversation with a professional. The data is only useful if you act on it.