What Your Radon Test Results Actually Mean for Your Home

If you have bought or sold a home in St. Louis over the past decade, you have almost certainly heard about radon. The EPA radon guidelines recommend action at 4.0 pCi/L, but many homeowners stop at that number. They see their radon test results, breathe a sigh of relief if it is under four, or panic if it is above. The reality is more complicated. I have performed hundreds of radon measurements across the region, and I can tell you that a single number does not tell the whole story. What matters is how that number was collected, what time of year it was taken, and what you do next.

Why St. Louis Homes Are Prone to Radon

Missouri radon levels rank among the highest in the country, and St. Louis sits in a part of the state that the radon map shows as Zone 1, meaning the average indoor radon level is expected to exceed 4.0 pCi/L. The geology here contains granite and shale that release radon gas as they break down. That gas moves through soil and finds its way into basements, crawl spaces, and slab foundations. I have tested homes in Kirkwood that hit 12 pCi/L and homes in the Central West End that barely registered 2.0. The variation is huge, and it depends on soil composition, foundation cracks, and even the way the house was built. The only way to know your own radon risk is to measure it directly. A radon map can tell you the area risk, but it cannot tell you what is happening inside your specific home.

The Difference Between a Radon Test Kit and Professional Testing

Most people first encounter radon through a radon test kit they buy at a hardware store or receive from a home inspector. These are short-term radon tests, usually charcoal canisters that sit in the lowest livable level of the house for two to seven days. They are useful for a quick screening, but they have real limitations. A short-term test measures radon levels at one moment in time. If the weather changes, if a window was left open, or if the furnace kicked on during the test, the result can swing wildly. I have seen a short-term test yield 2.5 pCi/L and a long-term radon test in the same house come back at 6.8 pCi/L. The short-term test gave a false sense of safety.

Professional radon testing, like what Air Sense Environmental does, uses a continuous radon monitor. That device sits in your home for anywhere from 48 hours to several months, taking readings every hour. A continuous radon monitor records the highs and lows, so you can see whether radon levels spike at night or stay steady. It also flags tampering, like a fan left running near the device. For a home purchase, a short-term radon test may be acceptable as a pass-fail check. For your own family’s health, I strongly recommend a long-term radon test, ideally three months or longer, because radon gas fluctuates with the seasons. Winter radon levels tend to be higher because homes are sealed tight, and the stack effect pulls soil gas indoors.

Interpreting Your Radon Test Results

When you receive your radon test results, you will see a number in picoCuries per liter of air. The EPA radon guidelines say that any result at or above 4.0 pCi/L should prompt action, and they recommend fixing the home when the level is between 2.0 and 4.0 pCi/L as well. I treat anything above 2.0 as worth a conversation. Radon exposure is cumulative, and there is no known safe level. The risk of lung cancer from radon increases linearly with exposure.

Your radon test results also include the test duration and the conditions during the test. Look at the minimum and maximum readings if you used a continuous radon monitor. A result that averages 3.5 pCi/L but spikes to 12 pCi/L every night for six hours is a different problem than a steady 3.5. The spike pattern suggests that radon is entering through a specific pathway, like a sump pump or a crack in the slab, and that a targeted radon mitigation could fix it. The steady 3.5 may require a more comprehensive radon reduction system.

I once tested a house in Maplewood where the radon test results averaged 4.2 pCi/L, but the hourly graph showed the level dropping to near zero every afternoon. The homeowner worked from home and was in the basement during the afternoon. The continuous radon monitor revealed that the drop coincided with the furnace fan running, which was depressurizing the basement and pulling in fresh air from the upstairs. The mitigation strategy was not a full radon reduction system. It was a simple seal around the furnace flue and a small fan to balance the pressure. That saved the homeowner thousands of dollars.

Common Misconceptions About Radon Mitigation

Some people assume that radon mitigation means ripping up the basement floor or installing an expensive ventilation system. In many cases, a radon reduction system is straightforward. A sub-slab depressurization system, which pulls radon gas from beneath the concrete and vents it above the roof line, can bring levels down from 15 pCi/L to well below 2.0. The cost varies by house, but it is usually less than a new furnace or a roof replacement.

Another misconception is that radon remediation is only for old homes with dirt floors. New construction in St. Louis can have high radon levels too. I have tested homes built in 2020 that had passive radon pipes installed but no fan. Without the fan, the radon levels were still above 4.0. A radon detector placed in those homes confirmed that the passive system alone was not enough. Adding a fan to the existing pipe turned it into an active radon reduction system that worked.

When to Retest

Your radon test results are a snapshot, not a permanent diagnosis. I recommend testing every two years, and always after any major renovation that changes the foundation, adds a basement bedroom, or alters the heating and cooling system. Also retest if you install a radon reduction system, to confirm it is working. Some homeowners test once when they buy the house and never think about it again. But radon gas can change over time. Soil settles, foundations crack, and new sump pumps create new entry points.

If you have a radon testing service handle the measurement, they can often keep your results on file and remind you when it is time to retest. Air Sense Environmental provides that service for their clients. They can also walk you through the radon test results and explain what they mean in plain language. That kind of support matters because radon is invisible and odorless, and it is easy to ignore until you understand the risk.

Final Thoughts on Radon in Homes

Radon in homes is a preventable health risk. The EPA radon guidelines give you a clear target, but they do not tell you how to get there. That is where professional judgment comes in. A good radon detector and a thorough testing protocol give you the data you need to make an informed decision. Whether you choose a short-term radon test for a quick check or a long-term radon test for a complete picture, the cost is small compared to the peace of mind. And if your radon test results show levels above 2.0 pCi/L, have a conversation with a qualified mitigation contractor. The fix is usually simpler than you think, and the benefit for your family’s health lasts a lifetime.