Understanding Radon Gas Risk in St. Louis Commercial and Multifamily Properties

When I first started working with commercial real estate in St. Louis, I assumed radon was mostly a concern for homeowners with basements. It took a few years of hands-on testing and mitigation work to realize how wrong that assumption was. The truth is that radon gas risk in commercial and multifamily buildings is often higher than in single-family homes, simply because of how these structures interact with the ground beneath them.

Radon comes from the natural decay of uranium in soil and rock. It moves up through the ground and enters buildings through cracks, joints, and gaps in the foundation. In a large building with a big footprint, there are more entry points and more potential for soil gas to accumulate. That is why property owners and managers in St. Louis need to take radon gas risk seriously, especially when dealing with HUD requirements or lender testing for a real estate transaction.

Why St. Louis Properties Are Vulnerable

St. Louis sits on a mix of limestone, shale, and glacial deposits. These geological layers can produce moderate to high levels of radon, depending on the specific location. The EPA has placed parts of the St. Louis area in Zone 1, meaning the predicted average indoor radon level exceeds the action level of 4 picocuries per liter. But the radon map is only a starting point. I have seen buildings in Zone 2 test well above that threshold, and I have seen buildings in Zone 1 come in clean. The only way to know is to test.

For commercial real estate and multifamily housing, the stakes are different than for a single-family home. Tenants, employees, and residents spend many hours inside these buildings. If radon levels are elevated, the long-term exposure can be a real health concern. That is why health department officials and environmental consultants recommend testing, even if local codes do not require it.

Testing Methods: From Simple Kits to Continuous Monitors

There are several ways to measure radon in a building. The most common approach for initial screening is a radon test kit that uses activated carbon. You place the kit in the lowest livable area, leave it for a few days, and send it to a lab. These kits are inexpensive and work well for a quick snapshot. But for commercial properties, especially those involved in a real estate transaction or HUD financing, a more thorough approach is often required.

Continuous radon monitors are electronic devices that record radon levels over time, typically 48 hours or longer. They provide hourly readings and can detect short-term spikes that a passive kit might miss. I have used both methods many times, and each has its place. For a lender testing scenario, the continuous monitor gives the kind of data that underwriters trust. It also helps in identifying whether the radon gas risk is steady or intermittent, which matters when designing a mitigation strategy.

Air Sense Environmental performs this kind of testing across St. Louis, and they emphasize that the test must be conducted under closed-building conditions to get accurate results. That means keeping windows shut, minimizing door use, and avoiding fans that could pull in outdoor air. It is a simple discipline, but one that many property managers overlook.

When Mitigation Becomes Necessary

If testing shows levels above the EPA action level, the next step is to install a radon mitigation system. For most commercial and multifamily buildings, the most effective method is sub-slab depressurization. This involves drilling a small hole through the concrete slab, creating a suction point beneath the foundation, and running a pipe from that point to a mitigation fan installed on the roof or outside the building. The fan creates negative pressure under the slab, pulling soil gas away from the building and venting it safely into the atmosphere.

I have seen this approach work in buildings with crawl spaces, basements, and even slab-on-grade foundations. The key is to seal all visible cracks and gaps in the concrete with a high-quality sealant, so the suction is directed where it is needed. A well-designed system can reduce indoor radon levels by 90 percent or more. But it is not a one-size-fits-all solution. The depth of the footing, the type of soil, and the layout of the building all affect how the system should be designed.

In some cases, especially in older multifamily housing, the foundation may have multiple sections with different soil conditions. That is when a professional assessment becomes invaluable. A single mitigation fan might not be enough; you may need multiple suction points. The cost scales with complexity, but the alternative is leaving residents and employees exposed to an invisible hazard.

HUD and Lender Testing Requirements

If you are involved in a real estate transaction that uses HUD financing, radon testing is often a requirement. HUD has specific protocols for how the test must be conducted, what type of equipment to use, and how long the test must run. Lender testing follows similar standards, especially when the property is being sold or refinanced. I have worked on deals where the radon test results became a sticking point, delaying the closing until a mitigation system was installed and a follow-up test confirmed safe levels.

These requirements exist because radon gas risk affects the long-term habitability and value of a property. A building with a known radon problem is harder to sell, harder to insure, and harder to lease. For commercial real estate, that translates directly into financial loss. For multifamily housing, it can lead to tenant turnover and liability concerns. Getting ahead of the issue with a pre-emptive test is almost always cheaper than dealing with it after a deal falls apart.

Practical Steps for Property Owners

If you own or manage a commercial or multifamily property in St. Louis, here are a few practical steps to consider:

• Check the EPA radon map for your specific address. Remember that the map is a guide, not a guarantee.
• Conduct a short-term test using either a radon test kit or a continuous radon monitor. Do this at least once, even if the building is new.
• If levels are above 4 pCi/L, consult a mitigation professional like Air Sense Environmental. They can design a system tailored to your building.
• Maintain the system after installation. Check the mitigation fan annually and replace it if needed. The sealant around foundation cracks may also need reapplication over time.
• Include radon testing as part of your indoor air quality management plan. It is one more way to protect the people who live and work in your building.

These steps are not complicated, but they require follow-through. I have seen too many property owners test once, get a borderline result, and then do nothing because they did not want to spend the money. That is a short-sighted approach. Radon levels can change over time as the ground shifts or as building materials settle. A building that tested fine five years ago might have a problem today.

The Role of Continuous Monitoring

For larger properties, or for buildings where radon gas risk is known to be high, continuous monitoring can be a smart investment. Instead of taking a single snapshot, you get a detailed picture of how radon levels fluctuate with weather, season, and building use. I have seen continuous monitors catch short-term spikes that a passive test would have missed entirely. That information can guide decisions about when to run the mitigation system more aggressively or when to schedule a follow-up test.

Some newer continuous radon monitors also measure temperature, humidity, and air pressure, giving a fuller view of indoor air quality. That data is useful for building operators who want to optimize ventilation and energy use at the same time. It is a small additional cost that often pays for itself through better building management.

Final Thoughts

Radon is not something you can see, smell, or taste. That makes it easy to ignore. But in St. Louis, where the geology creates real potential for elevated levels, ignoring radon gas risk is a gamble. Whether you are managing a multifamily housing complex, selling a commercial property, or overseeing a HUD-funded project, a few hundred dollars spent on testing today can save thousands in remediation and legal costs tomorrow.

The good news is that radon mitigation is well understood and highly effective. Sub-slab depressurization, combined with proper sealing and a reliable mitigation fan, can bring almost any building down to safe levels. The key is to test early, test often, and work with someone who knows the local soil conditions and building types. In St. Louis, that means working with a company like Air Sense Environmental, which has the experience and equipment to handle both testing and mitigation for commercial and multifamily properties.

If you are reading this and own a building that has never been tested, I would encourage you to start with a simple radon test kit. It costs less than a nice dinner out, and it could tell you something very important about the air your tenants or employees are breathing. That is a small price to pay for peace of mind.