Why Multifamily Radon Testing Matters for St. Louis Property Owners

If you own or manage a multifamily property in St. Louis, you probably spend a fair amount of time thinking about roofs, plumbing, and tenant turnover. Radon gas is less likely to make your to-do list. That is understandable. Radon is invisible, odorless, and tasteless. It does not announce itself the way a leaky faucet does. But ignoring it can carry serious consequences for your residents’ health and your liability as a building owner.

I have spent years working with commercial and multifamily properties across the region, and I can tell you that radon issues are not something to push to next year. The geology of St. Louis, with its limestone and glacial deposits, creates conditions where radon can seep into basements and lower-level units with surprising consistency. That is why multifamily radon testing should be part of your regular building maintenance, not a one-time checkbox before a sale.

How Radon Enters a Multifamily Building

Radon gas forms naturally when uranium in soil and rock breaks down. It moves up through the ground and into the air. In a single-family home, the path is usually straightforward: cracks in the foundation, gaps around pipes, or openings around sump pumps. In a multifamily building, the picture is more complicated. You might have a shared foundation, multiple slabs, and utility penetrations that run between units. The larger the footprint, the more entry points exist.

I have walked through buildings where the radon levels in one unit were three times higher than in the unit next door. That variability is common. It depends on soil composition, construction quality, and how the building sits on its lot. A short-term radon test in one unit does not tell you much about the rest of the building. That is one reason why professional multifamily radon testing requires a strategic sampling plan, not just a single detector placed in a random apartment.

The Role of HUD and Lender Testing

If you have dealt with HUD or a conventional lender on a multifamily property, you have probably encountered their radon requirements. The Department of Housing and Urban Development has specific guidelines for radon testing in multifamily housing, especially when federal financing or insurance is involved. Lenders, too, are paying closer attention. They want to know that the asset they are financing does not carry a hidden environmental liability.

I have seen deals stall because radon testing was done incorrectly or not at all. One property in St. Louis was set to close on a refinance, and the lender required radon test results for all ground-floor units. The initial test was a quick, DIY kit that came back borderline. The lender asked for a follow-up using continuous radon monitors, which gave hourly readings over several days. That data showed spikes at certain times of day, which indicated a problem that needed mitigation before the lender would sign off. That kind of scenario is becoming more common, not less.

Choosing the Right Testing Method

There are several ways to test a multifamily building, and each has its place. A short-term radon test is the most common for real estate transactions. It typically lasts two to seven days and uses a charcoal-based detector that you send to a lab. These are inexpensive and widely available. But they can be influenced by weather, building operations, and even how a tenant opens windows. For a more reliable picture, especially in larger buildings, continuous radon monitors give you real-time readings and can detect patterns that a single charcoal test might miss.

Long-term radon tests, which last three to twelve months, are better for understanding average exposure over the course of a year. Radon levels fluctuate seasonally. Heating season tends to create higher levels because buildings are closed up and the stack effect pulls air from the ground into the living space. A long-term test smooths out those seasonal swings and gives you a truer sense of what residents are breathing.

In my experience, the best approach for multifamily properties is a combination. Use short-term tests for initial screening or transaction work, and follow up with long-term monitoring in units that show elevated levels. That dual approach is what licensed radon testers typically recommend, and it is what I see working well for buildings in the St. Louis area.

What Happens When Levels Are High

If your multifamily radon testing reveals levels above the EPA action level of 4 picocuries per liter, you need a mitigation plan. The gold standard for most buildings is sub-slab depressurization. This involves installing a pipe through the foundation into the soil beneath, then using a fan to create negative pressure that pulls radon gas away from the building and vents it safely above the roofline.

I have worked on buildings where the existing slab had multiple cracks and the soil beneath was particularly porous. In those cases, a single suction point was not enough. We had to install multiple points and seal the slab with a radon-resistant construction membrane. That is not a weekend DIY job. It requires knowledge of building science and soil dynamics. A good mitigation contractor will do a pre-mitigation diagnostic test, often using smoke or a pressure gauge, to figure out where the radon is coming from and how to stop it.

Mitigation systems are not set-and-forget either. They need periodic maintenance and monitoring. Fans wear out. Seals degrade. I have seen buildings where the system was installed correctly but then a tenant finished a basement with drywall and flooring that blocked the suction. Regular follow-up testing is the only way to know the system is still working.

Indoor Air Quality and Tenant Health

Radon is the second leading cause of lung cancer in the United States, according to the EPA. That fact alone should be enough to get any property owner’s attention. But I find that many multifamily operators do not think about indoor air quality as a whole. They focus on mold, lead paint, or carbon monoxide. Radon tends to fall through the cracks because it is not something you can see or smell.

There is also a liability angle. If a tenant develops health problems and discovers that radon levels in their unit were never tested, or were tested and not disclosed, that can create legal exposure. Some states now require radon disclosure in rental agreements. While Missouri does not currently have a statewide disclosure law, smart property owners treat radon testing as a standard part of due diligence. It protects residents, and it protects the building owner from future claims.

I have seen buildings where the property manager proactively tested every ground-level unit and then installed mitigation systems in the ones that needed it. That kind of approach builds trust with tenants and can even become a selling point when you are trying to fill vacancies. People want to know that the air they breathe at home is safe.

Working with a Professional

Multifamily radon testing is not something you should hand to a maintenance staff member with a test kit and a clipboard. The testing standards are specific. The EPA’s National Radon Program provides protocols, but interpreting them for a building with fifty units, multiple HVAC zones, and varied occupancy patterns takes experience. Licensed radon testers understand how to place detectors, how many to use, and how long to run the test to get valid results.

In St. Louis, I have worked with Air Sense Environmental on several projects. They bring the kind of commercial-grade equipment, like continuous radon monitors, that gives you data you can rely on. They also handle HUD and lender testing, which has its own documentation requirements. If you are financing or refinancing a building, having a report from a recognized firm can mean the difference between a smooth closing and a delayed one.

A good tester will also walk you through what the results mean for your specific building. They will not just hand you a number and walk away. They will explain whether the levels are seasonal, whether a retest is advisable, and what kind of mitigation would be appropriate if needed. That consultative approach is worth the extra cost over a cheap online kit.

Practical Steps You Can Take Now

If you are reading this and wondering whether your building needs testing, here is a simple starting point. Check when the last radon test was done in your building. If it was more than two years ago, or if you have done any major work to the foundation or slab, schedule a new round of testing. Start with ground-floor units and any units that are below grade. Use a certified professional who can provide a report that meets HUD or lender standards if needed.

Talk to your tenants about radon. Let them know you are testing and why. That transparency goes a long way. If you find elevated levels, do not panic. Most problems can be solved with a properly designed mitigation system. The cost of mitigation is usually far less than the cost of a health crisis or a failed real estate transaction.

Radon is a manageable risk if you stay ahead of it. Multifamily radon testing, done right, gives you the information you need to protect your building, your residents, and your investment. It is one of those things that is easy to ignore until it becomes a problem. But in the multifamily world, problems tend to grow in scale and cost. A little prevention goes a long way.