What is Radon Testing?

Radon testing measures the amount of radon gas inside a home using a small test device placed over a set period, with results reported in picocuries per liter (pCi/L). Radon is a radioactive gas you cannot see, smell, or taste, so a test is the only way to know how much has built up indoors. This guide covers what radon testing is, what radon is and where it comes from, why the test matters, when to run one, how the process works, how to read the results, and what to do if the numbers come back high.

What Is Radon Testing?

Radon testing is the process of measuring radon gas concentration inside a home using a device that records levels over a set period, reported in picocuries per liter (pCi/L). The device sits in the lowest lived-in level of the home and collects data for anywhere from two days to twelve months, depending on the method. That data gets analyzed in a certified laboratory or read electronically by a continuous monitor, which produces a number you can compare against the federal action level.

Radon testing exists because radon gives no warning at all. A test is the only reliable way to detect radon in a home, since the gas produces no odor, color, or taste at any concentration. We build radon testing into our home inspections so buyers and owners get hard numbers instead of guesswork. Those numbers answer one question above all: whether the air inside carries a health risk worth acting on.

What Is Radon and Where Does It Come From?

Radon is a radioactive gas that forms naturally when uranium in soil and rock breaks down into radium, which then decays into radon. Radon comes from the ground beneath and around a home, then seeps upward through cracks in the foundation, gaps around pipes, sump pits, and openings in basement floors and walls. Radon that enters an enclosed space gets trapped and builds to concentrations far higher than the levels found in open outdoor air. The Environmental Protection Agency (EPA) puts the average outdoor radon level near 0.4 pCi/L and the average indoor level in American homes near 1.3 pCi/L.

Radon-bearing rock includes granite, shale, and other uranium-rich formations, which explains why some regions carry more radon potential than others. That potential ignores property lines, though. Missouri ranks among the higher-radon states, and roughly 10% of Missouri counties fall into the EPA Zone 1 category, the group with the highest predicted radon potential. High-risk pockets include St. Louis, Jefferson, St. Charles, and Greene counties, yet elevated readings turn up statewide, and your home can test high while the house next door tests low.

Where Is Radon Most Commonly Found?

Radon is most commonly found in the lowest levels of a home, such as basements, crawl spaces, and ground-floor rooms that sit closest to the soil. These lower levels contact the ground directly, so radon entering through foundation cracks reaches them first and concentrates there before diffusing upward. Basements finished into bedrooms, offices, or playrooms deserve the most attention, because people spend long hours in the exact spaces where radon collects.

What Are the Signs of Radon in Your House?

There are no signs of radon in your house, because radon is colorless, odorless, and tasteless and produces no immediate symptoms. You will not smell a strange odor, see a haze, or feel sick right after exposure. Radon harms health silently over years, which is why the gas earns its reputation as a hidden hazard. The absence of warning signs is the exact reason testing matters: a number on a report is the only signal you will ever get.

Why Is Radon Testing Important?

Radon testing is important because radon is the second leading cause of lung cancer in the United States and the leading cause among people who never smoked. The EPA attributes roughly 21,000 lung cancer deaths each year in the United States to radon exposure. The American Lung Association reports that one in seven of those radon-related lung cancer deaths occurs in people who never smoked. Testing turns an invisible, cancer-causing gas into a measurable number you can act on, which is the entire point.

The health link rests on hard evidence, not assumption. Researchers first confirmed it through studies of uranium miners exposed to high radon underground. One National Institute for Occupational Safety and Health (NIOSH) study of uranium miners recorded 371 lung cancer deaths where about 64 were expected from general population rates, roughly six times the normal figure. That evidence established radon as a Class A human carcinogen and shaped every testing guideline that followed. Long-term radon exposure drives the danger, not a brief afternoon in a basement.

How Long Can You Be Exposed to Radon Before You Get Sick?

You can be exposed to radon for years before you get sick, because radon causes gradual cellular damage rather than sudden illness. Radon produces no short-term symptoms, and the lung cancer risk builds over long stretches of steady exposure, usually many years. The amount of risk tracks the radon level and the total time spent breathing it, so higher concentrations and longer exposure raise the odds. No single day of exposure will make you sick, yet no level is fully risk-free either.

What Type of Home Is More Likely to Have Radon?

Homes with basements, crawl spaces, or slab-on-grade foundations built over uranium-rich soil are more likely to have radon, because the gas enters through direct contact with the ground. Age plays a smaller role than many people assume, since radon comes from the soil rather than from building materials, so a brand-new home can test just as high as a century-old one. New construction can include radon-resistant features, such as gravel and plastic sheeting under the slab plus a vent pipe, though even those homes need a test after completion to confirm the levels. Buyers weighing a new construction home should treat radon as a standard check, not an old-house problem.

Is Radon Testing Really Necessary?

Yes, radon testing is really necessary, because testing is the only way to detect a gas that is invisible, widespread, and linked to lung cancer. Elevated radon has turned up in homes in all three EPA radon zones, so no region is automatically safe. Testing is quick, low in cost, and capable of catching a problem long before it harms anyone in the household. A test either clears your home or points to a fixable issue, and both outcomes are worth knowing.

Is Radon Testing Mandatory?

Radon testing is not mandatory under federal law, and most states do not require it for existing homes or real estate transactions. Missouri follows that pattern: the state does not require radon testing and does not force sellers to disclose known radon levels, so buyers who want the data request the test themselves. Many lenders and buyers still ask for radon results during a sale, so the test often happens by choice even without a legal mandate. Skipping it leaves a health question unanswered on what is usually the largest purchase of a lifetime.

When Should You Do Radon Testing?

You should do radon testing when you buy or sell a home, before and after any major renovation, before finishing a basement into living space, and every two to five years as a baseline check. Each of these moments changes either your exposure or the home’s structure in ways that can shift radon levels. Buying or selling a home is the most common time to test, since a buyers inspection is the natural point to confirm the air is safe before money changes hands.

Renovations matter because breaking into a foundation or converting a basement can open new paths for soil gas. The EPA advises retesting after ground shifts, foundation settling, or repairs that disturb the barrier between soil and living space. Owners often pair radon with other indoor-air checks like mold testing to cover more ground in a single visit.

What Month Is Radon the Highest?

Radon is highest during the winter months, typically December through February, when homes stay sealed and heating systems run. Closed windows and doors trap soil gas indoors, and warm air rising inside a heated home creates a slight vacuum that pulls more radon up from the ground. Winter is therefore the most reliable season for a short-term test, because it captures levels closer to the yearly high. Testing in the heating season gives you a conservative, worst-case reading rather than a falsely low summer number.

How Does Radon Testing Work?

Radon testing works by placing a device in the lowest lived-in level of a home, leaving it undisturbed for a set period, and then measuring the radon it recorded. The device either captures the gas for lab analysis or reads it electronically in real time, and the final result comes back in picocuries per liter (pCi/L). We follow the placement, timing, and house-condition guidelines set by the EPA and InterNACHI so the reading reflects true levels. The home radon test we run reports hourly averages, peaks, and an overall level, which paints a fuller picture than a single number.

A professional radon test follows a clear sequence:

1. Place the test device in the lowest lived-in level, positioned 20 inches to 6 feet off the floor and at least 3 feet from windows and exterior doors.
2. Begin closed-house conditions at least 12 hours before the test starts, keeping windows and outside doors shut except for normal entry and exit.
3. Leave the device undisturbed for the full test period, from two days for a short-term test up to twelve months for a long-term test.
4. Retrieve the device and either send it to a certified laboratory or read the continuous monitor directly.
5. Compare the result in pCi/L against the EPA action level of 4.0 pCi/L to decide the next step.

What Are the Types of Radon Tests?

The two types of radon tests are short-term tests, which run from two to ninety days, and long-term tests, which run for more than ninety days and up to twelve months. Short-term tests deliver fast answers and use devices such as charcoal canisters, electret ion chambers, continuous radon monitors, and liquid scintillation detectors. Long-term tests deliver a more accurate year-round average and use alpha track detectors or electret detectors that account for daily and seasonal swings. The table below compares the two approaches on the factors that matter most.

Sources: U.S. Environmental Protection Agency; Centers for Disease Control and Prevention; California Department of Public Health; American Lung Association.

Who Does Radon Testing?

Radon testing is done by certified radon professionals or by homeowners using do-it-yourself test kits sold at hardware stores. Certified professionals bring calibrated equipment, controlled setup, and clear reporting, which the EPA recommends for any real estate transaction. Do-it-yourself kits cost less and work for a first screening, though they depend on the user following every instruction exactly. Our radon testing runs as part of the same visit as our broader inspection services, so the setup, timing, and reporting stay in trained hands.

How Accurate Is Radon Testing?

Radon testing is accurate when the device is placed correctly, the home stays under closed-house conditions, and the test runs for the full recommended period. A properly run test reflects true indoor radon within the limits of the device, and continuous monitors add reliability by recording hour-by-hour data. Accuracy depends far more on test conditions than on the device itself, so protocol is what separates a trustworthy result from a misleading one. Poor placement or disturbed conditions can push a reading too high or too low.

What Affects Radon Testing Results?

Several conditions affect radon testing results, and controlling them is what keeps a test honest. The main factors include:

• Closed-house conditions: opening windows and doors during the test lets radon escape and produces a falsely low reading.
• Device placement: setting the device near a sump pump, a floor crack, or an exterior wall can produce a falsely high reading.
• Weather: severe storms and high winds temporarily change radon levels, so a test should not run during or right after them.
• Season: winter readings tend to run higher than summer readings because homes stay sealed and heated.
• Height and drafts: a device set too close to the floor, a heat source, or a draft can skew the numbers.

Each factor traces back to one principle: the test measures the air people actually breathe under normal living conditions. Tampering with those conditions does not lower the real radon in the home, it only hides it, which defeats the purpose of testing in the first place.

Does Opening Windows Reduce Radon?

Opening windows reduces radon only temporarily, and it is not a real fix. Fresh air dilutes the gas while the windows stay open, but radon levels climb again within hours of closing up, because the soil keeps producing it. Opening windows during a test also breaks closed-house conditions and produces a false reading that hides the true level. Ventilation lowers radon slightly in the moment, yet only a permanent mitigation system reduces it for good.

What Do Your Radon Testing Results Mean?

Your radon testing results mean different things depending on where the number lands relative to the EPA action level of 4.0 pCi/L. A result at or above 4.0 pCi/L signals that you should take action to lower the radon, according to the EPA and the U.S. Surgeon General. A result between 2 and 4 pCi/L falls in a gray zone where the EPA advises you to consider action, while the World Health Organization (WHO) recommends action at an even lower 2.7 pCi/L. A reading below 2 pCi/L needs no action, though no level of radon is completely safe, so periodic retesting still makes sense.

Reading the number is only the first move. A short-term test that comes back at or above 4.0 pCi/L calls for a second test to confirm before you spend money on a fix, according to the EPA. That confirmation step protects you from acting on a single reading that a storm or an open window might have thrown off.

What Should You Do If Radon Levels Are High?

If radon levels are high, you should retest to confirm the result and then hire a certified radon mitigation contractor to install a reduction system. High radon is a fixable problem, and we provide the radon inspection and a clear report while pointing you toward the mitigation step, since the system installation is handled by a licensed mitigation specialist rather than by us. A radon mitigation system works by drawing soil gas from beneath the foundation and venting it above the roofline, which overpowers the natural vacuum pulling radon inside. Sealing cracks alone will not solve the problem, because sealing does nothing about the pressure difference that drives radon indoors.

Mitigation comes in two forms. A passive system uses a vent pipe and natural airflow to move radon out, while an active system adds a fan for extra pull when levels demand it. The EPA notes that existing mitigation technology can bring most homes down to 2 pCi/L or lower. After a system goes in, plan a re-inspection to confirm the fix worked, ideally within thirty days and then every two years afterward.

What Is the Fastest Way to Get Rid of Radon?

The fastest way to get rid of radon is a sub-slab depressurization system installed by a certified radon contractor. Sub-slab depressurization draws radon from under the foundation through a pipe and vents it outside, lowering levels within a day or two of activation. This active system, driven by an inline fan, ranks as the most effective and quickest mitigation method for most homes. Opening windows or running fans helps only in the moment, so a permanent system is the real answer for high levels.

Should You Buy a House If Radon Is High?

Yes, you can buy a house if radon is high, because high radon is a fixable problem rather than a dealbreaker. A radon mitigation system reliably lowers levels to a safe range, and the fix costs far less than most buyers expect next to the price of a home. Buyers commonly negotiate for the seller to install a system or provide a credit at closing, especially when a pre-listing inspection flagged the issue early. Walking away over radon alone rarely makes sense once you know the problem has a clear, permanent solution.

Frequently Asked Questions

Do Air Purifiers Get Rid of Radon?

Air purifiers do not get rid of radon in any meaningful way, because most purifiers filter particles rather than remove a gas at its source. Radon enters continuously from the soil, so filtering the air does nothing to stop the supply. The only proven fix is a mitigation system that vents soil gas outside before it accumulates. An air purifier may help with dust or allergens, yet it will not lower a high radon reading.

Can You Shower in Water With Radon?

You can shower in water with radon, but radon in water releases into the air during a shower and adds to what you breathe. The inhalation risk from radon released while showering outweighs the small risk from drinking the water, according to the EPA. Homes on private wells face this more than homes on treated municipal supplies, so well owners with high air readings should also test the water. Pairing air testing with well water testing gives well owners the full radon picture.

Which States Are Worst for Radon?

The states worst for radon include much of the Upper Midwest, the Appalachian region, and the northern Plains, where uranium-rich soil is common. States such as Iowa, Pennsylvania, Ohio, and North Dakota consistently report high average indoor levels. Elevated radon appears in every state, though, so a high-risk map never replaces an actual test of your own home. Local geology and your specific foundation matter more than any statewide average.

How Much Does It Cost to Fix Radon in a House?

The cost to fix radon in a house depends on the home’s size, foundation type, and the mitigation system required, so there is no single price. A simple sub-slab system in a small home costs less than a complex setup in a large home with multiple foundation types. A certified radon mitigation contractor provides an exact quote after assessing the property. The fix is a one-time investment that most homeowners find modest next to the value of the home it protects.

How Long Does Radon Testing Take?

Radon testing takes anywhere from two days to twelve months, depending on the test type you choose. A short-term test runs two to ninety days and suits real estate deadlines, while a long-term test runs beyond ninety days for a year-round average. Continuous monitors can produce a reliable short-term result in as little as forty-eight hours. Timing beyond that comes down to the method and how long it takes for each device to gather enough data.

How Often Should You Retest for Radon?

You should retest for radon every two to five years, and sooner after any major change to the home. Retesting makes sense after finishing a basement, repairing a foundation, or installing a mitigation system, since each can shift the levels. A home with a mitigation system should be retested within thirty days of installation and then every two years to confirm the fan and pipe still work. Living patterns matter too, so start testing a lower level once you begin spending real time down there.

The Bottom Line

Radon testing measures a hidden, radioactive gas that causes an estimated 21,000 lung cancer deaths a year in the United States, and it is the only way to know whether your home carries that risk. A short-term or long-term test, run under proper conditions, turns an invisible threat into a clear number, and a result at or above 4.0 pCi/L points to a fixable problem with a permanent solution. Testing is fast, the cost is low, and the peace of mind is real, whether you are buying, selling, or protecting the people under your roof.

We test homes throughout Mid-Missouri and deliver a report you can actually read and act on. If you have never tested your home, or it has been a few years, reach out to ARC Home Inspections and we will walk you through the process from scheduling to results. You can also call or text us anytime to ask questions before you book.