Radon testing measures the concentration of radon gas inside a building, typically your home, to determine whether levels are high enough to pose a lung cancer risk. The EPA recommends taking action if your home’s radon level reaches 4 picocuries per liter (pCi/L) or higher, and even suggests considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe level of exposure.
Radon is a naturally occurring radioactive gas that seeps up from the ground into buildings through cracks in foundations, gaps around pipes, and other openings. It’s colorless and odorless, so testing is the only way to know if it’s in your home. About 21,000 lung cancer deaths per year in the United States are attributed to residential radon exposure, making it the leading cause of lung cancer among nonsmokers.
Why Radon Is Dangerous
Radon itself isn’t the main problem. As it decays, it produces radioactive particles (primarily two forms of polonium) that you inhale. These particles lodge in the lining of your airways and emit high-energy alpha radiation. Alpha particles can’t penetrate skin, but when they’re sitting directly on lung tissue, they’re potent enough to damage DNA, trigger chromosomal abnormalities, and disrupt the normal cycle of cell growth and death. Over years, this repeated damage to the cells lining your airways, especially at points where the airways branch, can lead to cancer.
The risk scales with both the concentration of radon and how long you’re exposed. Someone living for decades in a home with moderately elevated radon faces a meaningful cumulative risk, which is why testing matters even if you feel fine and have no symptoms.
Short-Term vs. Long-Term Tests
There are two basic categories of radon tests, and they serve different purposes.
Short-term tests are the most common starting point. These are small passive devices, often charcoal pouches, that you place in your home for 2 to 4 days. After the testing period, you seal the device and mail it to a lab for analysis. Results come back within a few weeks. Short-term tests give you a snapshot of radon levels during a narrow window, which is useful for an initial screening but can be influenced by weather, barometric pressure, and how recently you’ve had windows open.
Long-term tests use a different technology called alpha track detectors. These stay in your home for three months to a full year before you send them to a lab. Because they average radon levels over a much longer period, they account for the natural seasonal fluctuations in radon and give a more accurate picture of your actual year-round exposure. If your short-term test comes back elevated, a long-term follow-up test can confirm whether the problem is persistent or was a temporary spike.
How to Test Your Home Correctly
Where and how you place the test device matters. The CDC recommends placing it in the lowest livable level of your home, which is usually the basement or ground floor. The device should sit at least three feet off the ground and in the middle of the room, not tucked in a corner, near a window, or close to exterior walls where drafts could skew the reading.
If you live in an apartment or condo, place the test in the lowest level within your unit. A first-floor unit in a building without a basement would test on the first floor.
For short-term tests, you need to maintain what’s called “closed house conditions” for 12 hours before the test begins and throughout the entire testing period. This means keeping all windows and doors shut except for normal entry and exit. You can run your heating and air conditioning as usual, but don’t operate whole-house fans or other systems that pull large volumes of outside air in. The goal is to let radon accumulate at its natural indoor rate so the test reflects what your home actually traps during typical living conditions.
DIY Kits vs. Professional Testing
DIY radon test kits are widely available at hardware stores and through state radon programs, often for under $20. These are the charcoal or alpha track devices described above. They’re straightforward to use and lab-analyzed, so the results are reliable as long as you follow the placement and closed-house instructions carefully.
Professional radon testers typically use continuous electronic monitors that log radon levels hour by hour over a 48-hour period or longer. These devices offer more granular data, can flag tampering (like someone opening windows during the test), and produce results on-site without waiting for a lab. Professional testing is common during real estate transactions, where both the buyer and seller want an independent, verifiable result. It costs more than a mail-in kit but provides a higher level of confidence in a single test.
For routine home screening, a DIY kit is a perfectly reasonable first step. If the result comes back near or above the action level, professional testing or a long-term follow-up test can confirm the finding before you commit to mitigation.
Understanding Your Results
Radon levels are measured in picocuries per liter of air (pCi/L) in the United States. The EPA’s action level is 4 pCi/L. To put that in context, the average outdoor radon concentration is about 0.4 pCi/L, so a reading of 4 pCi/L means your indoor air has roughly ten times the background level.
The EPA is clear that no level of radon is considered safe, which is why they recommend considering mitigation even between 2 and 4 pCi/L. Internationally, the World Health Organization sets a reference level of 300 Bq/m3 for homes (roughly equivalent to about 8 pCi/L), while workplace limits go up to 1,000 Bq/m3. Most countries align their residential guidelines close to the U.S. standard of 4 pCi/L.
If your first short-term test comes back below 2 pCi/L, your home is in a low-risk range, though retesting every few years is reasonable since conditions underground can shift. If the result falls between 2 and 4, a long-term follow-up test helps clarify your true average. Anything at 4 or above warrants mitigation.
What Happens if Levels Are High
The most common fix is called active soil depressurization. A contractor installs a pipe that runs from beneath your foundation slab to above the roofline, with a small fan that continuously pulls radon-laden air from under the house and vents it outside before it can enter your living space. The system runs quietly in the background and typically reduces indoor radon by 80% to 99%.
The EPA notes that most homes can be fixed for about the same cost as other common home repairs, though the exact price depends on your home’s size, foundation type, and the specific approach needed. Homes with crawl spaces, slab foundations, or basements each require slightly different installation methods.
After a mitigation system is installed, you should retest to confirm that levels have dropped below 4 pCi/L. Periodic retesting every two years or so ensures the system continues to function properly, since fans can wear out and foundation cracks can develop over time.
When Testing Matters Most
Every home should be tested at least once, regardless of geography. Radon levels can vary dramatically even between neighboring houses because they depend on localized soil composition, foundation integrity, and ventilation patterns. A home across the street might test at 1 pCi/L while yours reads 8.
Certain situations make testing especially important: buying or selling a home, finishing a previously unoccupied basement, after major foundation repairs, or if you’ve never tested and have lived in your home for years. Winter tends to produce the highest indoor radon readings because homes are sealed up tight, making it a good season to test if you want a conservative estimate of your exposure.