A radon inspection is a test that measures the concentration of radon gas inside a building, typically a home. Radon is a naturally occurring radioactive gas that seeps up from the ground through cracks in foundations, gaps around pipes, and other openings. It’s colorless and odorless, so testing is the only way to know whether it’s present at dangerous levels. The EPA recommends taking action if your home’s radon level reaches 4 pCi/L (picocuries per liter) or higher, and suggests considering fixes even at levels between 2 and 4 pCi/L, since there is no known safe level of exposure.
Why Radon Matters
Radon is the second leading cause of lung cancer after smoking. The gas breaks down into radioactive particles that lodge in lung tissue when inhaled over time. Because radon enters homes from the soil beneath them, any house in any region can have elevated levels, regardless of age or construction type. Two neighboring homes can have dramatically different readings. The only variable that matters is what’s happening underground beneath your specific foundation.
How the Test Works
Radon testing falls into two broad categories: short-term and long-term. Short-term tests measure radon for two to four days and give you a quick screening result. Long-term tests run for a minimum of 91 days, often six to twelve months, and capture seasonal fluctuations that short-term tests miss. Radon levels rise and fall with weather patterns, soil moisture, and how tightly your home is sealed, so a long-term test provides a more accurate picture of your actual exposure.
The testing devices themselves come in passive and active varieties. Passive devices, like charcoal canisters and alpha track detectors, sit in place and absorb radon without needing power. You mail them to a lab for analysis after the testing period. Active devices, called continuous radon monitors, run on electricity and record radon levels hour by hour. Professional inspectors typically use continuous monitors because they produce detailed data and can flag tampering or unusual conditions during the test. Research-grade and mid-grade active sensors deliver results within about 5% of the true value, while consumer-grade sensors can be off by nearly 10%, with accuracy dropping further at lower radon concentrations.
Regardless of device type, placement matters. The detector should sit at least 20 inches above the floor, away from drafts, exterior walls, high heat, and high humidity. Testing happens on the lowest livable level of the home, which is usually the basement or ground floor.
What You Need to Do Before a Test
Radon testing requires what’s called “closed-house conditions.” You need to close all exterior windows and doors at least 12 hours before the test starts and keep them closed for the entire testing period. You can enter and exit normally, but you shouldn’t leave doors open for extended periods or run whole-house fans that pull in outside air. These conditions prevent outdoor air from diluting the radon reading and producing a falsely low result.
If a professional is conducting the test, they’ll set up the monitor, verify closed-house conditions, and return to retrieve the device after the testing period. Many inspectors place tamper indicators on windows and doors to ensure conditions stayed consistent.
What the Results Tell You
Your radon report will show an average concentration in picocuries per liter (pCi/L). If a continuous monitor was used, you’ll also see hourly readings that show how levels fluctuated over the test period. Here’s how to interpret the number:
- Below 2 pCi/L: Low risk. No action required, though retesting every few years is reasonable since conditions underground can change.
- 2 to 4 pCi/L: The EPA recommends considering mitigation in this range. The risk is real, though lower than at higher concentrations.
- 4 pCi/L or above: The EPA’s action level. Mitigation is strongly recommended.
A single short-term test that comes back high is usually followed by a second short-term test or a long-term test to confirm the result before investing in mitigation.
Cost of a Radon Inspection
A standalone professional radon inspection costs between $146 and $714 nationally, with an average around $421. The price varies based on your location, the type of equipment used, and whether the inspector is certified through a national program. If you’re adding radon testing onto a standard home inspection during a real estate transaction, expect to pay an additional $90 to $250 on top of the general inspection fee.
DIY test kits are available at hardware stores for under $20, though they use passive technology and require mailing to a lab. They’re a reasonable first screening tool, but professional continuous monitors provide more reliable and detailed data, especially when results will factor into a home purchase.
Radon Inspections During a Home Sale
Radon testing is one of the most common add-ons during the home inspection phase of a real estate transaction. Buyers can include a radon inspection contingency in the purchase contract, a clause that requires satisfactory radon results before the sale moves forward. If levels come back at or above 4 pCi/L, the contingency gives the buyer several options: request that the seller install a mitigation system before closing, negotiate a price reduction, ask for a credit to cover future mitigation costs, or walk away from the deal entirely.
Sellers can accept, reject, or counter any radon contingency, and the cost of testing is typically the buyer’s responsibility, though that’s negotiable. The practical advice is straightforward: get the test done during the inspection window so you have leverage to negotiate before you’re locked in.
What Happens if Levels Are High
The most common fix is a system called active soil depressurization. A contractor installs a pipe through the foundation slab that connects to a fan, which continuously pulls radon from beneath the house and vents it above the roofline where it disperses harmlessly. The system runs quietly in the background and typically reduces indoor radon levels by 80% to 99%.
Installation costs average about $1,200 nationally, with most homeowners paying between $800 and $1,500 depending on the home’s foundation type and local market rates. The fan uses roughly as much electricity as a light bulb, so operating costs are minimal. After installation, you should retest to confirm the system is working and then check levels every two years or so to make sure performance holds.
Homes with crawlspaces, slab foundations, and basements can all be mitigated. Even homes with very high readings, well above 4 pCi/L, can almost always be brought down to safe levels with a properly designed system.