Radon is a radioactive gas released as uranium in soil and rock breaks down. It seeps into houses through foundation cracks, the sump pit, gaps around pipes — and once inside, it can build up to levels that matter. You can't see it, smell it, or taste it. The damage comes from breathing it over years: the EPA attributes roughly 21,000 lung cancer deaths a year in the US to radon, which makes it the second-leading cause of the disease overall and the leading cause among non-smokers. Those aren't scare numbers from a test-kit company; they're the EPA's own published figures.
The EPA's zone map colors counties by predicted average levels, and it misleads people in both directions. Homeowners in red zones assume they're doomed; homeowners in green zones assume they're safe. Both are guessing. The map is a county-level average built from geology, but radon is house-specific — it depends on your foundation, your soil contact, your cracks, even how your house drafts. Two houses on the same street routinely test very differently: one at 1.2 pCi/L, the next one at 9. The EPA's position cuts through all of it: test every home, whatever the map says. About 1 in 15 US homes tests at or above the action level, and they turn up in every zone and every state.
Place the kit in the lowest lived-in level. A finished basement, a ground-floor bedroom, a family room over the slab — those count. A crawl space or an upstairs bedroom doesn't. Set the kit 20 inches to 6 feet above the floor, away from exterior walls, windows, vents, and drafts. Keep it out of the kitchen and bathroom; humidity throws the reading off.
Close the house up. For a short-term test to count, windows and exterior doors stay shut — normal coming and going is fine — for 12 hours before the test starts and the whole time it runs, usually 2 to 4 days. This is the step people skip, and it's the step that makes the number mean anything.
Seal it and mail it the day the test ends. Charcoal kits start losing accuracy once the test period is over. Drop it in the mail the same day if you can, and write down the start and stop times the lab form asks for.
Confirm a high result before spending money. Short-term readings swing with weather and season. If the first test comes back at 4.0 pCi/L or higher, EPA protocol is a follow-up: either a second short-term test (and average the two) or a long-term test that runs 90 days or more. One dramatic number isn't a verdict.
Results come back in picocuries per liter, written pCi/L. For reference, average outdoor air is about 0.4 pCi/L, and the average US home sits around 1.3.
| Your result | What it means | What to do |
|---|---|---|
| Under 2.0 pCi/L | Below the national indoor average range; low risk | Nothing now. Retest every few years, and after renovations |
| 2.0 to 3.9 pCi/L | Above average. The EPA says consider fixing; the World Health Organization's reference level is 2.7 | Run a long-term test to confirm, then decide. Plenty of homeowners mitigate in this range |
| 4.0 pCi/L and up | The EPA action level | Fix it. Mitigation typically cuts levels by 50 to 99 percent |
No level of radon is officially "safe." Risk rises with concentration and with years of exposure, so the action levels are about practical risk reduction, not a bright line between safe and dangerous. The numbers also describe a long-term average, not an emergency: a house at 6 pCi/L is a problem to put on the schedule, not a reason to sleep at a hotel tonight.
The standard fix is called active soil depressurization — a fan and a PVC pipe that pull radon-bearing air from under the slab and vent it above the roofline, before it can enter the house. It's about a half-day job for a certified mitigator and commonly runs in the $1,200 to $2,000 range, depending on the house. Done right, it takes most homes below 2.0 pCi/L. You'll hear that sealing foundation cracks fixes radon, and sealing does help — as a supporting measure. The EPA is clear that sealing alone rarely brings a high level down and keeps it down; the fan is what does the work. One entry point worth a look either way is the sump pit, which is basically an open channel into the soil under your slab: a sealed cover matters there, and keeping the pump itself in working order is covered in our guide to testing your sump pump.
Radon also shows up in real estate deals constantly. Testing during a home sale is standard practice in many states, buyers ask for it, and sellers have to disclose results they know about. If you're the one buying, a $20 test belongs on the same list as the home inspection.
Treat radon the way you treat the house's other invisible gas, carbon monoxide: test, place the gear correctly, recheck on a schedule. Different gas, different hardware — CO needs detectors on the wall, and our guide to where to place carbon monoxide detectors covers that one. For radon, retest every couple of years, after any renovation that touches the foundation or adds living space downstairs, after a mitigation system goes in (to verify it actually worked), and if you start spending real time on a lower level you barely used before. A twice-a-year pass over these quiet checks is already built into our home safety inspection checklist.
Skipping the test because your county is "low risk." Zone maps are averages. Your house is a specific structure sitting on specific soil, and it's the specific one you sleep in.
Testing in the kitchen or bathroom. Steam and humidity interfere with charcoal kits. The lowest lived-in room is the right spot.
Leaving windows open. It dilutes the sample and manufactures the low result you were hoping for.
Acting on a single short-term test. Levels move around with weather and season. Confirm anything at or above 4.0 before hiring a mitigator — and don't let one low reading from a drafty week close the subject, either.
Trying to fix a high level with caulk. Sealing supports a mitigation system. On its own, it rarely solves a genuinely high level.
Yes, test — every home, map or no map. A kit costs about the same as a pizza, runs two to four days, and hands you a real number instead of a guess. Act at 4.0 pCi/L, think hard anywhere from 2.0 up, and retest every couple of years. It's the cheapest certainty you'll ever buy about the house you live in.
A do-it-yourself short-term kit runs about $10 to $30 including the lab fee, and state radon programs often sell them for less. Having a professional test — common during a real estate transaction — typically costs $150 to $300. Digital continuous monitors cost more up front, generally in the $100 to $250 range, but let you watch levels change over time.
There's no known completely safe level; risk climbs with concentration and years of exposure. The EPA sets its action level at 4.0 pCi/L and advises considering mitigation between 2.0 and 3.9. The practical goal is to get your home as low as reasonably possible, not to chase zero.
Radon enters from the ground, so it concentrates at the lowest level of a building and readings drop as you go up. Ground-floor and basement living spaces carry the real risk. An upper-floor apartment usually tests low, but a ground-floor unit in the same building can still test high — the test that matters is at the lowest level people actually live in.
Levels often run higher in winter, when the house is closed up and heated, so a winter test is a reasonable picture of the worse end of your range. Any season gives a usable result as long as you follow the closed-house rules for a short-term test. For the truest average, a long-term test spread across seasons beats any single snapshot.
A typical sub-slab depressurization system commonly runs $1,200 to $2,000 installed, with most of the variation coming from the size and layout of the house. Running costs are small — the fan uses about as much electricity as a light bulb. Fans eventually wear out, and a replacement generally lands in the low hundreds.