The short answer
Radon is an invisible radioactive gas that seeps out of soil into homes, and it causes an estimated 21,000 US lung cancer deaths a year. You cannot smell or see it, and the only way to know your level is a test that costs very little. EPA recommends fixing any home at 4 pCi/L or higher, and mitigation usually works.
Radon is the second leading cause of lung cancer in the US after smoking, and the leading cause among people who have never smoked.
EPA estimates radon causes about 21,000 lung cancer deaths each year in the US, roughly 2,900 of them in people who never smoked.
EPA recommends fixing your home if the level is 4 pCi/L or higher, and says to consider fixing it between 2 and 4 pCi/L.
Testing is cheap and simple: place a short-term kit in the lowest lived-in level, about three feet up and away from walls, then mail it to the lab.
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The full explanation.
Test your home. Here is exactly how.
This is the rare cancer risk with a clear, cheap, finishable action. You can be done in twenty minutes of effort plus a few days of waiting.
Get a kit. Contact your state or tribal radon program through EPA's radon zone map at epa.gov/radon. Some states hand out free or discounted kits. Otherwise buy one at a hardware store, online, or through National Radon Program Services at sosradon.org. A short-term kit usually costs about as much as a takeaway meal, lab fee included.
Place it properly. Put the detector in the lowest level of your home that you actually use. Set it about three feet off the floor, roughly in the middle of the room, away from walls, windows, drafts, vents and exterior doors. In an apartment, use the lowest level of your own unit.
Follow the timing on the packet. Short-term kits run 2 to 90 days. Long-term kits run more than 90 days and give a better picture of your year-round average, because radon levels swing with weather and season. Keep windows shut during a short-term test if the instructions say to.
Mail it in and read the number. The lab returns a result in picocuries per liter, written pCi/L.
What your number means
4 pCi/L or higher. EPA recommends having a radon reduction system installed. If your first result was a short-term test, run a second test and average the two before you commit.
Between 2 and 4 pCi/L. EPA says consider fixing. There is no level that is proven safe, and risk falls as the number falls.
Below 2 pCi/L. No action needed. Retest if you finish a basement, start using a lower level as living space, or make major changes to the building.
Retest at least every two years in any case, and after any significant renovation.
Why this is worth the twenty minutes
Radon is the second leading cause of lung cancer in the US after smoking, and the number one cause among people who have never smoked. EPA estimates about 21,000 lung cancer deaths a year in the US from radon, of which roughly 2,900 are in never-smokers.
It is invisible and has no smell. It is produced as uranium in soil and rock breaks down, and it seeps into buildings through cracks and gaps in floors, walls and foundations, and sometimes through well water. Tight, well-insulated homes can hold it. Levels vary house by house on the same street, so no neighbor's result and no regional map substitutes for testing your own home.
Radon and its decay products are classified in IARC Group 1, meaning the evidence that they cause cancer in people is strong. Unlike many Group 1 agents, radon's individual risk at common household levels is not trivial, which is why the response is a specific numeric action level rather than general advice.
Smoking multiplies it
EPA's own risk tables make the interaction hard to miss. At a household level of 4 pCi/L over a lifetime, about 62 in 1,000 people who smoke are estimated to develop lung cancer from radon, against about 7.3 in 1,000 people who never smoked. Radon raises risk for everyone; it raises it far more for people who smoke. If both apply to you, fixing the house and getting help to quit are the same project.
Fixing a high result
The standard fix is active sub-slab depressurization: a pipe through the floor slab into the soil beneath, with a fan that pulls radon out and vents it above the roofline. EPA calls it the most common and usually the most reliable method, and it typically cuts radon by 50 to 99 percent. Most homes can be brought to 2 pCi/L or below. The fan adds a modest amount to your electricity bill.
Use a qualified contractor. Check your state's licensing or certification requirements, or use the National Radon Proficiency Program at 1-800-269-4174 or the National Radon Safety Board at 1-866-329-3474. Retest within 30 days of the system starting up to confirm it worked, and keep the paperwork; it matters when you sell.
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Words to know
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Common questions
How do I actually get a test kit?
Start with your state or tribal radon program, which you can find through EPA's radon zone map at epa.gov/radon. Some states give kits away free or at a discount. Otherwise, kits are sold at hardware stores, online, and through National Radon Program Services at sosradon.org. A short-term kit typically costs in the range of a takeaway meal, including the lab fee. You can also hire a certified measurement professional, which is what most people do during a home sale.
Where do I put the detector, and for how long?
Put it in the lowest level of your home that you actually use, about three feet off the floor, away from walls, windows, drafts and exterior doors. In an apartment, use the lowest level of your own unit. Short-term kits run from 2 to 90 days; long-term kits run more than 90 days and give a better picture of your year-round average, because radon swings with season and weather. If a short-term test comes back at 4 pCi/L or higher, run a second test and average the two before deciding.
My result was 4.2 pCi/L. What now?
That is above EPA's action level, so the recommendation is to have a radon reduction system installed by a qualified contractor. Find one through your state radon program, or through the National Radon Proficiency Program at 1-800-269-4174 or the National Radon Safety Board at 1-866-329-3474. Retest within 30 days of the system being switched on to confirm it worked, and retest at least every two years afterwards.
I rent. Can I still do anything?
You can still test, and knowing the number is worth having. Fixing the building is the owner's decision, and some states have rules requiring landlords to disclose or address radon. Your state radon program can tell you what applies where you live. Testing before you sign a lease, or asking whether the building has been tested, is a reasonable question.
I don't have a basement. Do I still need to test?
Yes. Radon enters through cracks and openings in whatever the house sits on, including slab-on-grade foundations and crawl spaces. Homes on the same street can have very different levels, so a neighbor's result does not tell you yours. Every home should be tested regardless of construction, age or region.
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Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-08-18Next planned review: 2027-07-30
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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