The short answer
Radon is the leading cause of lung cancer among people who never smoked. The EPA action level is 4 pCi/L, home test kits are widely available, and most homes can be fixed.
Radon causes about 21,000 lung cancer deaths a year in the United States, with an uncertainty range of 8,000 to 45,000, and is the leading cause among people who never smoked.
The EPA action level is 4 pCi/L. At that level, out of 1,000 people, roughly 62 smokers and 7 never-smokers could get lung cancer from radon over a lifetime.
Average indoor radon is about 1.3 pCi/L and average outdoor about 0.4 pCi/L. EPA and CDC suggest considering action between 2 and 4 pCi/L.
Short-term tests run 2 to 90 days; long-term tests run more than 90 days. Some state radon programs offer kits free or discounted.
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52 sec · Captioned · Radon is the #2 cause of lung cancer — testing your home costs under $25.
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This page is for anyone who wants to test their home for radon and fix it if levels are high. It walks through the numbers, the test, and the repair.
What radon is and why it matters
Radon is a radioactive gas produced when uranium in soil and rock breaks down. It has no color, no smell, and no taste. It enters buildings through cracks in foundations, gaps around pipes, sump pits, and crawl spaces. Once inside, it can accumulate.
The EPA and the Surgeon General's office estimate radon causes about 21,000 lung cancer deaths each year in the United States, with an uncertainty range of 8,000 to 45,000. It is the second leading cause of lung cancer overall and the leading cause among people who have never smoked.
This is one of the few significant cancer risks you can measure in your own house and, if necessary, reduce.
The numbers that define the decision
- 4 pCi/L is the EPA action level. At or above it (after a confirming test), CDC says to contact a licensed professional about a radon reduction system.
- 2 to 4 pCi/L is the range where EPA advises considering action, noting that reducing levels below about 2 pCi/L is technically difficult.
- 1.3 pCi/L is the average indoor level in the United States.
- 0.4 pCi/L is the average outdoor level.
At a sustained 4 pCi/L, out of 1,000 people, the EPA estimates about 62 smokers and about 7 never-smokers could get lung cancer from radon over a lifetime. That gap is not a reason for smokers to test and others not to. Smoking combined with radon increases lung cancer risk further, so an elevated result matters even more for current and former smokers.
There is no level at which radon is known to be harmless. The 4 pCi/L figure is a practical action point, not a safety boundary.
How to test
Choose a test type. Short-term tests measure radon for 2 to 90 days. Long-term tests run more than 90 days and show your year-round average. Radon levels change from day to day, so a long-term test can give a better picture.
Get a kit. Buy one at a hardware store or online, or contact your state radon office. Some states offer free or discounted kits.
Place it correctly. Put the device in your basement or the lowest level of your home. In a multi-unit building, use the lowest level within your unit. Follow all the kit instructions, and leave the test in place for the full time they give.
Send it to the lab. Follow the directions on the kit for where and when to send it.
Reading your result
At or above 4 pCi/L: if you used a short-term test, take a second test, either short-term or long-term. If the average of two short-term tests, or any long-term test, is 4 pCi/L or higher, contact a licensed professional to install a radon reduction system.
Between 2 and 4 pCi/L: CDC says to consider installing a radon reduction system.
Below 2 pCi/L: CDC says no action is needed. Test again if you start living on a lower level of the home, and after major renovations, because both can change radon levels.
Fixing a house
CDC says lowering radon takes technical knowledge and special skills, so rely on a qualified professional. Contractors who do not specialize in radon reduction could make the problem worse. Ask your state radon office or a national radon proficiency program to help you find one.
The method used most is a vent pipe and fan system, called soil depressurization. It pulls radon from beneath the house and vents it outside. Sealing foundation cracks and other openings makes the system more effective. Similar systems can be installed in homes with crawl spaces.
On cost, CDC says it varies widely with how your home was built and how severe the problem is, and that most homes can be fixed for about the same cost as other common home repairs. Get one or more estimates from qualified contractors.
Test again after repairs, a few months after your home is fixed, to see whether radon levels dropped.
Who should treat this as a priority
Anyone who smokes or has smoked, because smoking combined with radon raises lung cancer risk further. Anyone who spends a lot of time in a basement or lower level, where radon is often higher. Anyone buying or selling a home, since EPA recommends a qualified tester in that case. And anyone who has never tested, because testing is the only way to know your level.
Sources
- EPA: A Citizen's Guide to Radon (PDF)
- EPA: Consumer's Guide to Radon Reduction, how to fix your home
- EPA: Basic Radon Facts (PDF)
- National Radon Program Services: testing devices
- National Cancer Institute — Radon and Cancer Fact Sheet
- CDC — Radon and Your Health
- CDC — Testing your home for radon
- CDC — Reducing radon in your home
- EPA: Health risk of radon
Common questions
My neighbor tested low. Do I still need to test?
Yes. Radon levels can differ from house to house, because the result depends on the soil beneath your foundation, the cracks and openings in your home, and how your house draws air. A neighbor's reading does not tell you yours. The only way to know your level is to test your own home.
My result was 3.2 pCi/L, below the action level. Should I do anything?
There is no threshold below which radon carries no risk; 4 pCi/L is an action level chosen for practicality, not a safety line. The EPA specifically suggests considering mitigation between 2 and 4 pCi/L, while noting that getting below about 2 pCi/L is technically difficult. CDC says to consider installing a radon reduction system when a result is between 2 and 4 pCi/L. Smoking combined with radon raises lung cancer risk further, so a household where anyone smokes has more reason to ask about it.
How much does it cost to fix a house?
CDC says cost varies widely with how your home was built and how high the radon is, and that most homes can be fixed for about the same cost as other common home repairs. Get one or more estimates from qualified contractors, and contact your state radon office for more information.
Does radon matter if nobody in my house smokes?
Yes. EPA calls radon the number one cause of lung cancer among people who have never smoked. The risk is lower than for smokers at the same exposure, about 7 per 1,000 never-smokers versus about 62 per 1,000 smokers over a lifetime at 4 pCi/L, but it is not close to zero. Smoking combined with radon raises the risk further.
Do I need to test if I live in an apartment or on an upper floor?
Radon enters from the soil, so levels are often higher in basements and lower levels. CDC says that in a multi-unit building you place the test device on the lowest level of your own unit. If you rent, ask your landlord whether the building has been tested.
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Written by: Cancer ExplainedSources last checked: 2026-08-11 what this meansLast updated: 2026-10-01Next planned review: 2027-07-30
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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, and this 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 then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. 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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