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 to test. EPA recommends fixing any home at 4 pCi/L or higher, and a licensed professional can install a radon reduction system.
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 low-cost and simple: place a kit in the basement or lowest level of your home, raised off the floor, then mail it to the lab.
On this page
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The whole article, with every detail and caution.
This page is a step-by-step guide for anyone who wants to check their home for radon, a natural gas that can cause lung cancer. It covers testing, what your result means, and how high levels get fixed.
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. Buy one at a hardware store or online, or hire a licensed professional to test. Your state radon program can point you to options.
Place it properly. CDC says to put the kit in the basement or the lowest level of your home. Raise it about three feet off the floor and place it toward the middle of the room.
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.
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 fixing your home at this level. CDC says to contact a licensed professional to install a radon reduction system.
Between 2 and 4 pCi/L. Consider installing a reduction system. CDC notes there is no known safe level of radon.
Below 2 pCi/L. CDC says no immediate action is needed. Even so, no level of radon is known to be safe.
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 forms as uranium in soil and rock breaks down. 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. Radon levels can vary greatly from home to home, so no neighbor's result substitutes for testing your own home.
Lung cancer is the main cancer clearly linked to radon. NCI notes that this link was first seen in underground uranium miners.
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 could develop lung cancer from radon, against about 7 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. Ask your contractor how well it is expected to work in your home.
Use a qualified contractor. Your state radon program can help you find one. CDC advises a licensed professional. Retest a few months after the system is installed to confirm it is working.
Sources
- EPA — Consumer's Guide to Radon Reduction: How to Fix Your Home (PDF)
- EPA — Radon program home page and state contacts
- CDC — Testing for Radon in Your Home
- NCI — Radon and Cancer
- EPA — A Citizen's Guide to Radon (PDF)
- EPA — What is EPA's Action Level for Radon and What Does it Mean?
- EPA — Find a Radon Test Kit or Measurement and Mitigation Professional
- American Cancer Society — Radon and Cancer
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. Kits are sold at hardware stores and online, and a licensed professional can also do the test.
Where do I put the detector, and for how long?
CDC says to put it in the basement or lowest level of your home, raised off the floor (about three feet) and toward the middle of the room. 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 varies from day to day and season to season. If a result is 4 pCi/L or higher, CDC says to contact a licensed professional about a radon reduction system.
My result was 4.2 pCi/L. What now?
That is above the 4 pCi/L level at which CDC says to contact a licensed professional to install a radon reduction system. Your state radon program can help you find one. After the fix, retest a few months later to confirm it worked.
I rent. Can I still do anything?
You can still test, and knowing the number is worth having. 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 can be a problem in homes with or without basements. Radon levels can vary greatly from home to home, so a neighbor's result does not tell you yours. EPA recommends testing all homes below the third floor, as the American Cancer Society also notes.
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Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-10-02Next 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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