The short answer
Radon is an invisible, odorless gas that seeps into homes from the ground. It is the second leading cause of lung cancer after smoking. You cannot see or smell radon, so testing your home is how you find out your level. If levels are high, a contractor can lower them.
Radon is an invisible, odorless radioactive gas that comes from the ground.
It is the second leading cause of lung cancer in the United States, after smoking.
Testing is how you find out your home's radon level.
If levels are high, a qualified contractor can help lower them.
Watch: The #2 cause of lung cancer is in some homes
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 know how radon in the home is linked to lung cancer, and what to do about it.
The simple version
Radon is a gas you cannot see, smell, or taste. It forms naturally in the ground and can seep into homes, where it can build up in the air.
Breathing in radon over many years can cause lung cancer. In fact, radon is the second leading cause of lung cancer in the United States, after smoking.
Because you cannot sense radon, testing your home is how you find out if it is a problem.
Where radon comes from
Radon comes from uranium, a natural element in soil and rock. As uranium slowly breaks down, it releases radon gas.
Outdoors, radon spreads out in the air and is not a concern. NCI says radon can enter homes through cracks in floors, walls, or foundations and collect indoors:
- Cracks in floors, walls, or the foundation
Once inside, radon can build up. EPA says every new home should also be tested after move-in, even if it was built radon-resistant.
How radon causes cancer
Radon is radioactive, which means it gives off small amounts of energy called radiation. When you breathe in radon, tiny radioactive particles can settle in your lungs.
Over the years, the radiation from these particles can damage the cells lining your lungs. This damage can build up and lead to lung cancer.
EPA says radon is the number one cause of lung cancer among people who do not smoke. Unlike many risks, radon exposure is something you can test for and fix.
Radon and smoking together
Radon and smoking are a dangerous combination. Each one raises lung cancer risk on its own. Together, they raise it far more than adding the two alone would suggest.
If you smoke and your home has high radon, the risk is especially high. Lowering radon and quitting smoking both help, and doing both offers the greatest benefit.
Test your home
You cannot see or smell radon, so testing is how you find out your home's level.
To test:
- You can buy a do-it-yourself test kit and follow the instructions.
- You can hire a qualified radon professional to test for you.
Radon is measured in units called picocuries per liter, or pCi/L. Higher numbers mean more radon. The 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, and your test kit or contractor can explain the results.
What to do about high levels
If your test shows high radon, do not panic. High levels can be lowered.
A qualified contractor can install a radon reduction system. This usually means a vent pipe and fan. They draw radon from under the home and release it outside, before it can build up indoors. Follow the instructions on your test kit closely.
Why radon is a risk you can act on
What sets radon apart from many cancer risks is how manageable it is. You cannot see or smell it, but you can measure it. If it is high, you can lower it.
Testing your home is a step you can take to find out your risk. If you smoke, lowering radon and quitting smoking together offer the greatest protection for your lungs. Your health care team and local health department can point you to reliable test kits and contractors. If it applies to you, they can also point you to lung cancer screening options.
Sources
Common questions
What is radon?
Radon is a radioactive gas that forms naturally when uranium in soil and rock breaks down. It has no color, smell, or taste. It can seep up from the ground into homes and other buildings, where it can build up in the air you breathe.
How does radon cause cancer?
When you breathe in radon, tiny radioactive particles can get into your lungs and give off radiation. Over many years, this radiation can damage lung cells and lead to lung cancer.
How common a cause of cancer is radon?
Radon is the second leading cause of lung cancer in the United States, after cigarette smoking. It is the leading cause of lung cancer among people who have never smoked.
How do I know if my home has radon?
You cannot see or smell radon, so testing is how you find out. You can use a test kit or hire a professional. The U.S. Surgeon General has urged Americans to test their homes.
What if my radon level is high?
If your level is high, a qualified contractor can install a radon reduction system. This usually involves a vent pipe and fan that pull radon from under the home and release it outside.
Does smoking make radon worse?
Yes. Radon and smoking together raise lung cancer risk much more than either one alone. If you smoke and also have high radon at home, lowering both is especially important.
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Last updated: 2026-10-16Next planned review: 2027-07-04
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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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