What Is Radon? The Invisible Gas Behind Lung Cancer, and How to Test for It

Radon is a radioactive gas that forms naturally when uranium in rocks and soil breaks down. It seeps up through the ground, gets into homes through cracks and gaps in the foundation, and can build up indoors. You can’t see, smell or taste it, but breathing high levels over many years can cause lung cancer. The only way to know whether your home has a problem is to test.

This explainer is research-based and draws on EPA, CDC and the National Cancer Institute (NCI). Our research method explains how we work.

Radon in plain terms

NCI describes radon as an invisible, odorless, tasteless gas released by the normal decay of uranium, thorium and radium in rocks and soil. It’s present in nearly all air, and everyone breathes a little of it every day. Outdoors it disperses to very low levels: EPA puts the average outdoor level at about 0.4 pCi/L. The trouble starts when it collects in enclosed spaces.

Radon is measured in picocuries per liter of air (pCi/L). Some numbers worth knowing, all from EPA:

Level	What it means
0.4 pCi/L	Average outdoor level
1.3 pCi/L	Average indoor level in US homes
2 to 4 pCi/L	Consider fixing your home
4 pCi/L or higher	Fix your home (EPA’s action level)

EPA, CDC and NCI all stress that there’s no known safe level. The 4 pCi/L figure is the point where EPA says to act, not a line below which radon is harmless.

Where radon comes from and how it gets inside

According to EPA’s Citizen’s Guide to Radon, soil gas is the main cause of radon problems in homes. Air pressure inside a house is usually lower than the pressure in the soil around the foundation, so the house acts a bit like a vacuum and draws radon in. EPA lists the usual entry points:

• Cracks in solid floors and walls
• Construction joints
• Gaps in suspended floors and around service pipes
• Cavities inside walls
• The water supply

Two smaller sources round it out. Radon can be released from well water during showering and other household use, though EPA says radon entering from soil is usually a much larger risk. And a small number of homes get radon from building materials, which EPA says rarely cause problems by themselves.

Which homes are affected?

Any home can have a radon problem: new or old, well sealed or drafty, with or without a basement. EPA estimates that nearly 1 in 15 US homes has an elevated level. NCI adds that indoor levels depend on the soil under each house and how easily radon enters, so homes next door to each other can test very differently. A neighbor’s result tells you little about yours.

Lower floors usually carry the most radon. NCI says basements and first floors typically have the highest levels because they’re closest to the ground.

Radon and lung cancer: what the agencies say

Radon gas decays into radioactive particles. When you breathe them in, they can get trapped in your lungs, and as they break down further they release small bursts of energy that can damage lung tissue (EPA). NCI says lung cancer is the only cancer proven to be associated with inhaling radon. CDC notes that lung cancer takes many years to develop and usually causes no symptoms until it’s advanced.

How big is the risk?

• About 21,000 deaths a year. EPA estimates radon causes about 21,000 lung cancer deaths a year in the US, with an uncertainty range of 8,000 to 45,000. NCI cites a range of 15,000 to 22,000.
• Second only to smoking. Radon is the second leading cause of lung cancer overall and, by EPA’s estimate, the leading cause among people who don’t smoke. EPA attributes about 2,900 of the yearly deaths to people who never smoked.
• Smoking multiplies it. NCI says radon and cigarette smoke together create a greater risk than either alone, and most radon-related lung cancer deaths occur among smokers.

EPA’s Citizen’s Guide shows how exposure and smoking interact. At a lifetime exposure of 4 pCi/L, it estimates that about 62 out of 1,000 people who smoke could get lung cancer, compared with about 7 out of 1,000 people who never smoked. Not everyone exposed to elevated radon will develop lung cancer, EPA notes, and your chances depend mostly on how much radon is in your home, how much time you spend there, and whether you smoke or have smoked.

What about children? EPA says there are no conclusive data on whether children face a greater risk from radon than adults. CDC adds that children may receive higher doses than adults at the same level because they breathe faster and have different lung sizes.

The science behind these numbers is solid by environmental-health standards. NCI explains that radon was first flagged when underground uranium miners died of lung cancer at high rates, and later studies that measured radon in the homes of people with and without lung cancer found a risk consistent with the miner data.

Important: If you’re worried about past radon exposure, or you smoke and have found a high level at home, talk to your doctor. CDC also points smokers to free help at 1-800-QUIT-NOW.

How to test for radon

EPA and the Surgeon General recommend testing all homes below the third floor. Testing is inexpensive, and CDC calls it the only way to know whether your level is high. You have two routes:

1. Do it yourself. Kits are sold at hardware stores and online, and CDC says your state radon office may offer free or discounted kits. You can also compare radon test kits on Amazon.
2. Hire a qualified tester. EPA suggests this if you’re buying or selling a home or would rather not do it yourself. Your state radon office can provide a list.

There are two kinds of tests:

• Short-term tests stay in your home for 2 to 90 days, depending on the device. They’re the quickest way to get a reading.
• Long-term tests run for more than 90 days. Because radon rises and falls from day to day and season to season, NCI calls a long-term test a better indicator of your average level.

EPA’s basic sequence is to start with a short-term test in the lowest lived-in level of the home, then follow up with a second test if the result is 4 pCi/L or higher before deciding to fix it.

CDC also recommends testing when you’re preparing to buy or sell, before and after renovations, and before you start spending more time on a lower level, such as turning a basement into a bedroom or office.

Why an air purifier won’t fix radon

It’s a reasonable question for anyone who owns an air purifier, and the answer is no. EPA’s technical summary on residential air cleaners says EPA does not recommend air cleaning to reduce the health risks of radon and its decay products. A purifier is built for particles (and, with enough carbon, some odors and gases). Radon keeps flowing in from the soil, so the fix has to stop it at the source. Our explainer on whether air purifiers work covers what they do remove, and even our basement air purifier picks are there for dust, spores and musty smells, not radon. We’d treat any product claim to the contrary as a red flag.

What works instead, according to EPA:

• Soil suction (soil depressurization). A vent pipe and fan pull radon from beneath the house and vent it outside, usually above the roofline. EPA calls it the main method and says some radon reduction systems can cut levels by up to 99%.
• A qualified contractor. EPA and CDC both say lowering high radon levels takes technical knowledge and special skills. Your state radon office can point you to qualified or state-certified contractors.
• Retesting. Test again after the work, and EPA suggests retesting every two years after that.

EPA says most homes can be fixed for about the same cost as other common home repairs, and that most homes today can be brought down to 2 pCi/L or below. Opening windows helps only temporarily; CDC calls natural ventilation a temporary strategy, not mitigation.

What we’d do

We’d test before worrying. A short-term kit in the lowest lived-in level costs little and answers the question for your house, not your neighborhood. If the result is 4 pCi/L or higher, we’d confirm it with a second test and then call a qualified mitigation contractor rather than reaching for a gadget. In our view, the 2 to 4 pCi/L range deserves a real look too, since EPA says to consider fixing it. And we’d skip any air cleaner marketed for radon; our guide on how to improve indoor air quality explains where filtration does help.