What Is Radon? The Complete Guide
Radon is a radioactive gas. It forms naturally when uranium in soil and rock breaks down. It has no smell, no color, and no taste. The only way to know if it is in your home is to test for it.
That is the short version. If you are the type who likes to actually understand what is going on in your house, read on.
Where radon comes from
Uranium is present in soil and rock almost everywhere on earth. As it decays, it produces radium. As radium decays, it produces radon. Radon is a gas, so it moves through soil, enters your home through foundation cracks, gaps around pipes, and construction joints, and accumulates indoors. Outdoors it disperses harmlessly into the atmosphere. Inside, it builds up.
The EPA estimates nearly 1 in 15 US homes has elevated radon levels. That number is probably an undercount, because most homes have never been tested.
Check your county's EPA zone on RadonLookup to understand your area's baseline risk.
Why radon matters
Radon is the second leading cause of lung cancer in the United States, responsible for around 21,000 deaths per year according to the EPA. It is the leading cause among non-smokers.
When radon decays indoors, it produces radioactive particles that can be inhaled and lodge in lung tissue, where they continue to emit radiation over time. The damage is cumulative. Short-term exposure at moderate levels is not an emergency. Years of exposure at elevated levels is a serious health risk.
The action levels
If you have never tested your home, that is the right place to start. A short-term kit costs around $15. See Tipper's picks.
How radon enters your home
Your home is typically at slightly lower pressure than the soil beneath it, which means air gets pulled in from the ground, and radon comes with it. It enters through cracks in concrete floors and walls, construction joints where the floor meets the wall, gaps around service pipes, sump pits, and crawl spaces with bare soil floors. A well-sealed home is not immune. Radon finds gaps that are invisible to the naked eye.
Foundation type matters a lot. Homes with basements or crawl spaces tend to accumulate more radon than homes on slabs, but slab homes are not automatically safe. And new construction is not safer than old. A tightly built new home can trap radon more efficiently than a drafty older one. Build date tells you nothing useful. Testing does.
If your home has a basement or crawl space, place your test kit there first. That is where concentrations are typically highest.
Radon levels and what they mean
Radon is measured in picocuries per liter of air, written as pCi/L. The US average indoor level is 1.3 pCi/L according to the EPA. The average outdoor level is 0.4 pCi/L. Here is how to read your result:
Well below the EPA action level.
No action needed. Retest every two years or after major renovations.
Below the action level, but worth watching.
A long-term test gives a more accurate seasonal average. Mitigation is optional but reasonable, especially in bedrooms.
At or above the EPA action level. Fix your home.
A certified contractor can install a mitigation system that reliably brings levels down below 2 pCi/L.
Significantly elevated. Take action sooner rather than later.
Do not wait for a long-term confirmation test at this level. Contact a certified contractor.
Use RadonLookup to see your county's average pCi/L and EPA zone, so you have context for your own result.
Who is most at risk
Everyone who breathes radon is affected by it, but some factors matter more than others. Smokers exposed to elevated radon face dramatically higher lung cancer risk. The combination is far more dangerous than either alone, and if anyone in your household smokes, testing is urgent rather than optional.
People who spend significant time on lower floors have greater exposure than those primarily on upper levels. Children may be more vulnerable due to higher respiration rates and more years of potential future exposure.
None of this means you should panic. It means you should test.
If anyone in your household smokes, move radon testing to the top of your home maintenance list.
How radon is fixed
The most common solution is sub-slab depressurization. A certified contractor installs a pipe through the foundation floor and a fan that draws radon from beneath the slab and vents it outside before it can enter the home. A typical system costs $800 to $2,000 depending on home size, foundation type, and local labor. It runs continuously on roughly the electricity of a light bulb.
Most homes see levels drop below 2 pCi/L after installation, and a reputable contractor will do a post-installation test to confirm it is working.
A note on DIY mitigation
If your test comes back above 4 pCi/L, find a certified contractor in your area using RadonLookup's contractor directory.
Testing: the only way to know
County zone data and state averages are useful context, but they do not tell you what is happening inside your specific home. Two houses on the same street can have radon levels that differ by a factor of ten. The variation comes down to foundation type, construction quality, soil composition, and ventilation. Your neighbor's result is interesting. Your result is what matters.
A short-term test kit costs around $15, takes 48 hours, and gives you a result within a week of mailing it to the lab. It is one of the most cost-effective health checks you can do for your home. Tipper has tested every home he has lived in. It takes less time than most things you will do this week.
See Tipper's picks for the best radon test kits at every price point, with a breakdown of short-term, long-term, and continuous monitors.