Radon is a naturally occurring radioactive gas that seeps into homes from the ground beneath them. It has no color, no smell, and no taste, which means you can’t detect it without a test. Radon is the second leading cause of lung cancer after smoking, and every home, regardless of age or construction type, can have elevated levels.
Where Radon Comes From
Radon forms underground when uranium and radium in soil and rock naturally break down through radioactive decay. These elements are present in virtually all soil, but certain rock types, particularly granite and some clays, contain much higher concentrations. As uranium decays into radium and then into radon, the gas migrates upward through gaps in the soil. Outdoors, it disperses harmlessly into the atmosphere. The problem starts when it moves upward into a building sitting on top of that soil.
How It Gets Inside Your Home
The primary way radon enters a home is through soil gas being pulled in through the foundation. Your house acts like a mild vacuum. Warm air rising inside the home (the “stack effect”), exhaust fans, and furnaces all create slightly lower air pressure indoors compared to the soil surrounding the foundation. That small pressure difference is enough to draw radon-laden soil gas inward.
The gas finds its way in through cracks in foundation slabs, gaps around pipes and wiring, joints between floors and walls, sump pump openings, and even through porous concrete or cinder block walls. A home doesn’t need visible damage for radon to enter. Microscopic gaps are sufficient.
Basements and ground-floor rooms typically have the highest concentrations because they’re closest to the soil, but radon can reach upper floors as well, especially in tightly sealed homes with limited ventilation.
Why Radon Is Dangerous
When you breathe in radon, it decays inside your lungs into radioactive particles (primarily forms of polonium). These particles release alpha radiation, a type of energy that travels only a short distance but hits hard. Alpha particles create dense tracks of damage when they collide with the DNA in lung cells, causing double-strand breaks and chromosomal damage that is far more difficult for cells to repair than damage from other types of radiation like X-rays.
Over years of exposure, this repeated DNA damage increases the likelihood that cells will mutate in ways that lead to lung cancer. The damaged cells can also send stress signals to neighboring healthy cells, causing further instability even in tissue that wasn’t directly hit by radiation.
Radon doesn’t cause any short-term symptoms. There’s no cough, no headache, no irritation. The damage accumulates silently over years, which is exactly why testing matters.
The Smoking Connection
Radon and cigarette smoke together are significantly more dangerous than either one alone. The combined effect is multiplicative, not just additive. At the EPA’s action level of 4 pCi/L, roughly 62 out of every 1,000 smokers exposed over a lifetime will develop lung cancer, compared to about 7 out of 1,000 for people who have never smoked. At very high levels like 20 pCi/L, smokers face a risk of about 260 per 1,000, while never-smokers face about 36 per 1,000.
Even for non-smokers, however, the risk is real. At 4 pCi/L, a lifetime of exposure gives a never-smoker roughly a 1-in-140 chance of developing lung cancer from radon alone. That’s comparable to other risks most people would take seriously.
What Levels Are Considered Unsafe
The EPA recommends taking action to reduce radon if your home tests at 4 pCi/L or higher. They also recommend considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe threshold for radon exposure. Any amount carries some risk; 4 pCi/L is simply the line where the EPA says the risk is high enough that action is clearly warranted.
The World Health Organization sets its reference level lower, at 300 Bq/m³ (about 8 pCi/L) for homes, while many European countries use even stricter guidelines. In the U.S., the average outdoor radon level is around 0.4 pCi/L, so any home reading significantly above that represents an indoor accumulation problem.
Does Your Location Matter?
Geography plays a real role. The EPA divides the country into three radon zones based on geology and measured indoor levels. Zone 1 areas, which include much of the northern Midwest, Appalachian region, and parts of the Mountain West, have the highest potential, with average indoor levels that may exceed 4 pCi/L. Zone 2 represents moderate potential (2 to 4 pCi/L averages), and Zone 3 represents lower potential (below 2 pCi/L).
But these are averages across counties, not guarantees for individual homes. A house in a low-risk zone can still test dangerously high depending on the specific soil beneath it, the foundation type, and how tightly the home is sealed. The EPA recommends testing every home, regardless of zone.
How to Test Your Home
Testing is straightforward and inexpensive. Two main options exist:
- Short-term test kits measure radon over 2 to 90 days and give you a quick snapshot. These are widely available at hardware stores and online for under $20. You place the device in the lowest livable level of your home, leave it undisturbed, and mail it to a lab for analysis.
- Long-term test kits measure radon for more than 90 days and reflect your home’s year-round average level. Because radon concentrations fluctuate with weather, soil moisture, and heating patterns, longer tests give a more accurate picture of your actual exposure.
If a short-term test comes back at or above 4 pCi/L, a follow-up test (either another short-term or a long-term test) confirms the result before you invest in a mitigation system.
How Radon Mitigation Works
If your home has high radon levels, the most common fix is a sub-slab depressurization system. A contractor installs a pipe through the foundation slab into the gravel or soil beneath it, then connects it to a small fan that runs continuously. The fan pulls radon-laden air from under the house and vents it above the roofline, where it disperses safely outdoors. This reverses the pressure dynamic that drew the gas inside in the first place.
Installation typically takes less than a day. Costs generally range from $800 to $2,500 depending on the home’s foundation type and local labor rates. A properly installed system can reduce radon levels by up to 99%, and most homeowners see their levels drop well below 2 pCi/L. The fan uses about as much electricity as a light bulb left on continuously. After installation, the EPA recommends retesting every two years to make sure the system is still performing.
For homes with crawl spaces rather than basements, a similar approach uses a heavy plastic membrane sealed over the soil, with a vent pipe and fan pulling gas from beneath it. Sealing cracks alone is not effective as a standalone fix, because radon finds new entry points, but sealing does improve the performance of an active venting system.