Radon is a naturally occurring radioactive gas that seeps into homes from the soil beneath them. It has no color, no odor, and no taste, which means it can accumulate indoors without anyone noticing. It’s estimated to cause about 21,000 lung cancer deaths per year in the United States, making it the second leading cause of lung cancer after smoking. Understanding how radon gets in, how to test for it, and how to reduce it can meaningfully lower your risk.
Where Radon Comes From
Radon starts with uranium, a naturally occurring element found in small amounts in most rocks and soil. Uranium slowly breaks down into radium, and radium breaks down into radon gas. Because this process happens underground, radon is constantly being generated in the earth beneath and around your home. As it forms, it migrates upward through gaps in the soil and can enter any building that sits on the ground.
During this decay process, radon releases several types of radiation. The most relevant for your health are alpha particles, which travel only a short distance but can damage lung tissue when you breathe radon in. Outside, radon disperses harmlessly into the atmosphere. Inside a house, it gets trapped and concentrates.
How Radon Gets Into Your Home
The main way radon enters a house is through the soil. Your home’s interior is typically at slightly lower air pressure than the surrounding soil, a phenomenon caused by warm air rising inside the house (sometimes called the stack effect), exhaust fans, and HVAC systems. That small pressure difference pulls soil gas, including radon, upward and into the building through any available opening.
Common entry points include cracks in basement floors and foundation walls, gaps around pipes and utility lines, joints where the floor meets the walls, and porous materials like cinder block. Any home can have elevated radon levels regardless of age, construction type, or geographic location, though some regions have naturally higher uranium concentrations in the soil.
Radon can also enter through well water. When you shower, flush toilets, or run the washing machine, dissolved radon escapes from the water into the air. The contribution is small, roughly one ten-thousandth of the water concentration, but homes relying on private wells with high radon levels may see a measurable effect. Most of the cancer risk from radon in water comes from breathing it in after it’s released into the air, not from drinking the water itself.
Why Radon Is a Serious Health Risk
Radon is the leading environmental cause of lung cancer. When you inhale radon gas, its radioactive decay products (tiny particles that form as radon breaks down further) lodge in the lining of your lungs. Over years, the alpha radiation these particles emit damages lung cells, which can eventually trigger cancer.
Smokers face a dramatically higher risk because tobacco smoke already irritates and damages the lungs, making them more vulnerable to radiation. For nonsmokers, the risk is lower but still real. There is no known safe level of radon exposure. The risk increases with both the concentration of radon in your home and the number of years you’re exposed to it.
The EPA Action Level: 4 pCi/L
Radon is measured in picocuries per liter of air (pCi/L). The EPA recommends fixing your home if radon levels reach 4 pCi/L or higher. Because no level of radon is considered completely safe, the EPA also suggests considering action for levels between 2 and 4 pCi/L. For context, outdoor air typically contains about 0.4 pCi/L.
Internationally, thresholds vary. The International Atomic Energy Agency sets a maximum reference level of 300 becquerels per cubic meter for homes, which translates to roughly 8 pCi/L. The EPA’s threshold is more conservative, reflecting the agency’s position that reducing exposure as much as possible is worthwhile.
How to Test Your Home
Testing is the only way to know your radon level. You can hire a certified radon tester or buy a test kit from a hardware store and do it yourself. Kits come in two types.
- Short-term kits measure radon over 2 to 90 days and give you a quick snapshot. These are useful as a first screening, but radon levels fluctuate with weather, season, and how you use your home.
- Long-term kits measure for more than 90 days and reflect your home’s year-round average. The longer the test runs, the more accurately it represents 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) helps confirm the result before you invest in mitigation. Tests should be placed in the lowest livable level of the home, typically the basement or ground floor, with windows and exterior doors kept closed as much as possible during the testing period.
How Radon Mitigation Works
The most common and effective fix is called active soil depressurization. A contractor drills a hole through your basement slab, inserts a PVC pipe down into the gravel or soil beneath, and runs that pipe up through the house and out through the roof. A fan mounted on the pipe continuously pulls soil gas from beneath the foundation and vents it safely above the roofline, where it disperses outdoors.
The system works by creating a low-pressure zone under the entire slab. As long as that pressure stays lower than what weather and household activities create inside the house, soil gas (and the radon in it) gets drawn into the pipe rather than up through cracks in the floor. Sealing visible cracks and gaps in the foundation improves performance but isn’t sufficient on its own without the active suction.
According to EPA estimates, professional installation typically costs between $800 and $2,500, with an average around $1,200. The price depends on your foundation type, the size of the home, and how powerful a fan is needed. Once installed, the system runs continuously and uses about as much electricity as a light bulb. Most systems reduce radon levels by 80% or more.
Radon-Resistant Features in New Homes
New homes can be built with radon-resistant construction at a fraction of what a retrofit costs. The approach includes five basic features that work together to block soil gas entry and make it easy to add active ventilation later if needed.
- Gravel layer: A 4-inch bed of coarse, clean gravel beneath the slab allows soil gases to circulate freely underneath the house rather than building up pressure.
- Plastic sheeting: A heavy-duty polyethylene sheet laid over the gravel prevents soil gas from passing up through the slab. It also keeps wet concrete from clogging the gravel when the foundation is poured.
- Vent pipe: A PVC pipe runs vertically from the gravel layer up through the house and out through the roof, providing a pathway for soil gas to escape. This pipe is labeled “Radon System.”
- Sealing: All cracks, joints, and openings in the concrete floor and foundation walls are sealed with caulk to minimize additional entry points.
- Electrical junction box: An outlet is installed in the attic so a vent fan can be added later if testing shows radon levels are still too high with the passive system alone.
With these features in place, adding a fan to convert a passive system into an active one is a simple, inexpensive upgrade. Many local building codes in high-radon areas now require some or all of these features in new construction.