What Is Radon Gas? Everything You Need to Know

What Is Radon?

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and water. It's part of the uranium decay chain — as uranium breaks down over millions of years, it produces radium, which then decays into radon gas. This process happens everywhere on Earth, in every type of soil and bedrock.

What makes radon particularly dangerous is that you can't see it, smell it, or taste it. Unlike carbon monoxide (which is also invisible but can produce symptoms), radon causes no immediate symptoms. The damage happens silently over years of exposure, as radioactive radon particles lodge in lung tissue and cause cellular damage that can lead to cancer.

Radon's chemical symbol is Rn, and it has an atomic number of 86. It's a noble gas, meaning it doesn't react chemically with other elements — but its radioactive decay products (called "radon daughters" or "radon progeny") are solid particles that can attach to dust and aerosols in the air. When you breathe these particles in, they can become trapped in your lungs and continue to emit radiation.

Where Does Radon Come From?

Radon originates from the natural radioactive decay of uranium, which is found in virtually all soils and rock formations worldwide. The concentration varies dramatically depending on local geology — areas with granite, shale, phosphate, and certain types of limestone tend to produce higher radon levels.

Once radon gas forms in the soil, it migrates upward through the ground toward the surface. Outdoors, radon disperses quickly into the atmosphere and remains at harmlessly low concentrations (typically 0.4 pCi/L). The problem occurs when radon enters an enclosed structure like a home, where it can accumulate to dangerous levels.

The EPA has mapped radon potential across the United States, dividing counties into three zones. Zone 1 counties have the highest predicted indoor radon levels (above 4 pCi/L), but elevated radon has been found in homes in every state. You cannot predict your home's radon level based on location alone — the only way to know is to test.

How Radon Enters Your Home

Radon enters homes through a process driven by simple physics. Your home acts like a vacuum relative to the soil around and beneath it. This "stack effect" — caused by warm air rising inside the home and creating slight negative pressure at the lowest level — draws soil gas (including radon) in through any available opening in the foundation.

Even homes with solid poured-concrete foundations have enough microscopic cracks, joints, and penetrations for significant radon entry. Here are the most common entry points:

Several factors can increase radon entry: heating and cooling systems that create negative pressure, exhaust fans, fireplaces, and even wind loading on one side of the house. Radon levels also tend to be higher during winter months when homes are sealed tight and heating systems increase the stack effect.

Health Risks of Radon Exposure

Radon is the second leading cause of lung cancer in the United States, after smoking. The EPA estimates that radon causes approximately 21,000 lung cancer deaths per year — more than drunk driving, house fires, drowning, and carbon monoxide poisoning combined.

When you inhale radon gas and its decay products, radioactive particles become trapped in your lung tissue. These particles emit alpha radiation, which damages the DNA in lung cells. Over years of exposure, this cumulative damage can cause lung cells to grow abnormally and develop into cancer.

The risk increases with higher radon concentrations and longer exposure times. Smokers exposed to radon face a dramatically higher risk — the combination of smoking and radon exposure is synergistic, meaning the combined risk is far greater than either risk alone. A smoker living in a home with 4 pCi/L of radon has approximately a 6.2% lifetime risk of dying from lung cancer, compared to 0.7% for a never-smoker at the same radon level.

For a deeper look at the health data, see our article on radon health risks and lung cancer statistics.

EPA Radon Guidelines & Action Levels

The U.S. Environmental Protection Agency (EPA) has established clear guidelines for radon levels in homes. Understanding these levels helps you interpret test results and decide what action to take.

It's important to understand that 4 pCi/L is not a "safe" threshold — it's an action level based on what's technically achievable with current mitigation technology and what's cost-effective for public health. The EPA has stated clearly: "There is no known safe level of radon exposure." Any reduction in radon reduces your risk.

Which Homes Are at Risk?

The short answer: every home. Radon has been found at elevated levels in homes in all 50 states and in every type of building. However, certain factors increase the likelihood of higher radon levels:

• •Geography: Homes in EPA Zone 1 (highest predicted radon) are most likely to have elevated levels, but high-radon homes exist in all three zones. States like Iowa, Pennsylvania, Ohio, Colorado, and Minnesota have particularly high prevalence.
• •Foundation type: Homes with basements tend to have higher radon levels because the below-grade living space is in direct contact with soil. Crawl spaces and slab-on-grade homes can also have high radon, just through different pathways.
• •Home age: Both old and new homes can have high radon. Older homes may have more cracks and gaps, while newer homes are often tighter and can accumulate radon more easily if no passive radon system was installed during construction.
• •Soil permeability: Sandy, gravelly, or fractured-rock soils allow radon to move more freely toward the surface and into homes. Dense clay soils may slow radon movement but don't eliminate it.
• •Neighbor's levels don't predict yours: Two identical houses next door to each other can have dramatically different radon levels. Soil conditions, foundation integrity, and home ventilation all play a role.

How to Test for Radon

Testing is the only way to know your home's radon level. The good news is that radon testing is simple, affordable, and widely available. The EPA recommends that every home be tested for radon, regardless of location or age.

DIY Test Kits ($10–$50)

The most affordable option. Place a charcoal canister or alpha-track detector in the lowest livable level of your home for 2 to 7 days, then mail it to a certified lab. Results typically arrive within a week. Short-term kits are great for initial screening, while long-term kits (90+ days) provide a more accurate annual average. Many state programs offer free or discounted kits.

Professional Testing ($125–$400)

A certified radon professional places a continuous radon monitor (CRM) in your home for 48+ hours. These devices measure radon levels every hour, detect tampering, and provide a detailed report. Professional testing is recommended for real estate transactions and for confirming elevated DIY results.

For a detailed comparison, see our DIY vs. professional radon testing guide.

When to Test

• ✓ When you move into any home (even a new one)
• ✓ Every 2 years as routine maintenance
• ✓ After any major renovation or addition
• ✓ If you install a new HVAC system
• ✓ Annually if you have a mitigation system
• ✓ Before buying or selling a home

What to Do If Your Radon Levels Are High

If your test results show radon levels at or above 4 pCi/L, don't panic — but do take action. Radon mitigation is a well-established technology that can reduce radon levels by up to 99%. Most residential systems are installed in a single day and cost between $800 and $2,500.

Learn more about systems and pricing in our complete guide to radon mitigation or check the radon mitigation cost guide.