What Is Radon and Why Is It Dangerous?

Radon is one of the most common and least understood environmental hazards in American homes. It has no color, no odor, and no taste — you cannot detect it without a test. Yet the EPA estimates radon causes approximately 21,000 lung cancer deaths in the United States each year, making it the second leading cause of lung cancer after cigarette smoking.

What Radon Is and Where It Comes From

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and groundwater. Uranium is present in trace amounts across virtually all soil types, which means radon is produced everywhere — not just in high-risk zones.

As uranium decays, it forms radium, which then decays into radon gas. Because radon is a gas at normal temperatures, it moves through soil particles and can migrate into the air above ground or into structures built on the ground.

The EPA divides the country into radon zones based on predicted average indoor radon levels, but high radon levels have been found in every state and in every type of home.

How Radon Enters Your Home

Radon enters homes primarily through the path of least resistance — cracks and openings in foundations. Common entry points include:

• Cracks in concrete slabs and foundation walls. Even hairline cracks allow radon to seep through under pressure.
• Construction joints. The gap between a wall and floor is a frequent entry point.
• Gaps around service pipes. Where plumbing and utility lines penetrate the foundation, small openings exist.
• Sump pits. Open sump baskets connect directly to the soil beneath the slab.
• Crawl space floors. Homes without sub-membrane barriers draw radon directly from exposed earth.

The driving force is pressure. The interior of a home is typically at slightly lower pressure than the soil beneath it, especially in winter when heating creates a stack effect. This pressure difference draws soil gases — including radon — upward into living spaces.

Well water is an additional source in homes on private wells drilled through radon-bearing rock formations. When water containing dissolved radon is used for showering or dishwashing, the gas is released into indoor air.

Health Risks: The Lung Cancer Link

When radon gas is inhaled, it decays into radioactive particles that can lodge in lung tissue. These particles continue to decay, releasing bursts of radiation that damage DNA in lung cells. Over years of exposure, this damage can lead to lung cancer.

The risk is dose-dependent — higher radon concentrations over longer periods increase cancer risk proportionally. Smokers exposed to elevated radon face a dramatically higher risk than nonsmokers, because tobacco damage compounds the effect of radiation-induced cell injury.

Radon-induced lung cancer typically develops over a period of years to decades. There are no immediate symptoms from radon exposure; it is a long-term cumulative hazard.

EPA Action Level: 4 pCi/L

The EPA recommends taking action to reduce radon levels in homes that test at or above 4 picocuries per liter (pCi/L). The average indoor radon level in U.S. homes is approximately 1.3 pCi/L; outdoor air typically measures around 0.4 pCi/L.

The EPA also notes that radon levels between 2 and 4 pCi/L represent a meaningful risk and that homeowners should consider mitigation in that range. There is no known “safe” level of radon — the goal is to reduce concentrations as low as reasonably achievable. For a fuller breakdown of what specific readings mean, see our guide on safe versus dangerous radon levels, and the radon testing guide covers how to measure your home.

A standard sub-slab depressurization system can typically reduce indoor radon levels by 80% to 99%, often bringing levels well below 2 pCi/L.

Who Is Most at Risk

While radon affects everyone in an exposed home, certain groups face elevated risk:

• Smokers and former smokers. The combined effect of smoking and radon exposure is multiplicative, not additive. A smoker in a high-radon home faces a risk up to 10 times higher than a nonsmoker in the same home.
• Children. Children have higher respiratory rates and spend more time at home, increasing cumulative exposure. Their developing cells may also be more susceptible to radiation damage.
• People who spend significant time in lower levels. Radon concentrations are highest near the foundation — basements, slab-on-grade main floors, and finished below-grade spaces.
• Long-term residents. Risk accumulates with duration. Someone who has lived in a high-radon home for 20 years faces substantially greater risk than a recent arrival.

Testing is the only way to know your exposure level. If you haven’t tested your home, find a local radon testing or mitigation contractor who can help.

Codes and Standards Worth Knowing

The EPA’s radon program sets the national action level (4 pCi/L) that drives nearly every mitigation decision, and the American Association of Radon Scientists and Technologists publishes the consensus standards that mitigation systems are designed and tested against, including ANSI/AARST RMS-LB for residential mitigation and MAH for measurement that mitigation systems are designed and tested against. A mitigator who is AARST-certified and references the current ANSI/AARST standard for your installation type is working at trade-association level, not freelancing.