Radon is a colorless, odorless, and tasteless naturally occurring radioactive gas that forms from the breakdown of uranium in soil, rock, and water. While present everywhere, it becomes hazardous when it seeps into a home and accumulates to high concentrations. Because uranium is distributed unevenly across the country, the concentration of radon varies significantly based on local geography and geology.
The Geological Basis for High Radon States
The potential for high indoor radon levels is fundamentally determined by two geological factors: the concentration of uranium in the underlying rock and soil, and the soil’s permeability. Uranium-rich rock formations, such as granite, dark shale, and phosphate rock, produce higher amounts of radium, which then decays to form radon gas. The soil’s ability to allow the gas to flow freely to the surface (permeability) is just as important as the concentration of the source material.
The U.S. Environmental Protection Agency (EPA) developed a map to categorize the country into three Radon Zones, with Zone 1 representing the highest risk. Zone 1 counties have a predicted average indoor radon screening level greater than 4 picocuries per liter (pCi/L). States with large areas in this highest-risk category include the upper Midwest and Northeast, such as Pennsylvania, Iowa, Minnesota, and Wisconsin.
High-risk areas also extend into mountain states like Colorado, Montana, and Idaho, where uranium deposits are more prevalent. Alaska, for example, has the highest average radon level in the country at 10.7 pCi/L, while Iowa’s average is 6.1 pCi/L. Even states that are not predominantly Zone 1 may contain high-risk regions due to localized geological anomalies.
The way a home interacts with the soil gas also plays a role in trapping the gas indoors. Tightly sealed homes and those built over highly permeable soil, such as coarse gravel or fractured bedrock, can draw in radon more efficiently through cracks in the foundation. Basements and first floors typically register the highest radon levels because of their proximity to the gas’s entry point from the ground.
Understanding the Health Hazards of Radon Exposure
Radon gas itself is mostly exhaled, but its danger arises from its rapid decay into solid, short-lived radioactive particles, known as radon progeny. Once inhaled, these decay products, specifically polonium-218 and polonium-214, can lodge in the cells lining the airways and lungs. These particles are alpha-emitters, releasing high-energy alpha radiation that damages the DNA of the lung cells. This damage can lead to genetic mutations and, over time, the development of lung cancer.
The risk is cumulative, increasing with the concentration of radon and the duration of exposure. Scientists estimate that radon exposure contributes to approximately 21,000 lung cancer deaths annually in the United States.
Radon is recognized as the leading cause of lung cancer among non-smokers and is the second leading cause after smoking. The risk is significantly amplified when combined with smoking, as the radioactive particles can attach to smoke particles, making them more likely to become embedded in lung tissue. Because there are no immediate symptoms of radon exposure, the first signs of a problem are often the symptoms of lung cancer, such as a persistent cough or shortness of breath.
Essential Steps for Testing and Mitigation
The only reliable way to know the radon level inside a home is through testing. Homeowners can use short-term test kits, which measure levels for two to seven days, or long-term kits, which collect data for 90 days or more. Because radon levels naturally fluctuate seasonally and daily, a long-term test provides a more accurate representation of the home’s average exposure.
The EPA recommends placing the test kit in the lowest lived-in area of the home, such as a basement or first-floor room, and away from drafts or high-heat areas. If a home’s test result registers 4 pCi/L or higher, the agency recommends taking action to reduce the concentration. Even levels between 2 pCi/L and 4 pCi/L warrant consideration for mitigation.
The most effective method for reducing indoor radon concentrations is sub-slab depressurization (ASD). This technique involves installing a vent pipe system and a continuously operating fan to create a negative pressure field beneath the foundation slab. The fan draws the radon gas from the soil before it can enter the home and safely exhausts it above the roofline. Sealing major cracks and other openings in the foundation is a complementary step performed during the installation of an ASD system, but it is not a sufficient standalone solution.