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High radon areas in the U.S. tend to be in the northern section of the country due to the types of soils found there. States with high concentrations of radon include Iowa, North Dakota, South Dakota, Maine, Minnesota, Illinois, Kansas, Montana, Idaho, Wyoming, Colorado, Pennsylvania, and Virginia. Most or all of these states fall in zone one, which is one designated for parts of the U.S. with the highest levels of radon — four pico Curies per Liter (pCi/L) or higher.
Zone two states, states with moderate radon levels of two to four pCi/L, include Alaska, Arizona, Nevada, Missouri, Utah, and Wisconsin. Significant parts of California, Oregon, and most of New England also fall in the zone two category. Zone three states are considered low risk, with less than two pCi/L.
Radon occurs naturally in the air almost everywhere because it is created when uranium breaks down in the soil. High radon areas of the U.S. have more uranium in the soil than others. Iowa, for instance, has the highest concentration in the country, because glaciers ground up granite rocks, a source of naturally-occurring radon, and deposited the mineral into the soil. This is also part of what makes farmland in the region so rich, however.
In high radon areas, the resulting gas transfers easily from the ground to buildings. The problem of radon build-up is not limited to older homes, and new construction also often suffers from increased levels of the gas. Radon can enter the home through cracks in floors or walls, construction joints, gaps in floors or around pipes, wall cavities, and through the water supply.
People who live in high radon areas are encouraged to have their homes tested. Since radon is not visible and does not have a taste or emit an odor, it is impossible to detect without a testing kit. Do-it-yourself testing packages are available at most hardware or home improvement stores, but many people choose to hire professionals.
According to the U.S. Surgeon General, radon is one of the leading causes of lung cancer, second only to smoking. The country's Environmental Protection Agency (EPA) estimates that 20,000 radon-related lung cancers occur in the U.S. every year, and recommends preventative measures be taken if a home is tested at four pCi/L or higher. The EPA also strongly encourages homeowners to take action if their home's radon level is two pCi/L or higher. Solutions may include a better ventilation system and sealing cracks in the foundation and walls.