Radon is a colorless, odorless gas that forms naturally when radioactive elements, such as uranium, decay. These radioactive elements can be found in different amounts in soil and rock throughout the world.
Though radon is present outdoors and indoors, it is often present in such small amounts that it does not pose a threat to human health. However, the American Cancer Society notes that prolonged exposure to high levels of radon, indoors or out can lead to lung cancer.
People who work underground, such as miners, are most likely to be exposed to high levels of radon, as are people who come in contact with phosphate fertilizers.
Kits to test for indoor radon levels are available at many retailers.