It is radioactive with a half-life of 3.8 days, decaying by the emission of alpha particles to polonium, bismuth, and lead in successive steps. The decay of radon-222 with emission of an alpha particle is followed within about an hour by a series of four further decays, two of them accompanied by emission of alpha particles and the other two accompanied by other types of radiation. The short-lived atoms into which a radon atom decays are actually isotopes of polonium, lead, and bismuth, but they are referred to collectively as radon daughters, or, by those sensitive to questions of gender, as radon progeny. The radon daughter atoms float around in the air during their few minutes of existence, often becoming attached to dust particles. Radon is a naturally occurring radioisotope.
Radioactivity is, and always has been a part of the earth.
Whereas the predecessors to radon in the chain are solids and will not migrate far from their place in the soil, radon is a gas and can migrate through a few feet of earth. Cohen says that on the average, about six atoms of radon emerge form every square inch of soil every second. Radon in outside air is diluted rapidly, but if it enters through a basement floor and is trapped in a tight house, it can reach high concentrations. It is generally assumed that no level of ionizing radiation is completely safe, but one must try to set a threshold of reasonably tolerable risk. The maximum permissible occupational exposure for persons working in radiation related occupations is 5 rem per year.