Everyone who lives on this planet is constantly exposed to naturally occurring ionizing radiation (background radiation). This has been true since the dawn of time. The average effective dose equivalent of radiation to which a person in the United States is exposed annually is estimated to be about 350 millirem. (A millirem is a unit that estimates the biological impact of a particular type of radiation absorbed in the body.)
Sources of background radiation include cosmic rays from the sun and stars; naturally occurring radioactive materials in rocks and soil; radionuclides (unstable radioactive counterparts to naturally stable atoms) normally incorporated into our body's tissues; and radon and its products, which we inhale. Radon exists as a gas and is present in soil from which it seeps into the air. Radon gets trapped inside buildings, especially if the ventilation is poor. Levels of environmental radiation depend upon geology, how we construct our dwellings, and altitude. For example, radiation levels from cosmic rays are greater for people on airplanes and those living on the Colorado plateau. This low-level background radiation is a part of the earth's natural environment and any degree of risk associated with it has not been demonstrated to date.
We are also exposed to ionizing radiation from man-made sources, mostly through medical procedures. On the average, doses from a diagnostic x-ray are much lower, in dose effective terms, than natural background radiation. Radiation therapy, however, can reach levels many times higher than background radiation but this is usually targeted only to the affected tissues. Besides extremely small amounts of ionizing radiation from color televisions and smoke detectors, there are small amounts of ionizing radiation in many building materials and mining and agricultural products, such as granite, coal, and potassium salt. People who smoke receive additional radiation from radionuclides in tobacco smoke.