Natural sources of radiation
Natural background radiation comes from four primary sources: cosmic radiation, solar radiation, terrestrial sources, and radon.
Radon-222 is produced by the decay of radium-226 which is present wherever uranium is found. Since radon is a gas, it seeps out of uranium-containing soils found across most of the world and may accumulate in well-sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certainly the most variable from location to location.
Radon-222 decays to Polonium, Lead, and Bismuth isotopes called Radon daughters or radon progeny, as shown in the decay scheme below. Half-lives and energies emitted are indicated.
The Earth, and all living things on it, are constantly bombarded by radiation from outside our solar system. This radiation interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. The portion of this ultraviolet radiation that is not absorbed by the atmosphere produces a suntan or a sunburn on people who have been in sunlight for extended periods of time.
Most materials on Earth contain some radioactive atoms, even if in small quantities. Most of the terrestrial non-radon-dose one receives from these sources is from gamma-ray emitters in the walls and floors when inside a house, or rocks and soil when outside. The major radionuclides of concern for terrestrial radiation are potassium, uranium, and thorium.