Cosmic Radiation
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
Includes ionizing and non-ionizing radiation from the sun.
Terrestrial Sources
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.
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.