Background radiation is the ionizing radiation from several natural radiation sources: sources in the Earth and from those sources that are incorporated in our food and water, which are incorporated in our body, and in building materials and other products that incorporate those radioactive sources; radiation sources from space (in the form of cosmic rays); and sources in the atmosphere which primarily come from both the radon gas that is released from the earth's surface and subsequently decays to radioactive atoms that become attached to airborne dust and particulates, and the production of radioactive atoms from the bombardment of atoms in the upper atmosphere by high-energy cosmic rays. Since 1945, until the cessation of above-ground nuclear weapons tests, it also comes from low levels of global radioactive contamination due to nuclear testing.
Natural background radiation
Natural background radiation comes from three primary sources: cosmic radiation, external terrestrial sources, and radon. The worldwide average background dose for a human being is about 2.4 mSv per year  (see Annex B: Exposures from natural radiation sources). This exposure is mostly from cosmic radiation and natural isotopes in the Earth.
The Earth, and all living things on it, are constantly bombarded by radiation from outside our solar system of positively charged ions from protons to iron nuclei. This radiation interacts in the atmosphere to create secondary radiation that rains down, including X-rays, muons, protons, alpha particles, pions, electrons, and neutrons. The dose from cosmic radiation is largely from muons, neutrons, and electrons.
The dose rate from cosmic radiation varies in different parts of the world based largely on the geomagnetic field and altitude.
External terrestrial sources
Radioactive material is found throughout nature. It occurs naturally in the soil, rocks, water, air, and vegetation. The major radionuclides of concern for terrestrial radiation are potassium, uranium and thorium. Each of these sources has been decreasing in activity since the birth of the Earth so that our present dose from potassium-40 is about ½ what it would have been at the dawn of life on Earth.
Radon gas seeps out of uranium-containing soils found across most of the world and may concentrate in well-sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certainly the most variable in the United States. Many areas of the world, including Cornwall and Aberdeenshire in the UK have high enough natural radiation levels that nuclear licensed sites cannot be built there—the sites would already exceed legal radiation limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste.
Man-made "background" radiation
Every above-ground nuclear detonation scatters a certain amount of radioactive contamination. Some of this contamination is local, rendering the immediate surroundings highly radioactive, while some of it is carried longer distances as nuclear fallout; some of this material is dispersed worldwide. Nuclear reactors may also release a certain amount of radioactive contamination. Under normal circumstances, a modern nuclear reactor releases miniscule amounts of radioactive contamination. However, reprocessing plants released waste, including plutonium, directly into the ocean. Major accidents, which have fortunately been relatively rare, have also released some radioactive contamination into the environment; this is the case, for example, with the Windscale fire (Sellafield accident) and the Chernobyl accident.
The amount of radioactive contamination released by human activity is rather small, in global terms, but the radiation background is also rather low. Some sources claim that the Earth's background radiation level has tripled since the beginning of the twentieth century. In fact, the total amount of radioactivity released by man is inconsequential to the large quantities of radioactivity in the natural environment (http://www.unscear.org/reports/2000_1.html) (Annex B).
Man-made radiation sources
The radiation from natural and man-made radiation sources are identical in their nature and their effects. These materials are distributed in the environment, and in our bodies, according to the chemical properties of the elements. The NRC, the EPA, and other US and international agencies, require that licensees limit radiation exposure to individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5 rem (50 mSv) per year, and 10 rem (100 mSv) in 5 years.
The exposure for an average person is about 360 millirems/year, 80 percent of which comes from natural sources of radiation. The remaining 20 percent results from exposure to man-made radiation sources, such as medical X-rays and a small fraction from atomic weapons tests.
In other contexts, background radiation may simply be any radiation that is pervasive. A particular example of this is the cosmic microwave background radiation, a nearly uniform glow that fills the sky in the microwave part of the spectrum; stars, galaxies and other objects of interest in radio astronomy stand out against this background.
In a laboratory, background radiation refers to the measured value from any sources that affect an instrument when a radiation source sample is not being measured. This background rate, which must be established as a stable value by multiple measurements, usually before and after sample measurement, is subtracted from the rate measured when the sample is being measured.
Background radiation for occupational doses measured for workers is all radiation dose that is not measured by radiation dose measurement instruments in potential occupational exposure conditions. This includes both "natural background radiation" and any medical radiation doses. This value is not typically measured or known from surveys, such that variations in the total dose to individual workers is not known. This can be a significant confounding factor in assessing radiation exposure effects in a population of workers who may have significantly different natural background and medical radiation doses. This is most significant when the occupational doses are very low.