Background radiation is (usually) the level of ionizing radiation measured at a point on or near the Earth's surface where no local human activity has produced additional radiation. This background radiation comes from natural sources in the earth, its atmosphere, or from space (in the form of cosmic rays). More recently, 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 3.5 mSv per year [1] (http://www.arpansa.gov.au/pubs/baseline/bg_rad.pdf), mostly from cosmic radiation and natural isotopes in the earth.

Cosmic radiation

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

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.

Artificial "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 minucule amounts of radioactive contamination. However, early designs paid little attention to this issue; some simply dumped waste plutonium directly into the ocean. Major accidents, which have fortunately been relatively rare, may also release 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 radioactive background is also rather low. Some sources claim that the Earth's background radiation level has tripled since the beginning of the twentieth century. The health and ecological effects of this increase are a subject of some dispute.

Man-made radiation sources

Natural and artificial radiation sources are identical in their nature and their effect. Above the background level of radiation exposure, the NRC requires that its licensees limit man-made 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 5,000 mrem (50 mSv) per year.

The exposure for an average person is about 360 millirems/year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to man-made radiation sources.

In more specialized contexts, background radiation may simply be any radiation that is measured but not of interest. 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.