Background radiation
Background radiation is the ionizing radiation emitted from a variety of natural and artificial radiation sources.  Primary contributions come from:
- Sources in the earth. These include sources in food and water, which are incorporated in the body, and in building materials and other products that incorporate those radioactive sources;
- Sources from space, in the form of cosmic rays;
- Sources in the atmosphere. One significant contribution comes from the radon gas that is released from the Earth's crust and subsequently decays into radioactive atoms that become attached to airborne dust and particulates. Another contribution arises from the radioactive atoms produced in the bombardment of atoms in the upper atmosphere by high-energy cosmic rays.
About 15% of background radiation dose to the general public comes from medical X-rays and nuclear medicine applied directly to patients.
About 3% of background radiation comes from other man-made sources such as:
- Smoke detectors
- Self-luminous dials and signs
- Global radioactive contamination due to historical nuclear weapons testing
- Nuclear power station or nuclear fuel reprocessing accidents (though these are rare)
- Normal operation of facilities used for nuclear power and scientific research
- Emissions from burning fossil fuels, such as power plants
- Emissions from nuclear medicine facilities and patients
- Emissions from the improper disposal or recycling of radioactive materials used in nuclear medicine
Accidental exposure to man-made radioactive substances can result in radiation exposure that is many times that received from background sources, whether natural or man-made. Additionally, radiation therapy can cause relatively high levels of exposure. However, when it comes to background radiation, naturally occurring sources are responsible for the vast majority of radiation exposure.

# Natural background radiation
Natural background radiation comes from two primary sources: cosmic radiation and terrestrial sources. The worldwide average background dose for a human being is about 2.4 millisievert (mSv) per year. This exposure is mostly from cosmic radiation and natural isotopes in the Earth. This is far greater than human-caused background radiation exposure, which in the year 2000 amounted to an average of about 0.01 mSv per year from historical nuclear weapons testing, nuclear power accidents and nuclear industry operation combined, and is greater than the average exposure from medical tests, which ranges from 0.04 to 1 mSv per year. Older coal-fired power plants without effective fly ash capture are one of the largest sources of human-caused background radiation exposure.
The level of natural background radiation varies depending on location, and in some areas the level is significantly higher than average. Such areas include Ramsar in Iran, Guarapari in Brazil, Kerala in India, and Yangjiang in China. In Ramsar a peak yearly dose of 260 mSv has been reported.

## Cosmic radiation
The Earth, and all living things on it, are constantly bombarded by radiation from outer space. This radiation primarily consists of positively charged ions from protons to iron nuclei derived from the sun and from other sources outside our solar system. This radiation interacts with atoms in the atmosphere to create secondary radiation, including X-rays, muons, protons, alpha particles, pions, electrons, and neutrons. The immediate dose from cosmic radiation is largely from muons, neutrons, and electrons, and this dose varies in different parts of the world based largely on the geomagnetic field and altitude. This radiation is much more intense in the upper troposphere, around 10 km altitude, and is thus of particular concern for airline crews and frequent passengers, who spend many hours per year in this environment. Here, the radiation exposure is not primarily due to the cosmic ray interaction with the thin atmosphere, but with the dense fuselage of the aircraft, causing relatively high background radiation in the cabin while the aircraft is at high altitude. Similarly, cosmic ray interaction with spacecraft components produces secondary radiation that causes higher background exposure in astronauts than in humans on the surface of Earth. Astronauts in low orbits, such as in the International Space Station or the Space Shuttle, are at low risk because the magnetic field of the Earth shields out most cosmic rays. Outside low Earth orbit, as experienced by the Apollo astronauts who travelled to the Moon, this background radiation is much more intense, and represents a considerable obstacle to potential future long term human exploration of the moon or Mars.
Cosmic rays also cause elemental transmutation in the atmosphere, in which secondary radiation generated by the cosmic rays combine with atomic nuclei in the atmosphere to generate different radioactive isotopes. Many so-called cosmogenic nuclides can be produced, but probably the most notable is carbon-14, which is produced by interactions with nitrogen atoms. These cosmogenic nuclides eventually reach the earth's surface and can be incorporated into living organisms. The production of these nuclides varies slightly with short-term variations in solar cosmic ray flux, but is considered practically constant over long scales of thousands to millions of years. The constant production, incorporation into organisms and relatively short half-life of carbon-14 are the principles used in radiocarbon dating of ancient biological materials such as wooden artifacts or human remains.

## 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 common elements with low-abundance radioactive isotopes, like potassium and carbon, or rare but intensely radioactive elements like uranium, thorium, radium and radon. Most of these sources have been decreasing, due to radioactive decay since the formation of the Earth, because there is no significant amount currently transported to the Earth. Thus, the present activity on earth from uranium-238 is only half as much as it originally was because of its 4.5 billion year half-life, and potassium-40 (half life 1.25 billion years) is only at about 8% of original activity. The effects on humans of the actual diminishment (due to decay) of these isotopes is minimal however. This is because humans evolved too recently for the difference in activity over a fraction of a half-life to be significant. Put another way, human history is so short in comparison to a half life of a billion years, that the activity of these long-lived isotopes has been effectively constant throughout our time on this planet.
In addition, many shorter half-life and thus more intensely radioactive isotopes have not decayed out of the terrestrial environment, however, because of natural on-going production of them. Examples of these are carbon-14 (cosmogenic), radium-226 (decay product of uranium-238) and radon-222 (a decay product of radium-226).

### Radiation inside the human body
Some of the essential elements that make up the human body, mainly potassium and carbon, have radioactive isotopes that add significantly to our background radiation dose. An average human contains about 30 milligrams of potassium-40 (40K) and about 10-8 grams of carbon-14 (14C). Excluding internal contamination by external radioactive material, the largest component of internal radiation exposure from biologically functional components of the human body is from potassium-40.  The decay of about 4,000 nuclei of 40K per second makes potassium the largest source of radiation in terms of number of decaying atoms.  The energy of beta particles produced by 40K is also about 10 times more powerful than the beta particles from 14C decay. There are about 1,200 beta particles per second produced by the decay of 14C. However, a 14C atom is in the genetic information of half the cells, while potassium is not a component of DNA. The decay of a 14C atom in DNA happens about 50 times per second, changing a carbon atom to one of nitrogen.

### Radon
Radon is a terrestrial source of radiation that is of particular concern because, although on average it is very rare, this intensely radioactive element can be found in high concentrations in many areas of the world, where it represents a significant health hazard. Radon is a decay product of uranium, which is relatively common in the earth's crust, but generally concentrated in ore-bearing rocks scattered around the world. Radon seeps out of these ores into the atmosphere or into ground water, and in these localities it can accumulate within dwellings and expose humans to high concentrations. The widespread construction of well insulated and sealed homes in the northern industrialized world has led to radon becoming the primary source of background radiation in some localities in northern North America and Europe. Some of these areas, including Cornwall and Aberdeenshire in the United Kingdom 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.
Radiation exposure from radon is indirect. Radon has a short half-life (4 days) and decays into other solid particulate radium-series radioactive nuclides. These radioactive particles are inhaled and remain lodged in the lungs, causing continued exposure. People in affected localities can receive up to 10 mSv per year background radiation. Radon is thus the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.

# Human-caused background radiation
Frequent above-ground nuclear explosions between the 1940s and 1960s scattered a substantial 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. The increase in background radiation due to these tests peaked in 1963 at about 0.15 mSv per year worldwide, or about 7% of average background dose from all sources. The Limited Test Ban Treaty of 1963 prohibited above-ground tests, thus by the year 2000 the worldwide dose from these historical tests has decreased to only 0.005 mSv per year.
Even so, in certain applications, such as space satellites, materials cannot practically be created by modern steel because background radiation is sufficient to disrupt its calibration.  In the 1990s, steel was exhumed from the underwater hull of the sunken SMS Kronprinz Wilhelm for this purpose because its pre-1945 steel was preserved from background radiation throughout the nuclear era.
Older coal-fired power plants without effective fly ash capture are one of the largest sources of human-caused background radiation exposure. When coal is burned, uranium, thorium and all the uranium daughters accumulated by disintegration  radium, radon, polonium  are released. The release of nuclear components from coal combustion far exceeds the entire U.S. consumption of nuclear fuels in nuclear generating plants. According to a 1978 article in Science magazine, "coal-fired power plants throughout the world are the major sources of radioactive materials released to the environment". Radioactive materials previously buried underground in coal deposits are released as fly ash or, if fly ash is captured, may be incorporated into concrete manufactured with fly ash. Radioactive materials are also released in gaseous emissions. The United Nations Scientific Committee on the Effects of Atomic Radiation estimates that per gigawatt-year (GWea) of electrical energy produced by coal, using the current mix of technology throughout the world, the population impact is approximately 0.8 lethal cancers per plant-year distributed over the affected population. With 400 GW of coal-fired power plants in the world, this amounts to some 320 deaths per year.
Nuclear reactors may also release a certain amount of radioactive contamination. Under normal circumstances, a modern nuclear reactor releases minuscule amounts of radioactive contamination. 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. In fact, the total amount of radioactivity released by humans is negligible in comparison natural background radiation.

# Artificial radiation sources
The radiation from natural and artificial 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 Nuclear Regulatory Commission, the United States Environmental Protection Agency, and other U.S. and international agencies, require that licensees limit radiation exposure to individual members of the public to 1 mSv (100 mrem) per year, and limit occupational radiation exposure to adults working with radioactive material to 50 mSv (5 rem) per year, and 100 mSv (10 rem) in 5 years.
The exposure for an average person is about 3.6 mSv/year, 80 percent of which comes from natural sources of radiation. The remaining 20 percent results from exposure to artificial radiation sources, such as medical X-rays, industrial sources like smoke detectors and a small fraction from nuclear weapons tests.

# Other usage
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.