Background radiation

Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is due to deliberate introduction of radiation sources.
Background radiation originates from a variety of sources, both natural and artificial. These include both cosmic radiation and environmental radioactivity from naturally occurring radioactive materials, as well as man-made medical X-rays, fallout from nuclear weapons testing and nuclear accidents.

Definition

Background radiation is defined by the International Atomic Energy Agency as "Dose or the dose rate attributable to all sources other than the one specified. A distinction is thus made between the dose which is already in a location, which is defined here as being "background", and the dose due to a deliberately introduced and specified source. This is important where radiation measurements are taken of a specified radiation source, where the existing background may affect this measurement. An example would be measurement of radioactive contamination in a gamma radiation background, which could increase the total reading above that expected from the contamination alone.
However, if no radiation source is specified as being of concern, then the total radiation dose measurement at a location is generally called the background radiation, and this is usually the case where an ambient dose rate is measured for environmental purposes.

Background dose rate examples

Background radiation varies with location and time, and the following table gives examples:

Radiation source	World	US	Japan	Remark
Inhalation of air	1.26	2.28	0.40	mainly from radon, depends on indoor accumulation
Ingestion of food and water	0.29	0.28	0.40
Terrestrial background radiation from ground	0.48	0.21	0.40	depends on soil and building material
Cosmic radiation from space	0.39	0.33	0.30	depends on altitude
sub total	2.40	3.10	1.50	sizeable population groups receive 10–20 mSv
Medical	0.60	3.00	2.30	worldwide figure excludes radiotherapy; US figure is mostly CT scans and nuclear medicine.
Consumer items	–	0.13		cigarettes, air travel, building materials, etc.
Atmospheric nuclear testing	0.005	–	0.01	peak of 0.11 mSv in 1963 and declining since; higher near sites
Occupational exposure	0.005	0.005	0.01	worldwide average to workers only is 0.7 mSv, mostly due to radon in mines; US is mostly due to medical and aviation workers.
Chernobyl accident	0.002	–	0.01	peak of 0.04 mSv in 1986 and declining since; higher near site
Nuclear fuel cycle	0.0002		0.001	up to 0.02 mSv near sites; excludes occupational exposure
Other	–	0.003		Industrial, security, medical, educational, and research
sub total	0.61	3.14	2.33
Total	3.01	6.24	3.83	millisieverts per year

Natural background radiation

Radioactive material is found throughout nature. Detectable amounts occur naturally in soil, rocks, water, air, and vegetation, from which it is inhaled and ingested into the body. In addition to this internal exposure, humans also receive external exposure from radioactive materials that remain outside the body and from cosmic radiation from space. The worldwide average natural dose to humans is about per year. This is four times the worldwide average artificial radiation exposure, which in 2008 amounted to about per year. In some developed countries, like the US and Japan, artificial exposure is, on average, greater than the natural exposure, due to greater access to medical imaging. In Europe, average natural background exposure by country ranges from under annually in the United Kingdom to more than annually for some groups of people in Finland.
The International Atomic Energy Agency states:

Terrestrial sources

Terrestrial background radiation, for the purpose of the table above, only includes sources that remain external to the body. The major radionuclides of concern are potassium, uranium and thorium and their decay products, some of which, like radium and radon are intensely radioactive but occur in low concentrations. 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 is only at about 8% of original activity. But during the time that humans have existed the amount of radiation has decreased very little.
Many shorter half-life isotopes have not decayed out of the terrestrial environment because of their on-going natural production. Examples of these are radium-226 and radon-222.
Thorium and uranium primarily undergo alpha and beta decay, and are not easily detectable. However, many of their daughter products are strong gamma emitters. Thorium-232 is detectable via a 239 keV peak from lead-212, 511, 583 and 2614 keV from thallium-208, and 911 and 969 keV from actinium-228. Uranium-238 manifests as 609, 1120, and 1764 keV peaks of bismuth-214. Potassium-40 is detectable directly via its 1461 keV gamma peak.
The level over the sea and other large bodies of water tends to be about a tenth of the terrestrial background. Conversely, coastal areas may have an additional contribution from dispersed sediment.

Airborne sources

The biggest source of natural background radiation is airborne radon, a radioactive gas that emanates from the ground. Radon and its isotopes, parent radionuclides, and decay products all contribute to an average inhaled dose of 1.26 mSv/a. Radon is unevenly distributed and varies with weather, such that much higher doses apply to many areas of the world, where it represents a significant health hazard. Concentrations over 500 times the world average have been found inside buildings in Scandinavia, the United States, Iran, and the Czech Republic. Radon is a decay product of uranium, which is relatively common in the Earth's crust, but more concentrated in ore-bearing rocks scattered around the world. Radon seeps out of these ores into the atmosphere or into ground water or infiltrates into buildings. It can be inhaled into the lungs, along with its decay products, where they will reside for a period of time after exposure.
Although radon is naturally occurring, exposure can be enhanced or diminished by human activity, notably house construction. A poorly sealed dwelling floor, or poor basement ventilation, in an otherwise well insulated house can result in the accumulation of radon within the dwelling, exposing its residents 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. Basement sealing and suction ventilation reduce exposure. Some building materials, for example lightweight concrete with alum shale, phosphogypsum and Italian tuff, may emanate radon if they contain radium and are porous to gas.
Radiation exposure from radon is indirect. Radon has a short half-life and decays into other solid particulate radium-series radioactive nuclides. These radioactive particles are inhaled and remain lodged in the lungs, causing continued exposure. Radon is thus assumed to be 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. However, the discussion about the opposite experimental results is still going on.
About 100,000 Bq/m³ of radon was found in Stanley Watras's basement in 1984. He and his neighbours in Boyertown, Pennsylvania, United States may hold the record for the most radioactive dwellings in the world. International radiation protection organizations estimate that a committed dose may be calculated by multiplying the equilibrium equivalent concentration of radon by a factor of 8 to 9 and the EEC of thoron by a factor of 40.
Most of the atmospheric background is caused by radon and its decay products. The gamma spectrum shows prominent peaks at 609, 1120, and 1764 keV, belonging to bismuth-214, a radon decay product. The atmospheric background varies greatly with wind direction and meteorological conditions. Radon also can be released from the ground in bursts and then form "radon clouds" capable of traveling tens of kilometers.

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 and larger nuclei derived from outside the Solar System. This radiation interacts with atoms in the atmosphere to create an air shower of 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. For example, the city of Denver in the United States receives a cosmic ray dose roughly twice that of a location at sea level. 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. During their flights airline crews typically get an additional occupational dose between per year and 2.19 mSv/year, according to various studies.
Similarly, cosmic rays cause 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 partially shielded by the magnetic field of the Earth, but also suffer from the Van Allen radiation belt which accumulates cosmic rays and results from the Earth's magnetic field. Outside low Earth orbit, as experienced by the Apollo astronauts who traveled 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 combines with atomic nuclei in the atmosphere to generate different nuclides. 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.
The cosmic radiation at sea level usually manifests as 511 keV gamma rays from annihilation of positrons created by nuclear reactions of high energy particles and gamma rays. At higher altitudes there is also the contribution of continuous bremsstrahlung spectrum.