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Background radiation is a measure of the level ofionizing radiation present in the environment at a particular location which isnot due to deliberate introduction of radiation sources.
Background radiation originates from a variety of sources, both natural and artificial. These include bothcosmic radiation andenvironmental radioactivity fromnaturally occurring radioactive materials (such asradon andradium), as well as man-made medicalX-rays,fallout fromnuclear weapons testing andnuclear accidents.
Background radiation is defined by theInternational Atomic Energy Agency as "Dose or the dose rate (or an observed measure related to the dose or dose rate) attributable to all sources other than the one(s) specified.[1] 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 thebackground radiation, and this is usually the case where an ambient dose rate is measured for environmental purposes.
Background radiation varies with location and time, and the following table gives examples:
| Radiation source | World[2] | US[3] | Japan[4] | Remark |
|---|---|---|---|---|
| Inhalation of air | 1.26 | 2.28 | 0.40 | mainly fromradon, depends on indoor accumulation |
| Ingestion of food and water | 0.29 | 0.28 | 0.40 | (potassium-40,carbon-14, etc.) |
| 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 (natural) | 2.40 | 3.10 | 1.50 | sizeable population groups receive 10–20 mSv |
| Medical | 0.60 | 3.00 | 2.30 | worldwide figure excludesradiotherapy; US figure is mostlyCT scans andnuclear 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;[2] US is mostly due to medical and aviation workers.[3] |
| 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 (artificial) | 0.61 | 3.14 | 2.33 | |
| Total | 3.01 | 6.24 | 3.83 | millisieverts per year |
Radioactive material is found throughout nature. Detectable amounts occur naturally insoil, rocks, water, air, and vegetation, from which it is inhaled and ingested into the body. In addition to thisinternal exposure, humans also receiveexternal exposure from radioactive materials that remain outside the body and from cosmic radiation from space. The worldwide average naturaldose to humans is about 2.4 mSv (240 mrem) per year.[2] This is four times the worldwide average artificial radiation exposure, which in 2008 amounted to about 0.6millisieverts (60 mrem) 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 tomedical imaging. In Europe, average natural background exposure by country ranges from under 2 mSv (200 mrem) annually in the United Kingdom to more than 7 mSv (700 mrem) annually for some groups of people in Finland.[5]
TheInternational Atomic Energy Agency states:
Terrestrial background radiation, for the purpose of the table above, only includes sources that remain external to the body. The majorradionuclides of concern arepotassium,uranium andthorium and their decay products, some of which, likeradium andradon are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, due toradioactive decay since the formation of the Earth, because there is no significant amount currently transported to the Earth. Thus, the present activity on Earth fromuranium-238 is only half as much as it originally was because of its 4.5 billion year half-life, andpotassium-40 (half-life 1.25 billion years) 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 (and thus more intensely radioactive) isotopes have not decayed out of the terrestrial environment because of their on-going natural production. Examples of these areradium-226 (decay product of thorium-230 in decay chain of uranium-238) andradon-222 (a decay product ofradium-226 in said chain).
Thorium and uranium (and their daughters) primarily undergoalpha andbeta decay, and are not easily detectable. However, many of theirdaughter products are strong gamma emitters.Thorium-232 is detectable via a 239 keV peak fromlead-212, 511, 583 and 2614 keV fromthallium-208, and 911 and 969 keV fromactinium-228. Uranium-238 manifests as 609, 1120, and 1764 keV peaks of bismuth-214 (cf. the same peak for atmospheric radon). Potassium-40 is detectable directly via its 1461 keV gamma peak.[7]
The level over the sea and other large bodies of water tends to be about a tenth of the terrestrial background. Conversely, coastal areas (and areas by the side of fresh water) may have an additional contribution from dispersed sediment.[7]
The biggest source of natural background radiation is airborneradon, a radioactive gas that emanates from the ground. Radon and itsisotopes, parentradionuclides, anddecay products all contribute to an average inhaled dose of 1.26 mSv/a (millisievertper year). Radon is unevenly distributed and varies with weather, such that much higher doses apply to many areas of the world, where it represents asignificant health hazard. Concentrations over 500 times the world average have been found inside buildings in Scandinavia, the United States, Iran, and the Czech Republic.[8] 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 theseores into the atmosphere or into ground water or infiltrates into buildings. It can be inhaled into the lungs, along with itsdecay 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.[citation needed] Basement sealing and suction ventilation reduce exposure. Some building materials, for examplelightweight concrete withalum shale,phosphogypsum and Italiantuff, may emanate radon if they containradium and are porous to gas.[8]
Radiation exposure from radon is indirect. Radon has a short half-life (4 days) and decays into other solid particulateradium-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 oflung cancer aftersmoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.[9][better source needed] However, the discussion about the opposite experimental results is still going on.[10][11]
About 100,000 Bq/m3 of radon was found inStanley Watras's basement in 1984.[12][13] He and his neighbours inBoyertown, Pennsylvania, United States may hold the record for the most radioactive dwellings in the world. International radiation protection organizations estimate that acommitted dose may be calculated by multiplying theequilibrium equivalent concentration (EEC) of radon by a factor of 8 to 9nSv·m3/Bq·h and the EEC ofthoron by a factor of 40nSv·m3/Bq·h.[2]
Most of the atmospheric background is caused by radon and its decay products. Thegamma spectrum shows prominent peaks at 609, 1120, and 1764 keV, belonging tobismuth-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.[7]
The Earth and all living things on it are constantly bombarded by radiation from outer space. This radiation primarily consists of positively charged ions fromprotons toiron and largernuclei derived from outside theSolar System. This radiation interacts with atoms in the atmosphere to create anair shower of secondary radiation, includingX-rays,muons,protons,alpha particles,pions,electrons, andneutrons. 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 thegeomagnetic field and altitude. For example, the city ofDenver in the United States (at 1650 meters elevation) receives a cosmic ray dose roughly twice that of a location at sea level.[14] This radiation is much more intense in the uppertroposphere, around 10 km altitude, and is thus of particular concern forairline 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 2.2 mSv (220 mrem) per year[15] and 2.19 mSv/year,[16] according to various studies.[17]
Similarly, cosmic rays cause higher background exposure inastronauts than in humans on the surface of Earth. Astronauts in loworbits, such as in theInternational Space Station or theSpace Shuttle, are partially shielded by themagnetic field of the Earth, but also suffer from theVan Allen radiation belt which accumulates cosmic rays and results from the Earth's magnetic field. Outside low Earth orbit, as experienced by theApollo astronauts who traveled to theMoon, this background radiation is much more intense, and represents a considerable obstacle to potential future long term human exploration of theMoon orMars.
Cosmic rays also causeelemental transmutation in the atmosphere, in which secondary radiation generated by the cosmic rays combines withatomic nuclei in the atmosphere to generate differentnuclides. Many so-calledcosmogenic nuclides can be produced, but probably the most notable iscarbon-14, which is produced by interactions withnitrogen 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 shorthalf-life of carbon-14 are the principles used inradiocarbon 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 ofpositrons created by nuclear reactions of high energy particles and gamma rays. At higher altitudes there is also the contribution of continuousbremsstrahlung spectrum.[7]
Two of the essential elements that make up the human body, namely potassium and carbon, have radioactive isotopes that add significantly to our background radiation dose. An average human contains about 17 milligrams ofpotassium-40 (40K) and about 24 nanograms (10−9 g) ofcarbon-14 (14C),[18] (half-life 5,730 years). Excluding internal contamination by external radioactive material, these two are the largest components of internal radiation exposure from biologically functional components of the human body. About 4,000 nuclei of40K[19] decay per second, and a similar number of14C. The energy ofbeta particles produced by40K is about 10 times that from the beta particles from14C decay.
14C is present in the human body at a level of about 3700 Bq (0.1 μCi) with abiological half-life of 40 days.[20] This means there are about 3700 beta particles per second produced by the decay of14C. However, a14C atom is in the genetic information of about half the cells, while potassium is not a component ofDNA. The decay of a14C atom inside DNA in one person happens about 50 times per second, changing a carbon atom to one ofnitrogen.[21]
The global average internal dose from radionuclides other than radon and its decay products is 0.29 mSv/a, of which 0.17 mSv/a comes from40K, 0.12 mSv/a comes from the uranium and thorium series, and 12 μSv/a comes from14C.[2]
Some areas have greater dosage than the country-wide averages.[22] In the world in general, exceptionally high natural background locales includeRamsar in Iran,Guarapari in Brazil,Karunagappalli in India,[23]Arkaroola in Australia[24] andYangjiang in China.[25]
The highest level of purely natural radiation ever recorded on the Earth's surface was 90 μGy/h on a Brazilian black beach (areia preta in Portuguese) composed ofmonazite.[26] This rate would convert to 0.8 Gy/a for year-round continuous exposure, but in fact the levels vary seasonally and are much lower in the nearest residences. The record measurement has not been duplicated and is omitted from UNSCEAR's latest reports. Nearby tourist beaches inGuarapari andCumuruxatiba were later evaluated at 14 and 15 μGy/h.[27][28] Note that the values quoted here are inGrays. To convert to Sieverts (Sv) a radiation weighting factor is required; these weighting factors vary from 1 (beta & gamma) to 20 (alpha particles).
The highest background radiation in an inhabited area is found inRamsar, primarily due to the use of local naturally radioactive limestone as a building material. The 1000 most exposed residents receive an average externaleffective radiation dose of 6 mSv (600 mrem) per year, six times theICRP recommended limit for exposure to the public from artificial sources.[29] They additionally receive a substantial internal dose from radon. Record radiation levels were found in a house where the effective dose due to ambient radiation fields was 131 mSv (13.1 rem) per year, and the internalcommitted dose fromradon was 72 mSv (7.2 rem) per year.[29] This unique case is over 80 times higher than the world average natural human exposure to radiation.
Epidemiological studies are underway to identify health effects associated with the high radiation levels in Ramsar. It is much too early to draw unambiguous statistically significant conclusions.[29] While so far support for beneficial effects of chronic radiation (like longer lifespan) has been observed in few places only,[29] a protective and adaptive effect is suggested by at least one study whose authors nonetheless caution that data from Ramsar are not yet sufficiently strong to relax existing regulatory dose limits.[30] However, the recent statistical analyses discussed that there is no correlation between the risk of negative health effects and elevated level of natural background radiation.[31]
Background radiation doses in the immediate vicinity of particles of high atomic number materials, within the human body, have a small enhancement due to thephotoelectric effect.[32]
Most of the natural neutron background is a product of cosmic rays interacting with the atmosphere. The neutron energy peaks at around 1 MeV and rapidly drops above. At sea level, the production of neutrons is about 20 neutrons per second per kilogram of material interacting with the cosmic rays (or, about 100–300 neutrons per square meter per second). The flux is dependent on geomagnetic latitude, with a maximum near the magnetic poles. At solar minimums, due to lower solar magnetic field shielding, the flux is about twice as high vs the solar maximum. It also dramatically increases during solar flares. In the vicinity of larger heavier objects, e.g. buildings or ships, the neutron flux measures higher; this is known as "cosmic ray induced neutron signature", or "ship effect" as it was first detected with ships at sea.[7]
Frequent above-ground nuclear explosions between the 1940s and 1960s scattered a substantial amount ofradioactive contamination. Some of this contamination is local, rendering the immediate surroundings highly radioactive, while some of it is carried longer distances asnuclear 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. TheLimited Test Ban Treaty of 1963 prohibited above-ground tests, thus by the year 2000 the worldwide dose from these tests has decreased to only 0.005 mSv per year.[36]
Thisglobal fallout has caused an estimated 200,000-460,000 deaths as of 2020.[37]
TheInternational Commission on Radiological Protection recommends limiting occupational radiation exposure to 50 mSv (5 rem) per year, and 100 mSv (10 rem) in 5 years.[38]
However,background radiation for occupational doses includes radiation that is not measured by radiation dose instruments in potential occupational exposure conditions. This includes both offsite "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.
At anIAEA conference in 2002, it was recommended that occupational doses below 1–2 mSv per year do not warrant regulatory scrutiny.[39]
Under normal circumstances, nuclear reactors release small amounts of radioactive gases, which cause small radiation exposures to the public. Events classified on theInternational Nuclear Event Scale as incidents typically do not release any additional radioactive substances into the environment. Large releases of radioactivity from nuclear reactors are extremely rare. To the present day, there were two majorcivilian accidents – theChernobyl accident and theFukushima I nuclear accidents – which caused substantial contamination. The Chernobyl accident was the only one to cause immediate deaths.
Total doses from the Chernobyl accident ranged from 10 to 50 mSv over 20 years for the inhabitants of the affected areas, with most of the dose received in the first years after the disaster, and over 100 mSv forliquidators. There were 28 deaths fromacute radiation syndrome.[40]
Total doses from the Fukushima I accidents were between 1 and 15 mSv for the inhabitants of the affected areas. Thyroid doses for children were below 50 mSv. 167 cleanup workers received doses above 100 mSv, with 6 of them receiving more than 250 mSv (the Japanese exposure limit for emergency response workers).[41]
The average dose from theThree Mile Island accident was 0.01 mSv.[42]
Non-civilian: In addition to the civilian accidents described above, several accidents at early nuclear weapons facilities – such as theWindscale fire, the contamination of theTecha River by the nuclear waste from theMayak compound, and theKyshtym disaster at the same compound – released substantial radioactivity into the environment. The Windscale fire resulted in thyroid doses of 5–20 mSv for adults and 10–60 mSv for children.[43] The doses from the accidents at Mayak are unknown.
TheNuclear Regulatory Commission, theUnited 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.
PerUNECE life-cycle assessment, nearly all sources of energy result in some level of occupational and public exposure toradionuclides as result of their manufacturing or operations. The following table uses man·Sievert/GW-annum:[44]
| Source | Public | Occupational |
|---|---|---|
| Nuclear power | 0.43 | 4.5 |
| Coal (modern) | 0.7 | 11 |
| Coal (older) | 1.4 | 11 |
| Natural gas | 0.1 | 0.02 |
| Oil | 0.0003 | 0.15 |
| Geothermal | 1–20 | 0.05 |
| Solar power | 0.8 | |
| Wind power | 0.1 | |
| Biomass | 0.01 |
Coal plants emit radiation in the form of radioactivefly ash which is inhaled and ingested by neighbours, and incorporated into crops. A 1978 paper fromOak Ridge National Laboratory estimated that coal-fired power plants of that time may contribute a whole-body committed dose of 19 μSv/a to their immediate neighbours in a radius of 500 m.[45] TheUnited Nations Scientific Committee on the Effects of Atomic Radiation's 1988 report estimated the committed dose 1 km away to be 20 μSv/a for older plants or 1 μSv/a for newer plants with improved fly ash capture, but was unable to confirm these numbers by test.[46] When coal is burned, uranium, thorium and all the uranium daughters accumulated by disintegration – radium, radon, polonium – are released.[47] 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.
The global average human exposure to artificial radiation is 0.6 mSv/a, primarily frommedical imaging. This medical component can range much higher, with an average of 3 mSv per year across the USA population.[3] Other human contributors include smoking, air travel, radioactive building materials, historical nuclear weapons testing, nuclear power accidents and nuclear industry operation.
A typical chest x-ray delivers 20 μSv (2 mrem) of effective dose.[48] A dental x-ray delivers a dose of 5 to 10 μSv.[49] ACT scan delivers an effective dose to the whole body ranging from 1 to 20 mSv (100 to 2000 mrem). The average American receives about 3 mSv of diagnostic medical dose per year; countries with the lowest levels of health care receive almost none. Radiation treatment for various diseases also accounts for some dose, both in individuals and in those around them.
Cigarettes containpolonium-210, originating from the decay products of radon, which stick totobacco leaves. Heavy smoking results in a radiation dose of 160 mSv/year to localized spots at the bifurcations of segmental bronchi in the lungs from the decay of polonium-210. This dose is not readily comparable to the radiation protection limits, since the latter deal with whole body doses, while the dose from smoking is delivered to a very small portion of the body.[50]
In a radiation metrology laboratory,background radiation refers to the measured value from any incidental sources that affect an instrument when a specific radiation source sample is being measured. This background contribution, which is 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.
This is in accordance with theInternational Atomic Energy Agency definition of background as being "Dose or dose rate (or an observed measure related to the dose or dose rate) attributable to all sources other than the one(s) specified.[1]
The same issue occurs with radiation protection instruments, where a reading from an instrument may be affected by the background radiation. An example of this is ascintillation detector used for surface contamination monitoring. In an elevated gamma background the scintillator material will be affected by the background gamma, which will add to the reading obtained from any contamination which is being monitored. In extreme cases it will make the instrument unusable as the background swamps the lower level of radiation from the contamination. In such instruments the background can be continually monitored in the "Ready" state, and subtracted from any reading obtained when being used in "Measuring" mode.
Regular Radiation measurement is carried out at multiple levels. Government agencies compile radiation readings as part of environmental monitoring mandates, often making the readings available to the public and sometimes in near-real-time. Collaborative groups and private individuals may also make real-time readings available to the public. Instruments used for radiation measurement include theGeiger–Müller tube and theScintillation detector. The former is usually more compact and affordable and reacts to several radiation types, while the latter is more complex and can detect specific radiation energies and types. Readings indicate radiation levels from all sources including background, and real-time readings are in general unvalidated, but correlation between independent detectors increases confidence in measured levels.
List of near-real-time government radiation measurement sites, employing multiple instrument types:
List of international near-real-time collaborative/private measurement sites, employing primarily Geiger-Muller detectors:
Our preliminary studies seem to indicate the presence of adaptive response in the cells of some Ramsar residents, but we do not claim to have seen hormetic effects in any of those studied. Given the apparent lack of ill effects among observed populations of these high dose rate areas, these data suggest that current dose limits may be overly conservative. However, the available data do not seem sufficient to cause national or international advisory bodies to change their current conservative radiation protection recommendations;
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