- Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.
Both Natural and human-made background radiation varies by location.
Average annual radiation exposure (millisievert) Radiation UNSCEAR Princeton Wa State MEXT Type Source World
Typical range USA USA Japan remark Natural Air 1.26 0.2-10.0a 2.29 2.00 0.40 mainly from Radon, (a)depend on indoor accumulation of radon gas Internal 0.29 0.2-1.0b 0.16 0.40 0.40 mainly from food (K-40, C-14, etc.) (b)Depend on diets Terrestrial 0.48 0.3-1.0c 0.19 0.29 0.40 (c)depend on soil and building material Cosmic 0.39 0.3-1.0d 0.31 0.26 0.30 (d)from sea level to high elevation sub total 2.40 1.0-13.0 2.95 2.95 1.50 Man made Medical 0.60 0.03-2.0 3.00 0.53 2.30 Fallout 0.007 0 - 1+ - - 0.01 peak at 1963 and spike at 1986. still high near test and accident sites. US; Fallout is included in others others 0.0052 0-20 0.25 0.13 0.001 average occupational exposure 0.7mSv, mining workers are high, population near Nuclear plant 0.02mSv sub total 0.6 0 to tens 3.25 0.66 2.311 Total 3.00 0 to tens 6.20 3.61 3.81
- figures are pre "2011 Fukushima Nuclear Plant Accident"
- Human-made values at UNCEAR are from Japan NIRS which summarized UNCEAR data.
The background radiation of the human environment is a combination of:
- Sources in the Earth. These include sources in water and food (banana equivalent dose), which are incorporated to the human body, to building materials, and to products that incorporate radioactive sources from nature;
- Sources from outer space, such as cosmic rays;
- Sources in the atmosphere, such as the radon gas released from the Earth's crust, which then decays into radioactive atoms that attach to airborne dust, and other particulate (granular, powder) materials, that human beings might ingest and inhale. Another factor is the radiation produced by the atomic bombardment of the upper atmosphere by high-energy cosmic rays.
Natural sources of background radiation account for most occurrences of human exposure to ionizing radiation; excluding 3.0 per cent for medical exposure, e.g. radiological imaging and radiation therapy, and other artificial sources of background radiation such as:
- Self-luminous dials and signs
- Global radioactive contamination consequent to nuclear weapons testing
- Nuclear power station and nuclear fuel reprocessing accidents
- Normal operation of nuclear power facilities and like scientific research
- Emissions from burning fossil fuels, such as those of coal-fueled power plants
- Emissions from nuclear medicine facilities and irradiated patients
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 radionuclides in the environment (including those within the body). This is far greater than human-caused background radiation exposure, which in the year 2000 amounted to an average of about 5 μSv 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, the northern Flinders Ranges in Australia and Yangjiang in China. In Ramsar a peak yearly dose of 260 mGy (not mSv) has been reported (compared with 0.06 mSv of a chest radiograph or up to 20 mSv of a CT scan). The highest levels of natural background radiation recorded in the world is from areas around Ramsar, particularly at Talesh-Mahalleh which is a very high background radiation area (VHBRA) having an effective dose equivalent several times in excess of ICRP-recommended radiation dose limits for radiation workers and up to 200 times greater than normal background levels. Most of the radiation in the area is due to dissolved radium-226 in water of hot springs along with smaller amounts of uranium and thorium due to travertine deposits. There are more than nine hot springs in the area with different concentrations of radioisotopes, and these are used as spas by locals and tourists. This high level of radiation does not seem to have caused ill effects on the residents of the area and even possibly has made them slightly more radioresistant, which is puzzling and has been called "radiation paradox". It has also been reported that residents have healthier and longer lives. On the basis of this and other evidence including the fact that life had originated in a much more irradiated environment, some scientists have questioned the validity of linear no-threshold model, on which all radiation regulations currently depend. Others point out that some level of radiation might actually be good for health and have a positive effect on population based on the controversial radiation hormesis model, by jump starting DNA repair mechanisms inside the cell. Background radiation doses in the immediate vicinities of particles of high atomic number materials, within the human body, have a small enhancement due to the photoelectric effect.
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 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. During their flights airline crews typically get an extra dose on the order of 2.2mSv (220 mrem) per year.
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 earths 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.
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 the long-lived elements 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 (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 nanograms (10−8 g) of carbon-14 (14C), which has a decay half-life of 5,730±40 years. 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. 14C is present in the human body at a level of 3700 Bq with a biological half-life of 40 days. 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 about half the cells, while potassium is not a component of DNA. The decay of a 14C atom inside DNA in one person happens about 50 times per second, changing a carbon atom to one of nitrogen.
Radon is a terrestrial source of ionizing 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 tests has decreased to only 0.005 mSv per year.
Older coal-fired power plants without effective fly ash capture are a large source 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. 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.
Under normal circumstances, a modern nuclear reactor releases minuscule amounts of radioactive contamination. While the radiation released in minor accidents varies, major accidents like Windscale fire (Sellafield accident), the Chernobyl accident, and the Fukushima I nuclear accidents release massive radioactive contamination into the environment.
Radiation levels at the stricken Fukushima I power plant have varied spiking up to 1,000 mSv/h (millisievert per hour), which is a level that can cause radiation sickness to occur at a later time following a one hour exposure. Significant release in emissions of radioactive particles took place following hydrogen explosions at three reactors, as technicians tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater. Concerns about the possibility of a large scale radiation leak resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading nuclear contamination. New Scientist has reported that emissions of radioactive iodine and cesium from the crippled Fukushima I nuclear plant have approached levels evident after the Chernobyl disaster in 1986. On March 24, 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures". See Radiation effects from Fukushima Daiichi nuclear disaster.
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. For average persons who have had no medical x-rays, only 3% of their annual radiation dose comes from artificial sources.
A standard medical x-ray's strength is about 2 mrem or 0.02 mSv but can be over ten times that, depending on the equipment used. A dental x-ray optimally has a dose as low as 0.0033 mSv but poor machines and technique can give doses as high as 0.11 mSv. The average American and European receives about 0.5 mSv of diagnostic medical dose per year; countries with lower levels of health care receive about one fifth of this dose.
Radiation treatment for various diseases also accounts for some dose, both in individuals and in those around them.
In other contexts, background radiation may simply be any radiation that is pervasive, whether ionizing or not. 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.
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- Background radiation description from the Radiation Effects Research Foundation
- Environmental and Background Radiation FAQ from the Health Physics Society
- Radiation Dose Chart from the American Nuclear Society
- Radiation Dose Calculator from the US Environmental Protection Agency
Radiation (Physics & Health) Main articles Electromagnetic radiation
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