on a hot summer day. Displayed background
gamma radiation level is 9.8
μR/h (0.82 mSv/a) This is very close to the world average background radiation of 0.87 mSv/a from cosmic and terrestrial sources. s used by early researchers first detected cosmic rays and other background radiation. They can be used to visualize the 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. The
International Atomic Energy Agency states: :"Exposure to radiation from natural sources is an inescapable feature of everyday life in both working and public environments. This exposure is in most cases of little or no concern to society, but in certain situations the introduction of health protection measures needs to be considered, for example when working with uranium and thorium ores and other Naturally Occurring Radioactive Material (
NORM). These situations have become the focus of greater attention by the Agency in recent years."
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 (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 are
radium-226 (decay product of thorium-230 in decay chain of uranium-238) and
radon-222 (a decay product of
radium-226 in said chain). Thorium and uranium (and their daughters) 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 (
cf. the same peak for atmospheric radon). Potassium-40 is detectable directly via its 1461 keV gamma peak. 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. 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 (EEC) of radon by a factor of 8 to 9 and the EEC of
thoron by a factor of 40 .
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 (at 1650 meters elevation) 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. (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 of 40K decay per second, and a similar number of 14C. The energy of
beta particles produced by 40K is about 10 times that from the beta particles from 14C decay. 14C is present in the human body at a level of about 3700 Bq (0.1 μCi) with a
biological half-life of 40 days. This means there are about 3700 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. 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 from 40K, 0.12 mSv/a comes from the uranium and thorium series, and 12 μSv/a comes from 14C. In the world in general, exceptionally high natural background locales include
Ramsar in Iran,
Guarapari in Brazil,
Karunagappalli in India,
Arkaroola in Australia and
Yangjiang in China. 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 of
monazite. 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 in
Guarapari and
Cumuruxatiba were later evaluated at 14 and 15 μGy/h. Note that the values quoted here are in
Grays. 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 in
Ramsar, primarily due to the use of local naturally radioactive limestone as a building material. The 1000 most exposed residents receive an average external
effective radiation dose of per year, six times the
ICRP recommended limit for exposure to the public from artificial sources. 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. 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.
Photoelectric Background radiation doses in the immediate vicinity of particles of high atomic number materials, within the human body, have a small enhancement due to the
photoelectric effect.
Neutron background 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. ==Artificial background radiation==