Radon is a
chemical elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
with symbol
Rn and
atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
86. It is a
radioactiveRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
, colorless, odorless, tasteless
noble gasThe noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
, occurring naturally as the decay product of
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
or
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
. Its most stable
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
,
222Rn, has a
half-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 3.8 days. Radon is one of the densest substances that remains a
gasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
under normal conditions and is considered a health hazard due to its radioactivity. Intense radioactivity also hindered chemical studies of radon and only a few compounds are known.
Radon is formed as part of the normal radioactive
decay chainIn nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations...
of uranium and thorium. Uranium and thorium have been around since the earth was formed and their
most common isotopeUranium is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The average atomic mass of natural uranium is 238.02891 u...
has a very long half-life (4.5 billion years). Uranium and thorium,
radiumRadium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now. As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung increasing your risk of developing lung cancer.
Radon is responsible for the majority of the public exposure to
ionizing radiationIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. It is often the single largest contributor to an individual's
background radiationBackground radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics and basements. It can also be found in some
spring watersA spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...
and hot springs.
Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence of
lung cancerLung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
. Thus, radon is considered a significant contaminant that affects
indoor air qualityIndoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....
worldwide. According to the
United States Environmental Protection AgencyThe U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
, radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA estimates.
Physical properties
Radon is a colorless and odorless gas, and therefore not detectable by human senses alone. At standard temperature and pressure, radon forms a monatomic gas with a density of 9.73 kg/m
3, about 8 times the density of the Earth's atmosphere at sea level, 1.217 kg/m
3. Radon is one of the heaviest gases at room temperature and is the heaviest of the noble gases. Although colorless at standard temperature and pressure, when cooled below its
freezing pointFreezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
of 202 K, radon has a brilliant
phosphorescencePhosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
which turns from yellow to orange-red as the temperature is lowered. Upon
condensationCondensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
, radon glows because of the intense radiation it produces.
Chemical properties
Being a noble gas, radon is chemically not very reactive. However, the 3.8 day half-life of radon-222 makes it useful in physical sciences as a natural
tracerA radioactive tracer, also called a radioactive label, is a substance containing a radioisotope that is used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as a cell or tissue...
.
Radon is a member of the zero-
valenceIn chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
elements that are called noble gases. It is inert to most common chemical reactions, such as combustion, because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. 1037 kJ/mol is required to extract one electron from its shells (also known as the first ionization energy). However, in accordance with
periodic trendsThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
, radon has a lower
electronegativityElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
than the element one period before it,
xenonXenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, and is therefore more reactive. Radon is sparingly soluble in water, but more soluble than lighter noble gases. Radon is appreciably more soluble in organic liquids than in water. Early studies concluded that the stability of radon
hydrateHydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
should be of the same order as that of the hydrates of
chlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
or
sulfur dioxideSulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, and significantly higher than the stability of the hydrate of
hydrogen sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
.
Because of its cost and radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all either
fluorideFluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
s or
oxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s. Radon can be oxidized by a few powerful oxidizing agents such as
fluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, thus forming
radon fluorideRadon difluoride is a compound of radon, a noble gas. Radon reacts readily with fluorine to form a solid compound, but this decomposes on attempted vaporization and its exact composition is uncertain. Calculations suggest that it may be ionic. The usefulness of radon compounds is limited because...
. It decomposes back to elements at a temperature of above 250 °C. It has a low
volatilityIn chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
and was thought to be . But because of the short half-life of radon and the radioactivity of its compounds, it has not been possible to study the compound in any detail. Theoretical studies on this molecule predict that it should have a Rn-F
bond distance- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
of 2.08
ǺThe angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
, and that the compound is thermodynamically more stable and less volatile than its lighter counterpart . The
octahedral moleculeIn chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
was predicted to have an even lower enthalpy of formation than the difluoride. The [RnF]
+ ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is believed to form by the reaction:
- Rn (g) + 2 (s) → (s) + 2 (g)
Radon oxides are among the few other reported compounds of radon. Radon
carbonylIn organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
RnCO has been predicted to be stable and to have a
linear molecular geometryIn chemistry, the Linear molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º. Linear organic molecules, e.g. acetylene, are often described by invoking sp orbital hybridization for the carbon centers. Many linear molecules exist, prominent...
. The molecules and RnXe were found to be significantly stabilized by
spin-orbit couplingIn quantum mechanics, the procedure of constructing eigenstates of total angular momentum out of eigenstates of separate angular momenta is called angular momentum coupling. For instance, the orbit and spin of a single particle can interact through spin-orbit interaction, in which case the...
. Radon caged inside a
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
has been proposed as a drug for tumors.
Isotopes
Radon has no
stable isotopeStable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...
s. However, 36 radioactive isotopes have been characterized, with their
atomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
es ranging from 193 to 228. The most stable isotope is
222Rn, which is a
decay productIn nuclear physics, a decay product is the remaining nuclide left over from radioactive decay. Radioactive decay often involves a sequence of steps...
of
226Ra, a decay product of
238UUranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
. Among the daughters of
222Rn is also the highly unstable isotope
218Rn.
There are three other radon isotopes that have a half-life of over an hour:
211Rn,
210Rn and
224Rn. The
220Rn isotope is a natural decay product of the most stable
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
isotope (
232Th), and is commonly referred to as thoron. It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly,
219Rn is derived from the most stable isotope of
actiniumActinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902...
(
227Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds. No radon isotopes significantly occur in the
neptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
(
237Np) decay series.
Progenies
222Rn belongs to the radium and uranium-238 decay chain, and has a half-life of 3.8235 days. Its four first products (excluding marginal decay schemes) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence:
- 222Rn, 3.8 days, alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
ing to...
- 218Po
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
, 3.10 minutes, alpha decayAlpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
ing to...
- 214Pb
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, 26.8 minutes, beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
ing to...
- 214Bi
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
, 19.9 minutes, beta decaying to...
- 214Po, 0.1643 ms, alpha decaying to...
At the next step,
214Po decays to
210Pb, which has a much longer half-life of 22.3 years. Its progenies are:
- 210Bi, 5.013 days, beta decaying to...
- 210Po, 138.376 days, alpha decaying to...
- 206Pb, stable.
The radon equilibrium factor is the ratio between the activity of all short-period radon progenies (which are responsible for most of radon's biological effects), and the activity that would be at equilibrium with the radon parent.
If a closed volume is constantly supplied with radon, the concentration of short-lived isotopes will increase until an equilibrium is reached where the rate of decay of each decay product will equal that of the radon itself. The equilibrium factor is 1 when both activities are equal, meaning that the decay products have stayed close to the radon parent long enough for the equilibrium to be reached, within a couple of hours. Under these conditions each additional pCi/L of radon will increase exposure, by 0.01 WL (see explanation of WL below). These conditions are not always met: in many homes, the equilibrium fraction is typically 40%; that is, there will be 0.004 WL of progeny for each pCi/L of radon in air.
210Pb takes much longer (decades) to come in equilibrium with radon, but, if the environment permits accumulation of dust over extended periods of time,
210lead and its decay products may contribute to overall radiation levels as well.
Because of their electrostatic charge, radon progenies adhere to surfaces or dust particles, whereas gaseous radon does not. Attachment removes them from the air, usually causing the equilibrium factor in the atmosphere to be less than one. The equilibrium factor is also lowered by air circulation or air filtration devices, and is increased by airborne dust particles, including cigarette smoke. In high concentrations, airborne radon isotopes contribute significantly to human health risk. The equilibrium factor found in epidemiological studies is 0.4.
History and etymology
Radon was the fifth radioactive element to be discovered, in 1900 by
Friedrich Ernst DornFriedrich Ernst Dorn was a German physicist who was the first to discover that a radioactive substance, later named radon, is emitted from radium.-Life and work:...
, after uranium, thorium, radium and polonium. In 1900 Dorn reported some experiments in which he noticed that radium compounds emanate a radioactive gas which he named
Radium Emanation (
Ra Em). Before that, in 1899,
PierrePierre Curie was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity and radioactivity, and Nobel laureate. He was the son of Dr. Eugène Curie and Sophie-Claire Depouilly Curie ...
and
Marie CurieMarie Skłodowska-Curie was a physicist and chemist famous for her pioneering research on radioactivity. She was the first person honored with two Nobel Prizes—in physics and chemistry...
observed that the "gas" emitted by radium remained radioactive for a month. Later that year, Robert B. Owens and
Ernest RutherfordErnest Rutherford, 1st Baron Rutherford of Nelson OM, FRS was a New Zealand-born British chemist and physicist who became known as the father of nuclear physics...
, at
McGill UniversityMohammed Fathy is a public research university located in Montreal, Quebec, Canada. The university bears the name of James McGill, a prominent Montreal merchant from Glasgow, Scotland, whose bequest formed the beginning of the university...
in
MontrealMontreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
, noticed variations when trying to measure radiation from thorium oxide. Rutherford noticed that the compounds of thorium continuously emit a radioactive gas which retain the radioactive powers for several minutes and called this gas "
emanation" (from Latin "emanare"—to elapse and "emanatio"—expiration), and later
Thorium Emanation (
Th Em). In 1901, he demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element. In 1903, similar emanations were observed from actinium by
André-Louis DebierneAndré-Louis Debierne was a French chemist and is considered the discoverer of the element actinium....
and were called
Actinium Emanation (
Ac Em).
Several names were suggested for these three gases:
exradio,
exthorio, and
exactinio in 1904;
radon,
thoron, and
akton in 1918;
radeon,
thoreon, and
actineon in 1919, and eventually
radon,
thoron, and
actinon in 1920. The likeness of the spectra of these three gases with those of argon, krypton, and xenon, and their observed chemical inertia led Sir
William RamsaySir William Ramsay was a Scottish chemist who discovered the noble gases and received the Nobel Prize in Chemistry in 1904 "in recognition of his services in the discovery of the inert gaseous elements in air" .-Early years:Ramsay was born in Glasgow on 2...
to suggest in 1904 that the "emanations" might contain a new element of the noble gas family.
In 1910, Sir William Ramsay and
Robert Whytlaw-GrayRobert H. Whytlaw-Gray was a chemist, born in London, England. He studied at University of Glasgow and University College London. He and William Ramsay isolated radon and studied its physical properties .-Biography:...
isolated radon, determined its density, and determined that it was the heaviest known gas. They wrote that "L'expression de l'émanation du radium est fort incommode," (the expression of radium emanation is very awkward) and suggested the new name niton (Nt) (from the Latin "nitens" meaning "shining") in order to emphasize the property of gases that cause the phosphorescence of some substances, and in 1912 it was accepted by the International Commission for Atomic Weights. In 1923, the International Committee for Chemical Elements and
International Union of Pure and Applied ChemistryThe International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
(IUPAC) chose among the names radon (Rn), thoron (Tn), and actinon (An). Later, when isotopes were numbered instead of named, the element took the name of the most stable isotope,
radon, while Tn was renamed
220Rn and An was renamed
219Rn. As late as the 1960s, the element was also referred to simply as
emanation. The first synthesized compound of radon, radon fluoride, was obtained in 1962.
The danger of high exposure to radon in mines, where exposures reaching 1,000,000
BqThe becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
/m
3 can be found, has long been known. In 1530,
ParacelsusParacelsus was a German-Swiss Renaissance physician, botanist, alchemist, astrologer, and general occultist....
described a wasting disease of miners, the
mala metallorum, and
Georg AgricolaGeorgius Agricola was a German scholar and scientist. Known as "the father of mineralogy", he was born at Glauchau in Saxony. His real name was Georg Pawer; Agricola is the Latinised version of his name, Pawer meaning "farmer"...
recommended ventilation in mines to avoid this mountain sickness (
Bergsucht).
In 1879, this condition was identified as lung cancer by Herting and Hesse in their investigation of miners from Schneeberg, Germany.
The first major studies with radon and health occurred in the context of
uranium miningUranium mining is the process of extraction of uranium ore from the ground. The worldwide production of uranium in 2009 amounted to 50,572 tonnes, of which 27% was mined in Kazakhstan. Kazakhstan, Canada, and Australia are the top three producers and together account for 63% of world uranium...
in the Joachimsthal region of
BohemiaBohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...
. In the US, studies and mitigation only followed decades of health effects on uranium miners of the
Southwestern United StatesThe Southwestern United States is a region defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas and Utah...
employed during the early
Cold WarThe Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
; standards were not implemented until 1971.
The presence of radon in indoor air was documented as early as 1950. Beginning in the 1970’s research was initiated to address sources of indoor radon, determinants of concentration, health effects, and approaches to mitigation. In the United States, the problem of indoor radon received widespread publicity and intensified investigation after a widely publicized incident in 1984. During routine monitoring at a Pennsylvania nuclear power plant, a worker was found to be contaminated with radioactivity. A high contamination of radon in his home was subsequently identified as responsible for the contamination.
Concentration units
All discussions of radon concentrations in the environment refer to
222Rn. While the average rate of production of
220Rn (from the thorium decay series) is about the same as
222Rn, the amount of
220Rn in the environment is much less than that of
222Rn because of the short half-life of
220Rn (1 minute versus 4 days).
Radon concentration is usually measured in the atmosphere, in
becquerelThe becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
per cubic meter (Bq/m
3), the
SI derived unitThe International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...
. Typical domestic exposures are about 100 Bq/m
3 indoors, and 10–20 Bq/m
3 outdoors.
It is often measured in picocuries per liter (pCi/L) in the USA, with 1 pCi/L=37 Bq/m
3.
In the mining industry, the exposition is traditionally measured in
working level (WL), and the cumulative exposition in
working level month (WLM): 1 WL equals any combination of short-lived
222Rn progeny (
218Po,
214Pb,
214Bi, and
214Po) in 1 liter of air that releases 1.3 × 10
5 MeV of potential alpha energy; one WL is equivalent to 2.08 × 10
−5 joules per cubic meter of air (J/m
3). The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m
3). One WLM is equivalent to 3.6 × 10
−3 J·h/m
3. An exposure to 1 WL for 1 working month (170 hours) equals 1 WLM cumulative exposure.
A cumulative exposition of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m
3.
Radon (
222Rn), when released into the air, decays to
210Pb and other radioisotopes, the levels of
210Pb can be measured. The rate of deposition of this radioisotope is weather dependent.
Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1000 Bq/m
3 (relatively high) concentration corresponds to 0.17 picogram per cubic meter. The average concentration of radon in the atmosphere is about 6 atoms of radon for each molecule in the air, or about 150 atoms in each ml of air. The radon activity of the Earth atmosphere originates from some tens of grams of radon, consistently replaced by decay of larger amounts of radium and uranium.
Natural
Radon is produced by the radioactive decay of radium-226, which is found in uranium ores; phosphate rock; shale’s; metamorphic minerals such as granite, gneiss, and schist; and, to a lesser degree, in common minerals such as limestone. Every square mile of surface soil, to a depth of 6 inches (2.6 km
2 to a depth of 15 cm), contains approximately 1 gram of radium, which releases radon in small amounts to the atmosphere On a global scale, it is estimated that 2,400 million curies (90 TBq) of radon are released from soil annually.
Radon concentration varies widely from place to place. In the open air, it ranges from 1 to 100 Bq/m
3, even less (0.1 Bq/m
3) above the ocean.
In caves or aerated mines, or ill-aerated houses, its concentration climbs to 20–2,000 Bq/m
3.
Radon concentration can be much higher in mining contexts. Ventilation regulations instruct to maintain radon concentration in uranium mines under the "working level", with 95th percentile levels ranging up to nearly 3 WL (546 pCi
222Rn per liter of air; 20.2 kBq/m
3, measured from 1976 to 1985).
The concentration in the air at the (unventilated)
GasteinBad Gastein is a spa town in the Austrian state of Salzburg, situated at the northern rim of the Hohe Tauern national park. It has 5,838 inhabitants. The name "Bad" means "spa", reflecting the town's history as a health resort. It is located at the head of the Gastein valley, about 1,000 metres ...
Healing Gallery averages 43 kBq/m
3 (1.2 nCi/L) with maximal value of 160 kBq/m
3 (4.3 nCi/L).
Radon mostly appears with the decay chain of the radium and
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
series (
222Rn), and marginally with the thorium series (
220Rn). The element emanates naturally from the ground, and some building materials, all over the world, wherever traces of uranium or
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
can be found, and particularly in regions with soils containing
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
or
shaleShale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, which have a higher concentration of uranium. However, not all granitic regions are prone to high emissions of radon. Being a rare gas, it usually migrates freely through faults and fragmented soils, and may accumulate in caves or water. Owing to its very short half-life (four days for
222Rn), radon concentration decreases very quickly when the distance from the production area increases. Radon concentration varies greatly with season and atmospheric conditions. For instance, it has been shown to accumulate in the air if there is a
meteorological inversionIn meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e...
and little wind.
High concentrations of radon can be found in some spring waters and hot springs. The towns of
Boulder, MontanaBoulder is a town in and the county seat of Jefferson County, Montana, United States. It is on the north bank of the Boulder River between Butte and Helena, slightly east of the Continental Divide, at the intersection of Interstate 15 and Montana Highway 69. The population was 1,300 at...
;
Misasais a town located in Tōhaku District, Tottori, Japan. It is also home to the official treasure of Sanbutsuji and the Okayama Hospital.The name "Misasa" originates from the belief that one who stays to enjoy three mornings in the town's famous hot springs will find all of his ailments cured.As of...
;
Bad KreuznachBad Kreuznach is the capital of the district of Bad Kreuznach, Rhineland-Palatinate, Germany. It is located on the Nahe river, a tributary of the Rhine...
,
GermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
; and the country of
JapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
have radium-rich springs which emit radon. To be classified as a radon mineral water, radon concentration must be above a minimum of 2 nCi/L (74 kBq/m
3). The activity of radon mineral water reaches 2,000 kBq/m
3 in Merano and 4,000 kBq/m
3 in Lurisia (Italy).
Natural radon concentrations in
Earth's atmosphereThe atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
are so low that radon-rich water in contact with the atmosphere will continually lose radon by volatilization. Hence, ground water has a higher concentration of
222Rn than
surface waterSurface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
, because radon is continuously produced by radioactive decay of
226Ra present in rocks. Likewise, the
saturated zoneAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
of a soil frequently has a higher radon content than the
unsaturated zoneThe vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater is at atmospheric pressure . Hence the vadose zone extends from the top of the ground surface to the water table...
because of
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
al losses to the atmosphere.
In 1971,
Apollo 15Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...
passed 110 km (68 mi) above the
Aristarchus plateauAristarchus is a prominent lunar impact crater that lies in the northwest part of the Moon's near side. It is considered the brightest of the large formations on the lunar surface, with an albedo nearly double that of most lunar features. The feature is bright enough to be visible to the naked eye,...
on the
MoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, and detected a significant rise in alpha particles thought to be caused by the decay of
222Rn. The presence of
222Rn has been inferred later from data obtained from the
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
alpha particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
spectrometer.
Radon is found in some
petroleumPetroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
. Because radon has a similar pressure and temperature curve to
propanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become radioactive because of decaying radon and its products.
Residues from the petroleum and
natural gasNatural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
industry often contain radium and its daughters. The sulfate scale from an
oil wellAn oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
can be radium rich, while the water, oil, and gas from a well often contains radon. Radon decays to form solid radioisotopes which form coatings on the inside of pipework.
Accumulation in houses
The phenomenon of heightened radon contamination in homes was discovered by chance in 1985 after the stringent radiation testing conducted at a nuclear power plant entrance revealed that Stanley Watras, an engineer entering the plant, was contaminated by radioactive substances. Typical domestic exposures are of approximately 100 Bq/m
3 indoors. Depending on how houses are built and ventilated, radon may accumulate in basements and dwellings. Radon can also seep into an indoor environment through cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. Radon concentrations in the same location may differ by a factor of two over a period of 1 hour. Also, the concentration in one room of a building may be significantly different than the concentration in an adjoining room.
The distribution of radon concentrations tends to be asymmetrical around the average, the larger concentrations have a disproportionately greater weight. Indoor radon concentration is usually assumed to follow a lognormal distribution on a given territory. Thus, the
geometric meanThe geometric mean, in mathematics, is a type of mean or average, which indicates the central tendency or typical value of a set of numbers. It is similar to the arithmetic mean, except that the numbers are multiplied and then the nth root of the resulting product is taken.For instance, the...
is generally used for estimating the "average" radon concentration in an area.
The mean concentration ranges from less than 10 Bq/m
3 to over 100 Bq/m
3 in some European countries. Typical
geometric standard deviationIn probability theory and statistics, the geometric standard deviation describes how spread out are a set of numbers whose preferred average is the geometric mean...
s found in studies range between 2 and 3, meaning (given the
68-95-99.7 ruleIn statistics, the 68-95-99.7 rule, or three-sigma rule, or empirical rule, states that for a normal distribution, nearly all values lie within 3 standard deviations of the mean....
) that the radon concentration is expected to be more than a hundred times the mean concentration for 2 to 3% of the cases.
The highest average radon concentrations in the United States are found in
IowaIowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...
and in the
Appalachian MountainThe Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
areas in southeastern Pennsylvania. Some of the highest readings ever have been recorded in the Irish town of
Mallow, County CorkMallow is the "Crossroads of Munster" and the administrative capital of north County Cork, in Ireland. The Northern Divisional Offices of Cork County Council are located in the town....
, prompting local fears regarding lung cancer. Iowa has the highest average radon concentrations in the United States due to significant glaciation that ground the granitic rocks from the
Canadian ShieldThe Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
and deposited it as soils making up the rich Iowa farmland. Many cities within the state, such as Iowa City, have passed requirements for radon-resistant construction in new homes.
In a few locations,
uranium tailingsUranium tailings are a waste byproduct of uranium mining. In mining, the raw uranium ore is brought to the surface and crushed into a fine sand...
have been used for landfills and were subsequently built on, resulting in possible increased exposure to radon.
Industrial production
Radon is obtained as a by-product of
uraniferous ores processing after transferring into 1% solutions of
hydrochloricHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
or
hydrobromic acidHydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at 124.3 °C and contains 47.6% HBr by weight, which is 8.89 mol/L. Hydrobromic acid has a pKa of −9, making it a...
s. The gas mixture extracted from the solutions contains , , He, Rn, , and
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s. The mixture is purified by passing it over copper at 720 °C to remove the and the , and then
KOHPotassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
and
Phosphorus pentoxide is a chemical compound with molecular formula P4O10 . This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.-Structure:...
are used to remove the acids and moisture by
sorptionSorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
. Radon is condensed by liquid nitrogen and purified from residue gases by sublimation.
Radon commercialization is regulated, but it is available in small quantities for the calibration of
222Rn measurement systems, at a price of almost $6,000 per milliliter of radium solution (which only contains about 15 picograms of actual radon at a given moment).
Radon is produced by a solution of radium-226 (half-life of 1600 years). Radium-226 decays by alpha-particle emission, producing radon which collects over samples of radium-226 at a rate of about 1 mm
3/day per gram of radium; equilibrium is quickly achieved and radon is produced in a steady flow, with an activity equal to that of the radium (50 Bq). Gaseous
222Rn (half-life of about four days) escapes from the capsule through diffusion.
Concentration scale
| Bq/m3 |
pCi/L |
Occurrence example |
| 1 |
~0.03 |
Radon concentration at the shores of large oceans is typically 1 Bq/m3.
Radon trace concentration above oceans or in Antarctica can be lower than 0.1 Bq/m3. |
| 10 |
0.27 |
Mean continental concentration in the open air: 10 to 30 Bq/m3.
Based on a series of surveys, the global mean indoor radon concentration is estimated to be 39 Bq/m3. |
| 100 |
2.7 |
Typical indoor domestic exposure. Most countries have adopted a radon concentration of 200–400 Bq/m3 for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (160 Bq/m3), then no action is necessary. A cumulated exposure of 230 Bq/m3 of radon gas concentration during a period of 1 year corresponds to 1 WLM. |
| 1,000 |
27 |
Very high radon concentrations (>1000 Bq/m3) have been found in houses built on soils with a high uranium content and/or high permeability of the ground. If levels are 20 picocuries radon per liter of air (800 Bq/m3) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. |
| 10,000 |
270 |
The "Working Level" in uranium mines corresponds to a 7000 Bq/m3 concentration.
The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m3 (about 1.2 nCi/L) with maximal value of 160 kBq/m3 (about 4.3 nCi/L). |
| 100,000 |
~2700 |
About 100,000 Bq/m3 (2.7 nCi/L) was measured in Stanley Watras's basement |
| 1,000,000 |
27000 |
Concentrations reaching 1,000,000 Bq/m3 can be found in unventilated uranium mines. |
Medical
An early-20th-century form of
quackeryQuackery is a derogatory term used to describe the promotion of unproven or fraudulent medical practices. Random House Dictionary describes a "quack" as a "fraudulent or ignorant pretender to medical skill" or "a person who pretends, professionally or publicly, to have skill, knowledge, or...
was the treatment of maladies in a radiotorium. It was a small, sealed room for patients to be exposed to radon for its "medicinal effects". The carcinogenic nature of radon due to its ionizing radiation became apparent later on. Radon's molecule-damaging radioactivity has been used to kill cancerous cells. It does not, however, increase the health of healthy cells. In fact, the ionizing radiation causes the formation of free radicals, which results in genetic and other cell damage, resulting in increased rates of illness, including
cancerCancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
.
Exposure to radon, a process known as
radiation hormesisRadiation hormesis is the hypothesis that low doses of ionizing radiation are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation...
, has been suggested to mitigate auto-immune diseases such as
arthritisArthritis is a form of joint disorder that involves inflammation of one or more joints....
. As a result, in the late 20th century and early 21st century, some "health mines" were established in
Basin, MontanaBasin is a census-designated place in Jefferson County, Montana, United States. It lies about southeast of the Continental Divide in a high narrow canyon along Interstate 15 about halfway between Butte and Helena. Basin Creek flows roughly north to south through Basin and enters the Boulder River...
which attracted people seeking relief from health problems such as arthritis through limited exposure to radioactive mine water and radon. However, the practice is discouraged because of the well-documented ill effects of high-doses of radiation on the body.
Radioactive water baths have been applied since 1906 in
JáchymovFor other places called Joachimsthal, see Joachimsthal Jáchymov . compl: "Sant Joachim's Sthal" is a spa town in north-west Bohemia in the Czech Republic belonging to the Karlovy Vary Region. It is situated at an altitude of 733 m above sea level in the eponymous St...
,
Czech RepublicThe Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
, but even before radon discovery they were used in
Bad GasteinBad Gastein is a spa town in the Austrian state of Salzburg, situated at the northern rim of the Hohe Tauern national park. It has 5,838 inhabitants. The name "Bad" means "spa", reflecting the town's history as a health resort. It is located at the head of the Gastein valley, about 1,000 metres ...
,
AustriaAustria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
. Radium-rich springs are also used in traditional
JapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese
onsenAn is a term for hot springs in the Japanese language, though the term is often used to describe the bathing facilities and inns around the hot springs. As a volcanically active country, Japan has thousands of onsen scattered along its length and breadth...
in
Misasais a town located in Tōhaku District, Tottori, Japan. It is also home to the official treasure of Sanbutsuji and the Okayama Hospital.The name "Misasa" originates from the belief that one who stays to enjoy three mornings in the town's famous hot springs will find all of his ailments cured.As of...
,
Tottori Prefectureis a prefecture of Japan located in the Chūgoku region. The capital is the city of Tottori. It is the least populous prefecture in Japan.- History :Before the Meiji Restoration, Tottori encompassed the old provinces of Hōki and Inaba...
. Drinking therapy is applied in
Bad BrambachBad Brambach is a municipality in the Vogtlandkreis district, in Saxony, Germany....
,
GermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. Inhalation therapy is carried out in Gasteiner-Heilstollen, Austria, in
Świeradów-ZdrójŚwieradów-Zdrój is a spa town in Lubań County, Lower Silesian Voivodeship, in south-western Poland near the border with the Czech Republic. The town is located in the Kwisa valley of the Jizera Mountains, a part of the Sudetes range. It lies approximately south of Lubań, and west of the...
, Czerniawa-Zdrój,
KowaryKowary is a town in Jelenia Góra County, Lower Silesian Voivodeship, in south-western Poland.It lies approximately south-east of Jelenia Góra, and south-west of the regional capital Wrocław.-History:...
, Lądek Zdrój, Poland, in Harghita Băi, Romania, and in
BoulderBoulder is a town in and the county seat of Jefferson County, Montana, United States. It is on the north bank of the Boulder River between Butte and Helena, slightly east of the Continental Divide, at the intersection of Interstate 15 and Montana Highway 69. The population was 1,300 at...
, United States. In the United States and Europe there are several "radon
spasA destination spa is a short term residential/lodging facility with the primary purpose of providing individual services for spa-goers to develop healthy habits. Historically many such spas were developed at the location of natural hot springs or sources of mineral waters...
," where people sit for minutes or hours in a high-radon atmosphere in the belief that low doses of radiation will invigorate or energize them.
Radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made in accelerators and
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s. Radon has been used in implantable seeds, made of gold or glass, primarily
used to treat cancersBrachytherapy , also known as internal radiotherapy, sealed source radiotherapy, curietherapy or endocurietherapy, is a form of radiotherapy where a radiation source is placed inside or next to the area requiring treatment...
.
The gold seeds were produced by filling a long tube with radon pumped from a radium source, the tube being then divided into short sections by crimping and cutting. The gold layer keeps the radon within, and filters out the alpha and beta radiations, while allowing the
gamma rayGamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s to escape (which kill the diseased tissue). The activities might range from 0.05 to 5 millicuries per seed (2 to 200 MBq). The gamma rays are produced by radon and the first short-lived elements of its decay chain (
218Po,
214Pb,
214Bi,
214Po).
Radon and its first decay products being very short-lived, the seed is left in place. After 12 half-lives (43 days), radon radioactivity is at 1/2000 of its original level. At this stage, the predominant residual activity originates from the radon decay product
210Pb, whose half-life (22.3 years) is 2000 times that or radon (and whose activity is thus 1/2000 of radon's), and its descendants
210Bi and
210Po, totalizing 0.03% of the initial seed activity.
In the early part of the 20th century in the USA, gold which was contaminated with
210Pb entered the jewelry industry. This was from gold seeds which had held
222Rn that had been melted down after the radon had decayed.
Wearing a contaminated ring could lead to a skin exposition of 10 to 100 rad/day (0.4 to 4 mSv/h)
Scientific
Radon emanation from the soil varies with soil type and with surface uranium content, so outdoor radon concentrations can be used to track air masses to a limited degree. This fact has been put to use by some atmospheric scientists. Because of radon's rapid loss to air and comparatively rapid decay, radon is used in
hydrologicHydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
research that studies the interaction between ground water and
streamA stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
s. Any significant concentration of radon in a stream is a good indicator that there are local inputs of ground water. Radon is also used in the dating of oil-containing soils because radon has a high affinity for oil-like substances.
Radon soil-concentration has been used in an experimental way to map buried close-subsurface geological faults because concentrations are generally higher over the faults. Similarly, it has found some limited use in prospecting for
geothermal gradientGeothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied...
s.
Some researchers have investigated changes in groundwater radon concentrations for
earthquake predictionAn earthquake prediction is a prediction that an earthquake of a specific magnitude will occur in a particular place at a particular time . Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month...
. Radon has a half-life of approximately 3.8 days, which means that it can be found only shortly after it has been produced in the radioactive decay chain. For this reason, it has been hypothesized that increases in radon concentration is due to the generation of new cracks underground, which would allow increased ground water circulation, flushing out radon. The generation of new cracks might not unreasonably be assumed to precede major earthquakes. In the 1970s and 1980s, scientific measurements of radon emissions near faults found that earthquakes often occurred with no radon signal, and radon was often detected with no earthquake to follow. It was then dismissed by many as an unreliable indicator. However, as of 2009, it is under investigation as a possible precursor by NASA.
Radon is a known pollutant emitted from
geothermal power stationsGeothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet and from radioactive decay of minerals...
, though it disperses rapidly, and no radiological hazard has been demonstrated in various investigations. The trend in geothermal plants is to reinject all emissions by pumping deep underground, and this seems likely to ultimately decrease such radon hazards further.
In the 1950s, radon has been used in industrial
radiographyRadiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
.
Health epidemiology
Radon-222 has been classified by
International Agency for Research on CancerThe International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....
as being carcinogenic to humans, and as a gas that can be inhaled, lung cancer is a particular concern for people exposed to high levels of radon for sustained periods of time. During the 1940s and 50s, when safety standards requiring expensive ventilation in mines were not widely implemented, radon exposure was linked to lung cancer among non-smoking miners of uranium and other hard rock materials in what is now the
Czech RepublicThe Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
, and later among miners from the Southwestern United States.
Since that time, ventilation and other measures have been used to reduce radon levels in most affected mines that continue to operate. In recent years, the average annual exposure of uranium miners has fallen to levels similar to the concentrations inhaled in some homes. This has reduced the risk of occupationally induced cancer from radon, although health issues may persist for those who are currently employed in affected mines and for those who have been employed in them in the past. As the relative risk for miners has decreased, so has the ability to detect excess risks among that population.
Radon exposure (actually radon progeny) has been linked to lung cancer in numerous case-control studies performed in the United States, Europe and China. There are approximately 21,000 deaths per year in the USA due to radon-induced lung cancers.
One of the most comprehensive radon studies performed in the United States by Dr.
R. William FieldR. William Field is an academic scholar and Professor in the of Epidemiology within the College of Public Health at the University of Iowa. He received a BS and MS degree in Biology from Millersville University of Pennsylvania and a PhD in Preventive Medicine from the College of Medicine at the...
and colleagues found a 50% increased lung cancer risk even at the protracted exposures at the EPA's action level of 4 pCi/L. North American and European Pooled analyses further support these findings. Many researchers have highlighted a theoretical possible increased risk of
leukemiaLeukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...
of radon, but empirical support for this has not emerged.
The effects of radon if ingested are similarly unknown, although studies have found that its biological half-life ranges from 30–70 minutes, with 90 percent removal at 100 minutes. In 1999 National Research Council investigated the issue of radon in drinking water. The risks associated with ingestion was considered almost negligible.
As well as being ingested through drinking water, radon is also released from water when temperature is increased, pressure is decreased and when water is aerated. Optimum conditions for radon release and exposure occur during showering. Water with a radon concentration of 10
4 pCi/L can increase the indoor airborne radon concentration by 1 pCi/L under normal conditions of water use.
Domestic-level exposure
Most models of residential radon exposure are based on studies of miners, and direct estimates of the risks posed to homeowners would be more desirable. Nonetheless, because of the difficulties of measuring the risk of radon relative to other contributors—namely smoking—models of their effect have often made use of them.
Radon has been considered the second leading cause of lung cancer and leading environmental cause of cancer mortality by the
United States Environmental Protection AgencyThe U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
. Others have reached similar conclusions for the United Kingdom and France. Radon exposure in homes and offices may arise from certain subsurface rock formations and also from certain building materials (e.g. some granites); greatest risk of radon exposure arises from buildings which are tight, insufficiently ventilated and have leaks that let in soil air from the ground into basements and dwelling rooms.
The actionable concentration of radon in a home varies depending on the organization doing the recommendation, for example, the United States Environmental Protection Agency encourages that action be taken at concentrations as low as 74 Bq/m
3 (2 pCi/L), and the
European UnionThe European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
recommends action be taken when concentrations reach 400 Bq/m
3 (11 pCi/L) for old houses and 200 Bq/m
3 (5 pCi/L) for new ones.
On 8 July 2010 the UK's Health Protection Agency issued new advice setting a "Target Level" of 100 Bq/m
3 whilst retaining an "Action Level" of 200 Bq/m
3.
Radon and smoking
Results from epidemiological studies indicate that the risk of lung cancer increases with exposure to residential radon. However, there are always major uncertainties in this studies. A classical and well-known example of source of error is smoking.
In addition, smoking is the most important risk factor for lung cancer. In the West, tobacco smoke is estimated to cause about 90% of all lung cancers. There is a tendency for other hypothetical lung cancer risks to drown in the risk of smoking. Results from epidemiological studies must always be interpreted with caution. According to the EPA, the risk of lung cancer for smokers is significant due to synergistic effects of radon and smoking. For this population about 62 people in a total of 1,000 will die of lung cancer compared to 7 people in a total of 1,000 for people who have never smoked.
Radon, like other known or suspected external risk factors for lung cancer, is a threat for smokers and former smokers. This was clearly demonstrated by the European pooling study. A commentary to the pooling study stated: "it is not appropriate to talk simply of a risk from radon in homes. The risk is from smoking, compounded by a synergistic effect of radon for smokers. Without smoking, the effect seems to be so small as to be insignificant."
A study of radiation from post
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radiotherapy shows that the simple models previously used to assess the combined and separate risks from radiation and smoking need to be developed. This is also supported by new discussion about the calculation method,
LNTThe linear no-threshold model is a method for predicting the long term, biological damage caused by ionizing radiation and is based on the assumption that the risk is directly proportional to the dose at all dose levels....
, which routinely has been used.
Radon and passive smoking
An important question is if also passive smoking can cause a similar synergy effect with residential radon. This has been insufficiently studied. The basic data for the European pooling study makes it impossible to exclude that such synergy effect is an explanation for the (very limited) increase in the risk from radon that was stated for non-smokers.
A study from 2001 which included 436 cases (never smokers who had lung cancer), and a control group (1649 never smokers) showed that exposure to radon increased the risk of lung cancer in never smokers. But the group that had been exposed to passive smoking at home appeared to bear the entire risk increase, while those who were not exposed to passive smoking did not show any increased risk with increasing radon level.
This needs confirmation by additional studies. Although the startling results from 2001 new studies seems not to have been implemented.
Testing and mitigation
There are relatively simple tests for radon gas, but these tests are not commonly done, even in areas of known systematic hazards. Radon test kits are commercially available. The short-term radon test kits used for screening purposes are inexpensive, in many cases free. The kit includes a collector that the user hangs in the lowest livable floor of the house for 2 to 7 days. The user then sends the collector to a laboratory for analysis. Long term kits, taking collections for up to one year, are also available. An open-land test kit can test radon emissions from the land before construction begins.
Radon levels fluctuate naturally, due to factors like transient weather conditions, so an initial test might not be an accurate assessment of a home's average radon level. Radon levels are at a maximum during the coolest part of the day when pressure differentials are greatest. Therefore, a high result (over 4 pCi/L) justifies repeating the test before undertaking more expensive abatement projects. Measurements between 4 and 10 pCi/L warrant a long term radon test. Measurements over 10 pCi/L warrant only another short term test so that abatement measures are not unduly delayed. Purchasers of real estate are advised to delay or decline a purchase if the seller has not successfully abated radon to 4 pCi/L or less.
Because the half-life of radon is only 3.8 days, removing or isolating the source will greatly reduce the hazard within a few weeks. Another method of reducing radon levels is to modify the building's ventilation. Generally, the indoor radon concentrations increase as ventilation rates decrease. In a well ventilated place, the radon concentration tends to align with outdoor values (typically 10 Bq/m
3, ranging from 1 to 100 Bq/m
3).
Radon levels in indoor air can be lowered in a number of ways, from sub-slab depressurization to increasing the ventilation rate of the building. The four principal ways of reducing the amount of radon accumulating in a house are:
- Sub-slab depressurization (soil suction) by increasing under-floor ventilation;
- Improving the ventilation of the house and avoiding the transport of radon from the basement into living rooms;
- Installing a radon sump system in the basement;
- Installing a positive pressurization or positive supply ventilation system.
According to the EPA's "A Citizen's Guide to Radon", the method to reduce radon "primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside", which is also called sub-slab depressurization, active soil depressurization, or soil suction. Generally indoor radon can be mitigated by sub-slab depressurization and exhausting such radon-laden air to the outdoors, away from windows and other building openings. "EPA generally recommends methods which prevent the entry of radon. Soil suction, for example, prevents radon from entering your home by drawing the radon from below the home and venting it through a pipe, or pipes, to the air above the home where it is quickly diluted" and "EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently" according to the EPA's "Consumer's Guide to Radon Reduction: How to fix your home".
Positive-pressure ventilation systems can be combined with a heat exchanger to recover energy in the process of exchanging air with the outside, and simply exhausting basement air to the outside is not necessarily a viable solution as this can actually draw radon gas
into a dwelling. Homes built on a crawl space may benefit from a radon collector installed under a "radon barrier" (a sheet of plastic that covers the crawl space).
For crawlspaces, the EPA states "An effective method to reduce radon levels in crawlspace homes involves covering the earth floor with a high-density plastic sheet. A vent pipe and fan are used to draw the radon from under the sheet and vent it to the outdoors. This form of soil suction is called submembrane suction, and when properly applied is the most effective way to reduce radon levels in crawlspace homes."
See also
- International Radon Project
The International Radon Project is a World Health Organization initiative to reduce the lung cancer risk around the world.The IRP released their guidance to member countries in September 2009 ....
- Lucas cell
- Radiation Exposure Compensation Act
The United States Radiation Exposure Compensation Act is a federal statute providing for the monetary compensation of people, including atomic veterans, who contracted cancer and a number of other specified diseases as a direct result of their exposure to atmospheric nuclear testing undertaken by...
- Radiohalo
Radiohalos or pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks. The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure...
External links
- Toxicological Profile for Radon, Draft for Public Comment, Agency for Toxic Substances and Disease Registry, September 2008
- Health Effects of Exposure to Radon: BEIR VI. Committee on Health Risks of Exposure to Radon (BEIR VI), National Research Council available on-line
- UNSCEAR 2000 Report to the General Assembly, with scientific annexes: Annex B: Exposures from natural radiation sources.
- Should you measure the radon concentration in your home?, Phillip N. Price, Andrew Gelman
Andrew Gelman is an American statistician, professor of statistics and political science, and director of the Applied Statistics Center at Columbia University. He earned an S.B. in mathematics and in physics from MIT in 1986 and a Ph.D...
, in Statistics: A Guide to the Unknown, January 2004.
- Radon and radon publications at the United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
- National Radon Program Services hosted by Kansas State University
Kansas State University, commonly shortened to K-State, is an institution of higher learning located in Manhattan, Kansas, in the United States...
- Radon Information from the UK Health Protection Agency
- Frequently Asked Questions About Radon at National Safety Council
The National Safety Council is a 501 nonprofit, nongovernmental public service organization dedicated to protecting life and promoting health. Headquartered in Itasca, Illinois, NSC is a member organization, founded in 1913 and granted a congressional charter in 1953...
- Home Buyer's and Seller's Guide to Radon An article by the International Association of Certified Home Inspectors (InterNACHI)
- Radon and Lung Health from the American Lung Association
- Radon's impact on your health – Lung Association
- The Geology of Radon, James K. Otton, Linda C.S. Gundersen, and R. Randall Schumann
- Map referring to radon concentrations in England and Wales
- EPA Federal Radon Mitigation Action Plan