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= Radon =
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Introduction
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Radon is a chemical element; it has symbol Rn and atomic number 86. It
is a radioactive noble gas and is colorless and odorless. Of the three
naturally occurring radon isotopes, only (222)Rn has a sufficiently
long half-life (3.825 days) for it to be released from the soil and
rock where it is generated. Radon isotopes are the immediate decay
products of radium isotopes. The instability of (222)Rn, its most
stable isotope, makes radon one of the rarest elements. Radon will be
present on Earth for several billion more years despite its short
half-life, because it is constantly being produced as a step in the
decay chains of (238)U and (232)Th, both of which are abundant
radioactive nuclides with half-lives of at least several billion
years. The decay of radon produces many other short-lived nuclides,
known as "radon daughters", ending at stable isotopes of lead. (222)Rn
occurs in significant quantities as a step in the normal radioactive
decay chain of (238)U, also known as the uranium series, which slowly
decays into a variety of radioactive nuclides and eventually decays
into stable (206)Pb. (220)Rn occurs in minute quantities as an
intermediate step in the decay chain of (232)Th, also known as the
thorium series, which eventually decays into stable (208)Pb.
Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens
at McGill University in Montreal, and was the fifth radioactive
element to be discovered. First known as "emanation", the radioactive
gas was identified during experiments with radium, thorium oxide, and
actinium by Friedrich Ernst Dorn, Rutherford and Owens, and
André-Louis Debierne, respectively, and each element's emanation was
considered to be a separate substance: radon, thoron, and actinon. Sir
William Ramsay and Robert Whytlaw-Gray considered that the radioactive
emanations may contain a new element of the noble gas family, and
isolated "radium emanation" in 1909 to determine its properties. In
1911, the element Ramsay and Whytlaw-Gray isolated was accepted by the
International Commission for Atomic Weights, and in 1923, the
International Committee for Chemical Elements and the International
Union of Pure and Applied Chemistry (IUPAC) chose radon as the
accepted name for the element's most stable isotope, (222)Rn; thoron
and actinon were also recognized by IUPAC as distinct isotopes of the
element.
Under standard conditions, radon is gaseous and can be easily inhaled,
posing a health hazard. However, the primary danger comes not from
radon itself, but from its decay products, known as radon daughters.
These decay products, often existing as single atoms or ions, can
attach themselves to airborne dust particles. Although radon is a
noble gas and does not adhere to lung tissue (meaning it is often
exhaled before decaying), the radon daughters attached to dust are
more likely to stick to the lungs. This increases the risk of harm, as
the radon daughters can cause damage to lung tissue. Radon and its
daughters are, taken together, often the single largest contributor to
an individual's background radiation dose, but due to local
differences in geology, the level of exposure to radon gas differs by
location. A common source of environmental radon is uranium-containing
minerals in the ground; it therefore accumulates in subterranean areas
such as basements. Radon can also occur in ground water, such as
spring waters and hot springs. Radon trapped in permafrost may be
released by climate-change-induced thawing of permafrosts, and radon
may also be released into groundwater and the atmosphere following
seismic events leading to earthquakes, which has led to its
investigation in the field of earthquake prediction. It is possible to
test for radon in buildings, and to use techniques such as sub-slab
depressurization for mitigation.
Epidemiological studies have shown a clear association between
breathing high concentrations of radon and incidence of lung cancer.
Radon is a contaminant that affects indoor air quality worldwide.
According to the United States Environmental Protection Agency (EPA),
radon is the second most frequent cause of lung cancer, after
cigarette smoking, causing 21,000 lung cancer deaths per year in the
United States. 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
policy-oriented estimates. Significant uncertainties exist for the
health effects of low-dose exposures.
Physical properties
=====================
Radon is a colorless, odorless, and tasteless gas and therefore is not
detectable by human senses alone. At standard temperature and
pressure, it forms a monatomic gas with a density of 9.73 kg/m3, about
8 times the density of the Earth's atmosphere at sea level, 1.217
kg/m3. It is one of the densest gases at room temperature (a few are
denser, e.g. CF3(CF2)2CF3 and WF6) and is the densest of the noble
gases. Although colorless at standard temperature and pressure, when
cooled below its freezing point of 202 K, it emits a brilliant
radioluminescence that turns from yellow to orange-red as the
temperature lowers. Upon condensation, it glows because of the intense
radiation it produces. It is sparingly soluble in water, but more
soluble than lighter noble gases. It is appreciably more soluble in
organic liquids than in water. Its solubility equation is as follows:
:
where is the molar fraction of radon, is the absolute temperature,
and and are solvent constants.
Chemical properties
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Radon is a member of the zero-valence elements that are called noble
gases, and is chemically not very reactive. The inert pair effect
stabilizes the 6s shell, making it unavailable for bonding--a
consequence only understood within relativistic quantum chemistry. The
3.8-day half-life of (222)Rn makes it useful in physical sciences as a
natural tracer. Because radon is a gas at standard conditions, unlike
its decay-chain parents, it can readily be extracted from them for
research.
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. Its first ionization energy--the minimum
energy required to extract one electron from it--is 1037 kJ/mol. In
accordance with periodic trends, radon has a lower electronegativity
than the element one period before it, xenon, and is therefore more
reactive. Early studies concluded that the stability of radon hydrate
should be of the same order as that of the hydrates of chlorine () or
sulfur dioxide (), and significantly higher than the stability of the
hydrate of hydrogen sulfide ().
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 fluorides or oxides. Radon can
be oxidized by powerful oxidizing agents such as fluorine, thus
forming radon difluoride (). It decomposes back to its elements at a
temperature of above , and is reduced by water to radon gas and
hydrogen fluoride: it may also be reduced back to its elements by
hydrogen gas. It has a low volatility and was thought to be . 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 of 2.08 ångströms (Å), and that the compound
is thermodynamically more stable and less volatile than its lighter
counterpart xenon difluoride (). The octahedral molecule Radon
hexafluoride was predicted to have an even lower enthalpy of formation
than the difluoride. The [RnF]+ ion is believed to form by the
following reaction:
: Rn (g) + 2 (s) → (s) + 2 (g)
For this reason, antimony pentafluoride together with chlorine
trifluoride and have been considered for radon gas removal in uranium
mines due to the formation of radon-fluorine compounds. Radon
compounds can be formed by the decay of radium in radium halides, a
reaction that has been used to reduce the amount of radon that escapes
from targets during irradiation. Additionally, salts of the [RnF]+
cation with the anions , , and are known. Radon is also oxidised by
dioxygen difluoride to at .
Radon oxides are among the few other reported compounds of radon; only
the trioxide () has been confirmed. The higher fluorides and have
been claimed, are calculated to be stable, but have not been
confirmed. They may have been observed in experiments where unknown
radon-containing products distilled together with xenon hexafluoride:
these may have been , , or both. Trace-scale heating of radon with
xenon, fluorine, bromine pentafluoride, and either sodium fluoride or
nickel fluoride was claimed to produce a higher fluoride as well which
hydrolysed to form . While it has been suggested that these claims
were really due to radon precipitating out as the solid complex
[RnF][NiF6]2−, the fact that radon coprecipitates from aqueous
solution with has been taken as confirmation that was formed, which
has been supported by further studies of the hydrolysed solution. That
[RnO3F]− did not form in other experiments may have been due to the
high concentration of fluoride used. Electromigration studies also
suggest the presence of cationic [HRnO3]+ and anionic [HRnO4]− forms
of radon in weakly acidic aqueous solution (pH > 5), the procedure
having previously been validated by examination of the homologous
xenon trioxide.
The decay technique has also been used. Avrorin et al. reported in
1982 that 212Fr compounds cocrystallised with their caesium analogues
appeared to retain chemically bound radon after electron capture;
analogies with xenon suggested the formation of RnO3, but this could
not be confirmed.
It is likely that the difficulty in identifying higher fluorides of
radon stems from radon being kinetically hindered from being oxidised
beyond the divalent state because of the strong ionicity of radon
difluoride () and the high positive charge on radon in RnF+; spatial
separation of molecules may be necessary to clearly identify higher
fluorides of radon, of which is expected to be more stable than due
to spin-orbit splitting of the 6p shell of radon (RnIV would have a
closed-shell 6s6p configuration). Therefore, while should have a
similar stability to xenon tetrafluoride (), would likely be much
less stable than xenon hexafluoride (): radon hexafluoride would also
probably be a regular octahedral molecule, unlike the distorted
octahedral structure of , because of the inert pair effect. Because
radon is quite electropositive for a noble gas, it is possible that
radon fluorides actually take on highly fluorine-bridged structures
and are not volatile. Extrapolation down the noble gas group would
suggest also the possible existence of RnO, RnO2, and RnOF4, as well
as the first chemically stable noble gas chlorides RnCl2 and RnCl4,
but none of these have yet been found.
Radon carbonyl (RnCO) has been predicted to be stable and to have a
linear molecular geometry. The molecules and RnXe were found to be
significantly stabilized by spin-orbit coupling. Radon caged inside a
fullerene has been proposed as a drug for tumors. Despite the
existence of Xe(VIII), no Rn(VIII) compounds have been claimed to
exist; should be highly unstable chemically (XeF8 is
thermodynamically unstable).
Radon reacts with the liquid halogen fluorides ClF, , , , , and to
form . In halogen fluoride solution, radon is nonvolatile and exists
as the RnF+ and Rn2+ cations; addition of fluoride anions results in
the formation of the complexes and , paralleling the chemistry of
beryllium(II) and aluminium(III). The standard electrode potential of
the Rn2+/Rn couple has been estimated as +2.0 V, although there is no
evidence for the formation of stable radon ions or compounds in
aqueous solution.
Isotopes
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Radon has no stable isotopes. Thirty-nine radioactive isotopes have
been characterized, with mass numbers ranging from 193 to 231. Six of
them, from 217 to 222 inclusive, occur naturally. The most stable
isotope is (222)Rn (half-life 3.82 days), which is a decay product of
(226)Ra, the latter being itself a decay product of (238)U. A trace
amount of the (highly unstable) isotope (218)Rn (half-life about 35
milliseconds) is also among the daughters of (222)Rn. The isotope
(216)Rn would be produced by the double beta decay of natural (216)Po;
while energetically possible, this process has however never been
seen.
Three other radon isotopes have a half-life of over an hour: (211)Rn
(about 15 hours), (210)Rn (2.4 hours) and (224)Rn (about 1.8 hours).
However, none of these three occur naturally. (220)Rn, also called
thoron, is a natural decay product of the most stable thorium isotope
((232)Th). It has a half-life of 55.6 seconds and also emits alpha
radiation. Similarly, (219)Rn is derived from the most stable isotope
of actinium ((227)Ac)--named "actinon"--and is an alpha emitter with a
half-life of 3.96 seconds.
The radium or uranium series
Daughters
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Rn belongs to the radium and uranium-238 decay chain, and has a
half-life of 3.8235 days. Its first four 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:
* Rn, 3.82 days, alpha decaying to...
* Po, 3.10 minutes, alpha decaying to...
* Pb, 26.8 minutes, beta decaying to...
* Bi, 19.9 minutes, beta decaying to...
* Po, 0.1643 ms, alpha decaying to...
* Pb, which has a much longer half-life of 22.3 years, beta decaying
to...
* Bi, 5.013 days, beta decaying to...
* Po, 138.376 days, alpha decaying to...
* Pb, 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 overall decay rate of the decay
products equals 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 'working
level' (WL, a measure of radioactivity commonly used in mining). These
conditions are not always met; in many homes, the equilibrium factor
is typically 40%; that is, there will be 0.004 WL of daughters for
each pCi/L of radon in the air. Pb takes much longer to come in
equilibrium with radon, dependent on environmental factors, but if the
environment permits accumulation of dust over extended periods of
time, 210Pb and its decay products may contribute to overall radiation
levels as well. Several studies on the radioactive equilibrium of
elements in the environment find it more useful to use the ratio of
other Rn decay products with Pb, such as Po, in measuring overall
radiation levels.
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 1. The equilibrium factor is also
lowered by air circulation or air filtration devices, and is increased
by airborne dust particles, including cigarette smoke. The equilibrium
factor found in epidemiological studies is 0.4.
History and etymology
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Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens
at McGill University in Montreal. It was the fifth radioactive element
to be discovered, after uranium, thorium, radium, and polonium. In
1899, Pierre and Marie Curie observed that the gas emitted by radium
remained radioactive for a month. Later that year, Rutherford and
Owens noticed variations when trying to measure radiation from thorium
oxide. Rutherford noticed that the compounds of thorium continuously
emit a radioactive gas that remains radioactive for several minutes,
and called this gas "emanation" (from , to flow out, and ,
expiration), and later "thorium emanation" ("Th Em"). In 1900,
Friedrich Ernst Dorn reported some experiments in which he noticed
that radium compounds emanate a radioactive gas he named "radium
emanation" ("Ra Em"). In 1901, Rutherford and Harriet Brooks
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 Debierne, and were called
"actinium emanation" ("Ac Em").
Several shortened names were soon suggested for the three emanations:
'exradio', 'exthorio', and 'exactinio' in 1904; 'radon' (Ro), 'thoron'
(To), and 'akton' or 'acton' (Ao) in 1918; 'radeon', 'thoreon', and
'actineon' in 1919, and eventually 'radon', 'thoron', and 'actinon' in
1920. (The name radon is not related to that of the Austrian
mathematician Johann Radon.) The likeness of the spectra of these
three gases with those of argon, krypton, and xenon, and their
observed chemical inertia led Sir William Ramsay to suggest in 1904
that the "emanations" might contain a new element of the noble-gas
family.
In 1909, Ramsay and Robert Whytlaw-Gray isolated radon and determined
its melting temperature and critical point. Because it does not
conform to expected periodic trends, their obtained melting point (the
only experimental value) was questioned in 1925 by Friedrich Paneth
and E. Rabinowitsch, but 'ab initio' Monte Carlo simulations from 2018
agree almost exactly with Ramsay and Gray's result. In 1910, they
determined its density (that showed it was the heaviest known gas) and
its position in the periodic table. They wrote that "" ("the
expression 'radium emanation' is very awkward") and suggested the new
name niton (Nt) (from , shining) to emphasize the radioluminescence
property, 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 Chemistry (IUPAC)
chose the name of the most stable isotope, radon, as the name of the
element. The isotopes thoron and actinon were later renamed (220)Rn
and (219)Rn. This has caused some confusion in the literature
regarding the element's discovery as while Dorn had discovered radon
the isotope, he was not the first to discover radon the element.
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. Even today, the word 'radon' may refer to either
the element or its isotope 222Rn, with 'thoron' remaining in use as a
short name for 220Rn to stem this ambiguity. The name 'actinon' for
219Rn is rarely encountered today, probably due to the short half-life
of that isotope.
The danger of high exposure to radon in mines, where exposures can
reach 1,000,000 Bq/m3, has long been known. In 1530, Paracelsus
described a wasting disease of miners, the 'mala metallorum', and
Georg Agricola recommended ventilation in mines to avoid this mountain
sickness ('Bergsucht'). In 1879, this condition was identified as lung
cancer by Harting and Hesse in their investigation of miners from
Schneeberg, Germany. The first major studies with radon and health
occurred in the context of uranium mining in the Joachimsthal region
of Bohemia. In the US, studies and mitigation only followed decades of
health effects on uranium miners of the Southwestern US employed
during the early Cold War; standards were not implemented until 1971.
In the early 20th century in the US, gold contaminated with the radon
daughter 210Pb entered the jewelry industry. This was from gold
brachytherapy seeds that had held 222Rn, which were melted down after
the radon had decayed.
The presence of radon in indoor air was documented as early as 1950.
Beginning in the 1970s, research was initiated to address sources of
indoor radon, determinants of concentration, health effects, and
mitigation approaches. In the US, 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 concentration of radon in his
home was subsequently identified as responsible.
Concentration units
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210Pb is formed from the decay of 222Rn. Here is a typical deposition
rate of 210Pb as observed in Japan as a function of time, due to
variations in radon concentration.
Discussions of radon concentrations in the environment refer to 222Rn,
the decay product of uranium and radium. While the average rate of
production of 220Rn (from the thorium decay series) is about the same
as that of 222Rn, the amount of 220Rn in the environment is much less
than that of 222Rn because of the short half-life of 220Rn (55
seconds, versus 3.8 days respectively).
Radon concentration in the atmosphere is usually measured in becquerel
per cubic meter (Bq/m3), the SI derived unit. Another unit of
measurement common in the US is picocuries per liter (pCi/L); 1 pCi/L
= 37 Bq/m3. Typical domestic exposures average about 48 Bq/m3 indoors,
though this varies widely, and 15 Bq/m3 outdoors.
In the mining industry, the exposure is traditionally measured in
'working level' (WL), and the cumulative exposure in 'working level
month' (WLM); 1 WL equals any combination of short-lived 222Rn
daughters (218Po, 214Pb, 214Bi, and 214Po) in 1 liter of air that
releases 1.3 × 105 MeV of potential alpha energy; 1 WL is equivalent
to 2.08 × 10−5 joules per cubic meter of air (J/m3). The SI unit of
cumulative exposure is expressed in joule-hours per cubic meter
(J·h/m3). One WLM is equivalent to 3.6 × 10−3 J·h/m3. An exposure to 1
WL for 1 working-month (170 hours) equals 1 WLM cumulative exposure.
The International Commission on Radiological Protection recommends an
annual limit of 4.8WLM for miners. Assuming 2000 hours of work per
year, this corresponds to a concentration of 1500 Bq/m3.
222Rn 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 1,000 Bq/m3 (relatively high)
concentration corresponds to 0.17 picogram per cubic meter (pg/m3).
The average concentration of radon in the atmosphere is about 6 molar
percent, or about 150 atoms in each milliliter of air. The radon
activity of the entire Earth's atmosphere originates from only a few
tens of grams of radon, consistently replaced by decay of larger
amounts of radium, thorium, and uranium.
Natural
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Radon concentration next to a uranium mine
Radon is produced by the radioactive decay of radium-226, which is
found in uranium ores, phosphate rock, shales, igneous and metamorphic
rocks such as granite, gneiss, and schist, and to a lesser degree, in
common rocks 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 about 1
gram of radium, which releases radon in small amounts to the
atmosphere. It is estimated that 2.4 billion curies (90 EBq) of radon
are released from soil annually worldwide. This is equivalent to some
15.3 kg.
Radon concentration can differ 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 ventilated mines, or poorly ventilated 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 (222)Rn per liter of air; 20.2 kBq/m(3), measured
from 1976 to 1985).
The concentration in the air at the (unventilated) Gastein 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 radium/uranium series (decay chain)
((222)Rn), and marginally with the thorium series ((220)Rn). The
element emanates naturally from the ground, and some building
materials, all over the world, wherever traces of uranium or thorium
are found, and particularly in regions with soils containing granite
or shale, which have a higher concentration of uranium. 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 (222)Rn), 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 inversion and little wind.
High concentrations of radon can be found in some spring waters and
hot springs. The towns of Boulder, Montana; Misasa; Bad Kreuznach,
Germany; and the country of Japan have radium-rich springs that emit
radon. To be classified as a radon mineral water, radon concentration
must be above 2 nCi/L (74 kBq/m(3)). The activity of radon mineral
water reaches 2 MBq/m(3) in Merano and 4 MBq/m(3) in Lurisia (Italy).
Natural radon concentrations in the Earth's atmosphere 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 (222)Rn than surface water, because radon is
continuously produced by radioactive decay of (226)Ra present in
rocks. Likewise, the saturated zone of a soil frequently has a higher
radon content than the unsaturated zone because of diffusional losses
to the atmosphere.
In 1971, Apollo 15 passed above the Aristarchus plateau on the Moon,
and detected a significant rise in alpha particles thought to be
caused by the decay of (222)Rn. The presence of (222)Rn has been
inferred later from data obtained from the Lunar Prospector alpha
particle spectrometer.
Radon is found in some petroleum. Because radon has a similar pressure
and temperature curve to propane, and oil refineries separate
petrochemicals based on their boiling points, the piping carrying
freshly separated propane in oil refineries can become contaminated
because of decaying radon and its products.
Residues from the petroleum and natural gas industry often contain
radium and its daughters. The sulfate scale from an oil well can be
radium rich, while the water, oil, and gas from a well often contains
radon. Radon decays to form solid radioisotopes that form coatings on
the inside of pipework.
Accumulation in buildings
===========================
Measurement of radon levels in the first decades of its discovery was
mainly done to determine the presence of radium and uranium in
geological surveys. In 1956, most likely the first indoor survey of
radon decay products was performed in Sweden, with the intent of
estimating the public exposure to radon and its decay products. From
1975 up until 1984, small studies in Sweden, Austria, the United
States and Norway aimed to measure radon indoors and in metropolitan
areas.
High concentrations of radon in homes were discovered by chance in
1984 after the stringent radiation testing conducted at the new
Limerick Generating Station nuclear power plant in Montgomery County,
Pennsylvania, United States revealed that Stanley Watras, a
construction engineer at the plant, was contaminated by radioactive
substances even though the reactor had never been fueled and Watras
had been decontaminated each evening. It was determined that radon
levels in his home's basement were in excess of 100,000 Bq/m3 (2.7
nCi/L); he was told that living in the home was the equivalent of
smoking 135 packs of cigarettes a day, and he and his family had
increased their risk of developing lung cancer by 13 or 14 percent.
The incident dramatized the fact that radon levels in particular
dwellings can occasionally be orders of magnitude higher than typical.
Since the incident in Pennsylvania, millions of short-term radon
measurements have been taken in homes in the United States. Outside
the United States, radon measurements are typically performed over the
long term.
In the United States, typical domestic exposures are of approximately
100 Bq/m3 (2.7 pCi/L) indoors. Some level of radon will be found in
all buildings. Radon mostly enters a building directly from the soil
through the lowest level in the building that is in contact with the
ground. High levels of radon in the water supply can also increase
indoor radon air levels. Typical entry points of radon into buildings
are cracks in solid foundations and walls, construction joints, gaps
in suspended floors and around service pipes, cavities inside walls,
and the water supply. Radon concentrations in the same place may
differ by double/half over one hour, and the concentration in one room
of a building may be significantly different from the concentration in
an adjoining room.
The distribution of radon concentrations will generally differ from
room to room, and the readings are averaged according to regulatory
protocols. Indoor radon concentration is usually assumed to follow a
log-normal distribution on a given territory. Thus, the geometric mean
is generally used for estimating the "average" radon concentration in
an area. The mean concentration ranges from less than 10 Bq/m3 to over
100 Bq/m3 in some European countries.
Some of the highest radon hazard in the US is found in Iowa and in the
Appalachian Mountain areas in southeastern Pennsylvania. Iowa has the
highest average radon concentrations in the US due to significant
glaciation that ground the granitic rocks from the Canadian Shield 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. The second highest readings
in Ireland were found in office buildings in the Irish town of Mallow,
County Cork, prompting local fears regarding lung cancer.
Since radon is a colorless, odorless gas, the only way to know how
much is present in the air or water is to perform tests. In the US,
radon test kits are available to the public at retail stores, such as
hardware stores, for home use, and testing is available through
licensed professionals, who are often home inspectors. Efforts to
reduce indoor radon levels are called radon mitigation. In the US, the
EPA recommends all houses be tested for radon. In the UK, under the
Housing Health & Safety Rating System, property owners have an
obligation to evaluate potential risks and hazards to health and
safety in a residential property. Alpha-radiation monitoring over the
long term is a method of testing for radon that is more common in
countries outside the United States.
Industrial production
=======================
Radon is obtained as a by-product of uraniferous ores processing after
transferring into 1% solutions of hydrochloric or hydrobromic acids.
The gas mixture extracted from the solutions contains , , He, Rn, ,
and hydrocarbons. The mixture is purified by passing it over copper at
to remove the and the , and then KOH and Phosphorus pentoxide are
used to remove the acids and moisture by sorption. 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. In 2008
it was priced at almost per milliliter of radium solution (which only
contains about 15 picograms of actual radon at any given moment).
Radon is produced commercially by a solution of radium-226 (half-life
of 1,600 years). Radium-226 decays by alpha-particle emission,
producing radon that collects over samples of radium-226 at a rate of
about 1 mm3/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. Radon sources have also
been produced for scientific purposes through the implantation of
radium-226 into solid stainless steel.
Concentration scale
=====================
Bq/m3 pCi/L Occurrence example
| **1**
~0.027 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, with changes in radon levels being used to
track foreign pollutants.
| **10**
0.27 Mean continental concentration in the open air: 10 to 30 Bq/m3.
An EPA survey of 11,000 homes across the USA found an average of 46
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.
| **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. Allowable
concentrations in uranium mines are approximately 1,220 Bq/m3 (33
pCi/L)
| **10,000**
270 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.
| ****
'Theoretical upper limit:' Radon gas (222Rn) at 100% concentration
(1 atmosphere, 0 °C); 1.538×105 curies/gram; 5.54×1019 Bq/m3.
Hormesis
==========
An early-20th-century form of quackery 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. Radon's
molecule-damaging radioactivity has been used to kill cancerous cells,
but it does not increase the health of healthy cells. The ionizing
radiation causes the formation of free radicals, which results in cell
damage, causing increased rates of illness, including cancer.
Exposure to radon has been suggested to mitigate autoimmune diseases
such as arthritis in a process known as radiation hormesis. As a
result, in the late 20th century and early 21st century, "health
mines" established in Basin, Montana, attracted people seeking relief
from health problems such as arthritis through limited exposure to
radioactive mine water and radon. 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 Jáchymov,
Czech Republic, but even before radon discovery they were used in Bad
Gastein, Austria. Radium-rich springs are also used in traditional
Japanese onsen in Misasa, Tottori Prefecture. Drinking therapy is
applied in Bad Brambach, Germany, and during the early 20th century,
water from springs with radon in them was bottled and sold (this water
had little to no radon in it by the time it got to consumers due to
radon's short half-life). Inhalation therapy is carried out in
Gasteiner-Heilstollen, Austria; Świeradów-Zdrój, Czerniawa-Zdrój,
Kowary, Lądek-Zdrój, Poland; Harghita Băi, Romania; and Boulder,
Montana. In the US and Europe, there are several "radon spas", where
people sit for minutes or hours in a high-radon atmosphere, such as at
Bad Schmiedeberg, Germany.
Nuclear medicine
==================
Radon has been produced commercially for use in radiation therapy, but
for the most part has been replaced by radionuclides made in particle
accelerators and nuclear reactors. Radon has been used in implantable
seeds, made of gold or glass, primarily used to treat cancers, known
as brachytherapy. 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 rays 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).
After 11 half-lives (42 days), radon radioactivity is at 1/2,048 of
its original level. At this stage, the predominant residual activity
of the seed originates from the radon decay product 210Pb, whose
half-life (22.3 years) is 2,000 times that of radon and its
descendants 210Bi and 210Po.
211Rn can be used to generate 211At, which has uses in targeted alpha
therapy.
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. Because of radon's rapid loss to air
and comparatively rapid decay, radon is used in hydrologic research
that studies the interaction between groundwater and streams. Any
significant concentration of radon in a river may be an indicator that
there are local inputs of groundwater.
Radon soil concentration has been used 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 gradients.
Some researchers have investigated changes in groundwater radon
concentrations for earthquake prediction. Increases in radon were
noted before the 1966 Tashkent and 1994 Mindoro earthquakes. 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 groundwater 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. As of 2009, it was under
investigation as a possible earthquake precursor by NASA; further
research into the subject has suggested that abnormalities in
atmospheric radon concentrations can be an indicator of seismic
movement.
Radon is a known pollutant emitted from geothermal power stations
because it is present in the material pumped from deep underground. It
disperses rapidly, and no radiological hazard has been demonstrated in
various investigations. In addition, typical systems re-inject the
material deep underground rather than releasing it at the surface, so
its environmental impact is minimal. In 1989, a survey of the
collective dose received due to radon in geothermal fluids was
measured at 2 man-sieverts per gigawatt-year of electricity produced,
in comparison to the 2.5 man-sieverts per gigawatt-year produced from
(14)C emissions in nuclear power plants.
In the 1940s and 1950s, radon produced from a radium source was used
for industrial radiography. Other X-ray sources such as (60)Co and
(192)Ir became available after World War II and quickly replaced
radium and thus radon for this purpose, being of lower cost and
hazard.
In mines
==========
(222)Rn decay products have been classified by the International
Agency for Research on Cancer as being carcinogenic to humans, and as
a gas that can be inhaled, lung cancer is a particular concern for
people exposed to elevated levels of radon for sustained periods.
During the 1940s and 1950s, 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 Republic, and later among
miners from the Southwestern US and South Australia. Despite these
hazards being known in the early 1950s, this occupational hazard
remained poorly managed in many mines until the 1970s. During this
period, several entrepreneurs opened former uranium mines in the US to
the general public and advertised alleged health benefits from
breathing radon gas underground. Health benefits claimed included
relief from pain, sinus problems, asthma, and arthritis, but the
government banned such advertisements in 1975, and subsequent works
have debated the truth of such claimed health effects, citing the
documented ill effects of radiation on the body.
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.
Residues from processing of uranium ore can also be a source of radon.
Radon resulting from the high radium content in uncovered dumps and
tailing ponds can be easily released into the atmosphere and affect
people living in the vicinity. The release of radon may be mitigated
by covering tailings with soil or clay, though other decay products
may leach into groundwater supplies.
Non-uranium mines may pose higher risks of radon exposure, as workers
are not continuously monitored for radiation, and regulations specific
to uranium mines do not apply. A review of radon level measurements
across non-uranium mines found the highest concentrations of radon in
non-metal mines, such as phosphorus and salt mines. However, older or
abandoned uranium mines without ventilation may still have extremely
high radon levels.
In addition to lung cancer, researchers have theorized a possible
increased risk of leukemia due to radon exposure. Empirical support
from studies of the general population is inconsistent; a study of
uranium miners found a correlation between radon exposure and chronic
lymphocytic leukemia, and current research supports a link between
indoor radon exposure and poor health outcomes (i.e., an increased
risk of lung cancer or childhood leukemia). Legal actions taken by
those involved in nuclear industries, including miners, millers,
transporters, nuclear site workers, and their respective unions have
resulted in compensation for those affected by radon and radiation
exposure under programs such as the compensation scheme for
radiation-linked diseases (in the United Kingdom) and the Radiation
Exposure Compensation Act (in the United States).
Domestic-level exposure
=========================
Radon has been considered the second leading cause of lung cancer in
the United States and leading environmental cause of cancer mortality
by the EPA, with the first one being smoking. Others have reached
similar conclusions for the United Kingdom and France. Radon exposure
in buildings may arise from subsurface rock formations and certain
building materials (e.g., some granites). The greatest risk of radon
exposure arises in buildings that are airtight, insufficiently
ventilated, and have foundation leaks that allow air from the soil
into basements and dwelling rooms. In some regions, such as Niška
Banja, Serbia and Ullensvang, Norway, outdoor radon concentrations may
be exceptionally high, though compared to indoors, where people spend
more time and air is not dispersed and exchanged as often, outdoor
exposure to radon is not considered a significant health risk.
Radon exposure (mostly radon daughters) has been linked to lung cancer
in case-control studies performed in the US, Europe and China. There
are approximately 21,000 deaths per year in the US (0.0063% of a
population of 333 million) due to radon-induced lung cancers. In
Europe, 2% of all cancers have been attributed to radon; in Slovenia
in particular, a country with a high concentration of radon, about 120
people (0.0057% of a population of 2.11 million) die yearly because of
radon. One of the most comprehensive radon studies performed in the US
by epidemiologist R. William Field 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. However, the conclusion that
exposure to low levels of radon leads to elevated risk of lung cancer
has been disputed, and analyses of the literature point towards
elevated risk only when radon accumulates indoors and at levels above
100 Bq/m3.
Thoron (220Rn) is less studied than Rn in regards to domestic exposure
due to its shorter half-life. However, it has been measured at
comparatively high concentrations in buildings with earthen
architecture, such as traditional half-timbered houses and modern
houses with clay wall finishes, and in regions with thorium- and
monazite-rich soil and sand. Thoron is a minor contributor to the
overall radiation dose received due to indoor radon exposure, and can
interfere with Rn measurements when not taken into account.
Action and reference level
============================
WHO presented in 2009 a recommended reference level (the national
reference level), 100 Bq/m3, for radon in dwellings. The
recommendation also says that where this is not possible, 300 Bq/m3
should be selected as the highest level. A national reference level
should not be a limit, but should represent the maximum acceptable
annual average radon concentration in a dwelling.
The actionable concentration of radon in a home varies depending on
the organization doing the recommendation, for example, the EPA
encourages that action be taken at concentrations as low as 74 Bq/m3
(2 pCi/L), and the European Union recommends action be taken when
concentrations reach 400 Bq/m3 (11 pCi/L) for old houses and 200 Bq/m3
(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/m3 whilst
retaining an "Action Level" of 200 Bq/m3. Similar levels (as in the
UK) are published by Norwegian Radiation and Nuclear Safety Authority
(DSA) with the maximum limit for schools, kindergartens, and new
dwellings set at 200 Bq/m3, where 100 Bq/m3 is set as the action
level.
Inhalation and smoking
========================
Results from epidemiological studies indicate that the risk of lung
cancer increases with exposure to residential radon. A well known
example of source of error is smoking, the main risk factor for lung
cancer. In the US, cigarette smoking is estimated to cause 80% to 90%
of all lung cancers.
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. It cannot be excluded that the risk of non-smokers should be
primarily explained by an effect of radon.
Radon, like other known or suspected external risk factors for lung
cancer, is a threat for smokers and former smokers. This was
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."
According to the European pooling study, there is a difference in risk
for the histological subtypes of lung cancer and radon exposure.
Small-cell lung carcinoma, which has a high correlation with smoking,
has a higher risk after radon exposure. For other histological
subtypes such as adenocarcinoma, the type that primarily affects
non-smokers, the risk from radon appears to be lower.
A study of radiation from post-mastectomy 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, the linear
no-threshold model, which routinely has been used.
A study from 2001, which included 436 non-smokers with lung cancer and
a control group of 1649 non-smokers without lung cancer, showed that
exposure to radon increased the risk of lung cancer in non-smokers.
The group that had been exposed to tobacco smoke in the home appeared
to have a much higher risk, while those who were not exposed to
passive smoking did not show any increased risk with increasing radon
exposure.
Absorption and ingestion from water
=====================================
The effects of radon if ingested are unknown, although studies have
found that its biological half-life ranges from 30 to 70 minutes, with
90% removal at 100 minutes. In 1999, the US National Research Council
investigated the issue of radon in drinking water. The risk associated
with ingestion was considered almost negligible; Water from
underground sources may contain significant amounts of radon depending
on the surrounding rock and soil conditions, whereas surface sources
generally do not. 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 in domestic living from
water occurred during showering. Water with a radon concentration of
104 pCi/L can increase the indoor airborne radon concentration by 1
pCi/L under normal conditions. However, the concentration of radon
released from contaminated groundwater to the air has been measured at
5 orders of magnitude less than the original concentration in water.
Ocean surface concentrations of radon exchange within the atmosphere,
causing 222Rn to increase through the air-sea interface. Although
areas tested were very shallow, additional measurements in a wide
variety of coastal regimes should help define the nature of 222Rn
observed.
Testing and mitigation
========================
A radon test kit
There are relatively simple tests for radon gas. In some countries
these tests are methodically done in areas of known systematic
hazards. Radon detection devices are commercially available. Digital
radon detectors provide ongoing measurements giving both daily,
weekly, short-term and long-term average readouts via a digital
display. Short-term radon test devices used for initial screening
purposes are inexpensive, in some cases free. There are important
protocols for taking short-term radon tests and it is imperative that
they be strictly followed. The kit includes a collector that the user
hangs in the lowest habitable floor of the house for two to seven
days. The user then sends the collector to a laboratory for analysis.
Long term kits, taking collections for up to one year or more, are
also available. An open-land test kit can test radon emissions from
the land before construction begins. Radon concentrations can vary
daily, and accurate radon exposure estimates require long-term average
radon measurements in the spaces where an individual spends a
significant amount of time.
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. The EPA has
advised purchasers of real estate 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/m3,
ranging from 1 to 100 Bq/m3).
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, 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. "[The] 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 the "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".
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 crawl spaces, the EPA states that "[a]n
effective method to reduce radon levels in crawl space 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 crawl space homes."
See also
======================================================================
* International Radon Project
* Lucas cell
* Pleochroic halo (aka radiohalo)
* Radiation Exposure Compensation Act
External links
======================================================================
* [
https://www.epa.gov/radon Radon] at the United States Environmental
Protection Agency
* [
https://radonmap.com/ Global Radon Map]
* [
http://www.periodicvideos.com/videos/086.htm Radon] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[
https://web.archive.org/web/20090713013203/http://www.lungne.org/site/c.ieJPISOvErH/b.4135285/k.B764/Radon.htm
Radon and Lung Health from the American Lung Association]
*
[
https://web.archive.org/web/20111002050058/http://www.usinspect.com/resources-for-you/house-facts/environmental-concerns-home/radon/geology-radon
The Geology of Radon], James K. Otton, Linda C.S. Gundersen, and R.
Randall Schumann
* [
https://www.nachi.org/radon.htm Home Buyer's and Seller's Guide to
Radon] An article by the International Association of Certified Home
Inspectors (InterNACHI)
*
[
https://web.archive.org/web/20010718212817/http://www.atsdr.cdc.gov/toxprofiles/tp145.html
Toxicological Profile for Radon], Draft for Public Comment, Agency for
Toxic Substances and Disease Registry, September 2008
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Radon
