Wikipedia

Radon

This article is about the chemical element. For other uses, seeRadon (disambiguation).Not to be confused withRadom,Radium,orRodan.

Radonis achemical element;it hassymbolRnandatomic number86. It is aradioactivenoble gasand is colorless and odorless. Of the three naturally occurring radon isotopes, onlyradon-222has a sufficiently longhalf-life(3.825 days) for it to be released from the soil and rock where it is generated. Radon isotopes are the immediatedecay productsofradiumisotopes. The instability of radon-222, 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 chain ofuranium-238,and that ofthorium-232,each of which is an extremely abundant radioactive nuclide with a half-life of several billion years. The decay of radon produces many other short-livednuclides,known as "radon daughters", ending at stable isotopes oflead.[3]Radon-222occurs in significant quantities as a step in the normal radioactivedecay chainof uranium-238, also known as theuranium series,which slowly decays into a variety of radioactive nuclides and eventually decays intolead-206,which is stable.Radon-220occurs in minute quantities as an intermediate step in the decay chain of thorium-232, also known as thethorium series,which eventually decays intolead-208,which is stable.

Radon,86Rn
Radon
Pronunciation/ˈreɪdɒn/​(RAY-don)
Appearancecolorless gas
Mass number[222]
Radon in theperiodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Xe
↑
Rn
↓
Og
astatine←radon→francium
Atomic number(Z)86
Groupgroup 18 (noble gases)
Periodperiod 6
Blockp-block
Electron configuration[Xe] 4f145d106s26p6
Electrons per shell2, 8, 18, 32, 18, 8
Physical properties
PhaseatSTPgas
Melting point202K​(−71 °C, ​−96 °F)
Boiling point211.5 K ​(−61.7 °C, ​−79.1 °F)
Density(at STP)9.73 g/L
when liquid (atb.p.)4.4 g/cm3
Critical point377 K, 6.28 MPa[1]
Heat of fusion3.247kJ/mol
Heat of vaporization18.10 kJ/mol
Molar heat capacity5R/2 = 20.786 J/(mol·K)
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
atT(K) 110 121 134 152 176 211
Atomic properties
Oxidation states0,+2, +6
ElectronegativityPauling scale: 2.2
Ionization energies
  • 1st: 1037 kJ/mol
Covalent radius150pm
Van der Waals radius220 pm
Color lines in a spectral range
Spectral linesof radon
Other properties
Natural occurrencefrom decay
Crystal structure​face-centered cubic(fcc)
Face-centered cubic crystal structure for radon

(predicted)
Thermal conductivity3.61×10−3W/(m⋅K)
Magnetic orderingnon-magnetic
CAS Number10043-92-2
History
DiscoveryErnest RutherfordandRobert B. Owens(1899)
First isolationWilliam RamsayandRobert Whytlaw-Gray(1910)
Isotopes of radon
Main isotopes[2] Decay
abun­dance half-life(t1/2) mode pro­duct
210Rn synth 2.4 h α 206Po
211Rn synth 14.6 h ε 211At
α 207Po
222Rn trace 3.8235 d α 218Po
224Rn synth 1.8 h β− 224Fr
Category: Radon
|references

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.[4]Radon and its daughters are, taken together, often the single largest contributor to an individual'sbackground radiationdose, but due to local differences in geology,[5]the level of exposure to radon gas differs from place to place. A common source is uranium-containing minerals in the ground, and therefore it accumulates in subterranean areas such as basements. Radon can also occur in some ground water likespringwaters and hot springs.[6]Radon trapped inpermafrostmay be released byclimate changeinducedthawing of permafrosts.[7]It is possible to test for radon in buildings, and to use techniques such as sub-slab depressurization formitigation.[8][9]

Epidemiologicalstudies have shown a clear link between breathing high concentrations of radon and incidence oflung cancer.Radon is a contaminant that affectsindoor air qualityworldwide. According to theUnited 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.[10]Significant uncertainties exist for the health effects of low-dose exposures.[11]

Characteristics

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Emission spectrumof radon, photographed byErnest Rutherfordin 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radium emanation (radon), while the outer two are ofhelium(added to calibrate the wavelengths).

Physical properties

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Radon is a colorless, odorless, and tasteless[12]gas and therefore is not detectable by human senses alone. Atstandard temperature and pressure,it forms amonatomic gaswith a density of 9.73 kg/m3,about 8 times the density of theEarth's atmosphereat sea level, 1.217 kg/m3.[13]It is one of the densest gases at room temperature (a few are denser, e.g.CF3(CF2)2CF3andWF6) and is the densest of the noble gases. Although colorless at standard temperature and pressure, when cooled below itsfreezing pointof 202 K (−71 °C; −96 °F), it emits a brilliantradioluminescencethat turns from yellow to orange-red as the temperature lowers.[14]Uponcondensation,it glows because of the intense radiation it produces.[15]It is sparinglysolublein water, but more soluble than lighter noble gases. It is appreciably more soluble inorganic liquidsthan in water. Its solubility equation is as follows,[16][17][18]

whereis the molar fraction of radon,is the absolute temperature, andandare solvent constants.

Chemical properties

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Radon is a member of the zero-valenceelements that are called noble gases, and is chemically not veryreactive.The 3.8-day half-life of radon-222 makes it useful in physical sciences as a naturaltracer.Because radon is a gas at standard conditions, unlike its decay-chain parents, it can readily be extracted from them for research.[19]

It isinertto most common chemical reactions, such ascombustion,because the outervalence shellcontains eightelectrons.This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[20]Itsfirst ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol.[21]In accordance withperiodic trends,radon has a lowerelectronegativitythan the element one period before it,xenon,and is therefore more reactive. Early studies concluded that the stability of radonhydrateshould be of the same order as that of the hydrates ofchlorine(Cl
2) orsulfur dioxide(SO
2), and significantly higher than the stability of the hydrate ofhydrogen sulfide(H
2S).[22]

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 eitherfluoridesoroxides.Radon can beoxidizedby powerful oxidizing agents such asfluorine,thus formingradon difluoride(RnF
2).[23][24]It decomposes back to its elements at a temperature of above 523 K (250 °C; 482 °F), and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements byhydrogengas.[25]It has a lowvolatilityand was thought to beRnF
2.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–Fbond distanceof 2.08ångströms(Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpartxenon difluoride(XeF
2).[26]Theoctahedral moleculeRnF
6was predicted to have an even lowerenthalpy of formationthan the difluoride.[27]The [RnF]+ionis believed to form by the following reaction:[28]

Rn (g) + 2[O
2]+
[SbF
6]−
(s) →[RnF]+
[Sb
2F
11]−
(s) + 2O
2(g)

For this reason,antimony pentafluoridetogether withchlorine trifluorideandN
2F
2Sb
2F
11have been considered for radon gas removal inuranium minesdue to the formation of radon–fluorine compounds.[19]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 duringirradiation.[25]Additionally, salts of the [RnF]+cation with the anionsSbF−
6,TaF−
6,andBiF−
6are known.[25]Radon is also oxidised bydioxygen difluoridetoRnF
2at 173 K (−100 °C; −148 °F).[25]

Radon oxides are among the few other reportedcompounds of radon;[29]only the trioxide (RnO
3) has been confirmed.[30]The higher fluoridesRnF
4andRnF
6have been claimed[30]and are calculated to be stable,[31]but their identification is unclear.[30]They may have been observed in experiments where unknown radon-containing products distilled together withxenon hexafluoride:these may have beenRnF
4,RnF
6,or both.[25]Trace-scale heating of radon with xenon, fluorine,bromine pentafluoride,and eithersodium fluorideornickel fluoridewas claimed to produce a higher fluoride as well whichhydrolysedto formRnO
3.While it has been suggested that these claims were really due to radon precipitating out as the solid complex [RnF]+
2[NiF6]2−,the fact that radoncoprecipitatesfromaqueous solutionwithCsXeO
3Fhas been taken as confirmation thatRnO
3was 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.Electromigrationstudies also suggest the presence of cationic [HRnO3]+and anionic [HRnO4]−forms of radon inweakly acidicaqueous solution (pH > 5), the procedure having previously been validated by examination of the homologous xenon trioxide.[30]

Thedecay techniquehas also been used. Avrorin et al. reported in 1982 that212Frcompounds 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.[32]

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 ofradon difluoride(RnF
2) and the high positive charge on radon in RnF+;spatial separation ofRnF
2molecules may be necessary to clearly identify higher fluorides of radon, of whichRnF
4is expected to be more stable thanRnF
6due tospin–orbitsplitting of the 6p shell of radon (RnIVwould have a closed-shell 6s2
6p2
1/2configuration). Therefore, whileRnF
4should have a similar stability toxenon tetrafluoride(XeF
4),RnF
6would likely be much less stable thanxenon hexafluoride(XeF
6):radon hexafluoridewould also probably be aregular octahedralmolecule, unlike the distorted octahedral structure ofXeF
6,because of theinert pair effect.[33][34]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.[34]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 RnCl2and RnCl4,but none of these have yet been found.[25]

Radoncarbonyl(RnCO) has been predicted to be stable and to have alinear molecular geometry.[35]The moleculesRn
2and RnXe were found to be significantly stabilized byspin-orbit coupling.[36]Radon caged inside afullerenehas been proposed as a drug fortumors.[37][38]Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist;RnF
8should be highly unstable chemically (XeF8is thermodynamically unstable). It is predicted that the most stable Rn(VIII) compound would be barium perradonate (Ba2RnO6), analogous to bariumperxenate.[31]The instability of Rn(VIII) is due to therelativisticstabilization of the 6s shell, also known as theinert pair effect.[31]

Radon reacts with the liquidhalogen fluoridesClF,ClF
3,ClF
5,BrF
3,BrF
5,andIF
7to formRnF
2.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 complexesRnF−
3andRnF2−
4,paralleling the chemistry ofberyllium(II) andaluminium(III).[25]Thestandard electrode potentialof the Rn2+/Rn couple has been estimated as +2.0 V,[39]although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.[25]

Isotopes

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Main article:Isotopes of radon

Radon has nostable isotopes.Thirty-nine radioactive isotopes have been characterized, withmass numbersranging from 193 to 231.[40][41]Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is222Rn (half-life 3.82 days), which is a decay product of226Ra,the latter being itself a decay product of238U.[42]A trace amount of the (highly unstable) isotope218Rn (half-life about 35milliseconds) is also among the daughters of222Rn. The isotope216Rn would be produced by thedouble beta decayof natural216Po; while energetically possible, this process has however never been seen.[43]

Three other radon isotopes have a half-life of over an hour:211Rn (about 15 hours),210Rn (2.4 hours) and224Rn (about 1.8 hours). However, none of these three occur naturally.220Rn, also called thoron, is a natural decay product of the most stable thorium isotope (232Th). It has a half-life of 55.6 seconds and also emitsalpha radiation.Similarly,219Rn is derived from the most stable isotope ofactinium(227Ac)—named "actinon" —and is an alpha emitter with a half-life of 3.96 seconds.[40]No radon isotopes occur significantly in theneptunium(237Np)decay series,though trace amounts of the isotopes221Rn (26 minutes) and217Rn (0.5 millisecond) are produced in minorbranches.

The radium or uranium series

Daughters

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Main article:Decay chain § Uranium series

222Rn belongs to the radium and uranium-238 decay chain, and has a half-life of 3.8235 days. Its first four products (excluding marginaldecay schemes) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence:[40]

  • 222Rn, 3.82 days,alpha decayingto...
  • 218Po,3.10 minutes, alpha decaying to...
  • 214Pb,26.8 minutes,beta decayingto...
  • 214Bi,19.9 minutes, beta decaying to...
  • 214Po, 0.1643 ms, alpha decaying to...
  • 210Pb, which has a much longer half-life of 22.3 years, beta decaying to...
  • 210Bi, 5.013 days, beta decaying to...
  • 210Po, 138.376 days, alpha decaying to...
  • 206Pb, stable.

The radon equilibrium factor[44]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 0.01working 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.[45]210Pb takes much longer (decades) to come in equilibrium with radon, 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.

Because of theirelectrostatic 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.[46]

History and etymology

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Apparatus used by Ramsay and Whytlaw-Gray to isolate radon.Mis a [[Capillary tube]], where approximately 0.1 mm3were isolated. Radon mixed with hydrogen entered the evacuated system through siphonA;mercury is shown in black.

Radon was discovered in 1899 byErnest RutherfordandRobert B. OwensatMcGill UniversityinMontreal.[47]It was the fifth radioactive element to be discovered, after uranium, thorium, radium, and polonium.[48][49][50][51]In 1899,PierreandMarie Curieobserved that the gas emitted by radium remained radioactive for a month.[52]Later that year, Rutherford and Owens noticed variations when trying to measure radiation from thorium oxide.[47]Rutherford noticed that the compounds of thorium continuously emit a radioactive gas that remains radioactive for several minutes, and called this gas "emanation" (fromLatin:emanare,to flow out, andemanatio,expiration),[53]and later "thorium emanation" ( "Th Em" ). In 1900,Friedrich Ernst Dornreported some experiments in which he noticed that radium compounds emanate a radioactive gas he named "radium emanation" ( "Ra Em" ).[54][55]In 1901, Rutherford andHarriet Brooksdemonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element.[56]In 1903, similar emanations were observed from actinium byAndré-Louis Debierne,[57][58]and were called "actinium emanation" ( "Ac Em" ).

Several shortened names were soon suggested for the three emanations:exradio,exthorio,andexactinioin 1904;[59]radon(Ro),thoron(To), andaktonoracton(Ao) in 1918;[60]radeon,thoreon,andactineonin 1919,[61]and eventuallyradon,thoron,andactinonin 1920.[62](The name radon is not related to that of the Austrian mathematicianJohann Radon.) The likeness of thespectraof these three gases with those of argon, krypton, and xenon, and their observed chemical inertia led SirWilliam Ramsayto suggest in 1904 that the "emanations" might contain a new element of the noble-gas family.[59]

In 1909, Ramsay andRobert Whytlaw-Grayisolated radon and determined itsmelting temperatureand approximatedensity.In 1910, they determined that it was the heaviest known gas.[63]They wrote that "L'expressionl'émanation du radiumest fort incommode"(" the expression 'radium emanation' is very awkward ") and suggested the new name niton (Nt) (fromLatin:nitens,shining) to emphasize the radioluminescence property,[64]and in 1912 it was accepted by theInternational Commission for Atomic Weights.In 1923, the International Committee for Chemical Elements andInternational 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 renamed220Rnand219Rn. 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.[65]

As late as the 1960s, the element was also referred to simply asemanation.[66]The first synthesized compound of radon, radon fluoride, was obtained in 1962.[67]Even today, the wordradonmay refer to either the element or its isotope222Rn, withthoronremaining in use as a short name for220Rn to stem this ambiguity. The nameactinonfor219Rn is rarely encountered today, probably due to the short half-life of that isotope.[65]

The danger of high exposure to radon in mines, where exposures can reach 1,000,000Bq/m3,has long been known. In 1530,Paracelsusdescribed a wasting disease of miners, themala metallorum,andGeorg Agricolarecommended ventilation in mines to avoid this mountain sickness (Bergsucht).[68][69]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 theJoachimsthalregion ofBohemia.[70]In the US, studies and mitigation only followed decades of health effects on uranium miners of theSouthwestern USemployed during the earlyCold War;standards were not implemented until 1971.[71]

In the early 20th century in the US, gold contaminated with the radon daughter210Pb entered the jewelry industry. This was from goldbrachytherapyseeds that had held222Rn, which were melted down after the radon had decayed.[72][73]

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.[74]

Occurrence

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See also:Radium and radon in the environment

Concentration units

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210Pb is formed from the decay of222Rn. Here is a typical deposition rate of210Pb as observed in Japan as a function of time, due to variations in radon concentration.[75]

All discussions of radon concentrations in the environment refer to222Rn. While the average rate of production of220Rn (from the thorium decay series) is about the same as that of222Rn, the amount of220Rn in the environment is much less than that of222Rn because of the short half-life of220Rn (55 seconds, versus 3.8 days respectively).[3]

Radon concentration in the atmosphere is usually measured inbecquerelper cubic meter (Bq/m3), theSI derived unit.Another unit of measurement common in the US ispicocuriesper liter (pCi/L); 1 pCi/L = 37 Bq/m3.[45]Typical domestic exposures average about 48 Bq/m3indoors, though this varies widely, and 15 Bq/m3outdoors.[76]

In the mining industry, the exposure is traditionally measured inworking level(WL), and the cumulative exposure inworking level month(WLM); 1 WL equals any combination of short-lived222Rn daughters (218Po,214Pb,214Bi, and214Po) in 1 liter of air that releases 1.3 × 105MeV of potential alpha energy;[45]1 WL is equivalent to 2.08 × 10−5joules per cubic meter of air (J/m3).[3]The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m3). One WLM is equivalent to 3.6 × 10−3J·h/m3.An exposure to 1 WL for 1 working-month (170 hours) equals 1 WLM cumulative exposure. A cumulative exposure of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m3.[77]

222Rn decays to210Pb and other radioisotopes. The levels of210Pb 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 0.17picogramper cubic meter (pg/m3). The average concentration of radon in the atmosphere is about 6×10−18molar percent,or about 150 atoms in each milliliter of air.[78]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.[79]

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.[5][80]Every square mile of surface soil, to a depth of 6 inches (2.6 km2to a depth of 15 cm), contains about 1 gram of radium, which releases radon in small amounts to the atmosphere.[3]It is estimated that 2.4 billion curies (90 EBq) of radon are released from soil annually worldwide.[81]This is equivalent to some 15.3 kilograms (34 lb).

Radon concentration can differ widely from place to place. In the open air, it ranges from 1 to 100 Bq/m3,even less (0.1 Bq/m3) above the ocean. In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000 Bq/m3.[82]

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 pCi222Rn per liter of air; 20.2 kBq/m3,measured from 1976 to 1985).[3] The concentration in the air at the (unventilated)GasteinHealing Gallery averages 43 kBq/m3(1.2 nCi/L) with maximal value of 160 kBq/m3(4.3 nCi/L).[83]

Radon mostly appears with the radium/uraniumseries (decay chain) (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 thorium are found, and particularly in regions with soils containinggraniteorshale,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 for222Rn), 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 ameteorological inversionand little wind.[84]

High concentrations of radon can be found in some spring waters and hot springs.[85]The towns ofBoulder, 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/m3).[86]The activity of radon mineral water reaches 2 MBq/m3in Merano and 4 MBq/m3in Lurisia (Italy).[83]

Natural radon concentrations in theEarth's atmosphereare so low that radon-rich water in contact with the atmosphere will continually lose radon byvolatilization.Hence,ground waterhas a higher concentration of222Rn thansurface water,because radon is continuously produced by radioactive decay of226Ra present in rocks. Likewise, thesaturated zoneof a soil frequently has a higher radon content than theunsaturated zonebecause ofdiffusionallosses to the atmosphere.[87][88]

In 1971,Apollo 15passed 110 km (68 mi) above theAristarchus plateauon theMoon,and detected a significant rise inalpha particlesthought to be caused by the decay of222Rn. The presence of222Rn has been inferred later from data obtained from theLunar Prospectoralpha particle spectrometer.[89]

Radon is found in somepetroleum.Because radon has a similar pressure and temperature curve topropane,andoil refineriesseparate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can becomecontaminatedbecause of decaying radon and its products.[90]

Residues from the petroleum andnatural gasindustry often contain radium and its daughters. The sulfate scale from anoil wellcan 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.[90]

Accumulation in buildings

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Typicallog-normalradon distribution in dwellings
Predicted fraction of U.S. homes having concentrations of radon exceeding the EPA's recommended action level of 4 pCi/L

High concentrations of radon in homes were discovered by chance in 1985 after the stringent radiation testing conducted at the newLimerick Generating Stationnuclear power plant revealed thatStanley Watras,a construction engineer at the plant, was contaminated by radioactive substances even though the reactor had never been fueled.[91]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.[12]Radon concentrations in the same place may differ by double/half over one hour. Also, the concentration in one room of a building may be significantly different from the concentration in an adjoining room.[3]The soil characteristics of the dwellings are the most important source of radon for the ground floor and higher concentration of indoor radon observed on lower floors. Most of the high radon concentrations have been reported from places nearfault zones;hence the existence of a relation between the exhalation rate from faults and indoor radon concentrations is obvious.[citation needed]

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 alog-normal distributionon a given territory.[92]Thus, thegeometric meanis generally used for estimating the "average" radon concentration in an area.[93]

The mean concentration ranges from less than 10 Bq/m3to over 100 Bq/m3in some European countries.[94]

Some of the highest radon hazard in the US is found inIowaand in theAppalachian Mountainareas in southeastern Pennsylvania.[95]Iowa has the highest average radon concentrations in the US due to significantglaciationthat ground the granitic rocks from theCanadian Shieldand deposited it as soils making up the rich Iowa farmland.[96]Many cities within the state, such asIowa 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 ofMallow, County Cork,prompting local fears regarding lung cancer.[97]

In a few places,uranium tailingshave been used forlandfillsand were subsequently built upon, resulting in possible increased exposure to radon.[3]

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 oftenhome inspectors.Efforts to reduce indoor radon levels are calledradon mitigation.In the US, the EPA recommends all houses be tested for radon. In the UK under the Housing Health & Safety Rating System (HHSRS) property owners have an obligation to evaluate potential risks and hazards to health and safety in a residential property.[98]

Industrial production

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Radon is obtained as a by-product ofuraniferous oresprocessing after transferring into 1% solutions ofhydrochloricorhydrobromic acids.The gas mixture extracted from the solutions containsH
2,O
2,He, Rn,CO
2,H
2Oandhydrocarbons.The mixture is purified by passing it over copper at 993 K (720 °C; 1,328 °F) to remove theH
2and theO
2,and thenKOHandP
2O
5are used to remove the acids and moisture bysorption.Radon is condensed by liquid nitrogen and purified from residue gases bysublimation.[99]

Radon commercialization is regulated, but it is available in small quantities for the calibration of222Rn measurement systems, at a price, in 2008, of almostUS$6,000(equivalent to $8,491 in 2023) per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment).[100] Radon is produced 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). Gaseous222Rn (half-life of about four days) escapes from the capsule throughdiffusion.[101]

Concentration scale

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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 inAntarcticacan 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/m3for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (150 Bq/m3), then no action is necessary. A cumulated exposure of 230 Bq/m3of 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. Allowable concentrations in uranium mines are approximately 1,220 Bq/m3(33 pCi/L)[102]
10,000 270 The concentration in the air at the (unventilated)Gastein Healing Galleryaverages 43 kBq/m3(about 1.2 nCi/L) with maximal value of 160 kBq/m3(about 4.3 nCi/L).[83]
100,000 ~2700 About 100,000 Bq/m3(2.7 nCi/L) was measured in Stanley Watras's basement.[103][104]
1,000,000 27000 Concentrations reaching 1,000,000 Bq/m3can be found in unventilated uranium mines.
~5.54 × 1019 ~1.5 × 1018 Theoretical upper limit:Radon gas (222Rn) at 100% concentration (1 atmosphere, 0 °C); 1.538×105curies/gram;[105]5.54×1019Bq/m3.

Applications

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Medical

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Main article:Radioactive quackery

An early-20th-century form ofquackerywas the treatment of maladies in aradiotorium.[106]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,[107]but it does not increase the health of healthy cells.[citation needed]The ionizing radiation causes the formation offree radicals,which results incell damage,causing increased rates of illness, includingcancer.

Exposure to radon has been suggested to mitigateautoimmune diseasessuch asarthritisin a process known asradiation hormesis.[108][109]As a result, in the late 20th century and early 21st century, "health mines" established inBasin, 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.[110]

Radioactive water baths have been applied since 1906 inJáchymov,Czech Republic, but even before radon discovery they were used inBad Gastein,Austria. Radium-rich springs are also used in traditional JapaneseonseninMisasa,Tottori Prefecture.Drinking therapy is applied inBad 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).[111]Inhalation therapy is carried out inGasteiner-Heilstollen,Austria;Świeradów-Zdrój,Czerniawa-Zdrój,Kowary,Lądek-Zdrój,Poland;Harghita Băi,Romania; andBoulder, 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 atBad Schmiedeberg,Germany.[109][112]

Radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made inparticle acceleratorsandnuclear reactors.Radon has been used in implantable seeds, made of gold or glass, primarily used to treat cancers, known asbrachytherapy. 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 thegamma raysto escape (which kill the diseased tissue). The activities might range from 0.05 to 5 millicuries per seed (2 to 200 MBq).[107]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 product210Pb, whose half-life (22.3 years) is 2,000 times that of radon and its descendants210Bi and210Po.

Scientific

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Radon emanation from the soil varies with soil type and with surface uranium content, so outdoor radon concentrations can be used to trackair massesto a limited degree. This fact has been put to use by some atmospheric scientists (Radon storm). Because of radon's rapid loss to air and comparatively rapid decay, radon is used inhydrologicresearch that studies the interaction between groundwater andstreams.Any significant concentration of radon in a stream is a good indicator that there are local inputs of groundwater.

Radon soil-concentration has been used in an experimental way to map buried close-subsurface geologicalfaultsbecause concentrations are generally higher over the faults.[113]Similarly, it has found some limited use in prospecting forgeothermal gradients.[114]

Some researchers have investigated changes in groundwater radon concentrations forearthquake prediction.[115][116][117]Increases in radon were noted before the1966 Tashkent[118]and1994 Mindoro[117]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.[119]As of 2009, it was under investigation as a possible precursor byNASA.[120]

Radon is a known pollutant emitted fromgeothermal power stationsbecause 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.[121]However, similar things can be said about trivial releases from operatingnuclear power plants.[citation needed]

In the 1940s and 1950s, radon was used forindustrial radiography.[122][123]Other X-ray sources, which became available after World War II, quickly replaced radon for this application, as they were lower in cost and had less hazard ofalpha radiation.

Health risks

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Main article:Health effects of radon

In mines

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Radon-222 decay products have been classified by theInternational Agency for Research on Canceras beingcarcinogenicto humans,[124]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,[125]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[126][127][128]andSouth Australia.[129]Despite these hazards being known in the early 1950s,[130]thisoccupational hazardremained 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,[131][132]but these were proven to be false[citation needed]and the government banned such advertisements in 1975.[133]

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.[134]As the relative risk for miners has decreased, so has the ability to detect excess risks among that population.[135]

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.[136]

In addition to lung cancer, researchers have theorized a possible increased risk ofleukemiadue to radon exposure. Empirical support from studies of the general population is inconsistent, and a study of uranium miners found a correlation between radon exposure andchronic lymphocytic leukemia.[137]

Miners (as well as milling and ore transportation workers) who worked in the uranium industry in the US between the 1940s and 1971 may be eligible for compensation under theRadiation Exposure Compensation Act(RECA). Surviving relatives may also apply in cases where the formerly employed person is deceased.

Not only uranium mines are affected by elevated levels of radon. Coal mines in particular are affected as well since coal may contain more uranium and thorium than commercially operational uranium mines.

Domestic-level exposure

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Prolonged exposure to higher concentrations of radon has been linked to an increase in lung cancer.[84]Since 1999, there have been investigations worldwide on how radon concentrations are estimated. In the United States alone averages have been recorded to be at least 40 Bq/m3.Steck et al. did a study on the variation between indoor and outdoor radon in Iowa and Minnesota. Higher radiation was found in a populated region rather than in unpopulated regions in Central America as a whole. In some northwestern Iowa and southwestern Minnesota counties, the outdoor radon concentrations exceed the national average indoor radon concentrations.[84]Despite the above average, both Minnesota and Iowa's numbers were exceptionally close, regardless of the distance. Accurate studies of radon exposure are needed to further understand the problems radon exposure can have on a community. Current research supports a link between radon exposure and poor health outcomes (i.e., an increased risk of lung cancer), but further research could support stricter radon limits both inside and outside of housing units.[84]

Radon exposure (mostly radon daughters) has been linked to lung cancer in numerous 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.[10][138]In Slovenia, a country with a high concentration of radon, about 120 people (0.0057% of a population of 2.11 million) yearly die because of radon.[139][140]One of the most comprehensive radon studies performed in the US by epidemiologistR. William Fieldand 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.[141]However, the discussion about the opposite results is still continuing,[142][143][144]especially a 2008 retrospective case-control study of lung cancer risk which showed substantial cancer rate reduction for radon concentrations between 50 and 123 Bq/m3.[145]

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.[134]Because of the difficulties of measuring the risk of radon relative to 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 EPA, with the first one beingsmoking.[146]Others have reached similar conclusions for the United Kingdom[134]and France.[147]Radon exposure in homes and offices may arise from certain subsurface rock formations, and also from 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.

Thoron (220Rn) was measured at comparatively high concentrations in buildings with earthen architecture, such as traditionalhalf-timbered housesand modern houses withclaywall finishes. Because of its short half-life, thoron occurs only close to the earthen surfaces as its sources whereas its progeny can be found throughout the indoor air of such buildings. Therefore, radiation exposure occurs at any location within such houses. In different dwellings with earthen architecture in Germany, a study found annual internal radiation doses due to the inhalation of thoron and its progeny of up to several milli-Sieverts.[148]

Action and reference level

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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/m3should 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.[149]

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),[76]and theEuropean Unionrecommends 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.[150]On 8 July 2010, the UK's Health Protection Agency issued new advice setting a "Target Level" of 100 Bq/m3whilst retaining an "Action Level" of 200 Bq/m3.[151]Similar levels (as in UK) are published by Norwegian Radiation and Nuclear Safety Authority (DSA)[152]with the maximum limit for schools, kindergartens, and new dwellings set at 200 Bq/m3,where 100 Bq/m3is set as the action level.[153]In all new housings preventative measures should be taken against radon accumulation.

Inhalation and smoking

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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.[154]

According to the EPA, the risk of lung cancer for smokers is significant due tosynergisticeffects 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.[10]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.[155]A commentary[155]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 thehistologicalsubtypes of lung cancer and radon exposure.Small-cell lung carcinoma,which has a high correlation with smoking, have a higher risk after radon exposure. For other histological subtypes such asadenocarcinoma,the type that primarily affects non-smokers, the risk from radon appears to be lower.[155][156]

A study of radiation from post-mastectomyradiotherapyshows that the simple models previously used to assess the combined and separate risks from radiation and smoking need to be developed.[157]This is also supported by new discussion about the calculation method, thelinear no-threshold model,which routinely has been used.[158]

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.[159]

Ingestion

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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 USNational Research Councilinvestigated the issue of radon in drinking water. The risk associated with ingestion was considered almost negligible.[160]Water from underground sources may contain significant amounts of radon depending on the surrounding rock and soil conditions, whereas surface sources generally do not.[161]

Ocean effects of radon

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The major importance of understanding222Rn flux from the ocean, is to know that the increase use of radon is also circulating and increasing in the atmosphere. Ocean surface concentrations have an exchange within the atmosphere, causing222Rn to increase through the air-sea interface.[162]Although areas tested were very shallow, additional measurements in a wide variety of coastal regimes should help define the nature of222Rn observed.[162] 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 occurred during showering. Water with a radon concentration of 104pCi/L can increase the indoor airborne radon concentration by 1 pCi/L under normal conditions.[80]

Testing and mitigation

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Main article:Radon mitigation
A digital radon detector
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.[10]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.[163]

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.[80]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.[10]

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.[3]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).[10]

The four principal ways of reducing the amount of radon accumulating in a house are:[10][164]

  • 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,[10]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".[165]

Positive-pressure ventilationsystems 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).[10][166] For crawl spaces, the EPA states "An 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."[165]

See also

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References

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