Radon#Medical

File:Radon spectrum.png of radon, photographed by Ernest Rutherford in 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radium emanation (radon), while the outer two are of helium (added to calibrate the wavelengths).]]

File:Radon decay in a cloud chamber.jpg

Radon is a colorless, odorless, and tasteless{{cite web |date=2016 |title=A Citizen's Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon |url=https://www.epa.gov/radon/citizens-guide-radon-guide-protecting-yourself-and-your-family-radon |publisher=Environmental Protection Agency}} 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.{{cite web |last=Williams |first=David R. |date=2007-04-19 |title=Earth Fact Sheet |publisher=NASA |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |access-date=2008-06-26}} 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 {{convert|202|K}}, it emits a brilliant radioluminescence that turns from yellow to orange-red as the temperature lowers.{{cite web |title=The Element Radon|website = It's Elemental |url=http://education.jlab.org/itselemental/ele086.html |access-date= |publisher=Jefferson Lab}} Upon condensation, it glows because of the intense radiation it produces.{{cite book |last=Thomas |first=Jens |date= 2002 |title= Noble Gases |publisher=Marshall Cavendish |isbn=978-0-7614-1462-9 |page=13 |url=https://books.google.com/books?id=T0Iiv0BJ1E0C&pg=PA13}} 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:{{Unbulleted list citebundle|{{cite book |last1=Gerrard |first1=W |title=Solubility Data Series |date=1979 |publisher=Pergamon Press |pages=264–271 |edition=Vol.2 |url= https://iupac.github.io/SolubilityDataSeries/volumes/SDS-2.pdf }}|{{cite book |last1=Battino |first1=R |title=Solubility Data Series |date=1979 |publisher= Pergamon Press |pages=227–234 |edition=Vol.2 |url=https://iupac.github.io/SolubilityDataSeries/volumes/SDS-2.pdf }}|{{cite journal |last1=Saito |first1=M |title=Determination of Radon Solubilities to 1,2-Dimethylbenzene, 1,3- Dimethylbenzene, 1,4-Dime thylbenzene, 1,3,5-Trimethylbenzene, 1, 2,4-Trimethylbenzene and 1-Isopropyl-4-methylbenzene |journal=Nippon Kagaku Kaishi |date=1999 |issue=6 |pages=363–368|doi=10.1246/nikkashi.1999.363 |url=https://www.jstage.jst.go.jp/article/nikkashi1972/1999/6/1999_6_363/_article/download/-char/ja|doi-access=free }}}}

: $\backslash chi\; =\; \backslash exp(B/T-A)$

where $\backslash chi$ is the molar fraction of radon, $T$ is the absolute temperature, and $A$ and $B$ are solvent constants.

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.{{rp|66}} The 3.8-day half-life of {{sup|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.{{cite web |last=Bader |first=Richard F. W. |url=http://miranda.chemistry.mcmaster.ca/esam/ |title=An Introduction to the Electronic Structure of Atoms and Molecules |publisher=McMaster University |access-date=2008-06-26}} Its first ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol.{{cite book |author=David R. Lide |title=CRC Handbook of Chemistry and Physics |edition=84th|publisher=CRC Press|location=Boca Raton, Florida|date=2003|chapter=Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions}} 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 ({{chem|Cl|2}}) or sulfur dioxide ({{chem|SO|2}}), and significantly higher than the stability of the hydrate of hydrogen sulfide ({{chem|H|2|S}}).{{cite journal |doi=10.1070/RC1982v051n01ABEH002787 |title=The Chemistry of Radon |date=1982 |author=Avrorin, V. V. |journal=Russian Chemical Reviews |volume=51 |issue=1 |page=12 |last2=Krasikova |first2=R. N. |last3=Nefedov |first3=V. D. |last4=Toropova |first4=M. A. |bibcode = 1982RuCRv..51...12A|s2cid=250906059 }}

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 ({{chem|RnF|2}}).{{Unbulleted list citebundle|{{cite journal |author=Stein, L. |date=1970 |journal=Science |volume=168 |doi=10.1126/science.168.3929.362 |title=Ionic Radon Solution |pmid=17809133 |issue=3929 |bibcode=1970Sci...168..362S |pages=362–4|s2cid=31959268 }}|{{cite journal |author=Pitzer, Kenneth S. |date=1975 |journal=Chemical Communications |volume=44 |pages=760–761 |title=Fluorides of radon and element 118 |doi=10.1039/C3975000760b |issue=18 |url=https://escholarship.org/uc/item/8xz4g1ff}}}} It decomposes back to its elements at a temperature of above {{Convert|523|K||abbr=}}, 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 {{chem|RnF|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–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 ({{chem|XeF|2}}).{{cite journal |doi=10.1021/jp9825516 |title=Chemical Bonding in XeF₂, XeF₄, KrF₂, KrF₄, RnF₂, XeCl₂, and XeBr₂: From the Gas Phase to the Solid State |date=1998 |author=Meng-Sheng Liao |author2=Qian-Er Zhang |journal=The Journal of Physical Chemistry A |volume=102 |page=10647 |issue=52 |bibcode=1998JPCA..10210647L}} The octahedral molecule Radon hexafluoride was predicted to have an even lower enthalpy of formation than the difluoride.{{cite journal |doi=10.1039/b212460m |title=Bonding in radon hexafluoride: An unusual relativistic problem? |date=2003 |author=Filatov, Michael |journal=Physical Chemistry Chemical Physics |volume=5 |page=1103 |last2=Cremer |first2=Dieter |issue=6 |bibcode=2003PCCP....5.1103F}} The [RnF]⁺ ion is believed to form by the following reaction:{{cite journal |doi=10.1016/S0022-1139(00)85275-6 |title=Noble-gas fluorides |date=1986 |author=Holloway, J. |journal=Journal of Fluorine Chemistry |volume=33 |issue=1–4 |page=149|bibcode=1986JFluC..33..149H }}

: Rn (g) + 2 {{chem|[O|2|]|+|[SbF|6|]|-}} (s) → {{chem|[RnF]|+|[Sb|2|F|11|]|-}} (s) + 2 {{chem|O|2}} (g)

For this reason, antimony pentafluoride together with chlorine trifluoride and {{Chem|N|2|F|2|Sb|2|F|11}} have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds.{{Ullmann |first1=Cornelius |last1=Keller |first2=Walter |last2=Wolf |first3=Jashovam |last3=Shani |title=Radionuclides, 2. Radioactive Elements and Artificial Radionuclides |doi=10.1002/14356007.o22_o15}} 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 {{chem|SbF|6|-}}, {{chem|TaF|6|-}}, and {{chem|BiF|6|-}} are known.{{cite journal |last1=Stein |first1=Lawrence |date=1983 |title=The Chemistry of Radon |journal=Radiochimica Acta |volume=32 |issue=1–3 |pages=163–171 |doi=10.1524/ract.1983.32.13.163|s2cid=100225806 }} Radon is also oxidised by dioxygen difluoride to {{chem|RnF|2}} at {{Convert|173|K||abbr=}}.

Radon oxides are among the few other reported compounds of radon;{{cite journal |title=The Chemistry of Radon |volume=51 |issue=1 |journal=Russian Chemical Reviews |date=1982 |page=12 |author=Avrorin, V. V. |author2=Krasikova, R. N. |author3=Nefedov, V. D. |author4=Toropova, M. A. |doi=10.1070/RC1982v051n01ABEH002787 |bibcode=1982RuCRv..51...12A|s2cid=250906059 }} only the trioxide ({{Chem|Rn|O|3}}) has been confirmed.{{cite book |last=Sykes |first=A. G. |title=Advances in Inorganic Chemistry |volume=46 |chapter=Recent Advances in Noble-Gas Chemistry |chapter-url=https://books.google.com/books?id=6iqXRtz6p3QC |access-date=2012-11-02 |date=1998 |publisher=Academic Press |isbn=978-0120236466 |pages=91–93}}> The higher fluorides {{chem|RnF|4}} and {{chem|RnF|6}} have been claimed, are calculated to be stable, but have not been confirmed.{{cite book |last1=Thayer |first1=John S. |title=Relativistic Methods for Chemists |volume=10 |year=2010 |page=80 |doi=10.1007/978-1-4020-9975-5_2|chapter=Relativistic Effects and the Chemistry of the Heavier Main Group Elements |isbn=978-1-4020-9974-8 |series=Challenges and Advances in Computational Chemistry and Physics }} They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been {{chem|RnF|4}}, {{chem|RnF|6}}, 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 {{chem|RnO|3}}. While it has been suggested that these claims were really due to radon precipitating out as the solid complex [RnF]{{su|p=+|b=2}}[NiF₆]²⁻, the fact that radon coprecipitates from aqueous solution with {{Chem|CsXeO|3|F}} has been taken as confirmation that {{chem|RnO|3}} 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.{{cite journal |last1=Avrorin |first1=V. V. |last2=Krasikova |first2=R. N. |last3=Nefedov |first3=V. D. |last4=Toropova |first4=M. A. |date=1982 |title=The Chemistry of Radon |url= |journal=Russian Chemical Reviews |volume=51 |issue=1 |pages=12–20 |doi=10.1070/RC1982v051n01ABEH002787 |bibcode=1982RuCRv..51...12A |s2cid=250906059 |access-date=}}

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 ({{chem|RnF|2}}) and the high positive charge on radon in RnF⁺; spatial separation of {{chem|RnF|2}} molecules may be necessary to clearly identify higher fluorides of radon, of which {{chem|RnF|4}} is expected to be more stable than {{chem|RnF|6}} due to spin–orbit splitting of the 6p shell of radon (Rn^IV would have a closed-shell 6s{{su|p=2}}6p{{su|b=1/2|p=2}} configuration). Therefore, while {{chem|RnF|4}} should have a similar stability to xenon tetrafluoride ({{chem|XeF|4}}), {{chem|RnF|6}} would likely be much less stable than xenon hexafluoride ({{chem|XeF|6}}): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of {{chem|XeF|6}}, because of the inert pair effect.{{cite journal |last1=Liebman |first1=Joel F. |date=1975 |title=Conceptual Problems in Noble Gas and Fluorine Chemistry, II: The Nonexistence of Radon Tetrafluoride |journal=Inorg. Nucl. Chem. Lett. |volume=11 |issue=10 |pages=683–685 |doi=10.1016/0020-1650(75)80185-1}}{{cite journal |last1=Seppelt |first1=Konrad |date=2015 |title=Molecular Hexafluorides |journal=Chemical Reviews |volume=115 |issue=2 |pages=1296–1306 |doi=10.1021/cr5001783|pmid=25418862 }} 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.{{cite journal |doi=10.1002/qua.963 |title=Prediction of the existence of radon carbonyl: RnCO |date=2002 |author=Malli, Gulzari L. |journal=International Journal of Quantum Chemistry |volume=90 |page=611 |issue=2}} The molecules {{chem|Rn|2}} and RnXe were found to be significantly stabilized by spin-orbit coupling.{{cite journal |doi=10.1002/(SICI)1097-461X(1998)66:2<131::AID-QUA4>3.0.CO;2-W |title=Relativistic pseudopotential calculations on Xe₂, RnXe, and Rn₂: The van der Waals properties of radon |date=1998 |author=Runeberg, Nino |journal=International Journal of Quantum Chemistry |volume=66 |page=131 |last2=Pyykkö |first2=Pekka |issue=2}} Radon caged inside a fullerene has been proposed as a drug for tumors.{{Unbulleted list citebundle|{{cite news |last=Browne |first=Malcolm W. |url=https://query.nytimes.com/gst/fullpage.html?res=9F0CE2DE1E3CF936A35750C0A965958260&sec=&spon=&pagewanted=all |title=Chemists Find Way to Make An 'Impossible' Compound |work=The New York Times |date=1993-03-05 |access-date=2009-01-30}}|{{Cite journal |last1=Dolg |first1=M. |last2=Küchle |first2=W. |last3=Stoll |first3=H. |last4=Preuss |first4=H. |last5=Schwerdtfeger |first5=P. |date=1991-12-20 |title=Ab initio pseudopotentials for Hg to Rn: II. Molecular calculations on the hydrides of Hg to At and the fluorides of Rn |journal=Molecular Physics |language=en |volume=74 |issue=6 |pages=1265–1285 |doi=10.1080/00268979100102951 |issn=0026-8976 |bibcode=1991MolPh..74.1265D}}}} Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; {{chem|RnF|8}} should be highly unstable chemically (XeF₈ is thermodynamically unstable).

Radon reacts with the liquid halogen fluorides ClF, {{chem|ClF|3}}, {{chem|ClF|5}}, {{chem|BrF|3}}, {{chem|BrF|5}}, and {{chem|IF|7}} to form {{chem|RnF|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 complexes {{chem|RnF|3|-}} and {{chem|RnF|4|2-}}, 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,{{cite journal |title=Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K |last=Bratsch |first=Steven G. |date=29 July 1988 |journal=Journal of Physical and Chemical Reference Data |volume=18 |issue=1 |pages=1–21 |bibcode=1989JPCRD..18....1B |doi=10.1063/1.555839 |s2cid=97185915 }} although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.

{{Main|Isotopes of radon}}

Radon has no stable isotopes. Thirty-nine radioactive isotopes have been characterized, with mass numbers ranging from 193 to 231.{{cite web|author=Sonzogni, Alejandro|title=Interactive Chart of Nuclides|url=http://www.nndc.bnl.gov/chart/|access-date=2008-06-06|publisher=Brookhaven National Laboratory|location=National Nuclear Data Center|archive-date=2011-07-21|archive-url=https://web.archive.org/web/20110721051025/http://www.nndc.bnl.gov/chart/|url-status=dead}}{{cite journal|last1=Neidherr|first1=D.|last2=Audi|first2=G.|last3=Beck|first3=D.|last4=Baum|first4=K.|last5=Böhm|first5=Ch.|last6=Breitenfeldt|first6=M.|last7=Cakirli|first7=R. B.|last8=Casten|first8=R. F.|last9=George|first9=S.|last10=Herfurth|first10=F.|last11=Herlert|first11=A.|date=19 March 2009|title=Discovery of {{sup|229}}Rn and the Structure of the Heaviest Rn and Ra Isotopes from Penning-Trap Mass Measurements|url=https://cds.cern.ch/record/1190495/files/PhysRevLett.102.112501.pdf|journal=Physical Review Letters|volume=102|issue=11|pages=112501–1–112501–5|bibcode=2009PhRvL.102k2501N|doi=10.1103/PhysRevLett.102.112501|pmid=19392194|last13=Kowalska|first21=L.|first12=A.|last12=Kellerbauer|last22=Stora|first22=T.|last21=Schweikhard|last20=Schwarz|first14=D.|first20=S.|last19=Rosenbusch|first19=M.|last18=Penescu|first18=L.|last17=Noah|first17=E.|last16=Naimi|first16=S.|last15=Minaya-Ramirez|first15=E.|first13=M.|last14=Lunney}} Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is {{sup|222}}Rn (half-life 3.82 days), which is a decay product of radium-226, the latter being itself a decay product of uranium-238.{{cite web|title=Principal Decay Scheme of the Uranium Series|url=http://www.gulflink.osd.mil/library/randrep/du/mr1018.7.appa.html|url-status=dead|archive-url=https://web.archive.org/web/20081025025424/http://www.gulflink.osd.mil/library/randrep/du/mr1018.7.appa.html|archive-date=2008-10-25|access-date=2008-09-12|publisher=Gulflink.osd.mil}} A trace amount of the (highly unstable) isotope {{sup|218}}Rn (half-life about 35 milliseconds) is also among the daughters of {{sup|222}}Rn. The isotope {{sup|216}}Rn would be produced by the double beta decay of natural {{sup|216}}Po; while energetically possible, this process has however never been seen.{{Cite journal

|last1=Tretyak |first1=V.I.

|last2=Zdesenko |first2=Yu.G.

|year=2002

|title=Tables of Double Beta Decay Data — An Update

|journal=At. Data Nucl. Data Tables |volume=80 |issue=1 |pages=83–116

|doi=10.1006/adnd.2001.0873

|bibcode=2002ADNDT..80...83T }}

Three other radon isotopes have a half-life of over an hour: {{sup|211}}Rn (about 15 hours), {{sup|210}}Rn (2.4 hours) and {{sup|224}}Rn (about 1.8 hours). However, none of these three occur naturally. {{sup|220}}Rn, also called thoron, is a natural decay product of the most stable thorium isotope ({{sup|232}}Th). It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, {{sup|219}}Rn is derived from the most stable isotope of actinium ({{sup|227}}Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds.

Image:Decay chain(4n+2, Uranium series).svg

{{Main|Decay chain#Uranium series}}

{{Sup|222}}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:

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

The radon equilibrium factor{{cite web |access-date=2009-07-07 |url=http://progenygrp.com/why_measure_rdps.htm |title=Why Measure RDPs? |url-status=dead |archive-url=https://web.archive.org/web/20150225020349/http://progenygrp.com/why_measure_rdps.htm |archive-date=2015-02-25}} 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. {{Sup|210}}Pb takes much longer to come in equilibrium with radon, dependent on environmental factors,{{Cite journal |last1=Joshi |first1=L. U. |last2=Rangarajan |first2=C. |last3=Sarada Gopalakrishnan |first3=Smt. |date=1969 |title=Measurement of lead-210 in surface air and precipitation |url=https://a.tellusjournals.se/articles/2832/files/submission/proof/2832-1-46460-1-10-20221018.pdf |journal=Tellus |volume=21 |issue=1|page=107 |doi=10.1111/j.2153-3490.1969.tb00423.x |bibcode=1969Tell...21..107J }} 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 {{Sup|222}}Rn decay products with {{Sup|210}}Pb, such as {{Sup|210}}Po, in measuring overall radiation levels.{{Unbulleted list citebundle|{{Cite journal|url=https://inis.iaea.org/collection/NCLCollectionStore/_Public/53/079/53079681.pdf |title=Radioactive lead in the environment and in the human body |last=Jaworowski |first=Z. |publisher=Institute of Nuclear Research |location=Warsaw, Poland |journal=At. Energy Rev. |date= 1969 |volume=7 |issue=1 }}|{{Cite journal|title=Polonium-210 and Lead-210 in the Terrestrial environment: A historical review |first1=Bertil R.R. |last1=Persson |first2=Elis |last2=Holm |doi=10.1016/j.jenvrad.2011.01.005 |pmid=21377252 |journal= J Environ Radioact |date=May 2011 |volume=102 |issue=5 |pages=420–9|bibcode=2011JEnvR.102..420P }}}}

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.{{cite book|url=https://books.google.com/books?id=YDRCCNibEqYC&pg=PA179|page=179|title=Health effects of exposure to radon, Volume 6 of BEIR (Series)|publisher=National Academies Press|date=1999|isbn=978-0-309-05645-8}}

Image:Radon apparatus.png, where approximately 0.1 mm³ were isolated. Radon mixed with hydrogen entered the evacuated system through siphon A; mercury is shown in black.]]

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.{{cite journal |title=Discovery of Radon |journal=Nature |volume=179 |page=912 |date=1957 |author=Partington, J. R. |doi=10.1038/179912a0 |issue=4566 |bibcode=1957Natur.179..912P|s2cid=4251991 |doi-access=free }}{{cite web |url=http://chemistry.about.com/library/weekly/aa030303a.htm |title=Timeline of Element Discovery |date=2008 |publisher=The New York Times Company |access-date=2008-02-28 |archive-date=2009-02-08 |archive-url=https://web.archive.org/web/20090208130034/http://chemistry.about.com/library/weekly/aa030303a.htm |url-status=dead }}{{Unbulleted list citebundle|{{cite journal |doi=10.1080/10256018808623931 |title=Zur Entdeckungsgeschichte des Radons |language=de |date=1988 |last1=Schüttmann |first1=W. |journal=Isotopenpraxis Isotopes in Environmental and Health Studies |volume=24 |issue=4 |page=158|bibcode=1988IIEHS..24..158S }}|{{cite journal |doi=10.1118/1.598902 |title=Rutherford, the Curies, and Radon |date=2000 |last1=Brenner |first1=David J. |journal=Medical Physics |volume=27 |issue=3 |page=618 |pmid=10757614 |bibcode=2000MedPh..27..618B }}}} In 1899, Pierre and Marie Curie observed that the gas emitted by radium remained radioactive for a month.{{cite journal |journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences |author=Curie, P. |author2=Curie, Mme. Marie |title=Sur la radioactivite provoquee par les rayons de Becquerel |language=fr |volume=129 |date= 1899 |pages=714–6}} Later that year, Rutherford and Owens noticed variations when trying to measure radiation from thorium oxide.{{cite journal |author=Rutherford, E. |author2=Owens, R. B. |title=Thorium and uranium radiation |journal=Trans. R. Soc. Can. |volume=2 |date=1899 |pages=9–12}}: "The radiation from thorium oxide was not constant, but varied in a most capricious manner", whereas "All the compounds of Uranium give out a radiation which is remarkably constant." Rutherford noticed that the compounds of thorium continuously emit a radioactive gas that remains radioactive for several minutes, and called this gas "emanation" (from {{langx|la|emanare}}, to flow out, and {{lang|la|emanatio}}, expiration),{{cite journal |author=Rutherford, E. |title=A radioactive substance emitted from thorium compounds |url=http://www.chemteam.info/Chem-History/Rutherford-half-life.html |journal=Phil. Mag. |volume=40 |date=1900 |issue=296 |pages=1–4|doi=10.1080/14786440009463821 }} 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").{{Unbulleted list citebundle|{{cite journal |journal=Abhandlungen der Naturforschenden Gesellschaft zu Halle |volume=22 |author=Dorn, Friedrich Ernst |page=155 |title=Über die von radioaktiven Substanzen ausgesandte Emanation |language=de |location=Stuttgart |date=1900 |url=http://publikationen.ub.uni-frankfurt.de/files/17242/E001458681_a.pdf}}|{{Cite journal |title = Die von radioactiven Substanzen ausgesandte Emanation |language=de |author = Dorn, F. E. |journal = Abhandlungen der Naturforschenden Gesellschaft zu Halle |date = 1900 |volume = 23 |pages = 1–15 |url=http://publikationen.ub.uni-frankfurt.de/files/17242/E001458681_a.pdf}}}} In 1901, Rutherford and Harriet Brooks demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element.{{cite journal |author=Rutherford, E. |author2=Brooks, H. T. |title=The new gas from radium |journal=Trans. R. Soc. Can. |volume=7 |date=1901 |pages=21–25}} In 1903, similar emanations were observed from actinium by André-Louis Debierne, and were called "actinium emanation" ("Ac Em").{{Unbulleted list citebundle|{{cite journal |author=Giesel, Fritz |title=Über den Emanationskörper aus Pechblende und über Radium |language=de |journal=Chemische Berichte |volume=36 |date=1903 |page=342 |doi=10.1002/cber.19030360177 |url=https://zenodo.org/record/1426068 }}|{{cite journal |author=Debierne, André-Louis |title=Sur la radioactivite induite provoquee par les sels d'actinium |language=fr |journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences |volume=136 |date=1903 |page=446 |url=http://gallica.bnf.fr/ark:/12148/bpt6k3091c/f446.table}}}}

Several shortened names were soon suggested for the three emanations: exradio, exthorio, and exactinio in 1904;{{cite journal |author=Ramsay, Sir William |author2=Collie, J. Norman |title=The Spectrum of the Radium Emanation |journal=Proceedings of the Royal Society |volume=73 |date= 1904 |pages=470–476 |doi=10.1098/rspl.1904.0064 |issue=488–496 |doi-access=free }} radon (Ro), thoron (To), and akton or acton (Ao) in 1918;{{cite journal |author=Schmidt, Curt |title=Periodisches System und Genesis der Elemente |language=de |journal=Zeitschrift für anorganische und allgemeine Chemie |volume=103 |date=1918 |pages=79–118 |doi=10.1002/zaac.19181030106 |url=https://zenodo.org/record/1428158 }} radeon, thoreon, and actineon in 1919,{{cite journal |title=Matière et lumière. Essai de synthèse de la mécanique chimique |language=fr |journal=Annales de Physique |series=IX |volume=11 |date=1919 |pages=5–108 |author=Perrin, Jean |doi=10.1051/anphys/191909110005 |author-link=Jean Baptiste Perrin |url=https://books.google.com/books?id=Vc9XAAAAYAAJ&q=rad%C3%A9on }} and eventually radon, thoron, and actinon in 1920.{{cite journal |author=Adams, Elliot Quincy |title=The Independent Origin of Actinium |journal=Journal of the American Chemical Society |volume=42 |date=1920 |page=2205 |doi=10.1021/ja01456a010 |issue=11 |bibcode=1920JAChS..42.2205A |url=https://zenodo.org/record/1428836 }} (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.{{cite journal |last1=Smits |first1=Odile R. |last2=Jerabek |first2=Paul |last3=Pahl |first3=Elke |last4=Schwerdtfeger |first4=Peter |date=2018 |title=A Hundred-Year-Old Experiment Re-evaluated: Accurate Ab Initio Monte Carlo Simulations of the Melting of Radon |url= |journal=Angewandte Chemie |volume=57 |issue=31 |publisher= |pages=9961–9964 |doi=10.1002/anie.201803353 |access-date=}} In 1910, they determined its density (that showed it was the heaviest known gas) and its position in the periodic table.{{cite journal |title=Some Physical Properties of Radium Emanation |author=R. W. Gray |author2=W. Ramsay |journal=J. Chem. Soc. Trans. |volume=1909 |pages=1073–1085 |date=1909|doi=10.1039/CT9099501073 |url=https://zenodo.org/record/1529110 }} They wrote that "{{Lang|fr|L'expression l'émanation du radium est fort incommode|italic=unset}}" ("the expression 'radium emanation' is very awkward") and suggested the new name niton (Nt) (from {{langx|la|nitens}}, shining) to emphasize the radioluminescence property,{{cite journal |title=La densité de l'emanation du radium |language=fr |author=Ramsay, W. |author2=Gray, R. W. |journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences |volume=151 |pages=126–128 |date=1910 |url=http://gallica.bnf.fr/ark:/12148/bpt6k31042/f126.table}} 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 Radon-220 and {{sup|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.{{cite journal |doi=10.1016/0022-1902(65)80255-X |date=1965 |title=Some physical and chemical properties of element 118 (Eka-Em) and element 86 (Em) |author=Grosse, A. V. |journal=Journal of Inorganic and Nuclear Chemistry |volume=27 |page=509 |issue=3}} The first synthesized compound of radon, radon fluoride, was obtained in 1962.{{cite journal |author=Fields, Paul R. |author2=Stein, Lawrence |author3=Zirin, Moshe H. |title=Radon Fluoride |journal=J. Am. Chem. Soc. |date=1962 |volume=84 |page=4164 |doi=10.1021/ja00880a048 |issue=21|bibcode=1962JAChS..84.4164F }} 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.{{cite journal |last1=Thornton |first1=Brett F. |last2=Burdette |first2=Shawn C. |date=22 August 2013 |title=Recalling radon's recognition |journal=Nature Chemistry |volume=5 |issue=9 |pages=804 |doi=10.1038/nchem.1731 |pmid=23965684 |bibcode=2013NatCh...5..804T |doi-access=free }}

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).{{Unbulleted list citebundle|{{Cite web|last=Masse |first=Roland |date=2002 |archive-url=https://web.archive.org/web/20071009164542/http://www.radon-france.com/pdf/historique.pdf |url=http://www.radon-france.com/pdf/historique.pdf |archive-date=October 9, 2007 |title=Le radon, aspects historiques et perception du risque |website=radon-france.com |language=fr |trans-title=Radon, historical aspects and perception of risk}}|{{Cite web|archive-url=https://web.archive.org/web/20090116120009/http://www.atsdr.cdc.gov/csem/radon/whosat_risk.html |url=http://www.atsdr.cdc.gov/csem/radon/whosat_risk.html |title=Radon Toxicity: Who is at Risk? |publisher=Agency for Toxic Substances and Disease Registry |date=2000 |archive-date=January 16, 2009}}}} In 1879, this condition was identified as lung cancer by Harting and Hesse in their investigation of miners from Schneeberg, Germany.{{Cite journal |last1=George |first1=A. C. |last2=Paschoa |first2=Anselmo Salles |last3=Steinhäusler |first3=Friedrich |date=2008 |title=World History Of Radon Research And Measurement From The Early 1900's To Today |url=https://pubs.aip.org/aip/acp/article-abstract/1034/1/20/860949/World-History-Of-Radon-Research-And-Measurement?redirectedFrom=fulltext |journal=AIP Conference Proceedings |publisher=AIP |volume=1034 |pages=20–33 |doi=10.1063/1.2991210|bibcode=2008AIPC.1034...20G }} The first major studies with radon and health occurred in the context of uranium mining in the Joachimsthal region of Bohemia.{{Cite book|last=Proctor |first=Robert N. |title=The Nazi War on Cancer |publisher=Princeton University Press |date=2000 |page= 99 |isbn=0-691-07051-2}} 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.{{Cite book|last1=Edelstein |first1=Michael R. |last2=William J. |first2=Makofske

|title=Radon's deadly daughters: science, environmental policy, and the politics of risk |publisher=Rowman & Littlefield |date=1998 |pages= 36–39 |isbn=0-8476-8334-6}}

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.{{Unbulleted list citebundle|{{cite web |title=Poster Issued by the New York Department of Health (ca. 1981) |publisher=Oak Ridge Associated Universities |date=2021-10-11 |url=https://www.orau.org/health-physics-museum/collection/health-physics-posters/other/poster-issued-by-the-new-york-department-of-health.html |access-date=2021-10-11}}|{{cite news |url=http://www.time.com/time/magazine/article/0,9171,838695,00.html |title=Rings and Cancer |access-date=2009-05-05 |magazine=Time |date=1968-09-13 |archive-date=2009-05-22 |archive-url=https://web.archive.org/web/20090522105043/http://www.time.com/time/magazine/article/0,9171,838695,00.html |url-status=dead }}}}

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.{{cite journal |last=Samet |first=J. M. |pmc=1003141 |pmid=1734594 |date=1992 |title=Indoor radon and lung cancer. Estimating the risks |volume=156 |issue=1 |pages=25–9 |journal=The Western Journal of Medicine}}

{{See also|Radium and radon in the environment}}

Image:Lead210inairatjapan.png |date=2006 |pmid=16181712 |issue=1 |doi=10.1016/j.jenvrad.2005.08.001 |volume=86 |last2=Sakaguchi |first2=A. |last3=Sasaki |first3=K. |last4=Hirose |first4=K. |last5=Igarashi |first5=Y. |last6=Kim |first6=C. |pages=110–31}}]]

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.{{cite news|url=http://www.epa.gov/radon/pdfs/402-r-03-003.pdf |archive-url=https://web.archive.org/web/20080227074413/http://www.epa.gov/radon/pdfs/402-r-03-003.pdf |archive-date=2008-02-27 |title=EPA Assessment of Risks from Radon in Homes|publisher= Office of Radiation and Indoor Air, US Environmental Protection Agency|date=June 2003}} 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.{{Cite journal |last1=Vaillant |first1=Ludovic |last2=Bataille |first2=Céline |date=2012-07-19 |title=Management of radon: a review of ICRP recommendations |journal=Journal of Radiological Protection |volume=32 |issue=3 |pages=R1–R12 |doi=10.1088/0952-4746/32/3/r1 |pmid=22809956 |bibcode=2012JRP....32R...1V |issn=0952-4746}}{{rp|R5}} 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.{{Cite journal |last1=Yang |first1=Handong |last2=Appleby |first2=Peter G. |date=2016-02-22 |title=Use of lead-210 as a novel tracer for lead (Pb) sources in plants |journal=Scientific Reports |volume=6 |pages=21707 |doi=10.1038/srep21707 |issn=2045-2322 |pmc=4761987 |pmid=26898637|bibcode=2016NatSR...621707Y }}

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{{e|-18}} molar percent, or about 150 atoms in each milliliter of air.{{cite web |url=http://www.us.lindegas.com/International/Web/LG/US/MSDS.nsf/NotesMSDS/Air+002/$file/Air+002.pdf |title=Health hazard data |publisher=The Linde Group |archive-url=https://web.archive.org/web/20130625060223/http://www.us.lindegas.com/International/Web/LG/US/MSDS.nsf/NotesMSDS/Air+002/$file/Air+002.pdf |archive-date=2013-06-25}} 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.{{cite web |access-date=2009-07-07 |url=http://www.laradioactivite.com/fr/site/pages/radon.htm |title=Le Radon. Un gaz radioactif naturel |language=fr |archive-date=2011-01-13 |archive-url=https://web.archive.org/web/20110113025038/http://www.laradioactivite.com/fr/site/pages/radon.htm |url-status=dead }}

Image:Radon Concentration next to Uranium Mine.PNG

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.{{cite book |author=Godish, Thad |title=Indoor Environmental Quality |date=2001 |publisher=CRC Press |isbn=978-1-56670-402-1}} Every square mile of surface soil, to a depth of 6 inches (2.6 km{{sup|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.Harley, J. H. in {{cite book |author1=Richard Edward Stanley |author2=A. Alan Moghissi |title=Noble Gases |url=https://books.google.com/books?id=RCxRAAAAMAAJ&q=%221600+pCi%2Fcm2%22&pg=PA659 |year=1975 |publisher=U.S. Environmental Protection Agency |page=111}} This is equivalent to some {{convert|15.3|kg}}.

Radon concentration can differ widely from place to place. In the open air, it ranges from 1 to 100 Bq/m{{sup|3}}, even less (0.1 Bq/m{{sup|3}}) above the ocean. In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000 Bq/m{{sup|3}}.{{cite journal |author=Sperrin, Malcolm |author2=Gillmore, Gavin |author3=Denman, Tony |date=2001 |title=Radon concentration variations in a Mendip cave cluster |journal=Environmental Management and Health |volume=12 |page=476 |doi=10.1108/09566160110404881 |issue=5 |url=http://eprints.kingston.ac.uk/1666/}}

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 {{sup|222}}Rn per liter of air; 20.2 kBq/m{{sup|3}}, measured from 1976 to 1985).

The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m{{sup|3}} (1.2 nCi/L) with maximal value of 160 kBq/m{{sup|3}} (4.3 nCi/L).{{cite journal |doi=10.2203/dose-response.05-025.Zdrojewicz |pmc=2477672 |pmid=18648641 |title=Radon Treatment Controversy, Dose Response |date=2006 |volume=4 |issue=2 |author=Zdrojewicz, Zygmunt |journal=Dose-Response |last2=Strzelczyk |first2=Jadwiga (Jodi) |pages=106–18}}

Radon mostly appears with the radium/uranium series (decay chain) ({{sup|222}}Rn), and marginally with the thorium series ({{sup|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 {{sup|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.{{Cite journal |last1=Steck |first1=D. J. |last2=Field |first2=R. W. |last3=Lynch |first3=C. F. |year=1999 |title=Exposure to atmospheric radon |journal=Environmental Health Perspectives |volume=107 |issue=2 |pages=123–127 |doi=10.1289/ehp.99107123 |pmc=1566320 |pmid=9924007 |s2cid=1767956 |doi-access=free|bibcode=1999EnvHP.107..123S }}

High concentrations of radon can be found in some spring waters and hot springs.{{cite web |url=http://www.cheec.uiowa.edu/misc/radon_occ.pdf |archive-url=https://web.archive.org/web/20060316062136/http://www.cheec.uiowa.edu/misc/radon_occ.pdf |url-status=dead |archive-date=2006-03-16 |title=Radon Occurrence and Health Risk |author=Field, R. William |publisher=Department of Occupational and Environmental Health, University of Iowa |access-date=2008-02-02}} 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{{sup|3}}).{{cite web |access-date=2009-07-07 |url=https://www.amtamassage.org/journal/winter03_journal/balneology.html |title=The Clinical Principles Of Balneology & Physical Medicine |url-status=dead |archive-url=https://web.archive.org/web/20080508064535/http://amtamassage.org/journal/winter03_journal/balneology.html |archive-date=May 8, 2008 }} The activity of radon mineral water reaches 2 MBq/m{{sup|3}} in Merano and 4 MBq/m{{sup|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 {{sup|222}}Rn than surface water, because radon is continuously produced by radioactive decay of {{sup|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.{{Unbulleted list citebundle|{{cite web |access-date=2008-06-28 |title=The Geology of Radon |url=http://energy.cr.usgs.gov/radon/georadon/3.html |publisher=United States Geological Survey |archive-date=2008-05-09 |archive-url=https://web.archive.org/web/20080509185452/http://energy.cr.usgs.gov/radon/georadon/3.html |url-status=dead }}|{{cite web |access-date=2008-06-28 |url=http://www.cosis.net/abstracts/EGU2008/08953/EGU2008-A-08953.pdf?PHPSESSID= |format=PDF |title=Radon-222 as a tracer in groundwater-surface water interactions |publisher=Lancaster University |archive-date=November 8, 2021 |archive-url=https://web.archive.org/web/20211108075203/https://www.cosis.net/abstracts/EGU2008/08953/EGU2008-A-08953.pdf?PHPSESSID= |url-status=dead }}}}

In 1971, Apollo 15 passed {{Cvt|110|km||abbr=}} above the Aristarchus plateau on the Moon, and detected a significant rise in alpha particles thought to be caused by the decay of {{sup|222}}Rn. The presence of {{sup|222}}Rn has been inferred later from data obtained from the Lunar Prospector alpha particle spectrometer.{{cite journal |last1=Lawson |first1=S. |last2=Feldman |first2=W. |last3=Lawrence |first3=D. |last4=Moore |first4=K. |last5=Elphic |first5=R. |last6=Belian |first6=R. |title=Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer |journal=J. Geophys. Res. |volume=110 |page=1029 |date=2005 |issue=E9 |doi=10.1029/2005JE002433 |bibcode=2005JGRE..110.9009L |doi-access=free }}

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.{{cite news |publisher=National Energy Board |access-date=2009-07-07 |url= http://www.neb-one.gc.ca/clf-nsi/rsftyndthnvrnmnt/sfty/sftydvsr/1994/nbs199401-eng.pdf |title=Potential for Elevated Radiation Levels In Propane |date=April 1994}}

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.

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,{{Cite thesis |last=Bengt |first=Hultqvist |title=Studies on naturally occurring ionizing radiations with special reference to radiation doses in swedish houses of various types |date=1956 |publisher=Stockholm College |page=125}} 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.

File:Radon Lognormal distribution.gif radon distribution in dwellings]]

File:US homes over recommended radon levels.gif

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.LaFavore, Michael. "Radon: The Quiet Killer." Funk & Wagnalls 1987 Science Yearbook. New York: Funk & Wagnalls, Inc., 1986. {{ISBN|0-7172-1517-2}}. 217–21. The incident dramatized the fact that radon levels in particular dwellings can occasionally be orders of magnitude higher than typical.{{cite web |date=April 22, 1997 |title=Nuclear reaction: why do citizens fear nuclear power? |url=https://www.pbs.org/wgbh/pages/frontline/shows/reaction/etc/script.html |website=www.pbs.org}} 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.Numerous references, see, for instance, [http://www.geology.cz/extranet/vav/geochemie-zp/radon/sympozia/2006/radon-2006-258-265.pdf Analysis And Modelling Of Indoor Radon Distributions Using Extreme Values Theory] or [http://www.geology.cz/extranet/vav/geochemie-zp/radon/sympozia/2006/radon-2006-252-257.pdf Indoor Radon in Hungary (Lognormal Mysticism)] for a discussion. Thus, the geometric mean is generally used for estimating the "average" radon concentration in an area.{{cite web |title=Data Collection and Statistical Computations |url=http://aprg.utoledo.edu/radon/datacoll.html |url-status=dead |archive-url=http://arquivo.pt/wayback/20160519081621/http://aprg.utoledo.edu/radon/datacoll.html |archive-date=2016-05-19 |access-date=2023-09-23 |website=University of Toledo}} The mean concentration ranges from less than 10 Bq/m³ to over 100 Bq/m³ in some European countries.{{citation |access-date=17 August 2013 |url=http://www.unscear.org/docs/reports/2006/09-81160_Report_Annex_E_2006_Web.pdf |publisher=United Nations |date=2008 |work=Report of the United Nations Scientific Committee on the Effects of Atomic Radiation (2006) |volume=2 |pages=209–210 |title=Annex E: Sources to effects assessment for radon in homes and workplaces}}

Some of the highest radon hazard in the US is found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania.{{cite web |last1=Price |first1=Phillip N. |last2=Nero |first2=A. |last3=Revzan |first3=K. |last4=Apte |first4=M. |last5=Gelman |first5=A. |last6=Boscardin |first6=W. John |title=Predicted County Median Concentration |publisher=Lawrence Berkeley National Laboratory |url=http://eetd.lbl.gov/IEP/high-radon/USgm.htm |access-date=2008-02-12 |archive-url=https://web.archive.org/web/20071231195400/http://eetd.lbl.gov/IEP/high-radon/USgm.htm |archive-date= 2007-12-31}} 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.{{cite web |url=http://www.cheec.uiowa.edu/misc/radon.html |title=The Iowa Radon Lung Cancer Study |author=Field, R. William |publisher=Department of Occupational and Environmental Health, University of Iowa |date = 2003}} 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.{{Cite news |url=https://www.rte.ie/news/2007/0920/93731-radon/ |title=Record radon levels found at Mallow office |date=2007-09-20 |work=RTE.ie |access-date=2018-09-09 |language=en}}

File:Stanowisko pomiaru radonu glebowego wf pw.jpg]]

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.{{Cite web|last=Featherstone|first=Sarah|date=10 March 2021|title=Dangers Of Radon Gas - Test & Guide For Landlords 2021|url=https://thebla.co.uk/dangers-of-radon-gas-test-guide-for-landlords-2021/|access-date=2021-05-16|language=en-GB}} Alpha-radiation monitoring over the long term is a method of testing for radon that is more common in countries outside the United States.

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 {{chem|H|2}}, {{chem|O|2}}, He, Rn, {{chem|CO|2}}, {{chem|H|2|O}} and hydrocarbons. The mixture is purified by passing it over copper at {{Convert|993|K||abbr=}} to remove the {{chem|H|2}} and the {{chem|O|2}}, 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.{{cite web |url=http://rn-radon.info/production.html |archive-url=https://web.archive.org/web/20081028133937/http://rn-radon.info/production.html |archive-date=2008-10-28 |title=Radon Production |publisher=Rn-radon.info |date=2007-07-24 |access-date=2009-01-30}}

Radon commercialization is regulated,{{Citation needed|date=February 2025}} but it is available in small quantities for the calibration of ²²²Rn measurement systems. In 2008 it was priced at almost {{US$|6000|2008}} per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment).{{cite web |title=SRM 4972 – Radon-222 Emanation Standard |url=https://www-s.nist.gov/srmors/view_detail.cfm?srm=4972 |url-status=dead |archive-url=https://web.archive.org/web/20200306035332/https://www-s.nist.gov/srmors/view_detail.cfm?srm=4972 |archive-date=6 March 2020 |access-date=2008-06-26 |publisher=National Institute of Standards and Technology}} 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.{{cite journal |author=Collé, R. |author2=R. Kishore |date=1997 |title=An update on the NIST radon-in-water standard generator: its performance efficacy and long-term stability |journal=Nucl. Instrum. Methods Phys. Res. A |volume=391 |pages=511–528 |bibcode=1997NIMPA.391..511C |doi=10.1016/S0168-9002(97)00572-X |issue=3 |url=https://zenodo.org/record/1259919}} Radon sources have also been produced for scientific purposes through the implantation of radium-226 into solid stainless steel.{{Cite web |last=Jörg |first=Florian |last2=Blaum |first2=Klaus |last3=Schweiger |first3=Christoph |last4=Simgen |first4=Hardy |date=January 4, 2023 |title=Production of 226Ra-implanted high-quality radon sources for detector characterization |url=https://cds.cern.ch/record/2845390/files/INTC-P-647.pdf |website=European Organization for Nuclear Research}}

{{Main|Radioactive quackery}}

An early-20th-century form of quackery was the treatment of maladies in a radiotorium.{{cite book |title=The Clinique, Volume 34 |publisher=Illinois Homeopathic Medical Association |date=1913 |url=https://books.google.com/books?id=KM5XAAAAMAAJ&q=%2Bradiotorium&pg=PA243 |access-date=2011-06-30}} 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,{{cite web |title=Radon seeds |url=https://www.orau.org/health-physics-museum/collection/brachytherapy/seeds.html |access-date=2009-05-05 |website=ORAU Museum of Radiation and Radioactivity}} but it does not increase the health of healthy cells.{{cn|date=October 2022}} 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.{{cite web |url=http://www.roadsideamerica.com/story/2143 |title=Radon Health Mines: Boulder and Basin, Montana |publisher= Roadside America |access-date=2007-12-04}}{{cite journal |author=Neda, T. |title=Radon concentration levels in dry CO₂ emanations from Harghita Băi, Romania, used for curative purposes |volume=277 |issue=3 |date=2008 |doi=10.1007/s10967-007-7169-0 |journal=Journal of Radioanalytical and Nuclear Chemistry |page=685 |last2=Szakács |first2=A. |last3=Mócsy |first3=I. |last4=Cosma |first4=C.|bibcode=2008JRNC..277..685N |s2cid=97610571 }} 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.{{cite journal |last1=Salak |first1=Kara |last2=Nordeman |first2=Landon |title=59631: Mining for Miracles |journal=National Geographic |date=2004 |url=http://ngm.nationalgeographic.com/ngm/0401/feature7/index.html |archive-url=https://web.archive.org/web/20080124233142/http://ngm.nationalgeographic.com/ngm/0401/feature7/index.html |url-status=dead |archive-date=January 24, 2008 |access-date=2008-06-26}}

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).{{Cite web |date=2004-08-18 |title=For that Healthy Glow, Drink Radiation! |url=https://www.popsci.com/scitech/article/2004-08/healthy-glow-drink-radiation/ |access-date=2022-09-17 |website=Popular Science |language=en-US}} 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.{{cite web |access-date=2008-06-26 |url=http://www.petros.cz/spa/spa_ja.asp |title=Jáchymov |publisher=Petros |url-status=dead |archive-url=https://web.archive.org/web/20020107060646/http://www.petros.cz/spa/spa_ja.asp |archive-date=January 7, 2002 }}

File:Radioactive Seeds (7845754328).jpg-containing seeds used in brachytherapy]]

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.{{cn|date=October 2022}}

²¹¹Rn can be used to generate ²¹¹At, which has uses in targeted alpha therapy.{{cite journal | last1=Crawford | first1=Jason R | last2=Kunz | first2=Peter | last3=Yang | first3=Hua | last4=Schaffer | first4=Paul | last5=Ruth | first5=Thomas J | title=²¹¹Rn/²¹¹At and ²⁰⁹At production with intense mass separated Fr ion beams for preclinical ²¹¹At-based α-therapy research | journal=Applied Radiation and Isotopes | publisher=Elsevier BV | volume=122 | year=2017 | issn=0969-8043 | doi=10.1016/j.apradiso.2017.01.035 | pages=222–228| pmid=28189025 | bibcode=2017AppRI.122..222C }}

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.{{Cite journal |last1=Lambert |first1=Gérard |last2=Polian |first2=Georges |last3=Taupin |first3=D. |date=1970-04-20 |title=Existence of periodicity in radon concentrations and in the large-scale circulation at lower altitudes between 40° and 70° south |url=http://doi.wiley.com/10.1029/JC075i012p02341 |journal=Journal of Geophysical Research |language=en |volume=75 |issue=12 |pages=2341–2345 |doi=10.1029/JC075i012p02341|bibcode=1970JGR....75.2341L }}{{efn|See radon storm.}} 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.{{Citation |last1=S. |first1=Sukanya |title=Radon Distribution in Groundwater and River Water |date=2023 |work=Environmental Radon: A Tracer for Hydrological Studies |pages=53–87 |editor-last=S. |editor-first=Sukanya |url=https://link.springer.com/chapter/10.1007/978-981-99-2672-5_3 |access-date=2024-10-15 |place=Singapore |publisher=Springer Nature |language=en |doi=10.1007/978-981-99-2672-5_3 |isbn=978-981-99-2672-5 |last2=Joseph |first2=Sabu |editor2-last=Joseph |editor2-first=Sabu}}

Radon soil concentration has been used to map buried close-subsurface geological faults because concentrations are generally higher over the faults.{{cite journal |author=Richon, P. |author2=Y. Klinger |author3=P. Tapponnier |author4=C.-X. Li |author5=J. Van Der Woerd |author6=F. Perrier |name-list-style=amp |date=2010 |title=Measuring radon flux across active faults: Relevance of excavating and possibility of satellite discharges |url=http://www.ipgp.fr/~klinger/page_web/biblio/publication/Richon_RadMeas2010%20.pdf |journal=Radiat. Meas. |volume=45 |pages=211–218 |doi=10.1016/j.radmeas.2010.01.019 |issue=2 |bibcode=2010RadM...45..211R |hdl=10356/101845 |access-date=2011-08-20 |archive-date=2013-06-26 |archive-url=https://web.archive.org/web/20130626115736/http://www.ipgp.fr/~klinger/page_web/biblio/publication/Richon_RadMeas2010%20.pdf |url-status=dead }} Similarly, it has found some limited use in prospecting for geothermal gradients.{{cite conference |last1=Semprini |first1=Lewis |last2=Kruger |first2=Paul |date=April 1980 |title=Radon Transect Analysis In Geothermal Reservoirs |conference=SPE California Regional Meeting, 9–11 April, Los Angeles, California |doi=10.2118/8890-MS |isbn=978-1-55563-700-2}}

Some researchers have investigated changes in groundwater radon concentrations for earthquake prediction.{{Unbulleted list citebundle|{{cite journal |author=Igarashi, G. |author2=Wakita, H. |date=1995 |title=Geochemical and hydrological observations for earthquake prediction in Japan |journal=Journal of Physics of the Earth |volume=43 |pages=585–598 |url=http://www.jstage.jst.go.jp/article/jpe1952/43/5/43_5_585/_pdf |doi=10.4294/jpe1952.43.585 |issue=5|doi-access=free }}|{{Cite journal |first1=Masayasu |last1=Noguchi |last2=Wakita |first2=Hiroshi |date=10 March 1977 |journal=Journal of Geophysical Research |doi=10.1029/JB082i008p01353 |title=A method for continuous measurement of radon in groundwater for earthquake prediction |pages= 1353–1357 |volume=82 |issue=8|bibcode=1977JGR....82.1353N }}}}{{cite journal |author=Richon, P. |author2=Sabroux, J.-C.|author3=Halbwachs, M.|author4=Vandemeulebrouck, J.|author5=Poussielgue, N.|author6=Tabbagh, J. |author7=Punongbayan, R. |date=2003 |title=Radon anomaly in the soil of Taal volcano, the Philippines: A likely precursor of the M 7.1 Mindoro earthquake (1994) |journal=Geophysical Research Letters |volume=30 |issue=9 |page=34 |doi=10.1029/2003GL016902|bibcode=2003GeoRL..30.1481R|s2cid=140597510 }} Increases in radon were noted before the 1966 Tashkent{{Cite book |editor-last=Cothern |editor-first=C.Richard | editor-last2=Smith | editor-first2=James E. |date=1987 |title=Environmental Radon |url=https://books.google.com/books?id=K7WvwZlc72MC&pg=PA53|series=Environmental Science Research |volume=35 | publisher=Springer Science & Business Media | publication-place=New York |isbn=978-0-306-42707-7|page=53}} 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.{{cite news |url=https://www.npr.org/templates/story/story.php?storyId=102804333 |title=Expert: Earthquakes Hard To Predict |newspaper=NPR.org |access-date=2009-05-05}} As of 2009, it was under investigation as a possible earthquake precursor by NASA;{{cite web |url=https://www.earthmagazine.org/article/earthquake-prediction-gone-and-back-again/ |title=EARTH Magazine: Earthquake prediction: Gone and back again |date=2012-01-05}} further research into the subject has suggested that abnormalities in atmospheric radon concentrations can be an indicator of seismic movement.{{Cite journal|doi=10.1038/s41598-024-61887-6 |last1=Tsuchiya |first1=Mayu |last2=Nagahama |first2=Hiroyuki |last3=Muto |first3=Jun |first4=Mitsuhiro |last4=Hirano |first5=Yumi |last5=Yasuoka |title=Detection of atmospheric radon concentration anomalies and their potential for earthquake prediction using Random Forest analysis |journal=Sci Rep |volume=14 |issue=11626 |date=2024|page=11626 |pmid=38821969 |bibcode=2024NatSR..1411626T |pmc=11143197 }}

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.{{cite web |title= Radon and Naturally Occurring Radioactive Materials (NORM) associated with Hot Rock Geothermal Systems |publisher= Government of South Australia—Primary Industries and Resources SA |access-date= 2013-07-16 |url= http://www.pir.sa.gov.au/__data/assets/pdf_file/0013/113341/090107_web.pdf |archive-url= https://web.archive.org/web/20120402134109/http://www.pir.sa.gov.au/__data/assets/pdf_file/0013/113341/090107_web.pdf |archive-date= 2012-04-02 |url-status= dead }} 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 carbon-14 emissions in nuclear power plants.{{Cite journal|url=https://www.iaea.org/sites/default/files/publications/magazines/bulletin/bull31-2/31205642131.pdf |title=Radiation versus radiation: Nuclear energy in perspective |journal=IAEA Bulletin |issue=2 |date=1989 |first1=Abel J. |last1=Gonzalez |first2=Jeanne |last2=Anderer}}

In the 1940s and 1950s, radon produced from a radium source was used for industrial radiography.{{Unbulleted list citebundle|{{cite journal |doi=10.1088/0950-7671/23/7/301 |title=Radon. Its Properties and Preparation for Industrial Radiography |date=1946 |author=Dawson, J. A. T. |journal=Journal of Scientific Instruments |volume=23 |page=138 |issue=7 |bibcode = 1946JScI...23..138D }}|{{cite journal |title= Use of radon for industrial radiography |first= A. |last= Morrison |journal= Canadian Journal of Research |date= 1945 |volume= 23f |issue= 6 |pages= 413–419 |doi= 10.1139/cjr45f-044 |pmid= 21010538 }}}} Other X-ray sources such as Cobalt-60 and Iridium-192 became available after World War II and quickly replaced radium and thus radon for this purpose, being of lower cost and hazard.{{Unbulleted list citebundle|{{Cite web|url=https://www.orau.org/health-physics-museum/collection/radioactive-sources/radium-industrial-radiography-source.html |website=ORAU Museum of Radiation and Radioactivity |title=Radium Industrial Radiography Source (ca. 1940s) |access-date=22 August 2024}}|{{Cite web|url=https://www.nde-ed.org/NDETechniques/Radiography/Introduction/history.xhtml |website=Iowa State University Center for Nondestructive Evaluation |title=History of Radiography |access-date=22 August 2024}}}}

{{Main|Health effects of radon}}

{{sup|222}}Rn decay products have been classified by the International Agency for Research on Cancer as being carcinogenic to humans,{{cite web |access-date=2008-06-26 |url=http://www.cancer.org/docroot/PED/content/PED_1_3x_Known_and_Probable_Carcinogens.asp |archive-url=https://web.archive.org/web/20031213030702/http://www.cancer.org/docroot/PED/content/PED_1_3x_Known_and_Probable_Carcinogens.asp |url-status=dead |archive-date=2003-12-13 |title=Known and Probable Carcinogens |publisher=American Cancer Society}} 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,{{cite book |title=A Century of X-rays and Radioactivity in Medicine |author=Mould, Richard Francis |date=1993 |isbn=978-0-7503-0224-1 |publisher=CRC Press}} 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{{Unbulleted list citebundle|{{Cite news |issn=0040-781X |title=Uranium Miners' Cancer |magazine=Time |access-date=2008-06-26 |date=1960-12-26 |url=http://www.time.com/time/magazine/article/0,9171,895156,00.html |archive-url=https://web.archive.org/web/20090115070225/http://www.time.com/time/magazine/article/0,9171,895156,00.html |url-status=dead |archive-date=January 15, 2009 }}|{{cite news |url=http://www.irsn.fr/FR/Larecherche/publications-documentation/Publications_documentation/BDD_publi/DRPH/LEADS/Documents/IRPA10-P2A-56.pdf |author=Tirmarche M. |author2=Laurier D. |author3=Mitton N. |author4=Gelas J. M. |title=Lung Cancer Risk Associated with Low Chronic Radon Exposure: Results from the French Uranium Miners Cohort and the European Project |access-date=2009-07-07 |archive-date=December 19, 2021 |archive-url=https://web.archive.org/web/20211219230252/https://www.irsn.fr/FR/Larecherche/publications-documentation/Publications_documentation/BDD_publi/DRPH/LEADS/Documents/IRPA10-P2A-56.pdf |url-status=dead }}|{{Cite journal |doi=10.1001/jama.1989.03430050045024 |volume=262 |last1=Roscoe |first1=R. J. |last2=Steenland |first2=K. |last3=Halperin |first3=W. E. |last4=Beaumont |first4=J. J. |last5=Waxweiler |first5=R. J. |title=Lung cancer mortality among nonsmoking uranium miners exposed to radon daughters| journal=Journal of the American Medical Association |date=1989-08-04 |pmid=2746814 |issue=5 |pages=629–633}}}} and South Australia.{{Cite journal |jstor = 3553403 |title = Radon Daughter Exposures at the Radium Hill Uranium Mine and Lung Cancer Rates among Former Workers, 1952–87 |last1 = Woodward |first1 = Alistair |date = 1991-07-01 |journal = Cancer Causes & Control |doi = 10.1007/BF00052136 |pmid = 1873450|volume = 2 |issue = 4 |pages = 213–220 |last2 = Roder |first2 = David |last3 = McMichael |first3 = Anthony J. |last4 = Crouch |first4 = Philip |last5 = Mylvaganam |first5 = Arul|s2cid = 9664907 }} Despite these hazards being known in the early 1950s,{{cite news |title = Uranium mine radon gas proves health danger (1952) |url = https://www.newspapers.com/clip/3853075/uranium_mine_radon_gas_proves_health/ |newspaper = The Salt Lake Tribune |date = 27 September 1952 |page = 13 |access-date = 2015-12-22}} 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,{{Unbulleted list citebundle|{{cite news |title = Radon gas mine health benefits advertisement (1953) |url = https://www.newspapers.com/clip/3869275/radon_gas_mine_health_benefits/ |newspaper = Greeley Daily Tribune |date = 27 March 1953 |page = 4 |access-date = 2015-12-22}}|{{cite web |title = Clipping from The Montana Standard |url = https://www.newspapers.com/clip/3869277/the_montana_standard/ |website = Newspapers.com |access-date = 2015-12-22}}}} but the government banned such advertisements in 1975,{{cite web |title = Government bans Boulder mine ads about radon health benefits (1975) |url = https://www.newspapers.com/clip/3869269/government_bans_boulder_mine_ads_about/ |website = Newspapers.com |access-date = 2015-12-22}} and subsequent works have debated the truth of such claimed health effects, citing the documented ill effects of radiation on the body.{{cite journal |last=Salak |first=Kara |author2=Nordeman, Landon |year=2004 |title=59631: Mining for Miracles |url=http://ngm.nationalgeographic.com/ngm/0401/feature7/index.html |url-status=dead |journal=National Geographic |publisher=National Geographic Society |archive-url=https://web.archive.org/web/20080124233142/http://ngm.nationalgeographic.com/ngm/0401/feature7/index.html |archive-date=January 24, 2008 |access-date=June 26, 2008}}

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.{{cite journal |author=Darby, S. |author2=Hill, D. |author3=Doll, R. |date=2005 |title=Radon: a likely carcinogen at all exposures |journal=Annals of Oncology |volume=12 |issue=10 |pages=1341–1351 |doi=10.1023/A:1012518223463 |pmid=11762803 |doi-access=free}} As the relative risk for miners has decreased, so has the ability to detect excess risks among that population.{{cite web |url=http://www.unscear.org/unscear/en/publications/2006_1.html |title=UNSCEAR 2006 Report Vol. I |publisher=United Nations Scientific Committee on the Effects of Atomic Radiation UNSCEAR 2006 Report to the General Assembly, with scientific annexes}}

File:Uranium waste near Rifle, Colorado.jpg. Waste from uranium mining has been allowed to settle and is exposed to the atmosphere, leading to the release of radon gas into the air and decay products into the groundwater.]]

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.{{cite journal

| url= http://www.rad-journal.org/helper/download.php?file=../papers/RadJ.2016.03.041.pdf

| title= Radon exhalation of the uranium tailings dump Digmai, Tajikistan

| author1= Schläger, M. |author2=Murtazaev, K. |author3= Rakhmatuloev, B. |author4= Zoriy, P.|author5= Heuel-Fabianek, B.

| year= 2016

| journal= Radiation and Applications

| volume=1 |pages=222–228

| doi=10.21175/RadJ.2016.03.041

| doi-access=free

}} The release of radon may be mitigated by covering tailings with soil or clay, though other decay products may leach into groundwater supplies.{{Cite web|url=https://www.energy.gov/lm/articles/uranium-mining-and-milling-near-rifle-colorado |website=Office of Legacy Management |via=Energy.gov |date=April 19, 2016 |title=Uranium Mining and Milling near Rifle, Colorado }}

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.{{Cite journal |last=Chen |first=Jing |date=April 2023 |title=A Review of Radon Exposure in Non-uranium Mines—Estimation of Potential Radon Exposure in Canadian Mines |journal=Health Physics |language=en |volume=124 |issue=4 |pages=244–256 |doi=10.1097/HP.0000000000001661 |issn=1538-5159 |pmc=9940829 |pmid=36607249}} However, older or abandoned uranium mines without ventilation may still have extremely high radon levels.{{Cite journal |last1=Miklyaev |first1=Petr S. |last2=Petrova |first2=Tatiana B. |last3=Maksimovich |first3=Nikolay G. |last4=Krasikov |first4=Alexey V. |last5=Klimshin |first5=Aleksey V. |last6=Shchitov |first6=Dmitriy V. |last7=Sidyakin |first7=Pavel A. |last8=Tsebro |first8=Dmitriy N. |last9=Meshcheriakova |first9=Olga Yu. |date=2024-02-01 |title=Comparative studies on radon seasonal variations in various underground environments: Cases of abandoned Beshtaugorskiy uranium mine and Kungur Ice Cave |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X23002394 |journal=Journal of Environmental Radioactivity |volume=272 |pages=107346 |doi=10.1016/j.jenvrad.2023.107346 |pmid=38043218 |bibcode=2024JEnvR.27207346M |issn=0265-931X}}

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,{{cite journal |pmid=16759978 |title=Incidence of leukemia, lymphoma, and multiple myeloma in Czech uranium miners: a case-cohort study |last1=Rericha |first1=V. |last2=Kulich |first2=M. |last3=Rericha |first3=R. |last4=Shore |first4=D. L. |last5=Sandler |first5=D. P. |date=2007 |volume=114 |journal=Environmental Health Perspectives |issue=6 |pmc=1480508 |pages=818–822 |doi=10.1289/ehp.8476}} 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).{{Cite journal |last1=Nunes |first1=Leonel J. R. |last2=Curado |first2=António |last3=da Graça |first3=Luís C. C. |last4=Soares |first4=Salete |last5=Lopes |first5=Sérgio Ivan |date=2022-03-25 |title=Impacts of Indoor Radon on Health: A Comprehensive Review on Causes, Assessment and Remediation Strategies |journal=International Journal of Environmental Research and Public Health |volume=19 |issue=7 |pages=3929 |doi=10.3390/ijerph19073929 |issn=1661-7827 |pmc=8997394 |pmid=35409610 |doi-access=free}} 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){{Cite book |last= |first= |url=https://books.google.com/books?id=shCh5KzE7xEC&q=%22compensation+scheme+for+radiation+linked+diseases%22&pg=PA20 |title=The Redfern Inquiry into human tissue analysis in UK nuclear facilities |date=2010-11-16 |publisher=The Stationery Office |isbn=9780102966183 |location= |pages= |language=en |quote= |via=}} and the Radiation Exposure Compensation Act (in the United States).{{cite web |last1= |date=July 21, 2004 |title=An Overview of the Radiation Exposure Compensation Program |url=http://www.gpo.gov/fdsys/pkg/CHRG-108shrg25152/html/CHRG-108shrg25152.htm |access-date=August 28, 2024 |website=www.gpo.gov |publisher=United States Senate and the U.S. Government Printing Office}}

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,{{cite web |date=February 27, 2024 |title=Health Risk of Radon |url=https://www.epa.gov/radon/health-risk-radon |access-date=August 15, 2024 |website=Environmental Protection Agency}} with the first one being smoking.{{cite journal |vauthors=Schabath MB, Cote ML |date=October 2019 |title=Cancer Progress and Priorities: Lung Cancer |journal=Cancer Epidemiol Biomarkers Prev |volume=28 |issue=10 |at=Radon |doi=10.1158/1055-9965.EPI-19-0221 |pmc=6777859 |pmid=31575553}} Others have reached similar conclusions for the United Kingdom and France.{{cite journal |author=Catelinois O. |author2=Rogel A. |author3=Laurier D. |last4=Billon |first4=Solenne |last5=Hemon |first5=Denis |last6=Verger |first6=Pierre |last7=Tirmarche |first7=Margot |date=2006 |title=Lung cancer attributable to indoor radon exposure in france: impact of the risk models and uncertainty analysis |journal=Environmental Health Perspectives |volume=114 |issue=9 |pages=1361–1366 |doi=10.1289/ehp.9070 |pmc=1570096 |pmid=16966089|bibcode=2006EnvHP.114.1361C }} Radon exposure in buildings may arise from subsurface rock formations and certain building materials (e.g., some granites).{{Cite book |last1=Todorović |first1=N. |title=Radon: geology, environmental impact and toxicity concerns |last2=Nikolov |first2=J. |last3=Petrović Pantić |first3=T. |last4=Kovačević |first4=J. |last5=Stojković |first5=I. |last6=Krmar |first6=M. |date=2015 |publisher=Nova Science Publishers, Inc. |isbn=978-1-63463-742-8 |editor-last1=Stacks |editor-first1=Audrey M. |pages=163–187 |chapter=Radon in Water - Hydrogeology and Health Implication}} 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.{{Cite journal |last1=Čeliković |first1=Igor |last2=Pantelić |first2=Gordana |last3=Vukanac |first3=Ivana |last4=Krneta Nikolić |first4=Jelena |last5=Živanović |first5=Miloš |last6=Cinelli |first6=Giorgia |last7=Gruber |first7=Valeria |last8=Baumann |first8=Sebastian |last9=Quindos Poncela |first9=Luis Santiago |last10=Rabago |first10=Daniel |date=2022-01-07 |title=Outdoor Radon as a Tool to Estimate Radon Priority Areas—A Literature Overview |journal=International Journal of Environmental Research and Public Health |volume=19 |issue=2 |pages=662 |doi=10.3390/ijerph19020662 |issn=1661-7827 |pmc=8775861 |pmid=35055485 |doi-access=free}}

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.{{cite web |url=http://www.epa.gov/radon/pubs/citguide.html |title=A Citizen's Guide to Radon |date=October 12, 2010 |work=www.epa.gov |publisher=United States Environmental Protection Agency |access-date=January 29, 2012}}{{Cite web |url=https://www.census.gov/quickfacts/fact/table/US/PST045221.html |title=QuickFacts |date=2022-07-01 |work=www.census.gov |publisher=United States Census Bureau |access-date=2023-03-08}} In Europe, 2% of all cancers have been attributed to radon;{{Cite journal |last1=Ngoc |first1=Le Thi Nhu |last2=Park |first2=Duckshin |last3=Lee |first3=Young-Chul |date=2022-12-21 |title=Human Health Impacts of Residential Radon Exposure: Updated Systematic Review and Meta-Analysis of Case–Control Studies |journal=International Journal of Environmental Research and Public Health |volume=20 |issue=1 |pages=97 |doi=10.3390/ijerph20010097 |doi-access=free |issn=1661-7827 |pmc=9819115 |pmid=36612419}} 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.{{Unbulleted list citebundle|{{Cite web|title=Žlahtni plin v Sloveniji vsako leto kriv za 120 smrti|url=https://www.24ur.com/novice/preverjeno/zlahtni-plin-v-sloveniji-vsako-leto-kriv-za-120-smrti.html|access-date=2021-11-02|website=www.24ur.com|language=sl}}|{{Cite web |url=https://www.stat.si/StatWeb/en/News/Index/9212 |date=2021-01-01 |title=Population, Slovenia, 1 January 2021 |publisher=Republic of Slovenia Statistical Office (Source: SURS) |access-date=2023-03-08 |work=www.stat.si |archive-date=2022-01-11 |archive-url=https://web.archive.org/web/20220111171853/https://www.stat.si/StatWeb/en/News/Index/9212 |url-status=dead }}}} 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.{{Cite report|archive-url=https://web.archive.org/web/20100528010149/http://deainfo.nci.nih.gov//advisory/pcp/pcp08-09rpt/PCP_Report_08-09_508.pdf |url=http://deainfo.nci.nih.gov//advisory/pcp/pcp08-09rpt/PCP_Report_08-09_508.pdf |title=Reducing Environmental Cancer Risk – What We Can Do Now |publisher=US Department of Health and Human Services |chapter=Exposure to Environmental Hazards from Natural Sources |pages=89–92 |date=April 2010 |archive-date=May 28, 2010}} However, the conclusion that exposure to low levels of radon leads to elevated risk of lung cancer has been disputed,{{Unbulleted list citebundle|{{cite journal |last=Fornalski |first=K. W. |author2=Adams, R. |author3=Allison, W. |author4=Corrice, L. E. |author5=Cuttler, J. M. |author6=Davey, Ch. |author7=Dobrzyński, L. |author8=Esposito, V. J. |author9=Feinendegen, L. E. |author10=Gomez, L. S. |author11=Lewis, P. |author12=Mahn, J. |author13=Miller, M. L. |author14=Pennington, Ch. W. |author15=Sacks, B. |author16=Sutou, S. |author17=Welsh, J. S. |pmid=26223888 |title=The assumption of radon-induced cancer risk |year=2015 |journal=Cancer Causes & Control |doi=10.1007/s10552-015-0638-9 |issue=26 |volume=10 |pages=1517–18|s2cid=15952263 }}|{{cite journal |last=Becker |first=K. |pmid=19330110 |title=Health Effects of High Radon Environments in Central Europe: Another Test for the LNT Hypothesis? |year=2003 |journal=Nonlinearity in Biology, Toxicology and Medicine |issue=1 |volume=1 |pages=3–35 |pmc=2651614|doi=10.1080/15401420390844447 }}|{{cite journal |author=Cohen B. L. |title=Test of the linear-no threshold theory of radiation carcinogenesis for inhaled radon decay products |journal=Health Physics |volume=68 |issue=2 |year=1995 |pmid=7814250 |url=http://www.phyast.pitt.edu/%7Eblc/LNT-1995.PDF |doi=10.1097/00004032-199502000-00002 |pages=157–74|s2cid=41388715 }}}} 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 {{Sup|222}}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,{{Cite journal|first1=Stefanie |last1=Gierl |first2=Oliver |last2=Meisenberg |first3=Peter |last3=Feistenauer |first4=Jochen |last4=Tschiersch |doi=10.1093/rpd/ncu076 |title=Thoron and thoron progeny measurements in German clay houses |journal=Radiation Protection Dosimetry |volume=160 |date=April 17, 2014 |issue=1–3 |pages= 160–163|pmid=24743764 }} and in regions with thorium- and monazite-rich soil and sand.{{Cite journal |last1=Ramola |first1=R.C. |last2=Prasad |first2=Mukesh |date=December 2020 |title=Significance of thoron measurements in indoor environment |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X20306998 |journal=Journal of Environmental Radioactivity |language=en |volume=225 |pages=106453 |doi=10.1016/j.jenvrad.2020.106453|pmid=33120031 |bibcode=2020JEnvR.22506453R }} Thoron is a minor contributor to the overall radiation dose received due to indoor radon exposure,{{Cite journal |last=Chen |first=Jing |date=2022 |title=Assessment of thoron contribution to indoor radon exposure in Canada |journal=Radiation and Environmental Biophysics |volume=61 |issue=1 |pages=161–167 |doi=10.1007/s00411-021-00956-0 |issn=0301-634X |pmc=8897316 |pmid=34973065|bibcode=2022REBio..61..161C }} and can interfere with {{Sup|222}}Rn measurements when not taken into account.

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.{{Cite book|url=http://whqlibdoc.who.int/publications/2009/9789241547673_eng.pdf |date=2009 |title=WHO Handbook on Indoor Radon |publisher=World Health Organization |archive-date=March 4, 2012 |archive-url=https://web.archive.org/web/20120304001907/http://whqlibdoc.who.int/publications/2009/9789241547673_eng.pdf |isbn=978-92-4-154767-3}}

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),{{cite web |title =Radiation Protection: Radon |publisher=United States Environmental Protection Agency |date=November 2007 |url=http://www.epa.gov/radiation/radionuclides/radon.html |access-date =2008-04-17}} 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.{{cite web |url=http://www.euro.who.int/__data/assets/pdf_file/0006/97053/4.6_-RPG4_Rad_Ex1-ed2010_editedViv_layouted.pdf |title=Radon Levels in Dwellings: Fact Sheet 4.6 |date=December 2009 |publisher=European Environment and Health Information System |access-date=2013-07-16 }} 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.{{cite web |title=HPA issues new advice on radon |publisher=UK Health Protection Agency |date=July 2010 |url=http://www.hpa.org.uk/NewsCentre/NationalPressReleases/2010PressReleases/100708Newadviceonradon/ |archive-url=https://web.archive.org/web/20100714170654/http://www.hpa.org.uk/NewsCentre/NationalPressReleases/2010PressReleases/100708Newadviceonradon/ |url-status=dead |archive-date=2010-07-14 |access-date=2010-08-13}} Similar levels (as in the UK) are published by Norwegian Radiation and Nuclear Safety Authority (DSA){{Cite web|title=Radon mitigation measures|url=https://dsa.no/en/radon/radon-mitigation-measures|access-date=2021-07-12|website=DSA|language=no}} 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.{{Cite web|url=https://www2.dsa.no/publication/strategy-for-the-reduction-of-radon-exposure-in-norway.pdf|title=Strategy for the reduction of radon exposure in Norway, 2010|accessdate=14 March 2023|archive-date=20 November 2021|archive-url=https://web.archive.org/web/20211120103812/https://www.dsa.no/publication/strategy-for-the-reduction-of-radon-exposure-in-norway.pdf|url-status=dead}}

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.{{cite web |title=What Are the Risk Factors for Lung Cancer? |url=https://www.cdc.gov/cancer/lung/basic_info/risk_factors.htm |website=Centers for Disease Control and Prevention |access-date=3 May 2020 |date=18 September 2019}}

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.{{cite journal |doi=10.1136/bmj.38308.477650.63 |pmid=15613366 |pmc=546066 |title=Radon in homes and risk of lung cancer: Collaborative analysis of individual data from 13 European case-control studies |journal=BMJ |volume=330 |issue=7485 |pages=223 |year=2005 |last1=Darby |first1=S. |last2=Hill |first2=D. |last3=Auvinen |first3=A. |last4=Barros-Dios |first4=J. M. |last5=Baysson |first5=H. |last6=Bochicchio |first6=F. |last7=Deo |first7=H. |last8=Falk |first8=R. |last9=Forastiere |first9=F. |last10=Hakama |first10=M. |last11=Heid |first11=I. |last12=Kreienbrock |first12=L. |last13=Kreuzer |first13=M. |last14=Lagarde |first14=F. |last15=Mäkeläinen |first15=I. |last16=Muirhead |first16=C. |last17=Oberaigner |first17=W. |last18=Pershagen |first18=G. |last19=Ruano-Ravina |first19=A. |last20=Ruosteenoja |first20=E. |last21=Rosario |first21=A. Schaffrath |last22=Tirmarche |first22=M. |last23=Tomášek |first23=L. |last24=Whitley |first24=E. |last25=Wichmann |first25=H.-E. |last26=Doll |first26=R. }} 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.{{cite web |first=R. William |last=Field |location=Charleston, South Carolina |url=https://www.aarst.org/images/PCPanelRadonTest.pdf |title=President's Cancer Panel, Environmental Factors in Cancer: Radon |date=December 4, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130829005508/http://www.aarst.org/images/PCPanelRadonTest.pdf |archive-date=August 29, 2013 |publisher=The American Association of Radon Scientists and Technologists (AARST)}}

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.{{cite journal |last1=Kaufman |first1=E. L. |last2=Jacobson |first2=J. S. |last3=Hershman |first3=D. L. |last4=Desai |first4=M. |last5=Neugut |first5=A. I. |date=2008 |title=Effect of breast cancer radiotherapy and cigarette smoking on risk of second primary lung cancer |journal=Journal of Clinical Oncology |volume=26 |issue=3 |pages=392–398 |doi=10.1200/JCO.2007.13.3033 |pmid=18202415|doi-access=free }} This is also supported by new discussion about the calculation method, the linear no-threshold model, which routinely has been used.{{cite journal |doi=10.1093/rpd/ncq141 |title=Review and evaluation of updated research on the health effects associated with low-dose ionising radiation |date=2010 |last1=Dauer |first1=L. T. |last2=Brooks |first2=A. L. |last3=Hoel |first3=D. G. |last4=Morgan |first4=W. F. |last5=Stram |first5=D. |last6=Tran |first6=P. |journal=Radiation Protection Dosimetry |volume=140 |issue=2 |pages=103–136 |pmid=20413418}}

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.{{cite journal |last1=Lagarde |first1=F. |last2=Axelsson |first2=G. |last3=Damber |first3=L. |last4=Mellander |first4=H. |last5=Nyberg |first5=F. |last6=Pershagen |first6=G. |date=2001 |title=Residential radon and lung cancer among never-smokers in Sweden |journal=Epidemiology |volume=12 |issue=4 |pages=396–404 |doi=10.1097/00001648-200107000-00009 |jstor=3703373 |pmid=11416777|s2cid=25719502 |doi-access=free }}

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;[http://www.nap.edu/openbook.php?isbn=0309062926 Risk Assessment of Radon in Drinking Water]. Nap.edu (2003-06-01). Retrieved on 2011-08-20. Water from underground sources may contain significant amounts of radon depending on the surrounding rock and soil conditions, whereas surface sources generally do not.{{cite web |url=http://water.epa.gov/lawsregs/rulesregs/sdwa/radon/basicinformation.cfm |title=Basic Information about Radon in Drinking Water |access-date=2013-07-24 }} 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.{{Cite web |last=Johnson |first=Jan |date=28 October 2019 |title=Answer to Question #13127 Submitted to "Ask the Experts" |url=https://hps.org/publicinformation/ate/q13127.html |access-date=2024-09-23 |website=Health Physics Society}}

Ocean surface concentrations of radon exchange within the atmosphere, causing ²²²Rn to increase through the air-sea interface.{{Cite journal|url=https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JC080i027p03828|doi=10.1029/JC080i027p03828|title=Radon 222 from the ocean surface|year=1975|last1=Wilkening|first1=Marvin H.|last2=Clements|first2=William E.|journal=Journal of Geophysical Research|volume=80|issue=27|pages=3828–3830|bibcode=1975JGR....80.3828W}} Although areas tested were very shallow, additional measurements in a wide variety of coastal regimes should help define the nature of ²²²Rn observed.

{{Main|Radon mitigation}}

File:Radon detector.jpg

Image:Radon test kit.jpg

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.{{cite web |url=https://hps.org/publicinformation/ate/q10299.html |title=Answer to Question #10299 Submitted to "Ask the Experts" |last=Baes |first=Fred |website=Health Physics Society |access-date=2016-05-19}}

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:{{cite web |author=World Health Organization |title=Radon and cancer, fact sheet 291 |url=https://www.who.int/mediacentre/factsheets/fs291/en/index.html |author-link=World Health Organization}}

• 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".{{cite web | url = http://www.epa.gov/radon/pubs/consguid.html#reductiontech| title = Consumer's Guide to Radon Reduction: How to fix your home| access-date = 2010-04-03| publisher = EPA}}

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).{{cite book |url=https://books.google.com/books?id=bspdQ8H2yUcC&pg=PT46 |page=46 |title=Building radon out a step-by-step guide on how to build radonresistant homes |publisher=DIANE Publishing |isbn=978-1-4289-0070-7}} 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."

{{Portal|Chemistry}}

• International Radon Project
• Lucas cell
• Pleochroic halo (aka radiohalo)
• Radiation Exposure Compensation Act

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{{Wiktionary}}

{{Wikiversity|Radon atom}}

• [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

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