View source for Radon

{{Use mdy dates|date=September 2024}}{{Hatnote group|{{About|the chemical element}}
{{Distinguish|Radom|Radium|Rodan}}
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{{Infobox radon}}
'''Radon''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Rn''' and [[atomic number]] 86. It is a [[radioactive decay|radioactive]] [[noble gas]] and is colorless and odorless. Of the three naturally occurring radon [[Isotope|isotopes]], only [[radon-222|{{sup|222}}Rn]] has a sufficiently long [[half-life]] (3.825 days) for it to be released from the soil and rock where it is generated. Radon isotopes are the immediate [[decay product]]s of [[radium]] isotopes. 

The instability of {{sup|222}}Rn, its most stable isotope, makes radon one of the rarest elements. Radon will be present on Earth for several billion more years despite its short half-life, because it is constantly being produced as a step in the decay chains of [[uranium-238|{{sup|238}}U]] and [[thorium-232|{{sup|232}}Th]], both of which are abundant radioactive nuclides with half-lives of at least several billion years. The decay of radon produces many other short-lived [[nuclide]]s, known as "radon daughters", ending at stable isotopes of [[lead]].<ref name="USPHS90">{{Cite web|url=http://www.bvsde.paho.org/bvstox/i/fulltext/toxprofiles/radon.pdf |title=Toxicological profile for radon |archive-url=https://web.archive.org/web/20160415161629/http://www.bvsde.paho.org/bvstox/i/fulltext/toxprofiles/radon.pdf |archive-date=2016-04-15 |work=[[Agency for Toxic Substances and Disease Registry]] |publisher=U.S. Public Health Service, In collaboration with U.S. Environmental Protection Agency|date= December 1990}}</ref> [[Radon-222|{{sup|222}}Rn]] occurs in significant quantities as a step in the normal radioactive [[decay chain]] of {{sup|238}}U, also known as the [[uranium series]], which slowly decays into a variety of radioactive nuclides and eventually decays into stable [[lead-206|{{sup|206}}Pb]]. [[Radon-220|{{sup|220}}Rn]] occurs in minute quantities as an intermediate step in the decay chain of {{sup|232}}Th, also known as the [[thorium series]], which eventually decays into stable [[lead-208|{{sup|208}}Pb]]. 

Radon was discovered in 1899 by [[Ernest Rutherford]] and [[Robert Bowie Owens|Robert B. Owens]] at [[McGill University]] in [[Montreal]], and was the fifth radioactive element to be discovered. First known as "emanation", the radioactive gas was identified during experiments with radium, [[thorium oxide]], and actinium by [[Friedrich Ernst Dorn]], Rutherford and Owens, and [[André-Louis Debierne]], respectively, and each element's emanation was considered to be a separate substance: radon, thoron, and actinon. Sir [[William Ramsay]] and [[Robert Whytlaw-Gray]] considered that the radioactive emanations may contain a new element of the noble gas family, and isolated "radium emanation" in 1909 to determine its properties. In 1911, the element Ramsay and Whytlaw-Gray isolated was accepted by the [[Commission on Isotopic Abundances and Atomic Weights|International Commission for Atomic Weights]], and in 1923, the International Committee for Chemical Elements and the [[International Union of Pure and Applied Chemistry|International Union of Pure and Applied Chemistry (IUPAC)]] chose radon as the accepted name for the element's most stable isotope, {{sup|222}}Rn; thoron and actinon were also recognized by IUPAC as distinct [[isotope|isotopes]] of the element.<ref name="ThorntonBurdette" />

A common source of environmental radon is uranium-containing minerals in the ground. Radon can also occur in ground water, such as [[Spring (hydrosphere)|spring]] waters and hot springs.<ref>{{cite web |title=Facts about Radon |publisher=Facts about |url=http://www.facts-about.org.uk/science-element-radon.htm |access-date=2008-09-07 |archive-url=https://web.archive.org/web/20050222004131/http://www.facts-about.org.uk/science-element-radon.htm |archive-date=2005-02-22}}</ref> Radon trapped in [[permafrost]] may be released by [[climate change | climate-change]]-induced [[Permafrost#Impacts of climate change|thawing of permafrosts]],<ref>{{cite web |url=https://www.cbc.ca/news/canada/north/radon-gas-exposure-permafrost-1.6351615 |title=Thawing permafrost can expose northerners to cancer-causing gas, study says |last1=Lamberink |first1=Liny |date=16 February 2022 |publisher=CBC News |website=cbc.ca |archive-url=https://web.archive.org/web/20220217220621/https://www.cbc.ca/news/canada/north/radon-gas-exposure-permafrost-1.6351615 |archive-date=17 February 2022 |url-status=live |access-date=22 February 2024}}</ref> and radon may also be released into groundwater and the atmosphere following seismic events leading to [[earthquake]]s, which has led to its investigation in the field of [[earthquake prediction]].<ref name="EARTHq" /> It is possible to test for radon in buildings, and to use techniques such as sub-slab depressurization for [[Radon mitigation|mitigation]].<ref name="Baraniuk">{{cite journal |last1=Baraniuk |first1=Chris |title=The race against radon |journal=Knowable Magazine |publisher=Annual Reviews |date=11 May 2022 |doi=10.1146/knowable-051122-1 |doi-broken-date=July 12, 2025 |doi-access=free |url=https://knowablemagazine.org/article/physical-world/2022/race-against-radon |access-date=17 May 2022}}</ref><ref>{{Cite web|date=October 28, 2005 |url=https://semspub.epa.gov/work/09/2099558.pdf |title=Skateland Sub-Slab Depressurization Testing Draft Technical Memorandum |publisher=Environmental Protection Agency}}</ref>

[[Epidemiology|Epidemiological]] studies have shown a clear association between breathing high concentrations of radon and incidence of [[lung cancer]].<ref>{{Unbulleted list citebundle|{{cite web |title=Health Risk of Radon |url=https://www.epa.gov/radon/health-risk-radon#head |website=[[United States Environmental Protection Agency]] |language=en |date=14 August 2014}}|{{cite journal |last1=Riudavets |first1=Mariona |last2=Garcia de Herreros |first2=Marta |last3=Besse |first3=Benjamin |last4=Mezquita |first4=Laura |title=Radon and Lung Cancer: Current Trends and Future Perspectives |journal=Cancers |date=27 June 2022 |volume=14 |issue=13 |page=3142 |doi=10.3390/cancers14133142|doi-access=free |pmid=35804914 |pmc=9264880 |bibcode=2022Cance..14.3142R }}|{{cite web |title=Radon |url=https://www.who.int/news-room/fact-sheets/detail/radon-and-health |website=www.who.int |publisher=[[World Health Organization]] |language=en}}}}</ref> Radon is a contaminant that affects [[indoor air quality]] worldwide. Because radon is denser than air it accumulates in basements and crawlspaces under dwellings.
According to the [[United States Environmental Protection Agency]] (EPA), radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA policy-oriented estimates.<ref name="epa" /> Significant uncertainties exist for the health effects of low-dose exposures.<ref name="JRR 2019">{{cite journal |last1=Dobrzynski |first1=Ludwik |last2=Fornalski |first2=Krzysztof W. |last3=Reszczyńska |first3=Joanna |date=23 November 2017 |title=Meta-analysis of thirty-two case–control and two ecological radon studies of lung cancer |journal=Journal of Radiation Research |doi=10.1093/jrr/rrx061  |doi-access=free |pmid=29186473 |pmc=5950923 |volume=59 |issue=2 |pages=149–163}}</ref> Due to local differences in geology, the level of exposure to radon gas differs by location.<ref name="Kusky">{{cite book |last=Kusky |first=Timothy M. |year=2003 |title=Geological Hazards: A Sourcebook |publisher=Greenwood Press |isbn=978-1-57356-469-4 |pages=236–239 |url=https://books.google.com/books?id=ZnARN4s-WRkC}}</ref>

== Characteristics ==

=== Physical properties ===
[[File:Radon spectrum.png|thumb|right|[[Emission spectrum]] of [[radon-222]] (radium emanation), photographed by [[Ernest Rutherford]] in 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radon-222, while the outer two are of [[helium]] (added to calibrate the wavelengths).]]

Radon is a colorless, odorless, and tasteless<ref name="guide">{{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 |archive-url=https://web.archive.org/web/20151004000255/http://www2.epa.gov/radon/citizens-guide-radon-guide-protecting-yourself-and-your-family-radon |archive-date=October 4, 2015 |publisher=Environmental Protection Agency}}</ref> 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&nbsp;kg/m<sup>3</sup>, about 8 times the density of the [[Atmosphere of Earth|Earth's atmosphere]] at sea level, 1.217&nbsp;kg/m<sup>3</sup>.<ref>{{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 |archive-date=May 8, 2013 |archive-url=https://web.archive.org/web/20130508021904/http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html }}</ref> It is one of the densest gases at room temperature (a few are denser, e.g. [[perfluorobutane|CF<sub>3</sub>(CF<sub>2</sub>)<sub>2</sub>CF<sub>3</sub>]] and [[tungsten hexafluoride|WF<sub>6</sub>]]) and is the densest of the noble gases. Radon is colorless at standard temperature and pressure. When cooled below its [[boiling point]] of {{Cvt|211.5|K}}, concentrated liquid radon emits radioluminescence of varying color; solidified radon emits a blue to yellow to red light when cooled further beyond its freezing point of {{convert|202|K}}.<ref name="ramsay-melting">{{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 }}</ref> Due to the hazards associated with high concentrations of radon, liquid and solid radon is almost never seen.<ref name=":0" /> It is sparingly [[soluble]] in water, but more soluble than lighter noble gases. It is appreciably more soluble in [[organic liquid]]s than in water. Its solubility equation is as follows:<ref>{{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 }}}}</ref>

: <math>\chi = \exp(B/T-A)</math>

where <math>\chi</math> is the molar fraction of radon, <math>T</math> is the absolute temperature, and <math>A</math> and <math>B</math> are solvent constants.

=== Chemical properties ===
[[File:Radon decay in a cloud chamber.jpg|thumb|Radon in a [[cloud chamber]]. The [[ionizing radiation]] of radon causes condensation to appear as cloud tracks in the chamber.]]
{{See also|Radon compounds}}
Radon is a member of the zero-[[Valence (chemistry)|valence]] elements that are called noble gases, and is chemically not very [[Reactivity (chemistry)|reactive]]. The [[inert pair effect]] stabilizes the 6s shell, making it unavailable for bonding—a consequence only understood within [[relativistic quantum chemistry]].<ref name="Thayer" />{{rp|66}} The 3.8-day half-life of {{sup|222}}Rn makes it useful in physical sciences as a natural [[Radioactive tracer|tracer]]. Because radon is a gas at standard conditions, unlike its decay-chain parents, it can readily be extracted from them for research.<ref name="Ullmann" />

It is [[Inert gas|inert]] to most common chemical reactions, such as [[combustion]], because the outer [[valence shell]] contains eight [[electron]]s. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.<ref>{{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}}</ref> Its [[first ionization energy]]—the minimum energy required to extract one electron from it—is 1037&nbsp;kJ/mol.<ref>{{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}}</ref> In accordance with [[Periodic table|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}}).<ref>{{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 }}</ref>

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 [[fluoride]]s or [[oxide]]s. Radon can be [[Oxidation|oxidized]] by powerful oxidizing agents such as [[fluorine]], thus forming [[radon difluoride]] ({{chem|RnF|2}}).<ref>{{Unbulleted list citebundle|{{cite journal |author=Stein, L. |date=1970 |journal=[[Science (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}}}}</ref> 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.<ref name="Stein" /> It has a low [[volatility (chemistry)|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 length|bond distance]] of 2.08&nbsp;[[ångström]]s (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart [[xenon difluoride]] ({{chem|XeF|2}}).<ref>{{cite journal |doi=10.1021/jp9825516 |title=Chemical Bonding in XeF<sub>2</sub>, XeF<sub>4</sub>, KrF<sub>2</sub>, KrF<sub>4</sub>, RnF<sub>2</sub>, XeCl<sub>2</sub>, and XeBr<sub>2</sub>: 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}}</ref> The [[Octahedral molecular geometry|octahedral molecule]] [[Radon hexafluoride|{{chem|RnF|6}}]] was predicted to have an even lower [[enthalpy of formation]] than the difluoride.<ref>{{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}}</ref> The [RnF]<sup>+</sup> [[ion]] is believed to form by the following reaction:<ref>{{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 }}</ref>

: 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 mining|uranium mines]] due to the formation of radon–fluorine compounds.<ref name="Ullmann">{{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}}</ref> 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]].<ref name="Stein" /> Additionally, salts of the [RnF]<sup>+</sup> cation with the anions {{chem|SbF|6|-}}, {{chem|TaF|6|-}}, and {{chem|BiF|6|-}} are known.<ref name="Stein">{{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 }}</ref> Radon is also oxidised by [[dioxygen difluoride]] to {{chem|RnF|2}} at {{Convert|173|K||abbr=}}.<ref name="Stein" />

Radon oxides are among the few other reported compounds of radon;<ref>{{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 }}</ref> only the trioxide ({{Chem|Rn|O|3}}) has been confirmed.<ref name="RnO3" /> The higher fluorides {{chem|RnF|4}} and {{chem|RnF|6}} have been claimed, are calculated to be stable, but have not been confirmed.<ref name="Thayer">{{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 }}</ref> 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.<ref name="Stein" /> 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 [[Hydrolysis|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}}[NiF<sub>6</sub>]<sup>2−</sup>, the fact that radon [[Coprecipitation|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 [RnO<sub>3</sub>F]<sup>−</sup> 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 [HRnO<sub>3</sub>]<sup>+</sup> and anionic [HRnO<sub>4</sub>]<sup>−</sup> forms of radon in [[Weak Acid|weakly acidic]] aqueous solution (pH &gt; 5), the procedure having previously been validated by examination of the homologous xenon trioxide.<ref name="RnO3" />

The [[decay technique]] has also been used. Avrorin et al. reported in 1982 that <sup>212</sup>[[francium|Fr]] compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of RnO<sub>3</sub>, but this could not be confirmed.<ref>{{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 }}</ref>

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<sup>+</sup>; 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 interaction|spin–orbit]] splitting of the 6p shell of radon (Rn<sup>IV</sup> 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 [[octahedral molecular geometry|regular octahedral]] molecule, unlike the distorted octahedral structure of {{chem|XeF|6}}, because of the [[inert pair effect]].<ref>{{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}}</ref><ref name="Seppelt">{{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 }}</ref> 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.<ref name="Seppelt"/> Extrapolation down the noble gas group would suggest also the possible existence of RnO, RnO<sub>2</sub>, and RnOF<sub>4</sub>, as well as the first chemically stable noble gas chlorides RnCl<sub>2</sub> and RnCl<sub>4</sub>, but none of these have yet been found.<ref name="Stein" />

Radon [[carbonyl]] (RnCO) has been predicted to be stable and to have a [[linear molecular geometry]].<ref>{{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}}</ref> The molecules {{chem|Rn|2}} and RnXe were found to be significantly stabilized by [[Angular momentum coupling|spin-orbit coupling]].<ref>{{cite journal |doi=10.1002/(SICI)1097-461X(1998)66:2<131::AID-QUA4>3.0.CO;2-W |title=Relativistic pseudopotential calculations on Xe<sub>2</sub>, RnXe, and Rn<sub>2</sub>: 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}}</ref> Radon caged inside a [[fullerene]] has been proposed as a drug for [[tumors]].<ref>{{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}}}}</ref> Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; {{chem|RnF|8}} should be highly unstable chemically<ref name="Thayer" /> (XeF<sub>8</sub> is thermodynamically unstable).

Radon reacts with the liquid [[interhalogen|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<sup>+</sup> and Rn<sup>2+</sup> 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).<ref name="Stein" /> The [[standard electrode potential]] of the Rn<sup>2+</sup>/Rn couple has been estimated as +2.0&nbsp;V,<ref>{{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 }}</ref> although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.<ref name="Stein" />

=== Isotopes ===
{{Main|Isotopes of radon}}

Radon has no [[stable isotope]]s. Thirty-nine radioactive isotopes have been characterized, with [[mass number]]s ranging from 193 to 231.<ref name="Sonzogni-2011">{{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/}}</ref><ref name="229Rn">{{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}}</ref> Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is {{sup|222}}Rn (half-life 3.82&nbsp;days), which is a decay product of [[radium-226|{{sup|226}}Ra]], the latter being itself a decay product of [[uranium-238|{{sup|238}}U]].<ref>{{cite web|title=Principal Decay Scheme of the Uranium Series|url=http://www.gulflink.osd.mil/library/randrep/du/mr1018.7.appa.html|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}}</ref> A trace amount of the (highly unstable) isotope {{sup|218}}Rn (half-life about 35&nbsp;[[millisecond]]s) 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.<ref name="Tretyak2002">{{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 }}</ref>

Three other radon isotopes have a half-life of over an hour: {{sup|211}}Rn (about 15&nbsp;hours), {{sup|210}}Rn (2.4&nbsp;hours) and {{sup|224}}Rn (about 1.8&nbsp;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.<ref name="Sonzogni-2011" /> <!-- No radon isotopes occur significantly in the neptunium (237Np) decay series, though trace amounts of the isotopes 221Rn (26 minutes) and 217Rn (0.5 millisecond) are produced in minor branches. -->
[[Image:Decay chain(4n+2, Uranium series).svg|thumb|upright=1.3|alt=Uranium series|The radium or uranium series]]

=== Daughters ===
{{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 scheme]]s) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence (only main decay branches shown):<ref name="Sonzogni-2011" />
* {{Sup|222}}Rn, 3.82 days, [[alpha decay]]ing to...
* {{Sup|218}}[[Polonium|Po]], 3.10 minutes, alpha decaying to...
* {{Sup|214}}[[Lead|Pb]], 26.8 minutes, [[beta decay]]ing to...
* {{Sup|214}}[[Bismuth|Bi]], 19.9 minutes, beta decaying to...
* {{Sup|214}}Po, 0.1643&nbsp;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<ref>{{cite web |access-date=2009-07-07 |url=http://progenygrp.com/why_measure_rdps.htm |title=Why Measure RDPs? |archive-url=https://web.archive.org/web/20150225020349/http://progenygrp.com/why_measure_rdps.htm |archive-date=2015-02-25}}</ref> 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&nbsp;''[[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&nbsp;WL of daughters for each pCi/L of radon in the air.<ref name="EPA03" /> {{Sup|210}}Pb takes much longer to come in equilibrium with radon, dependent on environmental factors,<ref>{{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 }}</ref> but if the environment permits accumulation of dust over extended periods of time, <sup>210</sup>Pb 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.<ref>{{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 }}}}</ref>

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.<ref>{{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}}</ref>

== History and etymology ==
[[Image:Radon apparatus.png|thumb|upright|Apparatus used by Ramsay and Whytlaw-Gray to isolate radon. '''M''' is a [[capillary tube]], where approximately 0.1&nbsp;mm<sup>3</sup> 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]].<ref name="Rutherford" /> It was the fifth radioactive element to be discovered, after uranium, thorium, radium, and polonium.<ref>{{cite journal |title=Discovery of Radon |journal=[[Nature (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 }}</ref> In 1899, [[Pierre Curie|Pierre]] and [[Marie Curie]] observed that the gas emitted by radium remained radioactive for a month.<ref>{{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}}</ref> Later that year, Rutherford and Owens noticed variations when trying to measure radiation from thorium oxide.<ref name="Rutherford">{{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."</ref> 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),<ref>{{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 }}</ref> 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").<ref>{{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}}}}</ref> In 1901, Rutherford and [[Harriet Brooks]] demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element.<ref>{{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}}</ref> In 1903, similar emanations were observed from actinium by [[André-Louis Debierne]], and were called "actinium emanation" ("Ac Em").<ref>{{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}}}}</ref>

Several shortened names were soon suggested for the three emanations: ''exradio'', ''exthorio'', and ''exactinio'' in 1904;<ref name="ramsay1904">{{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 }}</ref> ''radon'' (Ro), ''thoron'' (To), and ''akton'' or ''acton'' (Ao) in 1918;<ref>{{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 |bibcode=1918ZAACh.103...79S |url=https://zenodo.org/record/1428158 }}</ref> ''radeon'', ''thoreon'', and ''actineon'' in 1919,<ref>{{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 |url-access=subscription }}</ref> and eventually ''radon'', ''thoron'', and ''actinon'' in 1920.<ref>{{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 }}</ref> (The name radon is not related to that of the Austrian mathematician [[Johann Radon]].) The likeness of the [[Spectral line|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.<ref name="ramsay1904" />

In 1909, Ramsay and [[Robert Whytlaw-Gray]] isolated radon and determined its [[Melting point|melting temperature]] and [[critical point (thermodynamics)|critical point]].<ref name="ramsay-melting"/> 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 quantum chemistry methods|ab initio]]'' Monte Carlo simulations from 2018 agree almost exactly with Ramsay and Gray's result.<ref name=":0">{{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 |pages=9961–9964 |doi=10.1002/anie.201803353 |pmid=29896841 }}</ref> In 1910, they determined its [[density]] (that showed it was the heaviest known gas) and its position in the periodic table.<ref name="ramsay-melting"/> 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,<ref name="ramsay">{{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}}</ref> and in 1912 it was accepted by the [[Commission on Isotopic Abundances and Atomic Weights|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|{{sup|220}}Rn]] 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.<ref name="ThorntonBurdette" />

As late as the 1960s, the element was also referred to simply as ''emanation''.<ref>{{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}}</ref> The first synthesized compound of radon, radon fluoride, was obtained in 1962.<ref>{{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 }}</ref> Even today, the word ''radon'' may refer to either the element or its isotope <sup>222</sup>Rn, with ''thoron'' remaining in use as a short name for <sup>220</sup>Rn to stem this ambiguity. The name ''actinon'' for <sup>219</sup>Rn is rarely encountered today, probably due to the short half-life of that isotope.<ref name="ThorntonBurdette">{{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 |page=804 |doi=10.1038/nchem.1731 |pmid=23965684 |bibcode=2013NatCh...5..804T |doi-access=free }}</ref>

The danger of high exposure to radon in mines, where exposures can reach 1,000,000&nbsp;[[Becquerel|Bq]]/m<sup>3</sup>, 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'').<ref name="Masse-2002">{{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}}}}</ref> In 1879, this condition was identified as lung cancer by Harting and Hesse in their investigation of miners from Schneeberg, Germany.<ref name="George-2008">{{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 |url-access=subscription }}</ref> The first major studies with radon and health occurred in the context of uranium mining in the [[Jáchymov|Joachimsthal]] region of [[Bohemia]].<ref>{{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}}</ref> In the US, studies and mitigation only followed decades of health effects on uranium miners of the [[Southwestern United States|Southwestern US]] employed during the early [[Cold War]]; standards were not implemented until 1971.<ref>{{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}}</ref>

In the early 20th century in the US, gold contaminated with the radon daughter <sup>210</sup>Pb entered the jewelry industry. This was from gold [[brachytherapy]] seeds that had held <sup>222</sup>Rn, which were melted down after the radon had decayed.<ref>{{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 }}}}</ref>

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.<ref>{{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]]}}</ref><ref name="George-2008" />

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

=== Concentration units ===
[[Image:Lead210inairatjapan.png|thumb|upright=1.55|<sup>210</sup>Pb is formed from the decay of <sup>222</sup>Rn. Here is a typical deposition rate of <sup>210</sup>Pb as observed in Japan as a function of time, due to variations in radon concentration.<ref>{{cite journal |title=Radon |author= Yamamoto, M. |journal=[[Journal of Environmental Radioactivity]] |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}}</ref>]]

Discussions of radon concentrations in the environment refer to <sup>222</sup>Rn, the decay product of uranium and radium. While the average rate of production of <sup>220</sup>Rn (from the thorium decay series) is about the same as that of <sup>222</sup>Rn, the amount of <sup>220</sup>Rn in the environment is much less than that of <sup>222</sup>Rn because of the short half-life of <sup>220</sup>Rn (55 seconds, versus 3.8 days respectively).<ref name="USPHS90" />

Radon concentration in the atmosphere is usually measured in [[becquerel]] per cubic meter (Bq/m<sup>3</sup>), the [[SI derived unit]]. Another unit of measurement common in the US is [[Curie (unit)|picocurie]]s per liter (pCi/L); 1&nbsp;pCi/L = 37&nbsp;Bq/m<sup>3</sup>.<ref name="EPA03">{{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}}</ref> Typical domestic exposures average about 48&nbsp;Bq/m<sup>3</sup> indoors, though this varies widely, and 15&nbsp;Bq/m<sup>3</sup> outdoors.<ref name="EPA radon" /><!-- values converted from pCi/L values in ref -->

In the mining industry, the exposure is traditionally measured in ''[[working level]]'' (WL), and the cumulative exposure in ''working level month'' (WLM); 1&nbsp;WL equals any combination of short-lived <sup>222</sup>Rn daughters (<sup>218</sup>Po, <sup>214</sup>Pb, <sup>214</sup>Bi, and <sup>214</sup>Po) in 1 liter of air that releases 1.3&nbsp;×&nbsp;10<sup>5</sup>&nbsp;MeV of potential alpha energy;<ref name="EPA03" /> 1 WL is equivalent to 2.08&nbsp;×&nbsp;10<sup>−5</sup> joules per cubic meter of air (J/m<sup>3</sup>).<ref name="USPHS90" /> The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m<sup>3</sup>). One WLM is equivalent to 3.6&nbsp;×&nbsp;10<sup>−3</sup> J·h/m<sup>3</sup>. An exposure to 1&nbsp;WL for 1 working-month (170 hours) equals 1&nbsp;WLM cumulative exposure. The [[International Commission on Radiological Protection]] recommends an annual limit of 4.8WLM for miners.<ref>{{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}}</ref>{{rp|R5}} Assuming 2000 hours of work per year, this corresponds to a concentration of 1500 &nbsp;Bq/m<sup>3</sup>.

<sup>222</sup>Rn decays to <sup>210</sup>Pb and other radioisotopes. The levels of <sup>210</sup>Pb can be measured. The rate of deposition of this radioisotope is weather-dependent.<ref>{{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 |article-number=21707 |doi=10.1038/srep21707 |issn=2045-2322 |pmc=4761987 |pmid=26898637|bibcode=2016NatSR...621707Y }}</ref>

Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1,000&nbsp;Bq/m<sup>3</sup> (relatively high) concentration corresponds to 0.17&nbsp;[[pico-|picogram]] per cubic meter (pg/m<sup>3</sup>). The average concentration of radon in the atmosphere is about 6{{e|-18}} [[molar percent]], or about 150 atoms in each milliliter of air.<ref>{{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}}</ref> 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.<ref>{{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 }}</ref>

=== Natural ===
[[Image:Radon Concentration next to Uranium Mine.PNG|thumb|upright=1.1|Radon concentration next to a uranium mine]]

Radon is produced by the radioactive decay of radium-226, which is found in uranium ores, phosphate rock, shales, igneous and metamorphic rocks such as granite, gneiss, and schist, and to a lesser degree, in common rocks such as limestone.<ref name="Kusky" /><ref name="Thad. Godish 2001">{{cite book |author=Godish, Thad |title=Indoor Environmental Quality |date=2001 |publisher=CRC Press |isbn=978-1-56670-402-1}}</ref> Every square mile of surface soil, to a depth of 6&nbsp;inches (2.6&nbsp;km{{sup|2}} to a depth of 15&nbsp;cm), contains about 1&nbsp;gram of radium, which releases radon in small amounts to the atmosphere.<ref name="USPHS90" /> It is estimated that 2.4 billion curies (90 EBq) of radon are released from soil annually worldwide.<ref name="StanleyMoghissi1975">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}}<!-- URL was: nepis.epa.gov/Exe/ZyNET.exe/9101F2OM.TXT --></ref> 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&nbsp;Bq/m{{sup|3}}, even less (0.1&nbsp;Bq/m{{sup|3}}) above the ocean. In the United States, the average outdoor radon level is estimated to be 15&nbsp;Bq/m{{sup|3}} (0.4&nbsp;pCi/L).<ref name="auto">{{cite web |title=Health Risk of Radon |url=https://www.epa.gov/radon/health-risk-radon#riskcharts |website=United States Environmental Protection Agency |publisher=USEPA |access-date=April 8, 2026}}</ref> In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000&nbsp;Bq/m{{sup|3}}.<ref>{{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/|url-access=subscription }}</ref>

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&nbsp;WL (546&nbsp;pCi {{sup|222}}Rn per liter of air; 20.2&nbsp;kBq/m{{sup|3}}, measured from 1976 to 1985).<ref name="USPHS90" />
The concentration in the air at the (unventilated) [[Bad Gastein|Gastein]] Healing Gallery averages 43&nbsp;kBq/m{{sup|3}} (1.2&nbsp;nCi/L) with maximal value of 160&nbsp;kBq/m{{sup|3}} (4.3&nbsp;nCi/L).<ref name="zdo">{{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}}</ref>

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 [[Inversion (meteorology)|meteorological inversion]] and little wind.<ref name="ehp.niehs.nih.gov">{{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 }}</ref>

High concentrations of radon can be found in some spring waters and hot springs.<ref>{{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 |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}}</ref> The towns of [[Boulder, Montana]]; [[Misasa, Tottori|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&nbsp;nCi/L (74&nbsp;kBq/m{{sup|3}}).<ref>{{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 |archive-url=https://web.archive.org/web/20080508064535/http://amtamassage.org/journal/winter03_journal/balneology.html |archive-date=May 8, 2008 }}</ref> The activity of radon mineral water reaches 2&nbsp;MBq/m{{sup|3}} in Merano and 4&nbsp;MBq/m{{sup|3}} in Lurisia (Italy).<ref name="zdo" />

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 [[aquifer|saturated zone]] of a soil frequently has a higher radon content than the [[vadose zone|unsaturated zone]] because of [[diffusion]]al losses to the atmosphere.<ref>{{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 }}|{{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= }}}}</ref>

In 1971, [[Apollo 15]] passed {{Cvt|110|km||abbr=}} above the [[Aristarchus (crater)|Aristarchus plateau]] on the [[Moon]], and detected a significant rise in [[alpha particle]]s 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.<ref>{{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 }}</ref>

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 [[radioactive contamination|contaminated]] because of decaying radon and its products.<ref name="neb-one1994">{{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}}</ref>

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.<ref name="neb-one1994" />

===Accumulation in buildings===
Measurement of radon levels in the first decades of its discovery was mainly done to determine the presence of radium and uranium in geological surveys. In 1956, most likely the first indoor survey of radon decay products was performed in Sweden,<ref>{{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}}</ref> 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.<ref name="George-2008" />
[[File:Radon Lognormal distribution.gif|thumb|upright=1.75|Typical [[Log-normal distribution|log-normal]] radon distribution in dwellings]]
[[File:US homes over recommended radon levels.gif|thumb|upright=1.35|Predicted fraction of U.S. homes having concentrations of radon exceeding the EPA's recommended action level of 4&nbsp;pCi/L]]

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&nbsp;Bq/m<sup>3</sup> (2.7&nbsp;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.<ref name="lung">LaFavore, Michael. "Radon: The Quiet Killer." ''[[Funk & Wagnalls]] 1987 Science Yearbook.'' New York: Funk & Wagnalls, Inc., 1986. {{ISBN|0-7172-1517-2}}. 217–21.</ref> The incident dramatized the fact that radon levels in particular dwellings can occasionally be [[Order of magnitude|orders of magnitude]] higher than typical.<ref>{{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}}</ref> 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.<ref name="George-2008" />

In the United States, typical domestic exposures are of approximately 50&nbsp;Bq/m<sup>3</sup> (1.3&nbsp;pCi/L) indoors.<ref name="auto"/> 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.<ref name="guide" /> 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.<ref name="USPHS90" />

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.<ref>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.</ref> Thus, the [[geometric mean]] is generally used for estimating the "average" radon concentration in an area.<ref>{{cite web |title=Data Collection and Statistical Computations |url=http://aprg.utoledo.edu/radon/datacoll.html |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}}</ref> The mean concentration ranges from less than 10&nbsp;Bq/m<sup>3</sup> to over 100&nbsp;Bq/m<sup>3</sup> in some European countries.<ref>{{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 |archive-date=October 9, 2022 |archive-url=https://ghostarchive.org/archive/20221009/http://www.unscear.org/docs/reports/2006/09-81160_Report_Annex_E_2006_Web.pdf }}</ref>

Some of the highest radon hazard in the US is found in [[Iowa]] and in the [[Appalachian Mountains|Appalachian Mountain]] areas in southeastern Pennsylvania.<ref>{{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 <!--Added by H3llBot--> |archive-date= 2007-12-31}}</ref> 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.<ref>{{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}}</ref> 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.<ref>{{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}}</ref>
[[File:Stanowisko pomiaru radonu glebowego wf pw.jpg|thumb|left|A fixed-location device to measure soil concentrations of radon at the [[Warsaw University of Technology]]]]
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 inspector]]s. 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.<ref>{{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}}</ref> Alpha-radiation monitoring over the long term is a method of testing for radon that is more common in countries outside the United States.<ref name="George-2008" />

=== Industrial production ===
Radon is obtained as a by-product of [[Uranium ore deposits|uraniferous ores]] processing after transferring into 1% solutions of [[hydrochloric acid|hydrochloric]] or [[hydrobromic acid]]s. The gas mixture extracted from the solutions contains {{chem|H|2}}, {{chem|O|2}}, He, Rn, {{chem|CO|2}}, {{chem|H|2|O}} and [[hydrocarbon]]s. 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 [[potassium hydroxide|KOH]] and [[Phosphorus pentoxide|{{chem|P|2|O|5}}]] are used to remove the acids and moisture by [[sorption]]. Radon is condensed by liquid nitrogen and purified from residue gases by [[sublimation (phase transition)|sublimation]].<ref>{{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}}</ref>

Radon commercialization is regulated,<ref>{{cite web | title=EPA's Draft Criteria for Radon Credentialing Organizations | website=US EPA | date=2017-07-28 | url=https://www.epa.gov/radon/epas-draft-criteria-radon-credentialing-organizations | access-date=2025-05-27}}</ref> but it is available in small quantities for the calibration of <sup>222</sup>Rn 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).<ref>{{cite web |title=SRM 4972&nbsp;– Radon-222 Emanation Standard |url=https://www-s.nist.gov/srmors/view_detail.cfm?srm=4972 |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]]}}</ref> 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&nbsp;mm<sup>3</sup>/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&nbsp;Bq). Gaseous <sup>222</sup>Rn (half-life of about four days) escapes from the capsule through [[diffusion]].<ref>{{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}}</ref> Radon sources have also been produced for scientific purposes through the implantation of radium-226 into solid [[stainless steel]].<ref>{{Cite web |last1=Jörg |first1=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}}</ref>

=== Concentration scale ===
{| class="wikitable" style="margin:auto;"
|-
! Bq/m<sup>3</sup>
! pCi/L
! Occurrence example
|-
|style="color: black; background:silver; text-align:right;"| '''1'''
| ~0.027
| Radon concentration at the shores of large oceans is typically 1&nbsp;Bq/m<sup>3</sup>.
Radon trace concentration above oceans or in [[Antarctica]] can be lower than 0.1&nbsp;Bq/m<sup>3</sup>,<ref>{{Cite journal |last1=Jun |first1=Sang-Yoon |last2=Choi |first2=Jung |last3=Chambers |first3=S.D. |last4=Oh |first4=Mingi |last5=Park |first5=Sang-Jong |last6=Choi |first6=Taejin |last7=Kim |first7=Seong-Joong |last8=Williams |first8=A.G. |last9=Hong |first9=Sang-Bum |date=November 2022 |title=Seasonality of Radon-222 near the surface at King Sejong Station (62°S), Antarctic Peninsula, and the role of atmospheric circulation based on observations and CAM-Chem model |url=https://linkinghub.elsevier.com/retrieve/pii/S0013935122013251 |journal=Environmental Research |language=en |volume=214 |issue=Pt 3 |article-number=113998 |doi=10.1016/j.envres.2022.113998|pmid=35940229 |bibcode=2022ER....21413998J |url-access=subscription }}</ref> with changes in radon levels being used to track foreign pollutants.<ref>{{Cite web |last=ANSTO |title=Air pollution in Antarctica |url=https://phys.org/news/2014-12-air-pollution-antarctica.html |access-date=2024-09-23 |website=phys.org |language=en}}</ref>
|-
|style="color: black; background:aqua; text-align:right;"| '''10'''
| 0.27
| Mean continental concentration in the open air: 10 to 30&nbsp;Bq/m<sup>3</sup>.
An EPA survey<ref>{{Cite journal |last=Marcinowski |first=F. |date=1992-12-01 |title=Nationwide Survey of Residential Radon Levels in the US |url=https://academic.oup.com/rpd/article-abstract/45/1-4/419/5091672?redirectedFrom=fulltext |journal=Radiation Protection Dosimetry |volume=45 |issue=1–4 |pages=419–424 |doi=10.1093/rpd/45.1-4.419 |issn=0144-8420|url-access=subscription }}</ref> of 11,000 homes across the USA found an average of 46&nbsp;Bq/m<sup>3</sup>.
|-
|style="color: black; background:lime; text-align:right;"| '''100'''
| 2.7
| Typical indoor domestic exposure. Most countries have adopted a radon concentration of 200–400&nbsp;Bq/m<sup>3</sup> for indoor air as an Action or Reference Level.<ref name="Masse-2002" /> 
|-
|style="color: black; background:yellow; text-align:right;"| '''1,000'''
| 27
| Very high radon concentrations (>1000&nbsp;Bq/m<sup>3</sup>) have been found in houses built on soils with a high uranium content and/or high permeability of the ground. If levels are 20 picocuries radon per liter of air (800&nbsp;Bq/m<sup>3</sup>) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. Allowable concentrations in uranium mines are approximately 1,220&nbsp;Bq/m<sup>3</sup> (33&nbsp;pCi/L)<ref>{{cite book| title=The Mining Safety and Health Act – 30 CFR 57.0| publisher=United States Government| date=1977| url=http://www.msha.gov/30cfr/57.0.htm| access-date=2014-07-30| archive-url=https://web.archive.org/web/20140805040709/http://www.msha.gov/30cfr/57.0.htm| archive-date=2014-08-05}}</ref>
|-
|style="color: black; background:orange; text-align:right;"| '''10,000'''
| 270
| The concentration in the air at the (unventilated) [[Bad Gastein#Spa and Therapy|Gastein Healing Gallery]] averages 43&nbsp;kBq/m<sup>3</sup> (about 1.2&nbsp;nCi/L) with maximal value of 160&nbsp;kBq/m<sup>3</sup> (about 4.3&nbsp;nCi/L).<ref name="zdo" />
|-
|style="color: white; background:red; text-align:right;"| '''100,000'''
| ~2700
| About 100,000&nbsp;Bq/m<sup>3</sup> (2.7&nbsp;nCi/L) was measured in Stanley Watras's basement.<ref>{{Unbulleted list citebundle|{{cite conference |url=http://wpb-radon.com/Radon_research_papers/1995%20Nashville,%20TN/1995_14_Indoor%20Radon%20Concentration%20Data--Geographic%20and%20Geologic%20Distribution,%20Captial%20District,%20NY.pdf |title=Indoor Radon Concentration Data: Its Geographic and Geologic Distribution, an Example from the Capital District, NY |first1=John J. |last1=Thomas |first2=Barbara R. |last2=Thomas |first3=Helen M. |last3=Overeynder |date=September 27–30, 1995 |conference=International Radon Symposium |conference-url=http://internationalradonsymposium.org/ |publisher=American Association of Radon Scientists and Technologists |location=Nashville, TN |access-date=2012-11-28}}|{{cite book |last1=Upfal |first1=Mark J. |last2=Johnson |first2=Christine |title=Occupational, industrial, and environmental toxicology |date=2003 |publisher=Mosby |location=St. Louis, Missouri |isbn=978-0-323-01340-6 |chapter-url=http://toxicology.ws/Greenberg/Chapter%2065%20-%20Residential%20Radon.pdf |archive-url=https://web.archive.org/web/20130514202353/http://toxicology.ws/Greenberg/Chapter%2065%20-%20Residential%20Radon.pdf |archive-date=2013-05-14 |edition=2nd |chapter=65 Residential Radon |editor1-first=Michael I. |editor1-last=Greenberg |editor2-first=Richard J. |editor2-last=Hamilton |editor3-first=Scott D. |editor3-last=Phillips |editor4-first=Gayla J. |editor4-last=N. N. |access-date=28 November 2012}}}}</ref>
|-
|style="background:maroon; color:white; text-align:right;"| '''1,000,000'''
| 27000
| Concentrations reaching 1,000,000&nbsp;Bq/m<sup>3</sup> can be found in unventilated uranium mines.
|-
|style="background:black; color:white; text-align:right;"| '''{{nowrap|~5.54 × 10<sup>19</sup>}}'''
|style="background:#ddd;"| {{nowrap|~1.5 × 10<sup>18</sup>}}
|style="background:#ddd;"| ''Theoretical upper limit:'' Radon gas (<sup>222</sup>Rn) at 100% concentration (1 atmosphere, 0&nbsp;°C); 1.538×10<sup>5</sup> curies/gram;<ref>[https://www.ncbi.nlm.nih.gov/books/NBK158787/ Toxicological Profile for Radon], Table 4-2 (Keith S., Doyle J. R., Harper C., et al. Toxicological Profile for Radon. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2012 May. 4, CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION.) Retrieved 2015-06-06.</ref> 5.54×10<sup>19</sup> Bq/m<sup>3</sup>. 
|}

== Applications ==

=== Medical ===

==== Hormesis ====
{{Main|Radioactive quackery}}

An early-20th-century form of [[quackery]] was the treatment of maladies in a [[radiotorium]].<ref>{{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}}</ref> 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,<ref name="Radon seeds">{{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}}</ref> but it does not increase the health of healthy cells. The ionizing radiation causes the formation of [[free radicals]], which results in [[cell damage]], causing increased rates of illness, including [[cancer]].<ref>{{cite web | title=Does Radon Cause Cancer? | website=American Cancer Society | date=2022-10-31 | url=https://www.cancer.org/cancer/risk-prevention/radiation-exposure/radon.html | access-date=2025-05-27}}</ref><ref>{{cite book | last=Greenstock | first=C.L. | title=Advances in Radiation Biology | chapter=Free-Radical Processes in Radiation and Chemical Carcinogenesis | publisher=Elsevier | volume=11 | date=1984 | isbn=978-0-12-035411-5 | doi=10.1016/b978-0-12-035411-5.50012-5 | url=https://linkinghub.elsevier.com/retrieve/pii/B9780120354115500125 | access-date=2025-05-27 | pages=269–293}}</ref>

Exposure to radon has been suggested to mitigate [[autoimmune disease]]s such as [[arthritis]] in a process known as [[radiation hormesis]].<ref>{{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}}</ref><ref name="Hg">{{cite journal |author=Neda, T. |title=Radon concentration levels in dry CO<sub>2</sub> 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 }}</ref> 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.<ref>{{cite journal |last1=Salak |first1=Kara |last2=Nordeman |first2=Landon |title=59631: Mining for Miracles |journal=[[National Geographic (magazine)|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 |archive-date=January 24, 2008 |access-date=2008-06-26}}</ref>

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|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).<ref>{{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}}</ref> 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.<ref name="Hg" /><ref>{{cite web |access-date=2008-06-26 |url=http://www.petros.cz/spa/spa_ja.asp |title=Jáchymov |publisher=Petros |archive-url=https://web.archive.org/web/20020107060646/http://www.petros.cz/spa/spa_ja.asp |archive-date=January 7, 2002 }}</ref>

==== Nuclear medicine ====
[[File:Radioactive Seeds (7845754328).jpg|thumb|{{sup|222}}Rn- and [[Iodine-125|{{sup|125}}I]]-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 accelerator]]s and [[nuclear reactor]]s. 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 ray]]s to escape (which kill the diseased tissue). The activities might range from 0.05 to 5 millicuries per seed (2 to 200&nbsp;MBq).<ref name="Radon seeds" /> The gamma rays are produced by radon and the first short-lived elements of its decay chain (<sup>218</sup>Po, <sup>214</sup>Pb, <sup>214</sup>Bi, <sup>214</sup>Po).<ref name="nubase2020" />

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 <sup>210</sup>Pb, whose half-life (22.3 years) is 2,000 times that of radon and its descendants <sup>210</sup>Bi and <sup>210</sup>Po.<ref name="Radon seeds" /><ref name="nubase2020">{{NUBASE2020}}</ref>

<sup>211</sup>Rn can be used to generate <sup>211</sup>At, which has uses in [[targeted alpha therapy]].<ref>{{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=<sup>211</sup>Rn/<sup>211</sup>At and <sup>209</sup>At production with intense mass separated Fr ion beams for preclinical <sup>211</sup>At-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 }}</ref>

=== Scientific ===
Radon emanation from the soil varies with soil type and with surface uranium content, so outdoor radon concentrations can be used to track [[air mass]]es to a limited degree.<ref>{{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 }}</ref>{{efn|See [[radon storm]].}} Because of radon's rapid loss to air and comparatively rapid decay, radon is used in [[hydrology|hydrologic]] research that studies the interaction between groundwater and [[stream]]s. Any significant concentration of radon in a river may be an indicator that there are local inputs of groundwater.<ref>{{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|url-access=subscription }}</ref>

Radon soil concentration has been used to map buried close-subsurface geological [[fault (geology)|faults]] because concentrations are generally higher over the faults.<ref>{{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 }}</ref> Similarly, it has found some limited use in prospecting for [[geothermal gradient]]s.<ref>{{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}}</ref>

Some researchers have investigated changes in groundwater radon concentrations for [[earthquake prediction]].<ref>{{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|bibcode=1995JPE....43..585I |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 }}}}</ref><ref name="Mindoro">{{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 }}</ref> Increases in radon were noted before the [[1966 Tashkent earthquake|1966 Tashkent]]<ref>{{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}}</ref> and [[1994 Mindoro earthquake|1994 Mindoro]]<ref name="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.<ref>{{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}}</ref> As of 2009, it was under investigation as a possible earthquake precursor by [[NASA]];<ref name="EARTHq">{{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}}</ref> further research into the subject has suggested that abnormalities in atmospheric radon concentrations can be an indicator of seismic movement.<ref>{{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 }}</ref>

Radon is a known pollutant emitted from [[Geothermal power|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.<ref>{{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 }}</ref> In 1989, a survey of the [[collective dose]] received due to radon in geothermal fluids was measured at 2 man-[[sievert]]s per [[Kilowatt-hour#Multiples|gigawatt-year]] of electricity produced, in comparison to the 2.5 man-sieverts per gigawatt-year produced from [[carbon-14|{{sup|14}}C]] emissions in [[nuclear power plants]].<ref>{{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}}</ref>

In the 1940s and 1950s, radon produced from a radium source was used for [[industrial radiography]].<ref>{{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 }}}}</ref> Other X-ray sources such as [[Cobalt-60|{{sup|60}}Co]] and [[Iridium-192|{{sup|192}}Ir]] became available after World War II and quickly replaced radium and thus radon for this purpose, being of lower cost and hazard.<ref>{{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}}}}</ref><!--{{Unbulleted list citebundle|{{Cite journal|doi=10.1002/maco.19550060317 |title=Memorandum on gamma-ray sources for radiography. Prepared by a committee of the industrial radiology group, London, 1952 |date=1955 |last=Scheichl |first=L. |journal=Materials and Corrosion |issue=6 |pages=163-163}}|{{Cite journal|url=https://www.nature.com/articles/174726a0.pdf |journal=Nature |date=October 16, 1954 |volume=174 |title=Gamma-Ray Sources for Radiography}} Sources for continued availability of radon, radium in the 1950s as a gamma ray source - along with others-->

== Health risks ==
{{Main|Health effects of radon}}

=== In mines ===
{{sup|222}}Rn decay products have been classified by the [[International Agency for Research on Cancer]] as being [[carcinogenic]] to humans,<ref>{{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 |archive-date=2003-12-13 |title=Known and Probable Carcinogens |publisher=[[American Cancer Society]]}}</ref> 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,<ref>{{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}}</ref> 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<ref>{{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 |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 }}|{{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}}}}</ref> and [[South Australia]].<ref>{{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 }}</ref> Despite these hazards being known in the early 1950s,<ref>{{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}}</ref> 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,<ref>{{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}}}}</ref> but the government banned such advertisements in 1975,<ref>{{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}}</ref> and subsequent works have debated the truth of such claimed health effects, citing the documented ill effects of radiation on the body.<ref>{{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 |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}}</ref>

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.<ref name="Darby05">{{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}}</ref> As the relative risk for miners has decreased, so has the ability to detect excess risks among that population.<ref name="UNSCEAR06">{{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}}</ref>
[[File:Uranium waste near Rifle, Colorado.jpg|thumb|A tailing pond near [[Rifle, Colorado]]. 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.<ref name="OLM" />]]
Residues from processing of uranium ore can also be a source of radon. Radon resulting from the high radium content in uncovered dumps and [[Uranium tailings|tailing]] ponds<ref name="USPHS90" /> can be easily released into the atmosphere and affect people living in the vicinity.<ref>{{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
}}</ref> The release of radon may be mitigated by covering tailings with soil or clay, though other decay products may leach into [[groundwater]] supplies.<ref name="OLM">{{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 }}</ref>

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]].<ref>{{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|bibcode=2023HeaPh.124..244C }}</ref> However, older or abandoned uranium mines without ventilation may still have extremely high radon levels.<ref>{{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 |article-number=107346 |doi=10.1016/j.jenvrad.2023.107346 |pmid=38043218 |bibcode=2024JEnvR.27207346M |issn=0265-931X|url-access=subscription }}</ref>

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]],<ref>{{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}}</ref> 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]]).<ref name="Nunes-2022">{{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 |page=3929 |doi=10.3390/ijerph19073929 |issn=1661-7827 |pmc=8997394 |pmid=35409610 |bibcode=2022IJERP..19.3929N |doi-access=free}}</ref> 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)<ref>{{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=978-0-10-296618-3 |location= |pages= |language=en |quote= |via=}}</ref> and the [[Radiation Exposure Compensation Act]] (in the United States).<ref>{{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}}</ref>

=== Domestic-level exposure ===
Radon has been considered the second leading cause of lung cancer in the United States and leading environmental cause of cancer mortality by the EPA,<ref>{{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=[[United States Environmental Protection Agency|Environmental Protection Agency]]}}</ref> with the first one being [[smoking]].<ref>{{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}}</ref> Others have reached similar conclusions for the United Kingdom<ref name="Darby05" /> and France.<ref name="Catelinois">{{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 }}</ref> Radon exposure in buildings may arise from subsurface rock formations and certain building materials (e.g., some granites).<ref name="Todorovic">{{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}}</ref> 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.<ref name="RECR" /> In some regions, such as [[Niška Banja]], Serbia and in [[Ullensvang Municipality|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.<ref>{{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 |page=662 |doi=10.3390/ijerph19020662 |issn=1661-7827 |pmc=8775861 |pmid=35055485 |bibcode=2022IJERP..19..662C |doi-access=free}}</ref>

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.<ref name="epa">{{cite web |url=http://www.epa.gov/radon/pubs/citguide.html |archive-url=https://web.archive.org/web/20030419085501/http://www.epa.gov/radon/pubs/citguide.html |archive-date=April 19, 2003 |title=A Citizen's Guide to Radon |date=October 12, 2010 |work=[[United States Environmental Protection Agency]] |access-date=January 29, 2012}}</ref><ref>{{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}}</ref> In Europe, 2% of all cancers have been attributed to radon;<ref name="Ngoc-2022">{{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 |page=97 |doi=10.3390/ijerph20010097 |doi-access=free |issn=1661-7827 |pmc=9819115 |pmid=36612419 |bibcode=2022IJERP..20...97N }}</ref> 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.<ref>{{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 }}}}</ref> 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&nbsp;pCi/L. North American and European pooled analyses further support these findings.<ref name=RECR>{{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&nbsp;– 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}}</ref> However, the conclusion that exposure to low levels of radon leads to elevated risk of lung cancer has been disputed,<ref>{{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 |article-number=15401420390844447 |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 (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|bibcode=1995HeaPh..68..157C |s2cid=41388715 }}}}</ref> and analyses of the literature point towards elevated risk only when radon accumulates indoors<ref name="Nunes-2022" /> and at levels above 100&nbsp;Bq/m<sup>3</sup>.<ref name="Ngoc-2022" />

Thoron (<sup>220</sup>Rn) 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 [[Timber framing#Half-timbering|half-timbered houses]] and modern houses with [[clay]] wall finishes,<ref>{{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 }}</ref> and in regions with thorium- and [[monazite]]-rich soil and sand.<ref name="Ramola-2020">{{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 |article-number=106453 |doi=10.1016/j.jenvrad.2020.106453|pmid=33120031 |bibcode=2020JEnvR.22506453R |url-access=subscription }}</ref> Thoron is a minor contributor to the overall radiation dose received due to indoor radon exposure,<ref>{{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 }}</ref> and can interfere with {{Sup|222}}Rn measurements when not taken into account.<ref name="Ramola-2020" />

==== Action and reference level ====
WHO presented in 2009 a recommended reference level (the national reference level), 100&nbsp;Bq/m<sup>3</sup>, for radon in dwellings. The recommendation also says that where this is not possible, 300&nbsp;Bq/m<sup>3</sup> 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.<ref>{{Cite book|url=https://www.who.int/publications/i/item/9789241547673 |date=1 January 2009 |title=WHO Handbook on Indoor Radon: a public health perspective |publisher=World Health Organization |isbn=978-92-4-154767-3}}</ref>

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&nbsp;Bq/m<sup>3</sup> (2&nbsp;pCi/L),<ref name="EPA radon">{{cite web |title =Radiation Protection: Radon |publisher=[[United States Environmental Protection Agency]] |date=November 2007 |url=http://www.epa.gov/radiation/radionuclides/radon.html |archive-url=https://web.archive.org/web/20080116100743/http://www.epa.gov/radiation/radionuclides/radon.html |archive-date=January 16, 2008 |access-date =2008-04-17}}</ref> and the [[European Union]] recommends action be taken when concentrations reach 400&nbsp;Bq/m<sup>3</sup> (11&nbsp;pCi/L) for old houses and 200&nbsp;Bq/m<sup>3</sup> (5&nbsp;pCi/L) for new ones.<ref>{{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 }}</ref> On 8 July 2010, the UK's Health Protection Agency issued new advice setting a "Target Level" of 100&nbsp;Bq/m<sup>3</sup> whilst retaining an "Action Level" of 200&nbsp;Bq/m<sup>3</sup>.<ref name="HPA radon">{{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/ |archive-date=2010-07-14 |access-date=2010-08-13}}</ref> Similar levels (as in the UK) are published by Norwegian Radiation and Nuclear Safety Authority (DSA)<ref>{{Cite web|title=Radon mitigation measures|url=https://dsa.no/en/radon/radon-mitigation-measures|access-date=2021-07-12|website=DSA|language=no}}</ref> with the maximum limit for schools, kindergartens, and new dwellings set at 200&nbsp;Bq/m<sup>3</sup>, where 100&nbsp;Bq/m<sup>3</sup> is set as the action level.<ref>{{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|access-date=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}}</ref>

==== Inhalation and smoking ====
Results from epidemiological studies indicate that the risk of lung cancer increases with exposure to residential radon. One well known potential source of error in these studies is [[smoking]], which is the main risk factor for lung cancer. In the US, [[cigarette]] smoking is estimated to cause 80% to 90% of all lung cancers.<ref>{{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}}</ref>

Radon, like other known or suspected external risk factors for lung cancer, is a threat for smokers and former smokers. According to the EPA, the risk of lung cancer for smokers is significantly higher when they are exposed to radon due to the [[Synergy|synergistic]] effects of radon with 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.<ref name="epa" /> A 2005 review article by Darby and colleagues analyzing 13 different case studies across Europe 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."<ref name="bmj38308">{{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 |page=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. }}</ref> According to Darby, there is a difference in risk for the [[Histology|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.<ref name="bmj38308" /><ref>{{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 |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)}}</ref>

A 2008 study of radiation from post-[[mastectomy]] [[radiotherapy]] showed that the simple models previously used to assess the combined and separate risks from radiation and smoking were in need of development.<ref>{{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 }}</ref> The frequently used [[linear no-threshold model]] that describes how the body responds to and is damaged by radiation is one of these models.<ref>{{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 }}</ref>

==== Absorption and ingestion from water ====
The [[biological half-life]] of ingested radon ranges from 30 to 70 minutes, with 90% removal at 100 minutes. In 1999, the US [[National Research Council (United States)|National Research Council]] investigated the issue of radon in drinking water. The risk associated with ingestion was considered almost negligible.<ref>[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.</ref> The [[World Health Organization]] (WHO) recommends a maximum water contamination level of 100&nbsp;Bq/L, while the US Environmental Protection Agency recommends a maximum of 11.1&nbsp;Bq/L. The WHO also advises a maximum annual dose of 1&nbsp;mSv<sup>-1</sup> induced by radon from drinking water.<ref>{{Cite journal |last=Pourshabanian |first=Mina |last2=Nasseri |first2=Simin |last3=Nodehi |first3=Ramin Nabizadeh |last4=Hosseini |first4=Sara Sadat |last5=Mahvi |first5=Amir Hossein |date=2023-08-05 |title=Radon measurement and age-independent effective dose attributed to ingestion of bottled water in Iran: sensitivity analysis |url=https://pmc.ncbi.nlm.nih.gov/articles/PMC10404218/ |journal=Scientific Reports |volume=13 |issue=1 |page=12717 |doi=10.1038/s41598-023-39679-1 |issn=2045-2322 |pmc=10404218 |pmid=37543701}}</ref>

Water from underground sources may contain significant amounts of radon depending on the surrounding rock and soil conditions, whereas surface sources generally do not.<ref>{{cite web |url=http://water.epa.gov/lawsregs/rulesregs/sdwa/radon/basicinformation.cfm |archive-url=https://web.archive.org/web/20100820104554/http://water.epa.gov/lawsregs/rulesregs/sdwa/radon/basicinformation.cfm |archive-date=August 20, 2010 |title=Basic Information about Radon in Drinking Water |access-date=2013-07-24 }}</ref> 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 10<sup>4</sup>&nbsp;pCi/L can increase the indoor airborne radon concentration by 1&nbsp;pCi/L under normal conditions.<ref name="Thad. Godish 2001" /> 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.<ref>{{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}}</ref><!--The ocean surface only carries about {{val|e=-4}} <sup>226</sup>Ra, where measurements of <sup>222</sup>Rn concentration have been 1% over various continents.<ref name="agupubs.onlinelibrary.wiley.com" />--> 

Ocean surface concentrations of radon exchange within the atmosphere, causing <sup>222</sup>Rn to increase through the air-sea interface.<ref name="agupubs.onlinelibrary.wiley.com">{{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|url-access=subscription}}</ref> Although areas tested were very shallow, additional measurements in a wide variety of coastal regimes should help define the nature of <sup>222</sup>Rn observed.

=== Testing and mitigation ===
{{Main|Radon mitigation}}

[[File:Radon detector.jpg|alt=radon detector|thumb|A digital radon detector]]
[[Image:Radon test kit.jpg|thumb|A radon test kit]]
There are relatively simple tests for radon gas. In some countries these tests are methodically done in areas of known systematic hazards. Radon detection devices are commercially available. Digital radon detectors provide ongoing measurements giving both daily, weekly, short-term and long-term average readouts via a digital display. Short-term radon test devices used for initial screening purposes are inexpensive, in some cases free. There are important protocols for taking short-term radon tests and it is imperative that they be strictly followed. The kit includes a collector that the user hangs in the lowest habitable floor of the house for two to seven days. The user then sends the collector to a laboratory for analysis. Long term kits, taking collections for up to one year or more, are also available. An open-land test kit can test radon emissions from the land before construction begins.<ref name="epa" /> 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.<ref>{{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}}</ref>

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.<ref name="Thad. Godish 2001" /> Therefore, a high result (over 4&nbsp;pCi/L) justifies repeating the test before undertaking more expensive abatement projects. Measurements between 4 and 10&nbsp;pCi/L warrant a long-term radon test. Measurements over 10&nbsp;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&nbsp;pCi/L or less.<ref name="epa" />

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.<ref name="USPHS90" /> In a well-ventilated place, the radon concentration tends to align with outdoor values (typically 10&nbsp;Bq/m<sup>3</sup>, ranging from 1 to 100&nbsp;Bq/m<sup>3</sup>).<ref name="epa" />

The four principal ways of reducing the amount of radon accumulating in a house are:<ref name="epa" /><ref name="WHO291">{{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}}</ref>
* 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.<ref name="epa" /> 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".<ref name="epa.gov">{{cite web | url = http://www.epa.gov/radon/pubs/consguid.html#reductiontech| archive-url = https://web.archive.org/web/20150924134052/http://www.epa.gov/radon/pubs/consguid.html| archive-date = September 24, 2015| title = Consumer's Guide to Radon Reduction: How to fix your home| access-date = 2010-04-03| publisher = EPA}}</ref>

[[Positive pressure ventilation|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).<ref name="epa" /><ref>{{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}}</ref> 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."<ref name="epa.gov" />

[[Computational fluid dynamics]] (CFD) modelling has been used to quantify the performance of sub-slab air chamber systems under both depressurization and forced-air ventilation configurations. Models validated against experimental data from a real building show that depressurization systems reduce indoor radon concentrations by 45–90% relative to a sealed chamber, with smaller buildings achieving the greatest reductions and efficiency gains diminishing at higher extraction flow rates. Forced-air ventilation systems, by contrast, can generate overpressure within the chamber if the inlet flow exceeds the outlet capacity, which promotes radon ingress and may increase indoor concentrations. These findings support the optimization of mitigation system design by balancing radon reduction against construction complexity and [[energy consumption]].<ref>{{cite journal
 |last1=Sicilia |first1=Isabel
 |last2=Frutos |first2=Borja
 |last3=Sainz |first3=Carlos
 |last4=Alonso |first4=Carmen
 |last5=Martín-Consuegra |first5=Fernando
 |title=Analysis of a Radon Mitigation System Using Sub-Slab Air Chamber: CFD Model Development and Validation in an Experimental Case
 |journal=Journal of Environmental Radioactivity
 |date=2026
 |volume=297
 |article-number=108046
 |doi=10.1016/j.jenvrad.2026.108046
 |doi-access=free
 |url=http://hdl.handle.net/10261/432004
}}</ref>

== See also ==
{{Portal|Chemistry}}
* [[International Radon Project]]
* [[Lucas cell]]
* [[Pleochroic halo]] (aka radiohalo)
* [[Radiation Exposure Compensation Act]]
{{Subject bar
|book1=Radon
|book2=Period 6 elements
|book3=Noble gases
|book4=Chemical elements (sorted&nbsp;alphabetically)
|book5=Chemical elements (sorted by number)
}}
== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

== External links ==
{{Commons}}
{{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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[[Category:Radon| ]]
[[Category:Chemical elements]]
[[Category:Hazardous materials]]
[[Category:Noble gases]]
[[Category:Building biology]]
[[Category:Soil contamination]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Carcinogens]]
[[Category:Industrial gases]]
[[Category:Building defects]]