Editing Radon (section)

== 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 |pages=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 |url-status=dead }}</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 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/}}</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 |url-status=dead |archive-date=2006-03-16 |title=Radon Occurrence and Health Risk |author=Field, R. William |publisher=Department of Occupational and Environmental Health, University of Iowa |access-date=2008-02-02}}</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 |url-status=dead |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 |url-status=dead }}|{{cite web |access-date=2008-06-28 |url=http://www.cosis.net/abstracts/EGU2008/08953/EGU2008-A-08953.pdf?PHPSESSID= |format=PDF |title=Radon-222 as a tracer in groundwater-surface water interactions |publisher=Lancaster University |archive-date=November 8, 2021 |archive-url=https://web.archive.org/web/20211108075203/https://www.cosis.net/abstracts/EGU2008/08953/EGU2008-A-08953.pdf?PHPSESSID= |url-status=dead }}}}</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 100&nbsp;Bq/m<sup>3</sup> (2.7&nbsp;pCi/L) indoors. Some level of radon will be found in all buildings. Radon mostly enters a building directly from the soil through the lowest level in the building that is in contact with the ground. High levels of radon in the water supply can also increase indoor radon air levels. Typical entry points of radon into buildings are cracks in solid foundations and walls, construction joints, gaps in suspended floors and around service pipes, cavities inside walls, and the water supply.<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 |url-status=dead |archive-url=http://arquivo.pt/wayback/20160519081621/http://aprg.utoledo.edu/radon/datacoll.html |archive-date=2016-05-19 |access-date=2023-09-23 |website=University of Toledo}}</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}}</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,{{Citation needed|date=February 2025}} 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 |url-status=dead |archive-url=https://web.archive.org/web/20200306035332/https://www-s.nist.gov/srmors/view_detail.cfm?srm=4972 |archive-date=6 March 2020 |access-date=2008-06-26 |publisher=[[National Institute of Standards and Technology]]}}</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 |last=Jörg |first=Florian |last2=Blaum |first2=Klaus |last3=Schweiger |first3=Christoph |last4=Simgen |first4=Hardy |date=January 4, 2023 |title=Production of 226Ra-implanted high-quality radon sources for detector characterization |url=https://cds.cern.ch/record/2845390/files/INTC-P-647.pdf |website=European Organization for Nuclear Research}}</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 |pages=113998 |doi=10.1016/j.envres.2022.113998|pmid=35940229 |bibcode=2022ER....21413998J }}</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}}</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| url-status=dead}}</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=9780323013406 |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 |url-status=dead |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>. 
|}