Radon, ₈₆Rn

Radon
Pronunciation	/ˈreɪdɒn/ ​(RAY-don)
Appearance	colorless gas
Mass number
Radon in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Xe ↑ Rn ↓ Og astatine ← radon → francium
Atomic number (Z)	86
Group	group 18 (noble gases)
Period	period 6
Block	p-block
Electron configuration	[Xe] 4f14 5d10 6s2 6p6
Electrons per shell	2, 8, 18, 32, 18, 8
Physical properties
Phase at STP	gas
Melting point	202 K ​(−71 °C, ​−96 °F)
Boiling point	211.5 K ​(−61.7 °C, ​−79.1 °F)
Density (at STP)	9.73 g/L
when liquid (at b.p.)	4.4 g/cm3
Critical point	377 K, 6.28 MPa[1]
Heat of fusion	3.247 kJ/mol
Heat of vaporization	18.10 kJ/mol
Molar heat capacity	5R/2 = 20.786 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 110 121 134 152 176 211
Atomic properties
Oxidation states	0, +2, +6
Electronegativity	Pauling scale: 2.2
Ionization energies	1st: 1037 kJ/mol
Covalent radius	150 pm
Van der Waals radius	220 pm
Spectral lines of radon
Other properties
Natural occurrence	from decay
Crystal structure	​face-centered cubic (fcc) (predicted)
Thermal conductivity	3.61×10−3  W/(m⋅K)
Magnetic ordering	non-magnetic
CAS Number	10043-92-2
History
Discovery	Ernest Rutherford and Robert B. Owens (1899)
First isolation	William Ramsay and Robert Whytlaw-Gray (1910)
Isotopes of radon v e
Main isotopes[2] Decay abun­dance half-life (t1/2) mode pro­duct 210Rn synth 2.4 h α 206Po 211Rn synth 14.6 h ε 211At α 207Po 222Rn trace 3.8235 d α 218Po 224Rn synth 1.8 h β− 224Fr
Category: Radon view talk edit | references

Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only radon-222, has a sufficiently long half-life (3.825 days) for it to be released from the soil and rock where it is generated. Radon isotopes are the immediate decay products of radium isotopes. The instability of radon-222, its most stable isotope, makes radon one of the rarest elements. Radon will be present on Earth for several billion more years, despite its short half-life, because it is constantly being produced as a step in the decay chain of uranium-238, and that of thorium-232, each of which is an extremely abundant radioactive nuclide with a half-life of several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead.^[3] Radon-222 occurs in significant quantities as a step in the normal radioactive decay chain of uranium-238, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into lead-206, which is stable. Radon-220 occurs in minute quantities as an intermediate step in the decay chain of thorium-232, also known as the thorium series, which eventually decays into lead-208, which is stable.

Under standard conditions, radon is gaseous and can be easily inhaled, posing a health hazard. However, the primary danger comes not from radon itself, but from its decay products, known as radon daughters. These decay products, often existing as single atoms or ions, can attach themselves to airborne dust particles. Although radon is a noble gas and does not adhere to lung tissue, meaning it is often exhaled before decaying, the radon daughters attached to dust are more likely to stick to the lungs. This increases the risk of harm, as the radon daughters can cause damage to lung tissue.^[4] Radon and its daughters are, taken together, often the single largest contributor to an individual's background radiation dose, but due to local differences in geology,^[5] the level of exposure to radon gas differs from place to place. A common source is uranium-containing minerals in the ground, and therefore it accumulates in subterranean areas such as basements. Radon can also occur in some ground water like spring waters and hot springs.^[6] Radon trapped in permafrost may be released by climate change induced thawing of permafrosts.^[7] It is possible to test for radon in buildings, and to use techniques such as sub-slab depressurization for mitigation.^[8][9]

Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency (EPA), radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA policy-oriented estimates.^[10] Significant uncertainties exist for the health effects of low-dose exposures.^[11]

Characteristics

Physical properties

Radon is a colorless, odorless, and tasteless^[12] gas and therefore is not detectable by human senses alone. At standard temperature and pressure, it forms a monatomic gas with a density of 9.73 kg/m³, about 8 times the density of the Earth's atmosphere at sea level, 1.217 kg/m³.^[13] It is one of the densest gases at room temperature (a few are denser, e.g. CF₃(CF₂)₂CF₃ and WF₆) and is the densest of the noble gases. Although colorless at standard temperature and pressure, when cooled below its freezing point of 202 K (−71 °C; −96 °F), it emits a brilliant radioluminescence that turns from yellow to orange-red as the temperature lowers.^[14] Upon condensation, it glows because of the intense radiation it produces.^[15] It is sparingly soluble in water, but more soluble than lighter noble gases. It is appreciably more soluble in organic liquids than in water. Its solubility equation is as follows,^[16][17][18]

${\displaystyle \chi =\exp(B/T-A),}$

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

Chemical properties

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

It is inert to most common chemical reactions, such as combustion, because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.^[20] Its first ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol.^[21] In accordance with periodic trends, radon has a lower electronegativity than the element one period before it, xenon, and is therefore more reactive. Early studies concluded that the stability of radon hydrate should be of the same order as that of the hydrates of chlorine (Cl
₂) or sulfur dioxide (SO
₂), and significantly higher than the stability of the hydrate of hydrogen sulfide (H
₂S).^[22]

Because of its cost and radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all either fluorides or oxides. Radon can be oxidized by powerful oxidizing agents such as fluorine, thus forming radon difluoride (RnF
₂).^[23][24] It decomposes back to its elements at a temperature of above 523 K (250 °C; 482 °F), and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by hydrogen gas.^[25] It has a low volatility and was thought to be RnF
₂. Because of the short half-life of radon and the radioactivity of its compounds, it has not been possible to study the compound in any detail. Theoretical studies on this molecule predict that it should have a Rn–F bond distance of 2.08 ångströms (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart xenon difluoride (XeF
₂).^[26] The octahedral molecule RnF
₆ was predicted to have an even lower enthalpy of formation than the difluoride.^[27] The ⁺ ion is believed to form by the following reaction:^[28]

Rn (g) + 2 ^+
−
(s) → ^+
−
(s) + 2 O
₂ (g)

For this reason, antimony pentafluoride together with chlorine trifluoride and N
₂F
₂Sb
₂F
₁₁ have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds.^[19] Radon compounds can be formed by the decay of radium in radium halides, a reaction that has been used to reduce the amount of radon that escapes from targets during irradiation.^[25] Additionally, salts of the ⁺ cation with the anions SbF⁻
₆, TaF⁻
₆, and BiF⁻
₆ are known.^[25] Radon is also oxidised by dioxygen difluoride to RnF
₂ at 173 K (−100 °C; −148 °F).^[25]

Radon oxides are among the few other reported compounds of radon;^[29] only the trioxide (RnO
₃) has been confirmed.^[30] The higher fluorides RnF
₄ and RnF
₆ have been claimed^[30] and are calculated to be stable,^[31] but their identification is unclear.^[30] They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been RnF
₄, RnF
₆, or both.^[25] Trace-scale heating of radon with xenon, fluorine, bromine pentafluoride, and either sodium fluoride or nickel fluoride was claimed to produce a higher fluoride as well which hydrolysed to form RnO
₃. While it has been suggested that these claims were really due to radon precipitating out as the solid complex ⁺
₂²⁻, the fact that radon coprecipitates from aqueous solution with CsXeO
₃F has been taken as confirmation that RnO
₃ was formed, which has been supported by further studies of the hydrolysed solution. That ⁻ 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 ⁺ and anionic ⁻ forms of radon in weakly acidic aqueous solution (pH > 5), the procedure having previously been validated by examination of the homologous xenon trioxide.^[30]

The decay technique has also been used. Avrorin et al. reported in 1982 that ²¹²Fr compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of RnO₃, but this could not be confirmed.^[32]

It is likely that the difficulty in identifying higher fluorides of radon stems from radon being kinetically hindered from being oxidised beyond the divalent state because of the strong ionicity of radon difluoride (RnF
₂) and the high positive charge on radon in RnF⁺; spatial separation of RnF
₂ molecules may be necessary to clearly identify higher fluorides of radon, of which RnF
₄ is expected to be more stable than RnF
₆ due to spin–orbit splitting of the 6p shell of radon (Rn^IV would have a closed-shell 6s²
6p²
_1/2 configuration). Therefore, while RnF
₄ should have a similar stability to xenon tetrafluoride (XeF
₄), RnF
₆ would likely be much less stable than xenon hexafluoride (XeF
₆): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of XeF
₆, because of the inert pair effect.^[33][34] Because radon is quite electropositive for a noble gas, it is possible that radon fluorides actually take on highly fluorine-bridged structures and are not volatile.^[34] Extrapolation down the noble gas group would suggest also the possible existence of RnO, RnO₂, and RnOF₄, as well as the first chemically stable noble gas chlorides RnCl₂ and RnCl₄, but none of these have yet been found.^[25]

Radon carbonyl (RnCO) has been predicted to be stable and to have a linear molecular geometry.^[35] The molecules Rn
₂ and RnXe were found to be significantly stabilized by spin-orbit coupling.^[36] Radon caged inside a fullerene has been proposed as a drug for tumors.^[37][38] Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; RnF
₈ should be highly unstable chemically (XeF₈ is thermodynamically unstable). It is predicted that the most stable Rn(VIII) compound would be barium perradonate (Ba₂RnO₆), analogous to barium perxenate.^[31] The instability of Rn(VIII) is due to the relativistic stabilization of the 6s shell, also known as the inert pair effect.^[31]

Radon reacts with the liquid halogen fluorides ClF, ClF
₃, ClF
₅, BrF
₃, BrF
₅, and IF
₇ to form RnF
₂. In halogen fluoride solution, radon is nonvolatile and exists as the RnF⁺ and Rn²⁺ cations; addition of fluoride anions results in the formation of the complexes RnF⁻
₃ and RnF²⁻
₄, paralleling the chemistry of beryllium(II) and aluminium(III).^[25] The standard electrode potential of the Rn²⁺/Rn couple has been estimated as +2.0 V,^[39] although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.^[25]

Isotopes

Radon has no stable isotopes. Thirty-nine radioactive isotopes have been characterized, with mass numbers ranging from 193 to 231.^[40][41] Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is ²²²Rn (half-life 3.82 days), which is a decay product of ²²⁶Ra, the latter being itself a decay product of ²³⁸U.^[42] A trace amount of the (highly unstable) isotope ²¹⁸Rn (half-life about 35 milliseconds) is also among the daughters of ²²²Rn. The isotope ²¹⁶Rn would be produced by the double beta decay of natural ²¹⁶Po; while energetically possible, this process has however never been seen.^[43]

Three other radon isotopes have a half-life of over an hour: ²¹¹Rn (about 15 hours), ²¹⁰Rn (2.4 hours) and ²²⁴Rn (about 1.8 hours). However, none of these three occur naturally. ²²⁰Rn, also called thoron, is a natural decay product of the most stable thorium isotope (²³²Th). It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, ²¹⁹Rn is derived from the most stable isotope of actinium (²²⁷Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds.^[40] 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.

Daughters

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

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

The radon equilibrium factor^[44] is the ratio between the activity of all short-period radon progenies (which are responsible for most of radon's biological effects), and the activity that would be at equilibrium with the radon parent.

If a closed volume is constantly supplied with radon, the concentration of short-lived isotopes will increase until an equilibrium is reached where the overall decay rate of the decay products equals that of the radon itself. The equilibrium factor is 1 when both activities are equal, meaning that the decay products have stayed close to the radon parent long enough for the equilibrium to be reached, within a couple of hours. Under these conditions, each additional pCi/L of radon will increase exposure by 0.01 working level (WL, a measure of radioactivity commonly used in mining). These conditions are not always met; in many homes, the equilibrium factor is typically 40%; that is, there will be 0.004 WL of daughters for each pCi/L of radon in the air.^{[45] 210}Pb takes much longer (decades) to come in equilibrium with radon, but, if the environment permits accumulation of dust over extended periods of time, ²¹⁰Pb and its decay products may contribute to overall radiation levels as well.

Because of 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.^[46]

History and etymology

Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens at McGill University in Montreal.^[47] It was the fifth radioactive element to be discovered, after uranium, thorium, radium, and polonium.^{[48][49][50][51]} In 1899, Pierre and Marie Curie observed that the gas emitted by radium remained radioactive for a month.^[52] Later that year, Rutherford and Owens noticed variations when trying to measure radiation from thorium oxide.^[47] Rutherford noticed that the compounds of thorium continuously emit a radioactive gas that remains radioactive for several minutes, and called this gas "emanation" (from Latin: emanare, to flow out, and emanatio, expiration),^[53] 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").^[54][55] In 1901, Rutherford and Harriet Brooks demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element.^[56] In 1903, similar emanations were observed from actinium by André-Louis Debierne,^[57][58] and were called "actinium emanation" ("Ac Em").

Several shortened names were soon suggested for the three emanations: exradio, exthorio, and exactinio in 1904;^[59] radon (Ro), thoron (To), and akton or acton (Ao) in 1918;^[60] radeon, thoreon, and actineon in 1919,^[61] and eventually radon, thoron, and actinon in 1920.^[62] (The name radon is not related to that of the Austrian mathematician Johann Radon.) The likeness of the spectra of these three gases with those of argon, krypton, and xenon, and their observed chemical inertia led Sir William Ramsay to suggest in 1904 that the "emanations" might contain a new element of the noble-gas family.^[59]

In 1909, Ramsay and Robert Whytlaw-Gray isolated radon and determined its melting temperature and approximate density. In 1910, they determined that it was the heaviest known gas.^[63] They wrote that "L'expression l'émanation du radium est fort incommode" ("the expression 'radium emanation' is very awkward") and suggested the new name niton (Nt) (from Latin: nitens, shining) to emphasize the radioluminescence property,^[64] and in 1912 it was accepted by the International Commission for Atomic Weights. In 1923, the International Committee for Chemical Elements and International Union of Pure and Applied Chemistry (IUPAC) chose the name of the most stable isotope, radon, as the name of the element. The isotopes thoron and actinon were later renamed ²²⁰Rn and ²¹⁹Rn. This has caused some confusion in the literature regarding the element's discovery as while Dorn had discovered radon the isotope, he was not the first to discover radon the element.^[65]

As late as the 1960s, the element was also referred to simply as emanation.^[66] The first synthesized compound of radon, radon fluoride, was obtained in 1962.^[67] Even today, the word radon may refer to either the element or its isotope ²²²Rn, with thoron remaining in use as a short name for ²²⁰Rn to stem this ambiguity. The name actinon for ²¹⁹Rn is rarely encountered today, probably due to the short half-life of that isotope.^[65]

The danger of high exposure to radon in mines, where exposures can reach 1,000,000 Bq/m³, has long been known. In 1530, Paracelsus described a wasting disease of miners, the mala metallorum, and Georg Agricola recommended ventilation in mines to avoid this mountain sickness (Bergsucht).^[68][69] In 1879, this condition was identified as lung cancer by Harting and Hesse in their investigation of miners from Schneeberg, Germany. The first major studies with radon and health occurred in the context of uranium mining in the Joachimsthal region of Bohemia.^[70] In the US, studies and mitigation only followed decades of health effects on uranium miners of the Southwestern US employed during the early Cold War; standards were not implemented until 1971.^[71]

In the early 20th century in the US, gold contaminated with the radon daughter ²¹⁰Pb entered the jewelry industry. This was from gold brachytherapy seeds that had held ²²²Rn, which were melted down after the radon had decayed.^[72][73]

The presence of radon in indoor air was documented as early as 1950. Beginning in the 1970s, research was initiated to address sources of indoor radon, determinants of concentration, health effects, and mitigation approaches. In the US, the problem of indoor radon received widespread publicity and intensified investigation after a widely publicized incident in 1984. During routine monitoring at a Pennsylvania nuclear power plant, a worker was found to be contaminated with radioactivity. A high concentration of radon in his home was subsequently identified as responsible.^[74]

Occurrenceedit

Concentration unitsedit

All discussions of radon concentrations in the environment refer to ²²²Rn. While the average rate of production of ²²⁰Rn (from the thorium decay series) is about the same as that of ²²²Rn, the amount of ²²⁰Rn in the environment is much less than that of ²²²Rn because of the short half-life of ²²⁰Rn (55 seconds, versus 3.8 days respectively).^[3]

Radon concentration in the atmosphere is usually measured in becquerel per cubic meter (Bq/m³), the SI derived unit. Another unit of measurement common in the US is picocuries per liter (pCi/L); 1 pCi/L = 37 Bq/m³.^[45] Typical domestic exposures average about 48 Bq/m³ indoors, though this varies widely, and 15 Bq/m³ outdoors.^[76]

In the mining industry, the exposure is traditionally measured in working level (WL), and the cumulative exposure in working level month (WLM); 1 WL equals any combination of short-lived ²²²Rn daughters (²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po) in 1 liter of air that releases 1.3 × 10⁵ MeV of potential alpha energy;^[45] 1 WL is equivalent to 2.08 × 10⁻⁵ joules per cubic meter of air (J/m³).^[3] The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m³). One WLM is equivalent to 3.6 × 10⁻³ J·h/m³. An exposure to 1 WL for 1 working-month (170 hours) equals 1 WLM cumulative exposure. A cumulative exposure of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m³.^[77]

²²²Rn decays to ²¹⁰Pb and other radioisotopes. The levels of ²¹⁰Pb can be measured. The rate of deposition of this radioisotope is weather-dependent.

Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1,000 Bq/m³ (relatively high) concentration corresponds to 0.17 picogram per cubic meter (pg/m³). The average concentration of radon in the atmosphere is about 6×10⁻¹⁸ molar percent, or about 150 atoms in each milliliter of air.^[78] The radon activity of the entire Earth's atmosphere originates from only a few tens of grams of radon, consistently replaced by decay of larger amounts of radium, thorium, and uranium.^[79]

Naturaledit

Radon is produced by the radioactive decay of radium-226, which is found in uranium ores, phosphate rock, shales, igneous and metamorphic rocks such as granite, gneiss, and schist, and to a lesser degree, in common rocks such as limestone.^[5][80] Every square mile of surface soil, to a depth of 6 inches (2.6 km² to a depth of 15 cm), contains about 1 gram of radium, which releases radon in small amounts to the atmosphere.^[3] It is estimated that 2.4 billion curies (90 EBq) of radon are released from soil annually worldwide.^[81] This is equivalent to some 15.3 kilograms (34 lb).

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

Radon concentration can be much higher in mining contexts. Ventilation regulations instruct to maintain radon concentration in uranium mines under the "working level", with 95th percentile levels ranging up to nearly 3 WL (546 pCi ²²²Rn per liter of air; 20.2 kBq/m³, measured from 1976 to 1985).^[3] The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m³ (1.2 nCi/L) with maximal value of 160 kBq/m³ (4.3 nCi/L).^[83]

Radon mostly appears with the radium/uranium series (decay chain) (²²²Rn), and marginally with the thorium series (²²⁰Rn). The element emanates naturally from the ground, and some building materials, all over the world, wherever traces of uranium or thorium are found, and particularly in regions with soils 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 ²²²Rn), radon concentration decreases very quickly when the distance from the production area increases. Radon concentration varies greatly with season and atmospheric conditions. For instance, it has been shown to accumulate in the air if there is a meteorological inversion and little wind.^[84]

High concentrations of radon can be found in some spring waters and hot springs.^[85] The towns of Boulder, Montana; Misasa; Bad Kreuznach, Germany; and the country of Japan have radium-rich springs that emit radon. To be classified as a radon mineral water, radon concentration must be above 2 nCi/L (74 kBq/m³).^[86] The activity of radon mineral water reaches 2 MBq/m³ in Merano and 4 MBq/m³ in Lurisia (Italy).^[83]

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 ²²²Rn than surface water, because radon is continuously produced by radioactive decay of ²²⁶Ra present in rocks. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone because of diffusional losses to the atmosphere.^[87][88]

In 1971, Apollo 15 passed 110 km (68 mi) above the Aristarchus plateau on the Moon, and detected a significant rise in alpha particles thought to be caused by the decay of ²²²Rn. The presence of ²²²Rn has been inferred later from data obtained from the Lunar Prospector alpha particle spectrometer.^[89]

Radon is found in some petroleum. Because radon has a similar pressure and temperature curve to propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become contaminated because of decaying radon and its products.^[90]

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.^[90]

Accumulation in buildingsedit

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High concentrations of radon in homes were discovered by chance in 1985 after the stringent radiation testing conducted at the new Limerick Generating Station nuclear power plant revealed that Stanley Watras, a construction engineer at the plant, was contaminated by radioactive substances even though the reactor had never been fueled.^[91] Typical domestic exposures are of approximately 100 Bq/m³ (2.7 pCi/L) indoors. Some level of radon will be found in all buildings. Radon mostly enters a building directly from the soil through the lowest level in the building that is in contact with the ground. High levels of radon in the water supply can also increase indoor radon air levels. Typical entry points of radon into buildings are cracks in solid foundations and walls, construction joints, gaps in suspended floors and around service pipes, cavities inside walls, and the water supply.^[12] Radon concentrations in the same place may differ by double/half over one hour. Also, the concentration in one room of a building may be significantly different from the concentration in an adjoining room.^[3] The soil characteristics of the dwellings are the most important source of radon for the ground floor and higher concentration of indoor radon observed on lower floors. Most of the high radon concentrations have been reported from places near fault zones; hence the existence of a relation between the exhalation rate from faults and indoor radon concentrations is obvious.^{[citation needed]}

The distribution of radon concentrations will generally differ from room to room, and the readings are averaged according to regulatory protocols. Indoor radon concentration is usually assumed to follow a log-normal distribution on a given territory.^[92] Thus, the geometric mean is generally used for estimating the "average" radon concentration in an area.^[93]

The mean concentration ranges from less than 10 Bq/m³ to over 100 Bq/m³ in some European countries.^[94]

Some of the highest radon hazard in the US is found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania.^[95] 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.^[96] 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.^[97]

In a few places, uranium tailings have been used for landfills and were subsequently built upon, resulting in possible increased exposure to radon.^[3]

Since radon is a colorless, odorless gas, the only way to know how much is present in the air or water is to perform tests. In the US, radon test kits are available to the public at retail stores, such as hardware stores, for home use, and testing is available through licensed professionals, who are often home inspectors. Efforts to reduce indoor radon levels are called radon mitigation. In the US, the EPA recommends all houses be tested for radon. In the UK under the Housing Health & Safety Rating System (HHSRS) property owners have an obligation to evaluate potential risks and hazards to health and safety in a residential property.^[98]

Industrial productionedit

Radon is obtained as a by-product of uraniferous ores processing after transferring into 1% solutions of hydrochloric or hydrobromic acids. The gas mixture extracted from the solutions contains H
₂, O
₂, He, Rn, CO
₂, H
₂O and hydrocarbons. The mixture is purified by passing it over copper at 993 K (720 °C; 1,328 °F) to remove the H
₂ and the O
₂, and then KOH and P
₂O
₅ are used to remove the acids and moisture by sorption. Radon is condensed by liquid nitrogen and purified from residue gases by sublimation.^[99]

Radon commercialization is regulated, but it is available in small quantities for the calibration of ²²²Rn measurement systems, at a price, in 2008, of almost US$6,000 (equivalent to $8,491 in 2023) per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment).^[100] Radon is produced by a solution of radium-226 (half-life of 1,600 years). Radium-226 decays by alpha-particle emission, producing radon that collects over samples of radium-226 at a rate of about 1 mm³/day per gram of radium; equilibrium is quickly achieved and radon is produced in a steady flow, with an activity equal to that of the radium (50 Bq). Gaseous ²²²Rn (half-life of about four days) escapes from the capsule through diffusion.^[101]

Concentration scaleedit

Bq/m3	pCi/L	Occurrence example
1	~0.027	Radon concentration at the shores of large oceans is typically 1 Bq/m3. Radon trace concentration above oceans or in Antarctica can be lower than 0.1 Bq/m3.
10	0.27	Mean continental concentration in the open air: 10 to 30 Bq/m3. Based on a series of surveys, the global mean indoor radon concentration is estimated to be 39 Bq/m3.
100	2.7	Typical indoor domestic exposure. Most countries have adopted a radon concentration of 200–400 Bq/m3 for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (150 Bq/m3), then no action is necessary. A cumulated exposure of 230 Bq/m3 of radon gas concentration during a period of 1 year corresponds to 1 WLM.
1,000	27	Very high radon concentrations (>1000 Bq/m3) have been found in houses built on soils with a high uranium content and/or high permeability of the ground. If levels are 20 picocuries radon per liter of air (800 Bq/m3) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. Allowable concentrations in uranium mines are approximately 1,220 Bq/m3 (33 pCi/L)[102]
10,000	270	The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m3 (about 1.2 nCi/L) with maximal value of 160 kBq/m3 (about 4.3 nCi/L).[83]
100,000	~2700	About 100,000 Bq/m3 (2.7 nCi/L) was measured in Stanley Watras's basement.[103][104]
1,000,000	27000	Concentrations reaching 1,000,000 Bq/m3 can be found in unventilated uranium mines.
~5.54 × 1019	~1.5 × 1018	Theoretical upper limit: Radon gas (222Rn) at 100% concentration (1 atmosphere, 0 °C); 1.538×105 curies/gram;[105] 5.54×1019 Bq/m3.

Applicationsedit

Medicaledit

Zdroj:https://en.wikipedia.org?pojem=Radon_gas
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