Radon

86 astatine ← radon → francium Xe ↑ Rn ↓ Uuo Periodic table
General
Name, symbol, number	radon, Rn, 86
Element category	noble gases
Group, period, block	18, 6, p
Appearance	colorless
Standard atomic weight	(222) g·mol−1
Electron configuration	[Xe] 4f14 5d10 6s2 6p6
Electrons per shell	2, 8, 18, 32, 18, 8 (Image)
Physical properties
Phase	gas
Melting point	202 K (−71.15 °C, −96 °F)
Boiling point	211.3 K (−61.85 °C, −79.1 °F)
Critical point	377 K, 6.28 MPa
Heat of fusion	3.247 kJ·mol−1
Heat of vaporization	18.10 kJ·mol−1
Specific heat capacity	(25 °C) 20.786 J·mol��1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 110 121 134 152 176 211
Atomic properties
Crystal structure	cubic face centered
Oxidation states	0
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: 1037 kJ·mol−1
Atomic radius (calc.)	120 pm
Covalent radius	145 pm
Miscellaneous
Magnetic ordering	non-magnetic
Thermal conductivity	(300 K) 3.61 m W·m−1·K−1
CAS registry number	10043-92-2
Most stable isotopes
Main article: Isotopes of radon iso NA half-life DM DE (MeV) DP 210Rn syn 2.4 h Alpha 6.404 216Po 211Rn syn 14.6 h Epsilon 2.892 211At Alpha 5.965 207Po 222Rn trace 3.8235 d Alpha 5.590 218Po 224Rn syn 1.8 h Beta 0.8 224Fr
References
This box: view • talk • edit

Radon (pronounced /ˈreɪdɒn/) is a chemical element with symbol Rn and atomic number 86. Radon is a colorless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health hazard. The most stable isotope, ²²²Rn, has a half-life of 3.8 days. While having been less studied by chemists due to its high radioactivity, there are a few known compounds of this generally unreactive element.

Radon is formed from the normal radioactive decay of uranium. Uranium has been around since the earth was formed and has a very long half-life (4.5 billion years), which is the amount of time required for one-half of uranium to break down. Uranium, radium, and thus radon, will continue to exist indefinitely at about the same levels as they do now.^[1]

Radon is responsible for the majority of the mean public exposition to ionizing radiations, it is often the single largest contributor to an individual's background radiation dose, and is certainly the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as basements. Radon can be found in some spring waters and hot springs.^[2]

Breathing high concentrations of radon can cause lung cancer. Thus, radon is considered as a significant contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency, radon could be the second most frequent cause of lung cancer, after cigarette smoking; and radon-induced lung cancer the 6th leading cause of cancer death overall, causing 21,000 lung cancer deaths per year in the United States.^[3]

History and etymology

Discovered in 1900 by Friedrich Ernst Dorn, radon was the fifth radioactive element to be discovered, after uranium, thorium, radium and polonium.^[4][5][6] In 1900 Dorn reported some experiments in which he noticed that radium compounds emanate a radioactive gas which he named Radium Emanation (Ra Em).^[7] Before that, in 1899, Pierre and Marie Curie observed that the "gas" emitted by radium remained radioactive for a month.^[8] Later that year, Robert B. Owens and Ernest Rutherford noticed variations when trying to measure radiation from thorium oxide.^[9] Rutherford noticed that the compounds of thorium continuously emit a radioactive gas which retain the radioactive powers for several minutes and called this gas "emanation" (from Latin "emanare"—to elapse and "emanatio"—expiration),^[10] and later Thorium Emanation (Th Em). In 1901, he demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element.^[11] In 1903, similar emanations were observed from actinium by André-Louis Debierne^[12][13] and were called Actinium Emanation (Ac Em).

Several names were suggested for these three gases: exradio, exthorio, and exactinio in 1904;^[14] radon, thoron, and akton in 1918;^[15] radeon, thoreon, and actineon in 1919,^[16] and eventually radon, thoron, and actinon in 1920.^[17] 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.^[14]

In 1910, Sir William Ramsay and Robert Whytlaw-Gray isolated radon, determined its density, and determined that it was the heaviest known gas.^[18] They wrote that "L'expression l'émanation du radium est fort incommode," (the expression of radium emanation is very awkward) and suggested the new name niton (Nt) (from the Latin "nitens" meaning "shining") in order to emphasize the property of gases that cause the phosphorescence of some substances,^[18] 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 among the names radon (Rn), thoron (Tn), and actinon (An). Later, when isotopes were numbered instead of named, the element took the name of the most stable isotope, radon, while Tn became ²²⁰Rn and An ²¹⁹Rn. As late as the 1960s, the element was also referred to simply as emanation.^[19] The first synthesized compound of radon, radon fluoride, was obtained in 1962.^[20]

The danger of high expositions to radon in mines, where expositions reaching 1,000,000 Bq/m³ can be found, has been known for long. In 1530, Paracelsus described a wasting disease of miners, the mala metallorum, and Georg Agricola recommends mines ventilation to avoid this mountain sickness (Bergsucht).^[21] · ^[22] In 1879, this condition was identified as lung cancer by Herting 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, first in the Joachimsthal region of Bohemia and then in the Southwestern United States during the early Cold War.

Characteristics

Physical form

Radon is a colorless and odorless gas, and therefore not readily detectable by a human.

At standard temperature and pressure, radon forms a monatomic gas with a density of 9.73 kg/m³,^[23] about 8 times the surface density of the Earth's atmosphere, 1.217 kg/m³,^[24] and is one of the heaviest gases at room temperature and the heaviest of the noble gases, excluding ununoctium. At standard temperature and pressure, radon is a colorless gas, but when it is cooled below its freezing point of 202 K (−71 °C; −96 °F), it has a brilliant phosphorescence which turns yellow as the temperature is lowered, and becomes orange-red as the air liquefies at temperatures below 93 K (−180.1 °C; −292.3 °F).^[25] Upon condensation, radon also glows because of the intense radiation it produces.^[]

Because it is also radioactive and is a relatively unreactive chemical element, radon has few uses and is seldom used in academic research.

Isotopes

Radon has no stable isotopes. There are 36 radioactive isotopes that have been characterized which range from an atomic mass of 193 to 228.^[26] The most stable isotope is ²²²Rn, which is a decay product of ²²⁶Ra. It has a half-life of 3.823 days and decomposes by alpha particle emission into ²¹⁸Po.^[26] Among the decay daughters of this decay chain is also the highly unstable isotope ²¹⁸Rn. The naturally occurring ²²⁶Ra is a product of the decay chain of ²³⁸U.^[27] The decay chain of ²³⁸U lists all the decay products of ²²²Rn.

There are three other isotopes that have a half life of over an hour: ²¹¹Rn, ²¹⁰Rn and ²²⁴Rn. The ²²⁰Rn isotope is a natural decay product of the most stable thorium isotope (²³²Th), named thoron. 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.^[26] No radon isotopes are part of the other major decay series, that of neptunium (²³⁷Np).

Chemistry

Radon is a member of the zero-valence elements that are called noble or inert gases. 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.^[28] For example, an energy of more than 248 kcal/mol is required to extract one electron from its shells (also known as the first ionization energy).^[29] However, due to periodic trends, radon has a lower electronegativity than the element one period before it, xenon, and is therefore more reactive. Radon is sparingly soluble in water, but more soluble than lighter noble gases. Radon is appreciably more soluble in organic liquids than in water. Early studies concluded that the stability of radon hydrate should be of the same order as that of the hydrates of chlorine (Cl₂) or sulfur dioxide (SO₂), and significantly higher than the stability of the hydrate of hydrogen sulphide (H₂S).^[30]

Because of its price 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 a few powerful oxidizing agents such as F₂, thus forming radon fluoride.^[31][32] It decomposes back to elements at a temperature of above 250°C. It has a low volatility and was thought to be RnF₂. But 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. However, theoretical studies on this molecule have predicted that it should have a Rn-F bond distance of 2.08 A, and that the compound is thermodynamically more stable and less volatile than its lighter counterpart XeF₂.^[33] The octahedral molecule RnF₆ was predicted to have an even lower enthalpy of formation than the difluoride.^[34] The [RnF]⁺ is believed to form by the reaction:^[35]

Rn (g) + 2 [O₂]⁺[SbF₆]^��� (s) → [RnF]⁺[Sb₂F₁₁]⁻ (s) + 2 O₂ (g)

Radon oxides are among the few other reported compounds of radon.^[36] The radon carbonyl RnCO has been predicted to be stable and to have a linear geometry.^[37] The molecules Rn₂ and RnXe were found to be significantly stabilized by spin-orbit couplings.^[38] Radon caged inside a fullerene has been proposed as a drug for tumors.^[39]

Radon progenies

222Rn belongs to the Radium and Uranium-238 decay chain. It decays with a half-life of 3.8235 days. Its four first progenies (excluding marginal decay schemes) are very short-lived, meaning that the corresponding disintegrations are correlated to the initial radon distribution :

• 218-Po, 3.10 minutes, Alpha decay.
• 214-Pb, 26.8 min, Beta decay.
• 214-Bi, 19.9 min, Beta decay.
• 214-Po, 0.1643 ms, Alpha decay.

At the next step, Polonium 214 decays to 210-Pb, which has a much longer half-life of 22.3 years.

The radon equilibrium factor is the ratio between the activity of all short-period radon progenies (which are responsible of most of the biological effect), and the activity of the radon parent. The equilibrium factor is 1 when both activities are equal, meaning that the decay products have stayed close to the radon parent. Progenies usually sediment, whereas gaseous radon does not, so that the equilibrium factor in the atmosphere is usually less than one. The equilibrium factor retained in epidemiological studies is 0.4.^[40]

The lead-210 has a half-life of 22.3 years. Its progenies are:

• 210-Bi, 5.013 days, Beta decay.
• 210-Po, 138.376 days, Alpha decay.
• Lead-206, stable.

Occurrence

Concentration units

All discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 from the thorium decay series) is about the same as radon-222, the amount of radon-220 entering the environment is much less than that of radon-222 because of the short half-life of radon-220 (1 minute versus 4 days).^[1]

Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1000 Bq/m3 (relatively high) concentration corresponds to 0.17 pico-gram per cubic meter. The average concentration of radon in the atmosphere is about 6 × 10^-20 atoms of radon for each molecule in the air, or about 150 atoms in each mL of air.^[42] All the radon activity of the Earth atmosphere is due to some tens of grams of radon.^[43]

Radon concentration is usually measured in the atmosphere, in becquerel per cubic meter (Bq/m3), the SI derived unit. Typical domestic exposures are of ≈ 100 Bq/m3 indoors, and 10-20 Bq/m3 outdoors.

It is often measured in pico-curie per liter (pCi/l), with 1 pCi/l = 37 Bq/m3 (or 5 pCi/l ≈ 200 Bq/m3).

In the mining industry, the exposition is traditionally measured in working level (WL), and the cumulative exposition in working level month (WLM) : 1 WL equals any combination of short-lived 222Rn progeny (218Po, 214Pb, 214Bi, and 214Po) in 1 liter of air that releases 1.3 x 10^5 MeV of potential alpha energy ; one WL is equivalent to 2.08 x 10^5 joules per cubic meter of air (J/m3).^[1] The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (Jh/m3). One WLM is equivalent to 3.6 x 10-3 Jh/m3. An exposure to 1 WL for 1 working month (170 hours) equals 1 WLM cumulative exposure.

A cumulative exposition of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m3.^[44]

The radon (²²²Rn) released into the air decays to ²¹⁰Pb and other radioisotopes, the levels of ²¹⁰Pb can be measured. The rate of deposition of this radioisotope is dependent on the weather.

Natural

Radon is a decay product of uranium, which is relatively common in the Earth's crust, but generally concentrated in ore-bearing rocks scattered around the world. Every square mile of surface soil, to a depth of 6 inches, contains approximately 1 gram of radium, which releases radon in small amounts to the atmosphere^[1] On a global scale, it is estimated that 2,400 million curies of radon are released from soil annually.

Radon concentration varies wildly 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 aerated mines, or ill-aerated houses, its concentration climbs to 20-2,000 Bq/m3.^[45] Radon concentration can be much higher in mining contexts. Due to ventilation regulation, typical radon concentration in uranium mines is usually maintained under the "working level", with 95th percentile levels ranging up to nearly 3 WL (546 pCi radon-222/L of air; 20,202 Bq/m3, measured from 1976 to 1985).^[1] 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).^[46]

Radon mostly appears with the decay chain of the radium and uranium series (²²²Rn), and marginally with the thorium series (²²⁰Rn). The element emanates naturally from the ground all over the world, wherever traces of uranium or thorium can be found, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. However, 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. Due to its very small half-life (four days for ²²²Rn), its concentration decreases very quickly when the distance from the production area increases. Its 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.^[47]

High concentrations of radon can be found in some spring waters and hot springs.^[48] The towns of Boulder, Montana; Misasa; Bad Kreuznach, Germany; and the country of Japan have radium-rich springs which emit radon. To be classified as a radon mineral water, radon concentration must be above a minimum of 2 nCi/l (74 Bq/l).^[49] The activity of radon mineral water reaches 2,000 Bq/l in Merano and 4,000 Bq/l in Lurisia (Italy).^[46]

Natural radon concentrations in 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 the 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.^[50][51]

In 1971, Apollo 15 passed 110 kilometres (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 radon-222. The presence of radon-222 (²²²Rn) has been inferred later from data obtained from the Lunar Prospector alpha particle spectrometer.^[52]

Accumulation

Radon is found in some petroleum. Because radon has a similar pressure and temperature curve as propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become partially radioactive due to radon decay particles. Residues from the oil and 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. The radon decays to form solid radioisotopes which form coatings on the inside of pipework. An oil processing plant, the area of the plant where propane is processed, is often one of the more contaminated areas of the plant as radon has a similar boiling point as propane.^[53]

Typical domestic exposures are of ≈ 100 Bq/m3 indoors. Depending on how houses are built and ventilated, radon may accumulate in basements and dwellings. Radon concentrations in the same location may differ by a factor of two over a period of 1 hour. Also, the concentration in one room of a building may be significantly different than the concentration in an adjoining room.^[1]

The distribution of radon concentrations tends to be asymmetrical around the average, the larger concentrations have a disproportionately greater weight. Indoor radon concentrations is usually assumed to follow a lognormal distribution on a given territory.^[54] Thus, the geometric mean is generally used for estimating the "average" radon concentration in an area.^[55] The mean concentration ranges from less than 10 Bq/m³ to over 100 Bq/m³ in some European countries.^[56] Typical geometric standard deviations found in studies range between 2 and 3, meaning (given the 68-95-99.7 rule) that the radon concentration is expected to be more than a hundred time the mean concentration for 2 to 3% of the cases.

The highest average radon concentrations in the United States are found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania.^[57] Some of the highest readings ever have been recorded in the Irish town of Mallow, County Cork, prompting local fears regarding lung cancer. Iowa has the highest average radon concentrations in the United States due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland.^[58] Many cities within the state, such as Iowa City, have passed requirements for radon-resistant construction in new homes. In a few locations, uranium tailings have been used for landfills and were subsequently built on, resulting in possible increased exposure to radon.^[1]

Industrial production

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 H₂, O₂, He, Rn, CO₂, H₂O and hydrocarbons. The mixture is purified by passing it over copper at 1000°K to remove the H₂ and the O₂, and then KOH and P₂O₅ are used to remove the acids and moisture by sorption. Radon is condensed by liquid nitrogen and purified from residue gases by sublimation.^[59]

Radon commercialization is regulated, but it is available in small quantities for the calibration of radon-222 measurement systems, at a price of almost $6,000 per milli-litre of radium solution (which only contains about 15 picograms of actual radon at a given moment).^[60] Radon is produced by a solution of radium-226 (half-life of 1 600 years). Radium-226 decays by alpha-particle emission, producing Radon which collects over samples of radium-226 at a rate of about 0.001 cm³/day per gram of radium ; equilibrium is quickly achieved and the radon is produced in a steady flow, with an activity equals that of the radium (50 Bq). Gaseous radon-222 (half-life of ~ four days) escapes from the capsule through diffusion.

Concentration scale

Bq/m³	Occurrence example
1	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	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	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 (160 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	Very high radon concentrations (>1000 Bq/m³) have been found in countries where houses are built on soils with a high uranium content and/or high permeability of the ground. For 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.
10.000	The "Working Level" in uranium mines corresponds to a 7000 Bq/m³ concentration. 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).[46]
100.000	The World Health Organization issued as guidelines (1988) that remedial action should be considered when the radon activity exceeded 100 kBq/m³ in a building, and remedial action should be considered without long delay if exceeding 400 kBq/m³. About 100,000 Bq/m³ (2,700 pCi/l) was found in Stanley Watras's basement
1.000.000	Expositions reaching 1,000,000 Bq/m³ can be found in unventilated uranium mines.

Applications

Medical

Arthritis

It has been said that exposure to radon gas mitigates auto-immune diseases such as arthritis.^[61] As a result, in the late 20th century and early 21st century, some "health mines" were established in Basin, Montana which attracted people seeking relief from health problems such as arthritis through limited exposure to radioactive mine water and radon. The practice is controversial because of the "well-documented ill effects of high-dose radiation on the body."^[62]

Bathing

Radioactive water baths have been applied since 1906 in Jáchymov, Czech Republic, but even before radon discovery they were used in Bad Gastein, Austria. Radium-rich springs are also used in traditional Japanese onsen in Misasa, Tottori prefecture. Drinking therapy is applied in Bad Brambach, Germany. Inhalation therapy is carried out in Gasteiner-Heilstollen, Austria, in Kowary, Poland and in Boulder, Montana, United States. In the United States and Europe there are several "radon spas," where people sit for minutes or hours in a high-radon atmosphere in the belief that low doses of radiation will invigorate or energize them.^[63]

Medical Radiography

The radon gas which is used as a cancer treatment in medicine is obtained from the decay of a radium chloride source. In the past, radium and radon have both been used for X-ray medical radiography, but they have fallen out of use as they are radiotoxic alpha radiation emitters which are expensive and have been replaced with iridium-192 and cobalt-60 since they are far better photon sources.

Radiotherapy

Radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made in accelerators and nuclear reactors.

Radon has been used in implantable seeds, made of gold or glass, primarily used to treat cancers. The gold seeds were produced by filling a long tube with radon pumped from a radium source, the tube being then divided into short sections by crimping and cutting. The gold layer keeps the radon within, and filters out the alpha and beta radiations, while allowing the gamma rays to escape (which kill the diseased tissue). The activities might range from 0.05 to 5 millicuries per seed (2 to 200 MBq).^[64] The gamma rays are produced by radon and the first short-lived elements of its Decay chain (218-Po, 214-Pb, 214-Bi, 214Po).

Radon and its first decay products being very short-lived, the seed is left in place. After 12 half-lives (43 days), radon radioactivity is at 1/2000 of its original level. At this stage, the predominant residual activity is due to the radon decay product lead-210, whose half-life (22.3 year) is 2000 times that or radon (and whose activity is thus 1/2000 or radon's), and its descendants 210-Bi and 210-Po, totalizing 0.03% of the initial seed activity.

In the early part of the 20th century in the USA, gold which was contaminated with lead-210 entered the jewelry industry. This was from gold seeds which had held radon-222 that had been melted down after the radon had decayed.^[65] · ^[66] Wearing a contaminated ring could lead to a skin exposition of 10 to 100 rad/day (0.4 to 4 mSv/h)^[67]

Scientific

Tracking of air masses

Radon emanation from the soil varies with soil type and with surface uranium content, so outdoor radon concentrations can be used to track air masses to a limited degree. This fact has been put to use by some atmospheric scientists. Because of radon's rapid loss to air and comparatively rapid decay, radon is used in hydrologic research that studies the interaction between ground water and streams. Any significant concentration of radon in a stream is a good indicator that there are local inputs of ground water. Radon is also used in the dating of oil-containing soils because radon has a high affinity of oil-like substances.

Geological faults

Radon soil-concentration has been used in an experimental way to map buried close-subsurface geological faults because concentrations are generally higher over the faults. Similarly, it has found some limited use in geothermal prospecting. Some researchers have also looked at elevated soil-gas radon concentrations, or rapid changes in soil or groundwater radon concentrations, for earthquake prediction.^[68] The theory is that compression around a fault about to rupture could produce radon emission, as if the ground were being squeezed like a sponge. 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.^[69] However, as of 2009, it is under investigation as a possible precursor by NASA.^[70]

Power

Radon is a known pollutant emitted from geothermal power stations, though it disperses rapidly, and no radiological hazard has been demonstrated in various investigations. The trend in geothermal plants is to reinject all emissions by pumping deep underground, and this seems likely to ultimately decrease such radon hazards further.

Health incidence of radon exposition

Cancer on miners

Radon is a common problem encountered during uranium mining, and significant excesses in deaths from lung cancer have been identified in epidemiology studies of uranium miners and other hard rock miners employed in the 1940s and 1950s.^{[71][72] [73]}

The first major studies with radon and health occurred in the context of uranium mining, first in the Joachimsthal region of Bohemia and then in the Southwestern United States during the early Cold War. Because radon is a product of the radioactive decay of uranium, underground uranium mines may have high concentrations of radon. Many uranium miners in the Four Corners region contracted lung cancer and other pathologies as a result of high levels of exposure to radon in the mid-1950s. The increased number of incidences of lung cancer was particularly pronounced among Native American and Mormon miners, because those groups normally have low rates of lung cancer.^[74] Safety standards requiring expensive ventilation were not widely implemented or policed during this period.^[75]

In studies of uranium miners, workers exposed to radon levels of 50 to 150 picocuries of radon per liter of air (2000–6000 Bq/m3) for about 10 years have shown an increased frequency of lung cancer.^[1] Statistically significant excesses in lung cancer deaths were present after cumulative exposures of less than 50 WLM.^[1] However, there are a number of confounding factors to consider, including exposure to other agents, ethnicity, smoking history, and work experience. The cases reported in these miners cannot be attributed solely to radon or radon daughters but may be due to exposure to silica, to other mine pollutants, to smoking, or to other causes.^[1][76] The majority of miners in the studies are smokers and all inhale dust and other pollutants in mines. Because radon and cigarette smoke both cause lung-cancer, and since the effect of smoking is far above that of radon, it is complicated to disentangle the effects of the 2 kinds of exposure : misinterpreting the smoking habit by a few percent can blur out the radon effect.^[77]

In recent years, the average annual exposure of uranium miners has fallen to levels similar to the concentrations inhaled in some homes. The power to detect any excess risks in miners nowadays is likely to be small, because the exposures are much smaller than in the early years of mining.^[78]

Health risks

Radon-222 has been classified by International Agency for Research on Cancer as being carcinogenic to humans.^[79] Raised lung cancer rates have been reported from a number of cohort and case-control studies of underground miners exposed to radon and its decay products. There is sufficient evidence for the carcinogenicity of radon and its decay products in humans for such expositions.^[80]

The primary route of exposure to radon and its progeny is inhalation. Radiation exposure from radon is indirect. The health hazard from radon does not come primarily from radon itself, but rather from the radioactive products formed in the decay of radon.^[1] The general effects of radon to the human body are caused by its radioactivity and consequent risk of radiation-induced cancer. Lung cancer is the only observed consequence of high concentration radon exposures ; both human and animal studies indicate that the lung and respiratory system are the primary targets of radon daughter-induced toxicity.^[1]

Radon has a short half-life (4 days) and decays into other solid particulate radium-series radioactive nuclides. Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.^[81] If the gas is inhaled, these radioactive particles are inhaled and may attach to the inner lining of the lung. The pattern of their deposition in the respiratory tract is dependent on whether they are attached to particles or not. These particles remain lodged in the lungs, and continue to decay, causing continued exposure by emitting alpha radiations. The radiation decay products can damage cells in the lung tissue,^[82] either create free radicals or cause DNA breaks,^[81] perhaps causing mutations that sometimes turn cancerous.

The risk of lung-cancer caused by smoking is much higher than the risk of lung-cancer caused by indoor radon. Radiation from radon has been attributed to increase of lung cancer among smokers too. It is generally believed that exposure to radon and cigarette smoking are synergistic; that is, that the combined effect exceeds the sum of their independent effects. This is because the daughters of radon often become attached to smoke and dust particles, and are then able to lodge in the lungs.^[83]

It is unknown whether radon causes other types of cancer, but recent studies suggest a need for further studies to assess the relationship between radon and leukemia.^[84][85]

The effects of radon, if found in food or drinking water, are unknown. Following ingestion of radon dissolved in water, the biological half-time for removal of radon from the body ranges from 30 to 70 minutes : more than 90% of the absorbed radon was eliminated by exhalation within 100 minutes ; by 600 minutes, only 1% of the absorbed amount remained in the body.^[1]

Effective dose caused by radon exposition

UNSCEAR recommends^[86] a reference value of 9 nSv (Bq.h.m^-3)^-1

This means that people living permanently (8760 h/year) in a high concentration of 1000 Bq/m3 receive a dose of 80 mSv/year.

Studies of miners exposed to radon and its decay products provide a direct basis for assessing their lung cancer risk. The BEIR VI report, entitled Health Effects of Exposure to Radon, reported an excess relative risk from exposure to radon that was equivalent to 1.8 per cent per megabecquerel hours per cubic metre (MBq h m–3) (95 per cent confidence interval: 0.3, 35) for miners with cumulative exposures below 30 MBq h m–3.^[78]

The excess relative risk from long-term residential exposure to radon at 100 Bq m–3 is considered to be about 0.16 (after correction for uncertainties in exposure assessment), with about a three-fold factor of uncertainty higher or lower than that value.^[78] In other words, the absence of ill effect (or even a positive hormesis effects) at 100 Bq/m3 are compatible with the known data.

In the absence of other causes of death, the absolute risks of lung cancer by age 75 years at usual radon concentrations of 0, 100, and 400 Bq/m3 would be about 0.4%, 0.5%, and 0.7%, respectively, for lifelong non-smokers, and about 25 times greater (10%, 12%, and 16%) for cigarette smokers.^[87]

The risk estimates correspond to a unit risk of approximately 3–6 × 10–5 per Bq/m3, assuming a lifetime risk of lung cancer of 3%. This means that a person living in an average European house with 50 Bq/m3 has a lifetime excess lung cancer risk of 1.5–3 × 10–3. Similarly, a person living in a house with a high radon concentration of 1000 Bq/m3 has a lifetime excess lung cancer risk of 30–60 × 10–3 (3–6%), implying a doubling of background lung cancer risk.^[88]

Studies on domestic exposures

Major source of natural radiation

The largest natural contributor to public radiation dose is radon, a naturally occurring, radioactive gas found in soil and rock^[89], which comprises approximately 55% of the annual background dose. Radon gas levels vary by locality and the composition of the underlying soil and rocks.

Radon (at concentrations encountered in mines) was recognized as carcinogenic in the 1980s, in view of the lung cancer statistics for miners cohorts. Although radon may present significant risks, thousands of people annually go to radon-contaminated mines for deliberate exposure to help with the symptoms of arthritis without any serious health effects.^[90]

The possible danger of radon exposure in dwellings was discovered in 1984 when Stanley Watras, an employee at the Limerick nuclear power plant in Pennsylvania, set off the radiation alarms on his way to work for two weeks while authorities searched for the source of the contamination. They found that the source was high levels of radon—about 100,000 Bq/m³ (2,700 pCi/L)—in his house's basement, and it was not related to the nuclear plant. The risks associated with living in his house were estimated to be equivalent to smoking 135 packs of cigarettes every day. Following this highly publicized event, national radon safety standards were set, and radon detection and ventilation became a standard homeowner concern,^[91] though typical domestic expositions are two to three order of magnitude lower (100 Bq/m3, or 2.5 pCi/l). Beginning with the late 1980s, this led to activists forming campaigns to raise awareness of radiation resulting from radon.^[92]

Radon as a terrestrial source of background radiation is of particular concern because, although on average it is very rare, this intensely radioactive element can be found in high concentrations in many areas of the world. Some of these areas, including Cornwall and Aberdeenshire in the United Kingdom have high enough natural radiation levels that nuclear licensed sites cannot be built there—the sites would already exceed legal radiation limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste.^[93]

People in affected localities can receive up to 10 mSv per year background radiation.^[93]

This led to a health policy problem : what is the health impact of domestic exposures to radon for the concentrations typically fond in some buildings (100 Bq/m3) ?

Detection methods

When the exposition to a carcinogenic substance is suspected, the cause-effect relationship on any given case can never be ascertained. Lung cancer occurs spontaneously, and there is no difference between a “natural″cancer and another one caused by radon (or smoking). Furthermore, it takes years for a cancer to develop, so that determining the past exposition of a case is usually very approximative. The health effect of radon can only be demonstrated through theory and statistical observations.

The Study design for epidemiological methods may be of three kinds :

• The best proofs are statistical observations of cohorts (predetermined population with known expositions and exhaustive follow-up), like the ones made on miners, or the studies of Hiroshima and Nagasaki survivors. They are efficient, but very costly when the population needs to be a large one. Such studies can only be used when the effect is strong enough, hence, for high exposures.
• Alternate proofs are case-control studies (the environment factors of a “case” population is individually determined, and compared to that of a “control″population, to see what the difference might have been, and which factors may be significant), like the ones that have been used to demonstrate the link between lung cancer and smoking. Such studies can identify key factors when the signal/noise ratio is strong enough, but are very sensitive to selection bias, and prone to the existence of confounding factors.
• Lastly, ecological studies may be used (where the global environment variables and their global effect on two different populations are compared). Such studies are “cheap and dirty” : they can be easily conducted on very large populations (the whole USA, in Dr Cohen's study), but are prone to the existence of confounding factors, and exposed to the ecological fallacy problem.

Furthermore, theory and observation must confirm each other, for a relationship to be accepted as a fully proven. Even when a statistical link between factor and effect appears significant, it must be backed by a theoretical explanation ; and a theory is not accepted as factual unless confirmed by observations.

Epidemiology studies of domestic expositions

Several ecological studies have been performed to assess possible relationships between selected cancers and estimated radon levels within particular geographic regions where environmental radon levels appear to be higher than other geographic regions.^[96] Results of such ecological studies are mixed; both positive and negative associations, as well as no significant associations, have been suggested.^[97]

The most direct way to assess the risks posed by radon in homes is through case-control studies.

The studies have not produced a definitive answer, primarily because the risk is likely to be very small at the low exposure encountered from most homes and because it is difficult to estimate radon exposures that people have received over their lifetimes. In addition, it is clear that far more lung-cancers are caused by smoking than are caused by radon.^[77]

Pooled epidemiologic radon studies^[98] have found an trend to increased lung cancer risk from radon below the EPA's action level of 4 pCi/L, but the data did not depart significantly from an absence of effect (an excess odds ratio of 0.00 fell within all confidence intervals of the study).

The most elaborate case-control epidemiologic radon study performed by R. William Field and colleagues identified a 50% increased lung cancer risk with prolonged radon exposure at the EPA's action level of 4 pCi/L.^[99] Iowa has the highest average radon concentrations in the nation and a very stable population which added to the strength of the study. For that study, the odds ratio was found to be increased slightly above the confidence interval (95% CI) for cumulative radon exposures above 17 WLM (6.2 pC/l = 230 Bq/m³ and above).

The results of a methodical ten-year-long, case-controlled study of residential radon exposure in Worcester County, Massachusetts, found an apparent 60% reduction in lung cancer risk amongst people exposed to low levels (0–150 Bq/m³) of radon gas; levels typically encountered in 90% of American homes—an apparent support for the idea of radiation hormesis.^[100] In that study, a significant result (95% CI) was obtained for the 75-150 Bq/m3 category. The study paid close attention to the cohort's levels of smoking, occupational exposure to carcinogens and education attainment. However, unlike the majority of the residential radon studies, the study was not population-based. Errors in retrospective exposure assessment could not be ruled out in the finding at low levels.

However, a recent retrospective case-control study of lung cancer risk showed substantial cancer rate reduction between 50 and 123 Bq per cubic meter relative to a group at zero to 25 Bq per cubic meter.^[101] This study is evidence for hormesis, but a single study all by itself cannot be regarded as definitive. Other studies into the effects of domestic radon exposure have not reported a hormetic effect; including for example the respected "Iowa Radon Lung Cancer Study" of Field et al. (2000), which also used sophisticated radon exposure dosimetry.^[99]

All these case-control studies should be taken with caution, since it is known that just like ecological studies, they are prone to Ecological fallacy and confounding factors, and cannot cannot prove causation. Such studies can be used to point the way, but further confirmation must come from cohort studies, animal testings, and biological models of the radon effect.

Dose-effect models for domestic exposures

The only dose-effect relationship available are those of miners cohorts (for much higher expositions), exposed to radon. Studies of Hiroshima and Nagasaki survivors are less informative (the exposition to radon is chronic, localized, and the ionizing radiations are alpha rays). Although low-exposed miners experienced exposures comparable to long-term residence in high radon houses, the mean cumulative exposure among miners is approximately 30-fold higher than that associated with long-term residency in a typical home. It can be concluded from miner studies that when the radon exposure in houses compares to that in mines, radon is a health hazard ; but in the 1980's very little was known on the dose-effect relationship, both theoretically and statistically.

No conclusive statistics being available for the levels of exposure usually found in homes, the risks posed by domestic exposures is usually estimated on the basis of observed lung-cancer deaths caused by higher exposures in mines, under the assumption that the risk of developing lung-cancer increases linearly as the exposure increases.^[77]

This was the basis for the model proposed by BEIR IV in the 1980's. Research have since been made, both on epidemiological studies and in the radiobiology field.

Cohort studies are impractical for the study of domestic radon exposition : The expected effect of small exposures being very small, the direct observation of this effect would require huge cohorts - the population of whole countries.

However, from the evidence now available, a threshold exposure, that is, a level of exposure below which there is no effect of radon, cannot be excluded.^[102]

Health policy on radon public exposure

Death toll attributed to radon

Radon exposure is thought to be the second major cause of lung cancer after smoking.^[90] Iowa has the highest average radon concentration in the United States; studies performed there have demonstrated a 50% increased lung cancer risk with prolonged radon exposure above the EPA's action level of 4 pCi/L.^[99][103]

Radon is thus the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.^[104] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes ²¹⁴Po and ²¹⁸Po are thought to cause the majority^[105] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.^[106]

In the United Kingdom, residential radon is, after cigarette smoking, the second most frequent cause of lung cancer deaths: 83.9% of deaths are attributed to smoking only, 1.0% to radon only, and 5.5% to a combination of radon and smoking.^[107] The United States Environmental Protection Agency (EPA) says that radon is the number one cause of lung cancer among non-smokers.^[108] Based on studies carried out by the National Academy of Sciences in the United States, radon is the second most common cause of lung cancer after cigarette smoking, accounting for 15,000 to 22,000 cancer deaths per year in the U.S.^[109] The Surgeon General of the United States has reported that over 20,000 Americans die each year of radon-related lung cancer.^[110]

Radon concentration guidelines

The World Health Organization issued as guidelines (1988) that remedial action should be considered when the radon activity exceeded 100 kBq/m3 in a building, and remedial action should be considered without long delay if exceeding 400 kBq/m3.^[1]

The European Union recommends that action should be taken starting from concentrations of 400 Bq/m³ (11 pCi/L) for old houses and 200 Bq/m³ (5 pCi/L) for new ones. After publication of the North American and European Pooling Studies, Health Canada proposed a new guideline that lowers their action level from 800 to 200 Bq/m³ (22 to 5 pCi/L).^[111] The United States Environmental Protection Agency (EPA) strongly recommends action for any house with a concentration higher than 148 Bq/m³ (4 pCi/L),^[82] and encourages action starting at 74 Bq/m³ (2 pCi/L).

EPA recommends that all homes should be monitored for radon. If testing shows levels less than 4 picocuries radon per liter of air (160 Bq/m3), then no action is necessary. For 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.^[1] For instance, since radon has a half-life of four days, opening the windows once a day can cut the mean radon concentration to one fourth of its level.

The EPA recommends homes be fixed if an occupant's long-term exposure will average 4 picocuries per liter (pCi/L) (148 Bq m⁻³) or higher.^[112]

The United States Environmental Protection Agency (EPA) estimates that one in 15 homes in the United States has radon levels above the recommended guideline of 4 picocuries per liter (pCi/L) (148 Bq/m³).^[82] EPA radon risk level tables including comparisons to other risks encountered in life are available in their citizen's guide.^[113] The EPA estimates that nationally, 8% to 12% of all houses are above their maximum "safe levels" (four picocuries per liter—the equivalent to roughly 200 chest x-rays). The United States Surgeon General and the EPA both recommend that all homes be tested for radon.

 Radon testing

There are relatively simple tests for radon gas, but these tests are not commonly done, even in areas of known systematic hazards.

ASTM E-2121 is a standard for reducing radon in homes as far as practicable below 4 picocuries per liter (pCi/L) in indoor air.^[114][115]

Radon test kits are commercially available. The kit includes a collector that the user hangs in the lowest livable floor of the house for 2 to 7 days. The user then sends the collector to a laboratory for analysis. The National Environmental Health Association provides a list of radon measurement professionals.^[116]

Long term kits, taking collections for up to one year, are also available. An open-land test kit can test radon emissions from the land before construction begins. The EPA and the National Environmental Health Association have identified 15 types of radon testing.^[117] A Lucas cell is one type of device.

Radon levels fluctuate naturally. An initial test might not be an accurate assessment of a home's average radon level. Transient weather can affect short term measurements.^[118] Therefore, a high result (over 4 pc/l) justifies repeating the test before undertaking more expensive abatement projects. Measurements between 4 and 10 pc/l warrant a long term radon test. Measurements over 10 pc/l warrant only another short term test so that abatement measures are not unduly delayed. Purchasers of real estate are advised to delay or decline a purchase if the seller has not successfully abated radon to 4 pc/l or less.

The National Environmental Health Association administers a voluntary National Radon Proficiency Program for radon professionals consisting of individuals and companies wanting to take training courses and examinations to demonstrate their competency.^[119] A list of mitigation service providers is available.^[120] Indoor radon can be mitigated by sealing basement foundations, water drainage, or by sub-slab de-pressurization. In severe cases, mitigation can use air pipes and fans to exhaust sub-slab gases to the outside. Indoor ventilation systems are more effective, but exterior ventilation can be cost-effective in some cases.

 Mitigation

Since radon concentrations vary substantially from day to day, single grab-type measurements are generally not very useful, except as a means of identifying a potential problem area, and indicating a need for more sophisticated testing.^[1]

Transport of radon in indoor air is almost entirely controlled by the ventilation rate in the enclosure. Generally, the indoor radon concentrations increase as ventilation rates decrease.^[1] In a well ventilated place, the radon concentration tends to align with outdoor values (typically 10 Bq/m3, ranging from 1 to 100 Bq/m3).

Radon levels in indoor air can be lowered in a number of ways, from sealing cracks in floors and walls to increasing the ventilation rate of the building. The five principal ways of reducing the amount of radon accumulating in a house are:^[121]

• Improving the ventilation of the house and avoiding the transport of radon from the basement into living rooms;
• Increasing under-floor ventilation;
• Installing a radon sump system in the basement;
• Sealing floors and walls; and
• Installing a positive pressurization or positive supply ventilation system.

The half-life for radon is 3.8 days, indicating that once the source is removed, the hazard will be greatly reduced within a few weeks.

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).

 Notes and references

 Notes

 See also

• International Radon Project
• Lucas cell
• Radon mitigation
• Radiation Exposure Compensation Act
• Radiohalo

 References

• Toxicological Profile for Radon, Draft for Public Comment, Agency for Toxic Substances and Disease Registry, September 2008
• Health Effects of Exposure to Radon: BEIR VI. Committee on Health Risks of Exposure to Radon (BEIR VI), National Research Council available on-line
• UNSCEAR 2000 Report to the General Assembly,with scientific annexes : Annex B: Exposures from natural radiation sources.
• Technical support document for the 1992 Citizen's guide to radon, [EPA 400-R-92-011, May 1992].
• Should you measure the radon concentration in your home?, Phillip N. Price, Andrew Gelman, in Statistics: A Guide to the Unknown, January 2004.

 External links

Wikimedia Commons has media related to: Radon

Look up radon in Wiktionary, the free dictionary.

• Radon and radon publications at the United States Environmental Protection Agency
• Radon Information Center
• Frequently Asked Questions About Radon at National Safety Council
• Simulation of Rn220 using the Monte Carlo method
• Home Buyer's and Seller's Guide to Radon An article by the International Association of Certified Home Inspectors (InterNACHI)
• Radon and Lung Health from the American Lung Association
• Radon's impact on your health - Lung Association

[hide] v • d • e Periodic table
H																															He
Li	Be																									B	C	N	O	F	Ne
Na	Mg																									Al	Si	P	S	Cl	Ar
K	Ca															Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr															Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Uub	Uut	Uuq	Uup	Uuh	Uus	Uuo
Alkali metals Alkaline earth metals Lanthanoids Actinoids Transition metals Other metals Metalloids Other nonmetals Halogens Noble gases