AskDefine | Define radon

Dictionary Definition

radon n : a radioactive gaseous element formed by the disintegration of radium; the heaviest of the inert gasses; occurs naturally (especially in areas over granite) and is considered a hazard to health [syn: Rn, atomic number 86]

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see Radon



  1. A radioactive chemical element (symbol Rn, formerly Ro) with atomic number 86, one of the noble gases.


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Extensive Definition

Radon () is the chemical element that has the symbol Rn and atomic number 86. Radon is a colorless, naturally occurring, radioactive noble gas that is formed from the decay of radium. It is one of the heaviest substances that are gases under normal conditions and is considered to be a health hazard. The most stable isotope, 222Rn, has a half-life of 3.8 days and is used in radiotherapy. While having been less studied by chemists due to its radioactivity, there are a few known compounds of this generally unreactive element.
Radon is a significant contaminant that affects indoor air quality worldwide. Radon gas from natural sources can accumulate in buildings and reportedly causes 21,000 lung cancer deaths per year in the United States alone. Radon is the second most frequent cause of lung cancer, after cigarette smoking, and radon-induced lung cancer is thought to be the 6th leading cause of cancer death overall.

History and etymology

Radon is the third discovered radioactive element (after radium and polonium). It was discovered in 1898 by Friedrich Ernst Dorn. In 1900 he reported some experiments in which he noticed that radium compounds emanate a radioactive gas which he named as Radium Emanation (Ra Em). But before that, in 1899, Pierre and Marie Curie observed that the "gas" emitted by radium remained radioactive for a month. That year, Robert B. Owens with Ernest Rutherford noticed variations when trying to measure radiation from thorium oxide. 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), and later Thorium Emanation (Th Em). In 1901 he demonstrated clearly that the emanations are radioactive, but credited the Curies for the discovery of the element. In 1903, similar emanations were observed from actinium by André-Louis Debierne and were called Actinium Emanation (Ac Em).
Several names were suggested for these three gases: exradio, exthorio and exactinio in 1904; radon, thoron and akton in 1918; radeon, thoreon and actineon in 1919, and eventually radon, thoron and actinon in 1920. The likeness of the spectra of these three gases with those of argon, krypton and xenon, and their observed chemical inertia lead Sir William Ramsay to suggest in 1904 that the "emanations" might contain a new element of the noble gas family. and in 1912 it was accepted by the International Commission for Atomic Weights. In 1923, the International Committee for Chemical Elements and IUPAC chose for the names: radon (Rn), thoron (Tn), and actinon (An). Later, when isotopes were numbered instead of named, the name of 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 simply as emanation.
The first synthesized compound of radon was obtained in 1962 and is radon fluoride.
The first major studies of the health concern 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 uranium, uranium mines may have high concentrations of radon and its highly radioactive daughter products. 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 incidence of lung cancer was particularly pronounced among Native American and Mormon miners, because those groups normally have low rates of lung cancer. Safety standards requiring expensive ventilation were not widely implemented or policed during that period.
The danger of radon exposure in dwellings was discovered in 1984 with the case of Stanley Watras, an employee at the Limerick nuclear power plant in Pennsylvania. Watras set off the radiation alarms on his way into work for two weeks straight while authorities searched for the source of the contamination. They were shocked to find that the source was astonishingly high levels of radon, around 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 event, which was highly publicized, national radon safety standards were set, and radon detection and ventilation became a standard homeowner concern.


Radon has no stable isotopes. There are 34 radioactive isotope that have been studied. These range from an atomic mass of 195 to 228. 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. 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. Hereby is this decay series (with half-lives):
238U (4.5 x 109 yr) → 234Th (24.1 days) → 234Pa (1.18 min) → 234U (250,000 yr) → 230Th (75,000 yr) → 226Ra (1,600 yr) → 222Rn (3.82 days) → 218Po (3.1 min) → 218At (1.5 s) → 218Rn (35 ms) → 214Pb (26.8 min) → 214Bi (19.7 min) → 214Po (164 µs) → 210Pb (22.3 yr) → 210Bi (5.01 days) → 210Po (138 days) → 206Pb (stable).
There are three other isotopes that have a half life of above 1 hour: 211Rn, 210Rn and 224Rn. The 220Rn isotope is a natural decay product of the most stable thorium isotope (232Th) for which was 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) — for which it was named “actinon” — and is an alpha emitter with half-life of 3.96 seconds.


At standard temperature and pressure, radon forms a monoatomic gas with a density of 9.73 kg/m3, about 8 times the surface density of the Earth's atmosphere, 1.217 kg/m3, 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 (202 K ; −71 °C ; −96 °F) it has a brilliant phosphorescence which turns yellow as the temperature is lowered, and becomes orange-red at the temperatures air liquefies (below 93 K ; −180 °C). Upon condensation, radon also glows because of the intense radiation it produces.
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.
Radon is a health hazard as exposure can cause lung cancer - in fact it is the second major cause of lung cancer after smoking. Because it is also radioactive and is a relatively unreactive chemical element, radon has few uses and is seldom used in academic research.


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, for example) because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. Nevertheless, due to periodic trends, radon has a lower electronegativity than the element above it, xenon, and thus is relatively more reactive.
Because of this price and its radioactivity, experimental chemical research is seldomly done on this element 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 thus forming radon fluoride. Among the few other reported compounds of radon are radon oxides.


The average concentration of radon in the atmosphere is about 6 atoms Rn for each molecule in the air (or about 150 atoms in each mL of air). It can be found in some spring waters and hot springs. The towns of Boulder, Montana, and Misasa; Bad Kreuznach, Germany, as well as the country of Japan boast radium-rich springs which emit radon. Radon emanates naturally from the ground all over the world, particularly in regions with soils containing granite or shale. However, not all granitic regions are prone to high emissions of radon. Radon emitted from the ground has been shown to accumulate in the air if there is a meteorological inversion and little wind. In some caves, increased radon concentration was observed.
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 somewhat 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 very 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. In 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.
Radon, along with other noble gases krypton and xenon, is also produced during the operation of nuclear power plants. A small fraction of it leaks out of the fuel, through the cladding and into the cooling water, from which it is scavenged. It is then routed to a holding tank where it remains for a large number of half-lives. It is finally purged to the open air through a tall stack which is carefully monitored for radiation level.
Radon collects over samples of radium 226 at the rate of around 0.001 cm3/day per g of radium. 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. Here is a graph of the deposition rate observed in Japan. 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 which had been melted down (after the radon had decayed). The daughters of the radon are still radioactive today.
In 1971, Apollo 15 passed 110 kilometers 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.
Depending on how houses are built and ventilated, radon may accumulate in basements and dwellings. The highest average radon concentrations in the U.S. are found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania. 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 nation due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland. Many cities within the state, such as Iowa City have passed requirements for radon resistant construction in all new homes. A recent study has noted that the counties surrounding Three Mile Island have the highest radon concentrations in the United States and that this may be the cause of the increased lung cancer noted in the region.
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 has proposed a new guideline that lowers their action level from 800 to 200 Bq/m³ (22 to 5 pCi/L). The United States Environmental Protection Agency (EPA) strongly recommends action for any house with a concentration higher than 148 Bq/m³ (4 pCi/L), and encourages action starting at 74 Bq/m³ (2 pCi/L). EPA radon risk level tables including comparisons to other risks encountered in life are available in their citizen's guide. 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 U.S. Surgeon General and EPA recommend all homes be tested for radon.



It has been claimed that exposure to radon gas mitigates auto-immune diseases such as arthritis. 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 was controversial because of the "well-documented ill effects of high-dose radiation on the body."
Radioactive water baths have been applied since 1906 in Jáchymov, Czech Republic, but even before radon discovery they were used in Bad Gastein, Austria. Hot radium-rich spring releasing radon is 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 a few "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.
In addition personal testimonies of arthritis relief and other benefits, there is some (very limited) scientific evidence for this belief, known as hormesis. However, the general scientific community finds it unsubstantiated. There is no known biological mechanism by which such an effect could occur. In addition, it conflicts with the internationally recognized standard that there is no safe threshold for radiation exposure and that exposure should be limited to that "as low as reasonably achievable" (ALARA).
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.


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.
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 even looked at elevated soil-gas radon concentrations, or rapid changes in soil or groundwater radon concentrations, as a predictor for earthquakes. Results have been generally unconvincing but may ultimately prove to have some limited use in specific locations.
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.

Testing and mitigation

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. 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. 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. A Lucas cell is one type of device.
Radon levels fluctuate naturally. An initial test might not be an accurate assessment of your home's average radon level. Transient weather can affect short term measurements. 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. A list of mitigation service providers is available. 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 air to the outside. Indoor ventilation systems are more effective, but exterior ventilation can be cost-effective in some cases. Modern construction that conserves energy by making homes air tight exacerbates the risks of radon exposure if radon is present in the home. Older homes with more porous construction are more likely to vent radon naturally. Ventilation systems can be combined with a heat exchanger to recover energy in the process of exchanging air with the outside. Homes built on a crawl space can benefit from a radon collector installed under a radon barrier (a sheet of plastic that covers the crawl space).


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