Radon
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| General | ||||||||||||||||||||||
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| Name, Symbol, Number | radon, Rn, 86 | |||||||||||||||||||||
| Chemical series | 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 | |||||||||||||||||||||
| Physical properties | ||||||||||||||||||||||
| Phase | gas | |||||||||||||||||||||
| Density | (0 °C, 101.325 kPa) 9.73 g/L | |||||||||||||||||||||
| 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 | |||||||||||||||||||||
| Heat capacity | (25 °C) 20.786 J·mol−1·K−1 | |||||||||||||||||||||
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| Atomic properties | ||||||||||||||||||||||
| Crystal structure | cubic face centered | |||||||||||||||||||||
| Oxidation states | 0 | |||||||||||||||||||||
| Electronegativity | no data (scale Pauling) | |||||||||||||||||||||
| Ionization energies | 1st: 1037 kJ/mol | |||||||||||||||||||||
| 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 | |||||||||||||||||||||
| Selected isotopes | ||||||||||||||||||||||
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| References | ||||||||||||||||||||||
Radon (IPA: /ˈreɪdɒn/) is a chemical element that has the symbol Rn and atomic number 86. Radon is a radioactive noble gas that is formed by the decay of radium. It is one of the heaviest gases and is considered to be a health hazard. The most stable isotope is 222Rn which has a half-life of 3.8 days and is used in radiotherapy. 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.[1]
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[edit] Notable characteristics
Essentially chemically inert but radioactive, radon is the heaviest noble gas and one of the heaviest gases at room temperature. 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).
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 it is continuously produced by radiocative 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.
[edit] History
Radon (named after radium) was discovered in 1900 by Friedrich Ernst Dorn, who called it radium emanation. In 1908 William Ramsay and Robert Whytlaw-Gray, named it niton (Latin nitens meaning "shining"; symbol Nt) and isolated it, determined its density, and determined that it was the heaviest known gas. It has been called "radon" since 1923.
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 have high concentrations of radon and its highly radioactive daughter products. Many Native Americans, Mormons, and other miners in the Four Corners region contracted lung cancer and other pathologies as a result of high levels of exposure to radon gas while mining uranium for the Atomic Energy Commission in the mid-1950s. Safety standards instituted required expensive ventilation and as such were not widely implemented or policed.
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 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.[2][3]
[edit] Occurrence
On average, there is one atom of radon in 1 x 1021 molecules of air. Radon can be found in some spring waters and hot springs.[4] The towns of Boulder, Montana, Misasa, Japan, and Bad Kreuznach, Germany 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. Depending on how houses are built and ventilated, radon may accumulate in basements and dwellings. The highest average radon concentrations in counties in the U.S. are found in Iowa and in the Appalachian Mountains areas in southeastern Pennsylvania. [5]
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.
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).[6] The United States Environmental Protection Agency (EPA) strongly recommends action for any house with a concentration higher than 148 Bq/m3 (4 pCi/L)[7], 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.[8] Nearly one in 15 homes in the U.S. has a high level of indoor radon according to their statistics. The U.S. Surgeon General and EPA recommend all homes be tested for radon.
Radon emitted from the ground has been shown to accumulate in the air if there is a meteorological inversion and little wind.[9]
[edit] Compounds
Some experiments indicate that fluorine can react with radon and form radon fluoride. Radon clathrates have also been reported.
[edit] Isotopes
There are twenty known isotopes of radon. The most stable isotope is 222Rn, which is a decay product (daughter product) of 226Ra, has a half-life of 3.823 days and emits alpha particles. 220Rn is a natural decay product of thorium and is called “thoron.” It has a half-life of 55.6 seconds and also emits alpha radiation. 219Rn is derived from actinium, is called “actinon,” is an alpha emitter and has a half-life of 3.96 seconds.
The full decay series of 238U which produces natural radon is as follows (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), 214Pb (26.8 min), 214Bi (19.7 min), 214Po (164 µs), 210Pb (22.3 yr), 210Bi (5.01 days), 210Po (138 days), 206Pb (stable).
[edit] Applications
[edit] Radon therapy
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 airborne radiation will invigorate or energize them. However, there is no scientific evidence for this belief, nor any 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).
Radioactive water baths have been applied since 1906 in Joachimsthal, 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.
[edit] Hydrologic studies
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.
[edit] Geologic studies
Some researchers have looked at elevated soil-gas radon concentrations, or rapid changes in soil 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 soil-concentration has been used in an experimental way to map close-subsurface geological faults, because concentrations are generally higher over the faults. Similarly it has found some limited use in geothermal prospecting.
[edit] Atmospheric studies
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.
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.
[edit] Health effects and epidemiology
The general effects of radon to the human body are caused by its radioactivity and consequent risk of radiation-induced cancer. As an inert gas, radon has a low solubility in body fluids which lead to a uniform distribution of the gas throughout the body.[10] Radon gas and its solid decay products are carcinogens. The greatest health risks come from exposure to the inhaled solid radon gas decay products that are produced during the radioactive decay of radon gas. Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.[11] Once the radioactive decay products are inhaled into the lung, they undergo further radioactive decay, releasing small bursts of energy in the form of alpha particles that can either cause DNA breaks or create free radicals.[11]
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.[12] The Surgeon General of the United States has reported that over 20,000 Americans die each year of radon-related lung cancer.[13] The United States Environmental Protection Agency (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.[14]
A case-control epidemiologic radon study demonstrated a 50% increased lung cancer risk with prolonged radon exposure at the EPA's action level of 4 pCi/L.[15] Iowa has the highest average radon concentrations in the nation and a very stable population which added to the strength of the study. Pooled epidemiologic radon studies[16][17] have also shown an increased lung cancer risk from radon below the EPA's action level of 4 pCi/L.
It is unknown whether radon causes other types of cancer, but recent studies suggest a need for further studies on the relationship between cancer not in the lungs as well as other non-cancer related illnesses such as multiple sclerosis.[18][19]
Radon accumulates in underground mines and caves. Good ventilation should therefore be maintained in mines, and in some countries, guides in tourist caves are classified as "radiation workers", whose time of exposure is monitored. Tourism of caves is not generally considered a significant hazard for the relatively brief visits by members of the general public.
[edit] 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.[20][21]
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.[22] 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.[23] 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.[24] 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.[25] A list of mitigation service providers is available.[26] 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).
[edit] Notable radon researchers and scientists
- Sarah Darby
- R. William Field
- Dan Krewski
- Daniel J. Steck