2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||radon, Rn, 86|
|Chemical series||noble gases|
|Group, Period, Block||18, 6, p|
|Atomic mass||(222) g/mol|
|Electron configuration||[Xe] 4f14 5d10 6s2 6p6|
|Electrons per shell||2, 8, 18, 32, 18, 8|
|Density||(0 °C, 101.325 kPa)
|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|
|Crystal structure||cubic face centered|
|Electronegativity||no data (Pauling scale)|
|Ionization energies||1st: 1037 kJ/mol|
|Atomic radius (calc.)||120 pm|
|Covalent radius||145 pm|
|Thermal conductivity||(300 K) 3.61 mW·m−1·K−1|
|CAS registry number||10043-92-2|
Radon ( IPA: /ˈreɪdɒn/) is a chemical element in the periodic table that has the symbol Rn and atomic number 86. A radioactive noble gas that is formed by the disintegration of radium, radon 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 gas can accumulate in buildings, and drinking water, and cause lung cancer , causing potentially 20,000 deaths in the European Union each year, with an estimated 20,000 additional deaths per year in the US. Radon is a significant contaminant that impacts indoor air quality worldwide.
Essentially chemically inert, but radioactive, radon is the heaviest noble gas and one of the heaviest gases at room temperature. (The heaviest known gas is Uranium hexafluoride, UF6.) At standard temperature and pressure radon is a colorless gas, but when it is cooled below its freezing point (202K ; -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 93K ; -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. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone due to diffusional losses to the atmosphere.
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. The same applies to the hot water spas of Misasa, Tottori, Japan, where water is naturally rich in radium and exhales radon. There is no scientific evidence for this belief, except possibly radiation hormesis, nor any known biological mechanism by which such an effect could occur.
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, streams and rivers. Any significant concentration of radon in a stream or river is a good indicator that there are local inputs of ground water.
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.
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.
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.
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.
Although some physicians once believed that radon can be used therapeutically, there is no evidence for this belief and radon is not currently in medical use, at least in the developed world.
Radon has also been used to remove tumors. A capsule is placed in the patient near the tumor where the radiation will kill the cancerous cells. The surrounding cells are safe because of the short half-life of radon.
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, who named it niton (Latin nitens meaning "shining"; symbol Nt), isolated it, determined its density and 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 American Southwest during the early Cold War. Because radon is a daughter-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 would later contract 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 (see Geiger counter) 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 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 cited as the number one cause of lung cancer after cigarette smoking and radon induced lung cancer is thought to be the 6th leading cause of cancer death overall.
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. The towns of Misasa, Japan, and Bad Kreuznach, Germany boast radium-rich springs which emit radon.
Radon exhausts naturally from the ground, particularly in certain regions, especially (but not only) regions with granitic soils. 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 European Union recommends that action should be taken starting from concentrations of 400 Bq/m3 for old houses and 200 Bq/m3 for new ones. Health Canada has a guideline from 1988 that recommends action when the annual average concentration in a normal living area exceeds 800 Bq/m3, although they are proposing a new guideline that lowers the action level to 200 Bq/m3. Source . The National Council on Radiation Protection and Measurement ( NCRP) in the US recommends action for any house with a concentration higher than 8 pCi/L. The United States Environmental Protection Agency ( EPA) strongly recommends action for any house with a concentration higher than 148 Bq/m3 (given as 4 pCi/L), and encourages action starting at 74 Bq/m3 (given as 2 pCi/L). EPA radon risk level tables including comparisons to other risks encountered in life are available in their citizen's guide. 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. Since 1985, millions of homes have been tested for radon in the U.S.
Radon emitted from the ground has been shown to accumulate in the air if there is a meteorological inversion and little wind. Concentrations may exceed legal guidelines for short periods. It is not clear that any health effects would be epidemiologically detectable.
Some experiments indicate that fluorine can react with radon and form radon fluoride. Radon clathrates have also been reported.
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).
Toxicity and precautions
Radon is a radiological poison and a carcinogen. Some of the daughter products from radioactive decay of radon (such as polonium) are also toxic. Since radon is a gas, its decay products form a very fine particle that is both toxic and radioactive. This can potentially stick in the lungs and do far more damage than the radon itself.
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 US alone according to the National Cancer Institute (USA). On January 13, 2005, the Surgeon General of the United States reported that over 20,000 Americans die each year of radon-related lung cancer. Moreover, radon decay products (e.g. polonium-210) is also be present in tobacco smoke. Radon is a daughter product of the decay of uranium - 238. The USEPA recommends homes be fixed if an occupant's long-term exposure will average 4 picocuries per liter (pCi/L) or higher.
No level of radon exposure is safe. Studies performed by R. William Field et al. at the University of Iowa have demonstrated a 50% increased lung cancer risk with prolonged radon exposure at the EPA's action level of 4 pCi/L. Recent pooled epidemiologic radon studies by Dan Krewski et al. (2005; 2006) and Sarah Darby et al. (2005) have also shown an increased lung cancer risk from radon below the U.S. EPA's action level of 4 pCi/L.
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. In the U.S., about one in every 15 homes has a radon level above this standard.
Radon test kits are commercially available. In the U.S., single test kits can cost about $10. The kit includes a collector that the user hangs in the basement for a few days (2 to 7). 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 USEPA and the National Environmental Health Association, have identified 15 types of radon testing.
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 (NEHA) administers a voluntary National Radon Proficiency Program (NRPP) 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).
Radon therapy is an unscientific disease treatment that has been historically used in some spa resorts around the world. Beneficial health effects of radon have never been clinically proved, and considering radon's toxicity and the associated risks for health (radon causes lung cancer) it is not advised to undertake radon therapy.
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.