Radon

For other uses, see Radon (disambiguation).

86 astatine ← radon → francium Xe ↑ Rn ↓ Uuo periodic table
General
Name, Symbol, Number	radon, Rn, 86
Chemical series	noble gases
Group, Period, Block	18, 6, p
Appearance	colorless
Atomic mass	(222) g/mol
Electron configuration	[Xe] 4f14 5d10 6s2 6p6
Electrons per shell	2, 8, 18, 32, 18, 8
Physical properties
Phase	gas
Melting point	202 K (-71 °C, -96 °F)
Boiling point	211.3 K (-61.7 °C, -79.1 °F)
Critical point	377 K, 6.28 MPa
Heat of fusion	3.247 kJ/mol
Heat of vaporization	18.10 kJ/mol
Heat capacity	(25 °C) 20.786 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 110 121 134 152 176 211
Atomic properties
Crystal structure	cubic face centered
Oxidation states	0
Electronegativity	no data (Pauling scale)
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 mW/(m·K)
CAS registry number	10043-92-2
Notable isotopes
Main article: Isotopes of radon iso NA half-life DM DE (MeV) DP 211Rn syn 14.6 h Epsilon 2.892 211At Alpha 5.965 207Po 222Rn 100% 3.824 d Alpha 5.590 218Po

Radon (chemical symbol Rn, atomic number 86) is a radioactive noble gas that is formed by the disintegration of radium. It is one of the heaviest gases and is considered a health hazard. The most stable isotope is Rn-222 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 [1]. Radon is a significant contaminant that impacts indoor air quality worldwide.

• It has been estimated that radon causes potentially 20,000 deaths in the European Union each year.

Discovery

Radon (named after radium) was discovered in 1900 by Friedrich Ernst Dorn, who called it radium emanation. William Ramsay and Robert Whytlaw-Gray isolated it in 1908 and named it niton (from the Latin word nitens, meaning "shining"). When they determined its density, they found that it was the heaviest gas known as of then. It has been called radon since 1923.

Occurrence

On average, there is one atom of radon in 1 x 10²¹ molecules of air. In addition, radon can be found in some spring waters and hot springs. The towns of Misasa, Japan, and Bad Kreuznach, Germany, boast radium-rich springs that emit radon.

In certain regions, radon exhausts naturally from the ground. Many of these regions have granitic soils, but 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.

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.

In the United States, the National Council on Radiation Protection and Measurement (NCRP) recommends action for any house with a radon concentration higher than 8 pCi/L. The U.S. Environmental Protection Agency (EPA) recommends action for any house with a radon concentration higher than 148 becquerels per cubic meter (Bq/m³) (given as 4 pico-curies per liter (pCi/L)), and encourages action starting at 74 Bq/m³. (EPA radon risk level tables, including comparisons to other risks encountered in life, are available in their citizen's guide [2].) According to their statistics, nearly one in 15 homes in the United States has a high level of indoor radon. The U.S. Surgeon General and EPA recommend that all homes be tested for radon. Since 1985, millions of homes have been tested for radon in the United States.

Health Canada has a 1988 guideline from 1988 that recommends action when the annual average concentration in a normal living area exceeds 800 Bq/m³, but they are proposing a new guideline that lowers the action level to 200 Bq/m³ [3]. 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.

Notable characteristics

In the periodic table, radon is at the bottom of the noble gas group, that is, group 18 (former group 8A or group 0). It lies in period 6, after astatine (At). Thus it is the heaviest noble gas (has the highest density among noble gases) and one of the heaviest gases at room temperature. (Currently, the densest known gas is uranium hexafluoride, UF₆.)

At ordinary temperatures and pressures, radon is colorless. It is essentially chemically inert, but radioactive. When cooled below its freezing point, it produces a bright phosphorescence that turns yellow as the temperature is reduced, and it then becomes orange-red at the temperature when air liquefies.

Natural radon concentrations in the Earth's atmosphere are so low that radon-rich water in contact with the atmosphere will continually lose the gas by volatilization. Consequently, groundwater has a higher concentration of Rn-222 than surface water. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone, as a result of diffusional losses to the atmosphere.

Isotopes

There are 20 known isotopes of radon, of which the most stable one is radon-222. It is a decay product (daughter product) of radium-226 and has a half-life of 3.823 days. As it decays, it emits alpha particles. Radon-220 is a natural decay product of thorium and is called thoron. It has a half-life of 55.6 seconds and also emits alpha rays. Radon-219 is derived from actinium and is called actinon. It emits alpha particles and has a half-life of 3.96 seconds.

Compounds

As an inert gas, radon forms few compounds. Some researchers have found that radon reacts with fluorine to form radon fluoride. Others have reported the production of radon clathrates, which are composed of cage-like molecules containing radon atoms. Nonetheless, the rapid radioactive decay of radon limits the usefulness of its compounds.

Applications

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, 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, and their 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.

Toxicity and Precautions

The first major studies of the health effects of radon were carried out 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.[4] Following this event, which was highly publicized, national radon safety standards were set and radon detection and ventilation became a standard homeowner concern.

The exact danger posed by indoor radon is debated among experts. Though radon is cited as the number one cause of lung cancer after cigarette smoking, the numbers are still relatively low and are often entangled with those of smoking (the combination of radon gas exposure and cigarette smoking drastically increases lung cancer rates). It is unknown why indoor radon seems to have only been a problem since the 1980s; some experts have hypothesized that it is the result of better housing construction which allows radon to accumulate rather than seep back into the natural environment.

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 dust 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). Moreover, radon can also be present in tobacco smoke, in the cases that phosphate matrix containing considerable concentrations of uranium is used for fertilizing the source tobacco. Radon is a daughter product of the decay of uranium; Many phosphate deposits have 30 to 100 times the concentrations of uranium as typical soils. The exposure to radioactivity from inhaled radon and its daughter products is thought to be the source of malignant changes.

Radon therapy

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 are 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.

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory - Radon

• USGS Periodic Table - Radon

• Leonard A. Cole, Element of Risk: The Politics of Radon (American Association for the Advancement of Science Press, 1993). (a scholarly source critical of U.S. and EPA domestic radon policy)

• Decay chains of some elements including Radon

External links

• WebElements.com - Radon
• U.S. Environmental Protection Agency — Indoor Air Radon
• Agency for Toxic Substances and Disease Registry — Radon Toxicity Case Study

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