Radon

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For the town in France, see Radon, Orne. For the mathematician, see Johann Radon.

Radon 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 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], causing potentially 20,000 deaths in the European Union each year. Radon is probably the most significant contaminant in indoor air quality worldwide.

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Compounds
6 Isotopes
7 Toxicity and Precautions
8 Radon therapy
9 References
10 External links

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Notable characteristics

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 it has a brilliant phosphorescence which turns yellow as the temperature is lowered and orange-red at the temperature air liquefies.

Natural radon concentrations in Earth's atmosphere are so low that natural waters in contact with the atmosphere will continually lose radon by volatilization. Hence, ground water 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 due to diffusional losses to the atmosphere.

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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. There is no scientific evidence for this belief, 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.

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 unconvincing.

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.

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History

Radon (named for 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 danger of radon exposure in dwellings was discovered in 1984 by Stanley Watras, an employee at the Limerick nuclear power plant in Pennsylvania. Mr. 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 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 35 packs of cigarettes every day.Template:Citeneeded

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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. 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.

The National Council on Radiation Protection and Measurement (NCRP) recommends action for any house with a concentration higher than 8 pCi/L.

The United States Environmental Protection Agency recommends action for any house with a concentration higher than 148 Bq/m³ (given as 4 pCi/L). 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.

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Compounds

Some experiments indicate that fluorine can react with radon and form radon fluoride. Radon clathrates have also been reported.

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Isotopes

There are twenty known isotopes of radon. The most stable isotope is radon-222, which is a decay product (daughter product) of radium-226, has a half-life of 3.823 days and 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, is called actinon, is an alpha emitter and has a half-life of 3.96 seconds.

The full decay series of uranium-238 which produces natural radon is as follows (with half-lives): uranium-238 (4.5 x 10⁹ y), thorium-234 (24.1 d), protactinium-234 (1.18 m), uranium-234 (250,000 y), thorium-230 (75,000 y), radium-226 (1,600 y), radon-222 (3.82 d), polonium-218 (3.1 m), lead-214 (26.8 m), bismuth-214 (19.7 m), polonium-214 (164 micro-s), lead-210 (22.3 y), bismuth-210 (5.01 d), polonium-210 (138 d), lead-206 (stable).

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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 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 appears to be the second most common cause of lung cancer after cigarette smoking. The exposure to radioactivity from inhaled radon and its daughter products is thought to be the source of malignant changes.

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Radon therapy

Radon was a popular additive in products like toothpaste, hair creams and even food items in the early 20th century, due to its supposed curative powers. Radon was subsequently removed when its carcinogenic properties were discovered. (disputed — see [[: talk:Radon#{{{1|Disputed}}}|talk page]]) However, there are still radon baths and hot springs in Austria, Germany and Japan. These baths promote radon by saying that it cures illnesses like hypertension and hemorrhoids. There are similar radon treatment centers in the U.S.[2] and elsewhere which are usually billed as 'health centers', though the health benefits of low-levels of radiation are seen as dubious, at best, by most radiologists.

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References

Los Alamos National Laboratory - 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)