Radon

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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 ^*, causing potentially 20,000 deaths in the European Union each year. Radon is a significant contaminant that impacts indoor air quality worldwide.

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 radon-rich water 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.

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.

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 135 packs of cigarettes every day.^*

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.

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.

Compounds

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

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).

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 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

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.^* 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.

References

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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