Science Fair Project Encyclopedia
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 houses and cause lung cancer , causing potentially 20,000 deaths in the European Union each year.
Essentially chemically inert, but radioactive, radon is the heaviest noble gas and one of the heaviest gases at room temperature. (The heaviest is tungsten hexafluoride, WF6.) 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.
Radon is sometimes produced by a few hospitals for therapeutic use by pumping its gas from a radium source and storing it in very small tubes which are called seeds or needles. This practice is being phased-out as hospitals get seeds from suppliers who make them with the desired activity levels. Such seeds—often using radioactive forms of cobalt and caesium—also last for several years, which is a logistical advantage.
Because of its 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 (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.
On average, there is one molecule of radon in 1 x 1021 molecules of air. Radon can be found in some spring waters and hot springs. The town of Misasa, Japan, boasts its radium-rich springs exhausting 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 exhausts 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 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.
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 radioactive 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).
Also, its solid decay products, and their respective products, tend to form a fine dust which can easily enter the airways and become permanently stuck in lung tissue, producing heavy localized exposure. Rooms where radium, actinium, or thorium are stored should be well-ventilated in order to prevent build-up in the air. The build-up of radon is a potential health hazard in uranium and some lead mines. Build-up of radon in homes has also been a more recent health concern and many lung cancer cases are attributed to radon exposure each year. Indoors radon is estimated to cause about 21,000 lung cancer deaths per year in the U.S. Radon is the second leading cause of lung cancer in the U.S. today. About 12% of lung cancers in the U.S. are potentially attributable to radon, although these statistics are, when examined closely, somewhat dubious. Because of the long developmental time for lung cancers in general, and the multiple sources for lung cancer, it is difficult (if not impossible) to full discern what impact domestic radon has on the overall number of lung cancers.
Radon is of greatly increased danger to smokers. The solid decay products of radon (polonium-218, polonium-214, and lead-210 are the most damaging) staying in the atmosphere in dust form can fix themselves on the microparticles in tobacco smoke, which then enter the lungs. Thus the best mitigation for the risks of radon is simply to give up smoking, as the risk of non-smokers developing radon-related lung cancers is considerably lower.
To reduce the danger in homes one can either i) increase ventilation or ii) improve damp-proofing. Ventilation requires air bricks and fans to remove underfloor air. Such ventilation can cost £1,000 to install and £50 a year to run. Alternatively installing an effective damp-proof membrane across the whole footprint of the ground floor can prevent radon rising into the dwelling space. Thick PVC plastic sheeting can do the job well and only cost £100 in a new build. In lieu of building work, a layer of foil or plastic sheeting may be installed underneath carpeting or linoleum. In areas of high concentration, if it is possible to identify fissures or vents then blocking these can be advantageous. About 1.2 million new homes have been built with radon-resistant features since 1990 in the U.S. To date, EPA estimates that as many as 650 future lung cancer deaths are prevented (lives saved) each year as a result of houses altered and new houses built with preventative features installed.
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. However, there are still radon baths and hot springs in 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 by most radiologists.
- Cancer Research UK study and building procedures to mitigate risks.
- 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)
- WebElements.com - Radon
- EnvironmentalChemistry.com - Radon
- Indoor Air - Radon (Rn)
- Radon Brochure SCK.CEN Belgian Nuclear Research Centre, Mol, Belgium
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