Name: Radon
Symbol: Rn
Atomic Number: 86
Atomic Weight: 222.000000
Family: Noble Gases
CAS RN: 10043-92-2
Description: Colorless, odorless, tasteless radioactive noble gas. When cooled below its freezing point it brightly phosphoresces
State (25C): Gas
Oxidation states: 0

Molar Volume: 50.5 cm3/mole
Valence Electrons: 6p6

Boiling Point:  211K,  -62C, -80F
Melting Point:
202K, -71C, -96F
Electrons Energy Level: 2, 8, 18, 32, 18, 8
Isotopes: 33
Heat of Vaporization: 16.4 kJ/mol
Heat of Fusion: 2.89 kJ/mol
Density: 9.73 g/L @ 273K & 1atm
Specific Heat: 0.09 J/gK
Atomic Radius: 1.34
Ionic Radius: unknown
Electronegativity: N/A (Pauling); 2.06 (Allrod Rochow)
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 Bohema 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 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, around 100,000 Bq.m-3, in the basement of his house 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.


1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10 6s2p6


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 ; -71C ; -96F) it has a brilliant phosphorescence which turns yellow as the temperature is lowered, and becomes orange-red at the temperatures air liquefies (below 93K ; -180C).

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10
6s2 6p6

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, because it is continuously produced by radiocative decay of 226Ra present in the rocks.   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.


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 emanates 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 highest statewide average radon concentrations are found in Iowa where over 70% of short-term screening measurements are over the EPA's action level of 4 pCi/L.  The highest regional radon concentrations occur in the counties surrounding Three Mile Island in PA.

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. After publication of the North American and European Pooling Studies Health Canada has proposed a new guideline that lowers their action level from 800 to 200 Bq/m3.  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 by Daniel J. Steck et al. to accumulate in the air if there is a meteorological inversion and little wind.


Some experiments indicate that fluorine can react with radon and form radon fluoride (RnF) and the element glows with a yellow light in the solid state..   Radon clathrates have also been reported.


There are thirty three 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) rarrow.gif (63 bytes) 234Th (24.1 days) rarrow.gif (63 bytes) 234Pa (1.18 min) rarrow.gif (63 bytes) 234U (250,000 yr) rarrow.gif (63 bytes) 230Th (75,000 yr) rarrow.gif (63 bytes) 226Ra (1,600 yr) rarrow.gif (63 bytes) 222Rn (3.82 days) rarrow.gif (63 bytes) 218Po (3.1 min) rarrow.gif (63 bytes) 214Pb (26.8 min) rarrow.gif (63 bytes) 214Bi (19.7 min) rarrow.gif (63 bytes) 214Po (164 s) rarrow.gif (63 bytes) 210Pb (22.3 yr) rarrow.gif (63 bytes) 210Bi (5.01 days) rarrow.gif (63 bytes) 210Po (138 days) rarrow.gif (63 bytes) 206Pb (stable).

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Rn196 196.002 3 ms
Rn197 197.002 65 ms
Rn198 197.999 64 ms
Rn199 198.998 0.62 seconds
Rn200 199.996 0.96 seconds
Rn201 200.996 7 seconds
Rn202 201.993 10 seconds
Rn203 202.993 45 seconds
Rn204 203.991 1.24 minutes
Rn205 204.992 2.8 minutes
Rn206 205.99 5.67 minutes
Rn207 206.9907 9.25 minutes
Rn208 207.9896 24.35 minutes
Rn209 208.9904 28.5 minutes
Rn210 209.9897 2.4 hours
Rn211 210.9906 14.6 hours
Rn212 211.9907 23.9 minutes
Rn213 212.9939 25 ms
Rn214 213.9953 0.27 us
Rn215 214.9987 2.3 us
Rn216 216.0003 45 us
Rn217 217.0039 0.54 ms
Rn218 218.0056 35 ms
Rn219 219.0095 3.96 seconds
Rn220 220.0114 55.6 seconds
Rn221 221.015 25 minutes
Rn222 222.0176 3.8235 days
Rn223 223.0218 23.2 minutes
Rn224 224.024 107 minutes
Rn225 225.0284 4.5 minutes
Rn226 226.031 7.4 minutes
Rn227 227.035 22.5 seconds
Rn228 228.038 65 seconds

Toxicity and Epidemiology

The general effects of radon to the human body are due to its radioactivity and consequent risk of radiation-induced cancer. As an inert gas, "radon has a low solubility in body fluids which lead to a uniform distribution of the gas throughout the body" (Lindgren, 1989). Radon gas and its solid decay products are carcinogens.   Some of the daughter products, especially 218Po and 214Po, from radioactive decay of radon present a radiologic hazard.  Depending on the size of the particles radon decay products can be inhaled into the lung where they undergo further radioactive decay releasing small bursts of energy in the form of alpha particles that can either cause double strand DNA breaks or create free radicals that can also damage the DNA.

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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 Natonal 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) are also present in tobacco smoke. Radon is a daughter product of the decay of  238U.  The USEPA recommends homes be fixed if an occupant's long-term exposure will average 4 picocuries per liter (pCi/L) (148 Bq m-3) or higher

Studies performed by R. William Field, Daniel J. Steck, Charles F. Lynch, Brian J. Smith and colleagues 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/.   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. The Darby study showed that the progressive risk to smokers at higher levels of radon is far more than that to non-smokers, with ex-smokers intermediate, although no mechanism was identified to account for this.

Testing and Mitigation

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.  A Lucas cell is one type of device.

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 Environmenal 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.  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

Radon therapy has been historically used in some spa resorts around the world. Beneficial health effects of radon therapy have never been clinically proved.

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.

atom.gif (700 bytes)

Radon Data

Atomic Radius (): 1.34
Atomic Volume cm3/mol : 50.5cm3/mol
Covalent Radius: unknown
Crystal Structure: Cubic face centered
Ionic Radius: unknown

Chemical Properties

Electrochemical Equivalents: unknown
Electron Work Function: unknown
Electronegativity: N/A (Pauling); 2.06 (Allrod Rochow)
Heat of Fusion: 2.89 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 10.748
Second Ionization Potential: unknown
Third Ionization Potential: unknown
Valence Electron Potential: unknown
Ionization Energy (eV): 10.745 eV

Physical Properties

Atomic Mass Average: 222
Boiling Point: 211K,  -62C, -80F
Melting Point: 202K, -71C, -96F
Heat of Vaporization: 16.4 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: unknown
Thermal Conductivity: 0.0000364 W/cmK
Density: 9.73 g/L @ 273K & 1atm
Enthalpy of Atomization: unknown
Enthalpy of Fusion: 2.89 kJ/mole
Enthalpy of Vaporization: 16.4 kJ/mole
Flammability Class: unknown
Molar Volume: 50.5 cm3/mole
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.09 J/gK
Vapor Pressure: unknown
Estimated Crustal Abundance: 410-13 milligrams per kilogram
Estimated Oceanic Abundance: 610-16 milligrams per liter

(From radium; called niton at first, L. nitens, shining) The element was discovered in 1900 by Dorn, who called it radium emanation. In 1908 Ramsay and Gray, who named it niton, isolated the element and determined its density, finding it to be the heaviest known gas. It is essentially inert and occpies the last place in the zero group of gases in the Periodic Table. Since 1923, it has been called radon.  Radon-222, from radium, has a half-life of 3.823 days and is an alpha emitter; Radon-220, emanating naturally from thorium and called thoron, has a half-life of 55.6 s and is also an alpha emitter. Radon-219 emanates from actinium and is called actinon. It has a half-life of 3.96 s and is also an alpha emitter. It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in tiny amounts into the atmosphere. Radon is present in some spring waters, such as those at Hot Springs, Arkansas. On the average, one part of radon is present ot 1 x 1021 part of air. At ordinary temperatures radon is a colorless gas; when cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. It has been reported that fluorine reacts with radon, forming a fluoride. Radon clathrates have also been reported. Radon is still produced for therapeutic use by a few hospitals by pumping it from a radium source and sealing it in minute tubes, called seeds or needles, for application to patient. This practice has been largely discontinued as hosptials can get the seeds directly from suppliers, who make up the seeds with the desired activity for the day of use. Radon is available at a cost of about $4/m. Care must be taken in handling radon, as with other radioactive materials. The main hazard is from inhalation of the lement and its solid daughters which are collected on dust in the air. Good ventilation should be provided where radium, thorium, or actinium is stored to prevent build-up of the element. Radon build-up is a health consideration in uranium mines. Recently radon build-up in homes has been a concern. Many deaths from lung cancer are caused by radon exposure. In the U.S. it is recommended that remedial action be taken if the air in homes exceeds 4 pCi/l.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond