Radon is a chemically inert gas in Group VIIIA (Group 18) and in the 6th period. It is the heaviest element of the group, other members of which are helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe).
At the end of the 19th century Pierre and Marie Curie noticed the air that comes into contact with radium compounds becomes radioactive, like the radium compounds themselves. Friedrich Dorn gave the correct explanation for this in 1900 when he showed that one of the disintegration products of radium is a gas - radon. In 1910 Ramsay proved that it was the heaviest of the so-called "rare" or inert gases.
Various 'radiums' labeled A, B, C, D, E, and F were discovered by Ernest Rutherford and Harriet Brooks in the solid deposit left on the insides of glass containers confining radon gas. These are actually radioactive isotopes of Po, Pb and Bi.
In February 1989 Chemical and Engineering News said that "In three short years, the radioactive gas radon has progressed from relative obscurity to a cause of high anxiety as an indoor air pollutant" (Hanson, D., Feb. 6, 1989, page 7). Why should this be so?
The trouble with radon is that it is radioactive. Radon--222 is part of the chain of events beginning with the decay of uranium--238. (Other isotopes of Rn are products of other decay series). Radon occurs naturally in our environment. Since it comes from natural uranium deposits, the amount depends on local geology, but it is believed to account for about 55% of normal background radioactivity. Furthermore, since the gas is inert, and has a relatively long half-life (3.82 days), it is not trapped by chemical processes in the soil or water, and it is free to bubble up from the ground and seep into underground mines or into homes through pores in block walls, cracks in the basement floor or walls, or around pipes. When breathed by humans occupying that space, the radon--222 isotope can decay inside the lungs to give polonium, a radioactive element that is not a gas and is not chemically inert.
222Rn 4He + 218Po
t½ = 3.82 days
218Po 4He + 214Pb
t½ = 3 minutes
Therefore, polonium-218 can lodge in body tissues where it undergoes alpha decay to give lead--214, itself a radioactive isotope. The range of an alpha particle is quite small, perhaps 0.7 mm (about the thickness of a sheet of paper). However, this is approximately the thickness of the epithelium cells of the lungs, so the radiation can damage these tissues and induce lung cancer.
Virtually every home in the U.S. is believed to have some level of radon gas. To test for the presence of the gas, you can purchase testing kits of various kinds. There is currently a great deal of controversy over the level of radon that is considered "safe". The U.S. Environmental Protection Agency has set a standard of 4 picocuries per liter of air as an "action level". There are some who believe 1.5 picocuries is close to the average level, and that only about 2% of the homes will be over 8 picocuries per liter. If your home shows higher levels of radon gas than this, you should probably have it tested further and perhaps take corrective actions such as sealing cracks around the foundation and in the basement. But keep in mind the relative risks involved. A 1.5 picocurie/liter level of radon leads to a lung cancer risk about the same as the risk of your dying in an accident in your home.
Taken from "Chemistry and Chemical Reactivity" by J. Kotz and K. Purcell, Saunders College Publishing, 2nd ed., 1991, page 272.