What is radon?
Radon is a radioactive gas. It is colorless, odorless, tasteless, and chemically inert. Testing is the only way to know how much is present.
Radon is formed by the natural radioactive decay of uranium in rock, soil, and water. Naturally existing, low levels of uranium occur widely in Earth's crust. It can be found in all 50 states. Once produced, radon moves through the ground to the air above. Some radon remains below the surface and dissolves in water that collects and flows under the ground's surface.
Radon has a half-life of about 3.8 days—half of a given quantity of it breaks down every 3.8 days. When radon undergoes radioactive decay, it emits ionizing radiation in the form of alpha particles. It also produces short-lived decay products, often called progeny or daughters, some of which are also radioactive. Unlike radon, the progeny are not gases and can easily attach to dust and other particles. Those particles can be transported by air and be inhaled.
The decay of progeny continues until stable, non-radioactive progeny are formed. At each step in the decay process, radiation is released.
Sometimes the term radon is used in a broad sense, referring to radon and its radioactive progeny all at once. When a test measures radiation from the progeny, rather than radon itself, the measurements are usually expressed in working level (WL) units. When radiation from radon is measured directly, the amount is usually expressed in picocuries per liter of air (pCi/L).
January 2005 Surgeon General Health Advisory "Indoor radon is the second leading cause of lung cancer in the US and breathing it over prolonged periods can present a significant health risk to families all over the country. It's important to know that this threat is completey preventable. Radon can be detected with a simple test and fixed through well-established venting techniques."
In June 2003 the U.S.EPA assessment of risks from radon in homes was that "there were about 21,000 annual lung cancer deaths" and yet, in 2013 that estimate has increased to 25,000. That statistic should be going down, not up.