<P> Radon is formed as part of the normal radioactive decay chain of uranium into Pb . Uranium has been present since the earth was formed and its most common isotope has a very long half - life (4.5 billion years), which is the time required for one - half of uranium to break down . Thus, uranium and radon, will continue to occur for millions of years at about the same concentrations as they do now . </P> <P> Radon is responsible for the majority of the mean public exposure to ionizing radiation . It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location . Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics, and basements . It can also be found in some spring waters and hot springs . </P> <P> According to a 2003 report EPA's Assessment of Risks from Radon in Homes from the United States Environmental Protection Agency, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon - induced U.S. lung cancer deaths per year--second only to cigarette smoking . Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant . </P> <P> Radon concentration is usually measured in the atmosphere in becquerels per cubic meter (Bq / m), which is an SI derived unit . As a frame of reference, typical domestic exposures are about 100 Bq / m indoors and 10 - 20 Bq / m outdoors . In the US, radon concentrations are often measured in picocuries per liter (pCi / l), with 1 pCi / l = 37 Bq / m . </P>

What are the side effects of radon poisoning