The biggest source of natural background radiation is airborne radon (Rn), a radioactive gas that emanates from the ground. Radon is a decay product of thorium and uranium, which are the most common radioactive elements in the Earth’s crust and the more ore-bearing are the rocks, the more concentration of these elements you find. Out of the 2.4 mSv more than half is caused by inhaled Radon and its isotopes. As explained before Radon is not evenly distributed but varies with the composition of the earth and also varies with the weather, such that much higher doses apply to many areas of the world, where it represents a significant health hazard. Concentrations over 500 times the world average have been found inside buildings in Scandinavia, United States, Iran, and Czech Republic.
Radon has a short half-life of just less than 4 days and decays into other solid particulate nuclides. If the gas is inhaled, the radon atoms decay in the airways or the lungs, resulting in radioactive polonium-218 and -214 and ultimately lead atoms attaching to the lung’s tissue. In September 2009, the World Health Organization released a comprehensive global initiative on radon that recommended a reference level of 100 Bq/m3 for radon. Based on the studies carried out by the National Academy of Science radon is the second leading cause of lung cancer after smoking and accounts for 15,000 – 22,000 cancer death per year in the US alone. EPA recommends that all homes should be monitored for radon. If testing shows levels less than 4 picocuries radon per litre of air (160 Bq/m3), then no action is necessary.