Internalized Α-particle Emitting Radionuclides

Radon is a natural radioactive gas produced by the decay of uranium and thorium, which are present in all rocks and soils in small quantities. There are several isotopes of radon, the most important of which are 222Rn (produced from 238U) and 220Rn (produced from thorium). 220Rn is also known as thoron because of its parent radionuclide. In the United Kingdom, it has been shown that 220Rn delivers much smaller doses to the public in indoor environments than 222Rn (The Independent Advisory Group on Ionising Radiation, 2009). Unlike 222Rn, 220Rn is not formed during the radioactive decay of 238U, and is hence not present at appreciable levels in uranium mines. The epidemiological evidence on the cancer risks from radon is derived largely from cohort studies of underground miners that had been exposed to high levels of radon in the past. More recently, a series of case–control studies of lower exposures to residential radon have also been conducted. The previous IARC Monograph on radon IARC (1988) states that radon is a cause of lung cancer in humans, based on clear excess lung cancer rates consistently observed in underground miners, and elevated lung cancer risks seen in experimental animals exposed to radon. In a subsequent evaluation by IARC (2001), additional epidemiological evidence of an increased lung cancer was also seen in case–control studies of residential radon. Although results from the 13 case–control studies available at that time were not conclusive, the Working Group noted that the risk estimates from a meta-analyses of eight such studies were consistent with estimates based on the underground miner data (Lubin & Boice, 1997). In a detailed evaluation of the health risks of radon by the Committee on the Biological Effects of Ionizing Radiation (BEIR) within the INTERNALIZED α-PARTICLE EMITTING RADIONUCLIDES