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Radon is a naturally occurring element; the isotope of primary health concern is 222Rn. The largest source of radonin the environment is widely distributed uranium and its decay products in the soil (Buttafuoco et al. 2007; UNSCEAR 2000; Weast 1980). Radon is a decay product of radium and part of the uranium decay chain (see Figure 4-1) (Buttafuoco et al. 2007; O’Neil et al. 2006; UNSCEAR 2000). Every square mile of surface soil, to a depth of 6 inches, contains approximately 1 gram of radium, which slowly releases radon to the atmosphere (Weast 1980) when conditions of secular equilibrium exist.