Inhalation of radon gas is the second largest cause of lung cancer worldwide, second only to cigarette smoking. In the 1960s, underground uranium miners began to show unusually high incidences of lung cancer. The cause was shown to be related to the duration of the miner's exposure to radioactive materials. To cause lung cancer, the radioactive material must enter the lungs as a gas. It then causes progressive damage to the bronchial epithelium or lining of the lungs. What is the gas and how does it originate? ssss1 shows the many radioactive isotopes that are produced by the decay of the common isotope of uranium (²³⁸U). Uranium miners would be exposed to all of these, but which one would they inhale into their lungs? Because radon possesses a stable electron configuration, it tends not to combine with other elements. Like most noble elements, under normal near surface conditions, it tends to exist as separate atoms in the form of a gas. In the confined space of poorly ventilated underground mines, radioactive decay in the uranium series produces sufficient concentrations of radon to significantly increase the incidence of lung cancer. The other property that makes radon‐222 so dangerous is its short half‐life (3.825 days). Within days, most of the radon inhaled by miners decays into polonium‐218 with the emission of alpha particles (⁴He nuclei). Subsequently, most of the radioactive²¹⁸Po decays within hours into lead‐210 with the release of more alpha particles. Lung damage leading to lung cancer largely results from continued rapid release of alpha particles over long periods of exposure. Scientific studies on radon exposure have been complicated by the fact that many miners were also smokers. It turns out that smoking and radon exposure act synergistically to multiply the risk of developing lung cancer.