Radon, with a half life of less than four days, is a highly radioactive noble gas. For this it is rather abundant, because it is formed from the decay of radium. Being much more heavy than air, it stays on the ground and can concentrate in poorly ventilated buildings that are build on radium-rich undergrounds, like granite. In this case, the radon contamination can become dangerous, whereas normal concentrations are considered to be safe. Dangerously high values often occur at the mining of uranium. Radon decays further to polonium.