Radon

Radon is a chemical element (atomic number 86). At normal pressures and temperatures, it's an odorless, colorless gas. It's one of the few gaseous elements that is radioactive. This puts people exposed to it at risk of serous radiation sickness.
Most radon on Earth is created as the result of the radioactive decay of radium, which in turn is part of the chain of elements in the radioactive decay chain of both uranium and thorium into lead. As such, it's common in places where even a small amount of radioactive waste has been disposed of, including mining waste from uranium mining. However, it can also occur in places that are naturally rich in radioactive minerals. In addition, it can accumulate in poorly ventilated buildings. In addition, it can decay into other radioactive elements that pose a higher risk of radiation exposure.
Radon is the element that provides most people with their highest exposure to background radiation, and is the second leading cause of lung cancer after smoking.
Radon at Wikipedia
Category:Chemistry