Radon

Radon (Rn) is a radioactive noble gass, and is a decay product from some isotopes of Uranium and Thorium. Radon is the next step from Radium in the decay chain, and is (unlike Radium) a gas in earth's temperatures. It flows into the atmosphere, and can in large dozes have unfavourable effects on creatures' health, such as neusea and increased risk of cancer. Some houses are build on foundations with high concentartions of Radium, which sucks for its inhabitants I guess. Radon is coloruless and odourless, so you can never know if you're having this season's hottest sickness, or if cancer's coming your way.