Radon

Radon is a heavy noble gas with no color, smell or taste. It forms continuously as uranium in rocks and soil decays, passing through radium along the way. Friedrich Ernst Dorn identified it in 1900 as a gas given off by radium. Its most common form, radon-222, has a half-life of about 3.8 days.

Outdoors, radon spreads out harmlessly. But it can seep through cracks into basements and lower floors and build up. When breathed in, it and its short-lived decay products irradiate the lungs, making radon the second leading cause of lung cancer after smoking. Cheap test kits measure indoor levels, and fans that vent air from beneath the floor fix most problems.