You can’t smell, touch, or see this gas, but it’s the No. 2 cause of lung cancer in the U.S. behind smoking. It’s made when natural uranium in rock, soil, and water breaks down. It gets into buildings through cracks in floors and walls, and around plumbing and electrical wire. The radioactive particles in radon damage your lungs when you breathe them in or swallow them. A simple test kit can see if you have high levels in your home.