This gas is the second-leading cause of lung cancer in the United States, and as many as 1 out of every 15 homes has unsafe levels. Radon comes from the natural breakdown of uranium in the ground under your house, and it’s just as likely to be a problem in newer homes as in older ones. Because radon is odorless and colorless, a test is the only way to know your risk. Pick up a test kit at the hardware store, or hire a professional to check your home. Repeat at least every two to three years.