Radon is an odorless, radioactive gas that can creep up in through your home's cracks. Radon is the second leading cause of lung cancer in the United States today. One in 15 homes in the U.S. has elevated radon levels. Testing is the only way to know the radon level of a home.
If a house has elevated radon levels, the problem is fixable. There are mitigation systems that draw up the radon beneath the slab and exhales it outside the house. If the house you're buying tests high for radon, it's reasonable to ask the seller to install a radon mitigation system before closing.