From Columbia Daily Review:

One in 15 U.S. homes contains high levels of a gas that is thought to be the second-leading cause of lung cancer — after smoking — and causes more than 21,000 deaths a year.

The colorless, odorless killer is called radon, and it is the product of the breakdown of uranium in soil. The gas can seep upward into cracks and holes in the foundations of buildings, where it can accumulate. Radon also can sneak into a home through well water, and, in a small percentage of buildings, the building materials themselves can contain radon.

According to the U.S. Environmental Protection Agency, radon accumulated inside a building reaches a dangerous quantity when it is measured at 4 picocuries per liter and the building’s inhabitants are exposed to the gas for years.

The risk of developing lung cancer from radon exposure is much higher for smokers.

Robert Dye, an environmental scientist with the EPA, said there is no clear profile of homes that are more likely to contain high levels of radon. The gas can occur in houses of any age and value and anywhere in the United States.

Dye said without testing your home for radon, “there’s no way to know if you have an elevated level.”