Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as a decay product of radium, according to Wikipedia.

The recommended safe level for radon gas indoors is 4.0 picocuries per liter. The U.S. Environmental Protection Agency claims that those that have extended exposure to a 4.0 pCi/L level of radon has a 7 in 1,000 chance of death, due to lung cancer.

“Wilson, the owner of the home inspection company that tests for radon, said the test for radon takes about 48 hours. The test is installed in a lower living space of the house on a wall that is not close to a window. It usually costs $125 to $150.” As reported by ColumbiaMissourian.com

Sources:
https://en.wikipedia.org/wiki/Radon
http://www.columbiamissourian.com/
http://homeinspectionallstar.com/