An environmental information page by Jim VanErmen ABR CRR

A colorless, odorless, tasteless gas 

you can't even see -  RADON!

"...Agency's updated calculation of a best estimate of annual lung cancer deaths from radon is about 21,000 (with an uncertainty range of 8,000 to 45,000)," EPA Assessment of Risks from Radon in Homes PDF
EPA has extensive information on RADON where they answers a number of technical questions on the subject.
 EPA'S  "Home Buyer's and Seller's Guide to Radon"
 PDF link to 44 page printable pamphlet

The Radon Project  at Columbia University and the Lawrence Berkeley National Laboratory.  Based on the information you provide, the Radon Project website will estimate the radon level in your house and how much risk of lung cancer this will cause, additionally it will provide a recommendation on the actions you may consider.  NOTE: To accurately determine Radon levels in your home you should rely on a home radon test performed in your home.
An OnLine source to purchase a radon testing kit:  RadonZone.com Radon Home Test & Radon Kits
Locally an indoor short term text kit is available at    Lowes Kidde Radon Test Kit, Model Item # 202280
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