Radon is colorless odorless gas which usually comes into a person’s house through the basement. The EPA has some good information on radon on their website. This site is very informative with lots of illustrations and examples.
Check out this video for an up-close perspective on this topic.
photo credit: <a href=”http://www.flickr.com/photos/rocbolt/7375791654/”>rocbolt</a> via <a href=”http://photopin.com”>photopin</a> <a href=”http://creativecommons.org/licenses/by-nc/2.0/”>cc</a>