Radon gas is an incredibly dangerous, radioactive gas. Not only is Radon gas colorless and odorless, but it is also toxic and can cause serious health problems. Radon gas is the second leading cause of lung cancer behind cigarette smoke. Radon gas comes from uranium in the soil which, as it breaks down naturally, creates radon gas. It then migrates with other soil gases into the home where it can enter the lungs. Testing is the only way to know what the radon gas concentrations are within your home. It is recommended that all homes be tested for the level of concentration of radon gas. If the radon gas test results determine that the radon gas levels within your home are at or above 4.0 pCi/L, the EPA recommends installing a radon gas mitigation system.
When testing for radon gas "closed house conditions" must be in place for 12 hours prior to beginning testing as well as for the entire duration of the testing, which is typically 48 hours for a short term test for a real estate transaction.
The following are the requirements for "closed house conditions":
Windows on all levels and external doors should be kept closed (except during normal entry and exit) during the measurement period. (To the extent possible, external doors should not be left open for more than a few minutes during the measurement period.)
Do not operate fans, especially whole house fans, or other machines that bring in air from the outside or push air from inside the home to the outside. Normal operation of the heating system(s) or air conditioning system(s) is permitted. (These air conditioning systems must only re-circulate interior air.)
For more detailed information on radon gas visit the EPA's "A Citizen's guide to Radon".