The EPA recommends testing every 2 years to ensure radon levels remain low . It is especially important to test for radon when buying a new home or when your home has never been tested before.
What is radon?
Radon is a cancer-causing, radioactive gas that you cannot see, smell, or taste. Radon is the second leading cause of lung cancer in the United States today and causes thousands of deaths each year . The risk of radon depends on the level of radon and length of exposure .
How does Radon get into your home?
Cracks in solid floors/walls
Gaps in suspended floors
Gas around service pipes
Cavities inside walls
1 out of 15 homes is estimated to have elevated radon levels 
Average indoor levels are estimated to be about 1.3pCi/L while outside air is about 0.4 pCi/L 
Radon levels can typically be reduced down to 2 pCi/L with mitigation efforts. Even very high radon levels can be reduced up to 99% . It is recommended to use a qualified or state certified radon contractor, you can find one using this link. Further information on how to fix radon can be found here. Testing should also be done again after the system is installed to ensure the levels have been reduced and every 2 years to be sure radon levels remain low.