The US EPA has established the "action level" for deciding when you need to do something about the radon in your home, school, or work place at 4 pCi/L. One pCi/L means that in one liter of air there will be 2.2 radioactive disintegrations (events) each minute. For example, at 4 pCi/L there will be approximately 12,672 radioactive disintegrations in one liter of air, during a 24-hour period. You can read or download a copy of the US EPA's "A citizen's guide to radon". This publication goes into greater detail describing the meaning of your radon test results and what your personal hazard from the exposure may be.