Radon gas is approximately 7.5 times heavier than air. It is however a noble gas with no chemical affinity but is easily influenced by air movements and pressure. In a house with forced air heating and cooling, radon gas can easily be distributed throughout the entire dwelling. When radon gas is discharged via a radon mitigation system above the roof, the radon concentration falls off dramatically with distance from the point of discharge. In fact, the radon gas concentration approaches background levels at 3-4 feet from the discharge point. EPA disallowed ground level discharge of radon primarily because of the potential for re-entrainment of the gas into the house and because of the possibility of children being exposed to high radon levels. The concentration of radon gas at the discharge point can be tens of thousands of picocuries per minute.
Read more about radon at https://www.epa.gov/radon
Read the Consumer's Guide to Radon Reduction.