Radon is a radio-active gas that come in the house through the floor cracks.
Radon is by-product of uranium and it emits from ground everywhere. It is in traces outside. As home is closed that gas coming in from ground tends to accumulate.
The gas is colorless, & odorless so inhaling happens without knowing.
The gas is heavy, so it is alreadys concentrated on the lowermost floor of the house.
Every home regardless of slab/basement gets radon. Some homes have higher radon levels posing health hazard.
As the gas is radio-active it poses health hazard.
Per EPA, It is 2nd commonest cause of lung cancer only after smoking.
EPA levels
0-2 pCi/L - Average level
2-4 pCi/L - Borderline level
4+ pCi/L - Requires mitigation action.
Mitigation action - Install pipe and exhaust system for concrete plate.