Radon is a naturally-occurring radioactive gas that is potentially harmful. The danger can be mitigated with a piping system that diverts the gas away from the living space. Many buyers and sellers know about radon and test for it, but the rules are different in the two states.
The Illinois Radon Awareness Act requires sellers to provide buyers with a pamphlet from the Illinois Emergency Management Agency about the dangers of radon. The seller does not, however, have to test the home for the gas, or pay to mitigate it if gas is found. If the seller has tested the home and radon is found, the Act states that this must be disclosed to a buyer. Just as any other issue with the home like an old furnace or leaky roof, the buyer can negotiate to have the seller take care of it.
Starting in 2013, the Illinois Radon Awareness Act also required that all newly constructed houses and apartments include radon mitigation systems. So radon testing and mitigation should only be a concern when buying or selling a home built before 2013.
Missouri has no legislation regarding radon testing, disclosure, or mitigation. Buyers who are concerned about radon should consider having the home tested before moving in.