If you’re buying a house, ask whether it’s been recently tested for radon. If it has, ask to see the results. If it hasn’t, it would be wise to have the home tested by a professional. It is common for the buyer to perform a radon test alongside the home inspection and can be a contingency of the purchase contract.
The EPA recommends that to improve indoor air quality, people should consider installing a mitigation system to reduce radon levels below 2.0. While 4.0 is the number that triggers seller action (usually), the lower the radon, the less cumulative impact on health.