You may or may not be aware that the decomposition of uranium in our soil gives off a gas called "Radon Gas". The reason radon gas is an important subject is because it is the #2 cause of lung disease in the US, with cigarette smoke being the first.
High levels of radon have been found in every state and the distribution of this gas is random...one home may have a high level of radon and the home right next to it might not. The good thing to note is that radon levels can be lowered. The EPA has established a rating at or higher than 4pCi/L (4 picocuries per liter) as a level that needs mitigation. Levels lower than this may still pose a risk and in most cases can still be mitigated to even lower levels.
When purchasing a home, the buyer can do a radon test as part of the home inspection. Should the reading be at or higher than 4pCi/L, then the seller is notified and in most cases, will proceed to mitigate for the buyer. After mitigation results are then provided to the buyer to confirm readings are lower than 4pCi/L.