Who is at risk?
Everyone is at risk without exception.
In addition more concern goes towards children whose lungs are still developing and those who spend long times in their home, for instance because they have a home office in their basement or for any other reason..

If I would look for excluding groups I could make a case that people living in boat- and tree-homes and people living above the third floor in residential structures most likely are not subject to elevated levels of radon during the times they stay in these places. Although I am aware of rare exceptions to this rule also.

A Statement by the U.S. Surgeon General

The following quote exemplifies that the problem of high radon levels in peoples homes should not be ignored:

"Indoor radon gas is a national health problem. Radon causes thousands of deaths each year. Millions of homes have elevated levels. Most homes should be tested for Radon. When elevated levels are confirmed, the problem should be corrected."
U.S. Surgeon General