Radon as a terrestrial source of background radiation is of particular concern because, although overall very rare, where it does occur it often does so in high concentrations. Some of these areas, including parts of Cornwall and Aberdeenshire have high enough natural radiation levels that nuclear licensed sites cannot be built there—the sites would already exceed legal limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste.[69][clarification needed] People in affected localities can receive up to 10 mSv per year background radiation.[69]