text: The largest natural contributor to public radiation dose is radon, a naturally occurring, radioactive gas found in soil and rock, which comprises approximately 55% of the annual background dose.
  Radon gas levels vary by locality and the composition of the underlying soil and rocks.
  Radon (at concentrations encountered in mines) was recognized as carcinogenic in the 1980s, in view of the lung cancer statistics for miners' cohorts.
  Although radon may present significant risks, thousands of persons annually go to radon-contaminated mines for deliberate exposure to help with the symptoms of arthritis without any serious health effects.
  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.
  People in affected localities can receive up to 10 mSv per year background radiation.
  This led to a health policy problem: what is the health impact of exposure to radon concentrations (100 Bq/m) typically found in some buildings?