text: The fact that radon is present in indoor air has been known since at least the 1950s and research into its effects on human health started in the early 1970s. The danger of radon exposure in dwellings received more widespread public awareness after 1984, as a result of a case of Stanley Watras, an employee at the Limerick nuclear power plant in Pennsylvania. Mr. Watras set off the radiation alarms (see Geiger counter) on his way into work for two weeks straight while authorities searched for the source of the contamination. They were shocked to find that the source was astonishingly high levels of radon in his basement and it was not related to the nuclear plant. The risks associated with living in his house were estimated to be equivalent to smoking 135 packs of cigarettes every day.
  Depending how houses are built and ventilated, radon may accumulate in basements and dwellings. The European Union recommends that mitigation should be taken starting from concentrations of 400 Bq/m for old houses, and 200 Bq/m for new ones.
  The National Council on Radiation Protection and Measurements (NCRP) recommends action for any house with a concentration higher than 8 pCi/L (300 Bq/m).
  The United States Environmental Protection Agency recommends action for any house with a concentration higher than 148 Bq/m (given as 4 pCi/L). Nearly one in 15 homes in the U.S. has a high level of indoor radon according to their statistics. The U.S. Surgeon General and EPA recommend all homes be tested for radon. Since 1985, millions of homes have been tested for radon in the U.S.
  By adding a crawl space under the ground floor, which is subject to forced ventilation the radon level in the house can be lowered.