Radon was first recognized as a risk for miners in 1879. The link to cancer was sealed in the 1950s and 1960s.
But indoor radon did not hit the headlines until 1984, after Stanley Watras, an electrical engineer at a Pennsylvania nuclear power plant, kept setting off radiation monitor alarms as he walked into the plant.
It turned out that he was not picking up the radiation at work. Instead, scientists said, radon levels in his home were about 2,700 picocuries per liter, or the equivalent of smoking about 135 packs of cigarettes a day.
Watras' house sat on a uranium deposit in a geological formation known as the Reading Prong, which runs through portions of Pennsylvania, New York and New Jersey.
Soon, other houses in the area were found to have similarly high levels of radon. The discoveries led to study after study showing disturbing results around the country. The reports seemed to be followed by reaction from those who disagreed.