Johns Hopkins Bloomberg School of Public Health researchers say that levels of radon in Pennsylvania homes – where 42 percent of readings surpass what the U.S. government considers safe – have been on the rise since 2004, around the time that the fracking industry began drilling natural gas wells in the state.The researchers, publishing online April 9 in Environmental Health Perspectives, also found that buildings located in the counties where natural gas is most actively being extracted out of Marcellus shale have in the past decade seen significantly higher readings of radon compared with buildings in low-activity areas. There were no such county differences prior to 2004. Radon, an odorless radioactive gas, is considered the second-leading cause of lung cancer in the world after smoking.