Nearly half of the homes in Pennsylvania have unsafe radon levels and that number has been on the rise since 2004 when the oil and gas industry began fracking in the state.
A study by Johns Hopkins Bloomberg School of Public Health researchers found that 42 percent of readings surpass what the U.S. government considers safe. They said the rising rates of the odorless gas tied to lung cancer could have been due to fracking, a process of injecting a slurry of water, sand and chemicals into wells in an area known as Marcellus shale to remove oil and gas.
"One plausible explanation for elevated radon levels in people's homes is the development of thousands of unconventional natural gas wells in Pennsylvania over the past 10 years," said study leader Brian S. Schwartz, a professor in the Department of Environmental Health Sciences at John Hopkins. "These findings worry us."
The study, conducted with Pennsylvania's Geisinger Health System, a health care network, analyzed more than 860,000 indoor radon measurements included in a Pennsylvania Department of Environment Protection database from 1989 to 2013. Many of the measurements came from assessments of homes that were either being bought or sold.
Along with the geography, water sources, and weather, the researchers looked at fracking levels.
Pennsylvania has been a hotspot for fracking since the boom started more than a decade ago, with 7,469 unconventional natural gas wells drilled in the state since 2005.
In the process of pumping billions of gallons of chemically laced water deep underground to extract the gas, heavy metals as well as organic and radioactive materials such as radium-226 are brought to the surface. It decays into radon and most radon exposure has been linked to the diffusion of gas from soil.
Along with the unsafe levels across the state, the study found that houses and buildings located in rural and suburban townships, where most of the gas wells are, had a 39 percent higher concentration of radon than those in cities.
"By drilling 7,000 holes in the ground, the fracking industry may have changed the geology and created new pathways for radon to rise to the surface," Joan A. Casey, a Robert Wood Johnson Foundation Health & Society Scholar at the University of California-Berkeley and an author on the study, said. "Now there are a lot of potential ways that fracking may be distributing and spreading radon."
The findings offer another troubling sign that fracking is leaving behind a myriad of environmental problems. Critics have blamed the industry for producing 800 billion gallons of wastewater each year, causing water and air pollution in communities near fracking sites and sparking earthquakes in Oklahoma and Ohio.
They also have attacked states and the federal government for failing to do enough to regulate the industry, prompting the Obama Administration earlier this month to issue the first-ever regulations for fracking on federal lands - which among other things requires companies to disclose the chemicals they are using.
Still, the latest findings on radon are likely to be disputed. The authors even acknowledged that the state of Pennsylvania recently took a comprehensive set of measurements near 34 gas wells, including air samples for radon near four wells, which did not show high levels of the radioactive gas.
Susan Rickens, a spokeswoman for the Pennsylvania Department of Environmental Protection, confirmed the state's study didn't find elevated levels near drill sites, though it didn't take samples of indoor air quality as the John Hopkins study did.
More broadly, Rickens said the state sees radon as a "public health concern," adding that that all 67 counties have shown "high levels" of the gas.
"While our study didn't signal a problem, we will take a look at the John Hopkins study in-depth," she told CBS News. "It may signal areas where we need to do more study."
The oil and gas industry, which has sued to stop the new federal fracking regulations, also criticized the radon study.
In a blog post, Nicole Jacobs of oil and gas advocacy group Energy in Depth said the researchers' "own data debunk their conclusions: their highest radon readings were in counties in Reading Prong, which have absolutely no wells - not even conventional."
Reading Prong, the authors note, has historically high bedrock uranium concentrations, which could explain its high radon levels.
"They gloss over the fact that Pennsylvania has long had naturally occurring concentrations of radon, and simply blame fracking," she wrote. "Further, study after study, including a major study from the Pennsylvania DEP, has found no credible link between oil and gas development and radon exposure. Considering all these facts, it looks like the researchers were more interested in gleaning anti-fracking headlines than producing credible science."
Schwartz countered that the industry was "cherry picking" results in attempt to discredit the study and he and the other researchers had been "fair in the balance of our claims."