(CBS SF) – A preliminary study co-authored by a University of California researcher claims levels of cancer-causing radon gas has increased in Pennsylvania homes as fracking has expanded in the state.

According to USA Today, the study authored by Joan Casey of UC Berkeley and UCSF and Brian Schwartz of Johns Hopkins University is a preliminary “first look” into a possible connection between fracking and radon.

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Researchers analyzed more than 860,000 indoor radon measurements in Pennsylvania homes from 1989 to 2013. They found that radon levels in the state began to rise around 2004, when hydraulic fracturing took off.

“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,” Casey said in a Johns Hopkins University statement. “Now there are a lot of potential ways that fracking may be distributing and spreading radon.”

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The authors also found radon concentrations were 21 percent higher in buildings that used well water, which could indicate radon entering homes through showers and faucets. Fracking involves blasting millions of gallons of water, sand and chemicals deep underground to break apart shale deposits and release natural gas.

The Marcellus Shale Coalition, a top fracking industry group, disputed the findings in a statement to USA Today. “It’s unfortunate, yet not unexpected, that some anti-shale activists continue to peddle profoundly flawed and unsubstantiated claims, such as this, based purely on hypothetic and perhaps pre-determined narrative-driven ’cause and effect’ conclusions with the goal of generating fear,” the statement read in part.

Casey and Schwartz also acknowledge the findings conflicted with a study by Pennsylvania officials that found radon levels weren’t higher than expected in the air near fracking facilities and in natural gas coming out of the ground. The authors said additional studies are needed.

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Radon is the second leading cause of lung cancer, behind smoking.