This Jan. 17, 2013 file photo shows a fracking site in New Milford, Pa., Jan. 17, 2013. (Richard Drew/AP Photo)

Levels of cancer-causing radon have reportedly been on the rise in Pennsylvania ever since fracking picked up in the state.

Researchers at the Johns Hopkins Bloomberg School of Public Health say there is an alarming correlation between the unusually high levels of the colorless, odorless radioactive gas indoors and hydraulic fracturing, or fracking.

While pursuing her PhD, environmental health scientist Joan Casey and her colleagues at Bloomberg wanted to determine the sources of radon in Pennsylvania homes.

“We decided to do the study because historically Pennsylvania has had this big radon problem. We were doing house studies in the state for about the past decade. When the unconventional natural gas industry moved into the state, people were concerned," Casey said in an interview with Yahoo News.

Fracking in Bradford County, Pa., on May 9, 2012. Hydraulic fracturing for natural gas has been controversial, and more communities have come to see the environmental and health issues related to fracking as outweighing the economic benefits. (Katie Orlinsky/Getty Images)

Their findings, which appeared in the Environmental Health Perspectives journal on Thursday, show that radon levels are generally higher in areas of the state with fracking operations underway.

Fracking is the process of drilling and blasting high-pressure fluids into the ground to break shale rocks so that they release natural gas.

Radon is emitted from certain elements in soil and rock — uranium, thorium, and radium — as they decay. And there is a lot of uranium in the bedrock throughout Pennsylvania, as Casey points out.

If breathed in, radioactive particles in the gas can damage the cells in a person’s lungs, leading to cancer.

Casey says the Reading Prong in the eastern part of the state has bedrock with the highest levels of uranium in the country; fortunately, the Marcellus Shale — the country’s largest natural gas field — does not run through it, so it has not been subject to fracking.

Jesse Coleman, a researcher at Greenpeace, says the new study further confirms just how dangerous it would be for the practice to continue expanding without more regulation, given the adverse health effects it has already had on communities.

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“It’s very scary. We’ve known that there are a number of contamination issues from fracking. This is another one that has been put on the back burner for a long time,” Coleman told Yahoo News.

Casey and her colleagues analyzed data from nearly 2 million radon readings from every county in the state, reported NBC News. These measurements focused on 866,735 buildings, mostly homes, between 1987 and 2013.

A Cabot Oil and Gas natural gas drill is viewed at a hydraulic fracturing site on January 18, 2012 in South Montrose, Pennsylvania. (Spencer Platt/Getty Images)

"We evaluated associations of radon concentrations with geology, water source, building characteristics, season, weather, community socioeconomic status, community type, and unconventional natural gas development measures based on drilled and producing wells,” they wrote.

The scientists discovered that radon levels spiked around 2004 — when fracking started in the state — particularly in homes near the unconventional wells drilled for the controversial process.

Pennsylvania’s Department of Environmental Protection says about 40 percent of homes in the state have elevated radon levels.

The researchers point out that their study does not directly link radon with fracking and that other factors — such as homes being more tightly sealed — may be at play, though it does not seem as plausible.

The National Cancer Institute says radon is the second-leading cause of lung cancer in the United States and kills 15,000 to 22,000 people each year.