A new study published Thursday reported a disturbing correlation between unusually high levels of radon gas in mostly residences and an oil and gas production technique known as fracking that has become the industry standard over the past decade.Writing in the journal Environmental Health Perspective, researchers analyzed levels of radon — a colorless, odorless gas that is radioactive and has been linked to lung cancer — in 860,000 buildings from 1989 to 2013. They found that those in the same areas of the state as the fracking operations generally showed higher readings of radon. About 42 percent of the readings were higher than what is considered safe by federal standards. Moreover, the researchers discovered that radon levels spiked overall in 2004, at about the same time fracking activity began to pick up.

Hydraulic fracturing or “fracking” involves drilling 6,500 to 10,000 feet below the surface, and scientists theorize that radon trapped in rocks there is releasedinto the atmosphere.

“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,” one of the authors, Joan A. Casey from the John Hopkins Bloomberg School of Public Health, warned.

Researchers have also reported concerns about similar radon releases in parts of Colorado where fracking is also booming. In a study published by the U.S. Geological Survey in 2012, government researchers found that the radiation in water in the Appalachian Basin in parts of New York and Pennsylvania near fracking work in the Marcellus Shale was unusually high.

The natural gas industry has pointed out that fracking has been conducted safely for years. In response to the Pennsylvania study, Energy in Depth, a program of the Independent Petroleum Association of America issued a statement pointing out issues with the research. It noted that the state’s own department of environmental protection has reported that there is “little potential for additional radiation exposure to the public due to the use of natural gas extracted from geological formations” in Pennsylvania, that the state has had issues with naturally occurring radon long before shale development, and that some of the highest radon readings were in counties in where there are no wells.

“In other words, the authors want to remind readers to look at this as an exploratory analysis because they were missing a few key items to make their claims.  But that didn’t stop them from blaming fracking as if they did,” Energy in Depth’s Nicole Jacobs wrote in a blog post published Thursday afternoon.