A study published last week says the increase in radioactive material in Pennsylvania homes can be linked to the increase in hydraulic fracturing, or fracking, activity in the state, ThinkProgress reported.

Scientists from the Johns Hopkins Bloomberg School of Public Health said radon levels in Pennsylvania homes had been increasing since 2004, “around the same time the state’s Department of Environmental Protection (DEP) began rapidly increasing the number of permits it issued for unconventional gas drilling.”

The report also said there had been no “similar increases” in levels of the odorless, carcinogenic, radioactive gas in homes before 2004.

The study found that buildings using well water had a radon concentration 21 percent higher than buildings using municipal water. It also found that homes in townships, where there are more fracking wells than in larger metropolitan areas, had a 39 percent higher radon concentration than those in cities.

Currently, the DEP estimates that nearly 40 percent of Pennsylvania homes contain radon levels above the recommended concentration.

The report’s authors pointed out that the increase in radon levels at the same time as the increase in fracking only represents a correlation, and that the study does not definitively prove fracking caused the rise.

“We’re not convinced this industry is playing a role [in increased radon levels],” study leader Brian Schwartz told ThinkProgress. “All we’re saying is these findings provide no reassurance that the industry is not playing a role.

What the study does prove, however, is that more research needs to be done on the effects of fracking. In addition to a radon program in Pennsylvania, the researchers recommended “future studies to understand the impact of drilling on radon levels.”

Between groundwater contamination, earthquakes, and increases in radioactive gas, it’s becoming more and more apparent just how dangerous fracking really is.