Expectant moms who live close to fracking sites are significantly more likely to give birth to underweight babies, according to a comprehensive new study out of Pennsylvania.

And the closer they are to the well, the greater the risk, say researchers in the journal Science Advances. The researchers found that infants born within about a half-mile of a fracking site were 25 percent more likely to weigh 5.5 pounds or less, reports Science.

The study found an increased risk for a radius up to about two miles (three kilometers, to be exact) from wells, but not much of a risk beyond that.

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The study is based on data from 1.1 million babies born in the fracking-heavy state between 2004 and 2013. Low birth weights are linked to later problems ranging from asthma to lower test scores, and the toxic chemicals used in the fracking process have long raised health concerns, notes the LA Times.

"I think I was surprised by the magnitude of the impact within the half-mile radius," co-author Michael Greenstone of the University of Chicago tells the Washington Post.

But, he adds, while fracking opponents may focus on that part of the study, supporters may focus on the lack of a risk for babies outside the three-kilometer radius.

Still, given that 30,000 US babies a year are born within a half-mile and 100,000 within two miles of a fracking site, the results are noteworthy, he says.

An industry spokesperson countered that the study fails to take into account factors such as smoking and alcohol use, and says "it's dangerously misleading and inflammatory to suggest that natural gas development has done anything but improve public health.” (The EPA is looking into fracking's effect on groundwater.)

This article originally appeared on Newser: Babies born near fracking wells are smaller