"These results suggest that hydraulic fracturing does have an impact on our health, though the good news is that this is only at a highly localized level. "

It is the first peer-reviewed research that shows large-scale evidence that fracking may negatively affect infant health. It was co-authored by economists from Princeton University, the University of Chicago and UCLA and based on a study of more than 1.1 million births between 2004 and 2013 in Pennsylvania, a major producer of natural gas from shale deposits.

The study finds that babies born to mothers who live 1 kilometer, or about half a mile, from fracking sites are 25 percent more likely to be born at low birth weights. Infants born below 5.5 pounds are at greater risk of infant mortality, attention deficit hyperactivity disorder, known as ADHD, and asthma, according to the authors.

The impacts on infants born 3 kilometers, or nearly 2 miles, from the sites were about one-half to one-third lower than those living 1 kilometer away. Beyond 3 kilometers, there were no observable impacts on infant health.

"These results suggest that hydraulic fracturing does have an impact on our health, though the good news is that this is only at a highly localized level," Janet M. Currie, professor of economics and public affairs at Princeton University, said in a release.

To be sure, few people live very close to these sites because most fracking occurs in remote rural areas in several parts of the United States. However, the authors note that oil and gas production is starting to encroach on more densely populated areas in some parts of the United States, including around Pittsburgh.