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People living near natural-gas wells were more than twice as likely to report upper-respiratory and skin problems than those farther away, says a major study Wednesday on the potential health effects of fracking.

Nearly two of every five, or 39%, of those living less than a kilometer (or two-thirds of a mile) from a well reported upper respiratory symptoms, compared to 18% living more than 2 kilometers away, according to a Yale University-led random survey of 492 people in 180 households with ground-fed water wells in southwestern Pennsylvania.

The disparity was even greater for skin irritation. While 13% of those within a kilometer of a well said they had rashes and other skin symptoms, only 3% of those beyond 2 kilometers said the same.

“This is the largest study to look at the overall health of people living near the wells,” says lead author and University of Washington environmental health professor Peter Rabinowitz, who did the research while at Yale. The study focused on Washington County, part of the Marcellus Shale where hydraulic fracturing, or fracking, is widely used to extract natural gas.

“It suggests there may be more health problems in people living closer to natural gas wells,” but it doesn’t prove that the wells caused their symptoms, he says, adding more research is needed.

Fracking, combined with horizontal drilling, has spurred a U.S. boom in oil and natural-gas production. It blasts huge amounts of water – mixed with sand and chemicals – deep underground to break apart shale deposits and extract gas and oil from the rock’s pores.

Prior peer-reviewed studies have linked fracking to possible birth defects, higher lung disease risks, methane contamination in drinking water and elevated endocrine-disrupting chemical activity in groundwater. Some environmental groups, including the Natural Resources Defense Council, oppose fracking, saying it has insufficient safeguards.