Wyoming’s southwestern region was found to have an unsafe level of smog-causing ozone, a designation the U.S. Environmental Protection Agency linked to a boom in oil and gas drilling in the state.

The EPA included the Upper Green River Basin in its list of areas nationwide exceeding an ozone standard set in 2008 during the administration of President George W. Bush. Wyoming until today was the only state where all counties had met the federal ozone limit.

“It’s undoubtedly the oil and gas drilling going on out there,” John Walke, clean-air director for the New York-based Natural Resources Defense Council, said in an interview. Walke criticized President Barack Obama for throwing out a proposal by the EPA to lower that ozone standard.

Ozone, a precursor to smog, is caused by emissions from vehicle traffic, industrial activities or related projects such as road construction. Gas escaping from wells or pipelines contains volatile organic compounds, which also can cause smog. MORE …