The U.S. Environmental Protection Agency may have to go “back to the drawing board” in its effort to reduce ozone pollution in upwind states if the tenor of an industry challenge heard by the U.S. Supreme Court on Wednesday is any indication.

During arguments on the EPA’s recent “Good Neighbor Plan,” several of the conservative justices expressed sympathy with the argument that the agency failed to address potential legal defects in its effort to reduce gaseous pollutants from upwind states under the federal Clean Air Act.