A federal appeals court panel on Tuesday struck down a 2018 Trump administration rule that had relieved states of their obligation to curb air pollution that causes smog in downwind states hundreds of miles away.

The ruling requires the Environmental Protection Agency to draw up a new plan for addressing the nation's long-standing problems with ground-level ozone, or smog, to meet the requirements of the Clean Air Act. It's a task that the Trump administration has made far more difficult by rolling back the restrictions on coal power plant pollution in the Clean Power Plan, the Obama administration's signature policy on climate change.