Remember chlorofluorocarbons? The chemicals that made that hole in the ozone layer? After CFCs were banned, companies switched to hydrofluorocarbons. Uh oh! HFCs may not punch holes in the ozone layer, but they are potent greenhouse gases. D.C. Circuit (2017): An EPA rule that prohibits the use of HFCs for CFCs applies only to companies that have not already switched to HFCs. EPA: Okey dokey, everyone can use HFCs. D.C. Circuit (2020, over a dissent): Not without notice and comment rulemaking, they can’t.