Hydrofluoroolefin (HFO) makers Honeywell and Chemours are jointly asking the U.S. Supreme Court to consider their arguments in favor of an EPA hydrofluorocarbon (HFC) regulation. They are contesting a federal appeals court decision last August in a case lodged by Mexifluor Chem and Arkema, which make the refrigerant HFC-134a. That ruling overturned a 2015 EPA rule requiring manufacturers to replace HFCs with chemicals that are less-potent greenhouse gases, such as HFOs. Now, Honeywell and Chemours are telling the nation’s high court that the appellate decision creates uncertainty in the marketplace that will last for years until EPA can issue a new regulation restricting HFCs. The HFO makers and the environmental group Natural Resources Defense Council, which filed a separate petition to the high court, contend the 2017 ruling illegally prevents EPA from regulating manufacturers that replace ozone layer-depleting compunds with HFCs or other substitutes that are toxic, flammable, or environmentally harmful. The Supreme Court will decide in the coming months whether to hear the case.