Hydrofluorocarbons (HFCs) are greenhouse gases containing carbon, hydrogen, and fluorine. They are entirely human-made and do not occur in the natural environment. HFCs are used as refrigerant to promote cooling in appliances such as refrigerators and air conditioners. Decades ago, appliances used chlorofluorocarbons (CFCs) as refrigerants, which wreaked havoc on the ozone layer and were phased out after the 1987 Montreal Protocol. They were replaced by hydrofluorocarbons (HFCs), which do not deplete the ozone, but they do have up to 9,000 times greater warming capacity than carbon dioxide.