Compounds containing carbon and fluorine, which are classified under the Kyoto Protocol as among the basket of gases that contribute to greenhouse gas emissions. Natural geological emissions have been responsible for the PFCs that have accumulated in the atmosphere over a very long time. However, the largest current source is aluminium production. With the phase-out of ozone depleting substances, PFCs have been introduced into several specialised applications. But while PFCs are used to replace ozone depleting compounds, they also contribute to global warming.