Because H is the lightest element, atmospheric H can readily be lost to space via Jeans escape, an irreversible process that drives Earth's net mass loss. Photolysis of heavier compounds not prone to escape, such as CH or HO, can also liberate H from the upper atmosphere and contribute to this process. Another major sink of free atmospheric H is photochemical oxidation by hydroxyl radicals (•OH), which forms water.
Anthropogenic sinks of H include synthetic fuel production through the Fischer-Tropsch reaction and artificial nitrogen fixation through the Haber-Bosch process to produce nitrogen fertilizers.