They can be a one-time pump or a pump that can be fitted with a replacement cartridge. Most pumps use carbon dioxide and standard-threaded 16g CO2 canisters. Carbon dioxide leaks out of a rubber inner tube more rapidly than air - despite its larger molecule size, the CO2 molecule is significantly more soluble than Oxygen and Nitrogen in rubber and as such can cause a tire to deflate far quicker than if filled with air.