Propane requires more pressure than gasoline to become a liquid.
Natural gas requires even more pressure than propane.
So much in fact, that it's compressed, but not to the point of liquefaction, when used as a fuel Exposed to the open air, these fuels are gaseous and highly inflammable. If you've cooked with gas, you'll know what that means. Lighting the burner after the pilot light's been extinguished can produce a mini-explosion. The properties of LPG and natural gas differ somewhat. Natural gas is lighter than air and, when exposed to the air, usually dissipates quickly.