Originally Posted by jemoryl

There is a lot of false information being thrown around here. CO2 is not a 'smaller' molecule than N2, it simply diffuses through butyl rubber at a faster rate due to various subtle interactions.

The reason why CO2 is used in a cartridge is simple: when compressed to a certain pressure at room temperature it condenses into a liquid - this is what is present inside your little cylinder. Air (or nitrogen) would still be a gas at the same pressure/temperature; since a liquid is more dense than a gas, you can get 12 or 16 grams of CO2 in a small package. You could not do this were the substance to remain gaseous. When expanded to a lower pressure the liquid changes into a gas to inflate the tire (and this absorbs heat, which is why the inflator gets cold).

If you have taken chemistry or physics you may have been exposed to 'phase diagrams' for various substances: these show the state of the substance as a function of various variables (e.g. pressure, temperatue, etc.). CO2 is also common, non-toxic and will not harm rubber.