The gas is Inert in so much as it wont react with the metal, what will happen is the gas decomposes whilst welding to leave carbon monoxideShouldn't change to monoxide from dioxide, MIG is metal INERT gas welding after all
Some info here in the chart at the base of the page
http://www.elcosh.org/document/1003...t%3A+Welding+Fumes+and+Gases.html?show_text=1
I was struggling to find much info on the net but I seem to remember it from a safety course years ago, the main point is what ever you are welding it will give off some potentially bad fumes so make sure its well ventilated and you have a fire extinguisher to hand