The electric arc, used in MMA, MIG-MAG, TIG, and other welding processes, causes visible and invisible fumes to be given off by the melting flux when the arc is burning. These fumes can be harmful if inhaled through the nose and mouth of the welder. The content of the fumes may be either asphyxiating or toxic.

Asphyxiating fumes

Asphyxiating fumes are largely due to the shielding gases used around the arc. They are invisible and produce their effect by displacement of the surrounding air and thus, of its oxygen. (The air which we breathe consists mainly of nitrogen and oxygen, roughly in the ratio of four parts nitrogen to one part oxygen.) Asphyxiation begins when the oxygen content of the air falls below about 18%.

The gases usually used in welding are argon, carbon dioxide, helium and their mixtures. Argon and carbon dioxide are heavier than air (the mixtures are known under trade names). A mixture is selected to give the best welding conditions for a particular process. A simple mixture is the rutile coating for mild steel electrodes in the MMA welding of mild steel. The rutile gives a smooth arc, which is easily managed, but it gives off some hydrogen, which is absorbed by the molten metal and leads to hydrogen cracking under certain conditions of stress.