Gaseous fume is process dependent, and doesn't originate from the welding consumable or parent plate. Processes most likely to give gaseous fume are those with an open arc, such as MIG, TIG and Plasma Cutting.
Welding fume is an unavoidable by-product of welding. It consists of particulate fume, the part you can see and gaseous fume, which you cannot see, but can sometimes smell. All welding processes generate welding fume, the type and amount varying from process to process; MMA, MIG, FCAW and Plasma cutting tend to produce most, while TIG, Plasma welding, Laser welding and Submerged Arc give very little under normal circumstances, although TIG can give high levels of gaseous fume.
What is Gaseous Fume?
Gaseous fume consists of either one or more specific pollutant gases, mixed in the air around the welding area. As it is present at gaseous form it can easily enter the lungs. Whether the fume is likely to cause damage, depends largely on precisely what the gas, or gases are and on the concentration breathed in and the length of time of exposure to the specific pollutant(s).