@thorninmud
I’m not so sure about “vaporizing” metals; arc welding wouldn’t be so useful if it was actually vaporizing metal in the joints that the welder is trying to fill (or it would be more of a cutting operation than welding – such as plasma-arc cutting, for example, in which metal is concurrently melted and blown away by the arc). The temperatures are certainly high enough, but the shielding prevents the vaporization and atmospheric loss of metal you’re concerned with. In shielded metal arc welding, submerged arc welding and metal inert gas (mig) welding the shielding flux does generate a lot of smoke, but it’s not ‘metal’ (other than in the strict chemical sense that carbon and other elements, for example, are ‘metals’). And tungsten inert gas welding (tig) doesn’t even have a flux cover; that’s a process that heats metal to the melting point and adds compatible filler metal under the shielding gas. Again, no ‘vaporization’ of the metal.
You’re right that oxy-acetylene welding uses less heat than any of those processes, though, and has no shielding flux or gas for the welder to be concerned with.
And you’re also correct that the contaminants that can be found in metals, especially during the cutting process, and most especially when plasma-arc or acetylene torches are used, can put a lot of unwanted elements in the air. It’s especially helpful to wear a respirator for cutting operations. During normal welding operations, the welder would prep the joint area by grinding or machining to ‘clean’ base metals, unless the weld area is covered by a compatible coating (such as red oxide primer) that is designed to be welded through with no adverse health or mechanical drawbacks.
Finally, while no one wants to breathe any welding fumes that they don’t have to, breathing the fumes from ‘normal’ welding of galvanized steel is not particularly harmful.