From a frame perspective i don't think welding is a big deal. There is very little actual welding in a frame. Meaning that while obviously it is one of the key steps, it is a fairly trivial amount of work. When I am welding I am using about 20-30 amps of a total 150, that tops out around 30 amps input. So I am using about 5 amps input, about the same as a loaded electric drill.

The shield gas used is inert. The welding wire is just more metal.

Now behind all this stuff you have the electronics industry, the electrical power industry (my welding juice is all nuclear), and the steel industry etc... But what we all do with that stuff is pretty benign.

The paint used is probably a bigger at source of production problem than anything else.

Aluminum is a lot more consumptive than 4130. So maybe we are talking about running several electric drills for a while...