not being a welder i dont know about the chemistry of it all and some interesting comments have been made, one thing though it does show how important H&S can be in saving lives, without knowledge of what could go wrong as pointed out, the lives of the person welding and those around could be in danger.
we use gas bottles at work (no flame) for gassing up the camera peds, these are locked into a portable purpose built (for want of a better name) sackbarrow. There's been mention of in the past of cylinders falling over and the tap getting sheared or damaged somehow and the cylinder acting like an apollo rocket

I always look at these sackbarrows and wonder if they are stable enough not to fall or be pushed over.
though i have to say even qualified welders lose the plot sometimes as experienced on a site i was on in the 70's
they were mucking about and connected a cylinder(dont know of what) to a hose and a length of pipe, a bolt was inserted in the other end of the pipe and a lever was hit with a lump of metal.........well the bolt shot off across the site floor through a double glazed window across the road and through an office window opposite, a short time later the police entered the site and everyone including ourselves had disappeared to work on another floor

never knew if anyone got caught for it or not but i was shocked at the power behind one of those cylinders!