I used to work in a shop rebuilding underground coal mining equipment.  Lots of smoke, burning grease and coal dust, air arc gouging, welding over paint, you name it.  Our ventilation was a tall ceiling and closed doors.  We exclusively used FCAW-G, Lincoln's "Ultracore" electrode.  On the packaging it said in reference to the high manganese levels in the fume "if used indoors, local exhuast is required.  If used outdoors, a respirator may be required"  Now I was the only guy in the shop to use, or even own a respirator, but if the electrode maker says to use a respirator outdoors, the stuff can't be too good for you.  Long term manganese exposure leads to all sorts of fun stuff that will get your doctor that new boat he has always wanted, but it will get you brain damage, shaky hands, drooling, memory loss etc.  I tried to get one of my young co-workers to think about using a respirator one time.  I was switching filters and gave him the old one to inspect.  He tore it apart and shook out all of the dust that had accumulated and had a nice little pile of the stuff.  To all of you tough guy weldors out there, I have heard from many lawyers that welding fume is " the next asbestos" when it comes to personal injury litigation.  The cost of a respirator is fifteen bucks.  No they aren't comfortable, but neither is drooling on your shaking hands trying to remember what your kids names are.  Me, I'm saving up for an ultra sexy PAPR from 3M.