1st..
Welding stainless isn't poisonous, but it does give off hexavalent chromium, which is a carcinogen.
Essentially, you'll want to wear a respirator under your helmet. Hexavalent chromium is what gives stainless steel it's corrosion resistance. It is given off during welding, grinding, and some dipping processes. When welding chromium will rise in the smoke cloud, and will drop back down in the area. Just make sure not to keep food in the area, and wash your hands after working with it. Also, it can cause skin irritation and will be on the cloths you weld in.
Essentially, everything in this world will kill you, just use your noggin, and maybe a respirator when welding stainless, galvanized and zinc plated parts.
Also, clean metal is the best thing for welding. Dirt, paint oxidation and anything else cause weld contamination.