I have been welding for 47 years and there isn't any metal that doesn't give off some toxic fumes. Lead is the worst but others are bad also. Mild steel sheet metal is not so bad if it doesn't have any coatings on it such as Galvanise or Epoxy coatings, even some paints are bad. My advice is to clean the metal by grinding off all coatings and vent the area and weld it...GlennK
Yes, galvanized is bad, but you can grind the galvanizing off on both sides and weld it with little or no zinc fumes. you only have to grind the weld area approx. 1 1/2 inches on both sides and it should be pretty safe. I think some of the epoxy coatings are worse than the galvanized. At any rate clean it good and you should be ok, orjust go and buy raw sheet metal from the steel supply and then you have no worries. If I worried about every piece of steel I welded I probably wouldn't have welded any thing together and I have welded almost every alloy known to man and welded nearly every working day. Tig..Mig. stick..Oxy-acetylene..Silver braze...Nearly all of it..I am 66 years old and retired and still welding in my garage and building chassis and what ever else I need to weld... I loved it when I started and I still love it. One day it may kill me but til then...Weld On....GlennK..
As said above welding fumes of any kind aren't much fun. Steel is ok, zinc galvanize fumes aren't toxic - no long term health risk - but breating the fumes will cause the "galvanize fever" - a few hours after breathing them you sill swear you have the flu! Goes away over night. Cadmium plating is another story. It will do permanent damage so be very careful if it. Best bet is to do as advised above and clean any coating off the steel well clear of the heat affected zone and you won't have any problems. Also, if you leave the coatings, they will pollute the weld and the joint will be significantly weakened. Also, use a fan to keep fresh air circulated into your environment reqardless of what you are welding. Can get a great box fan @ Walmart for about $20.
One thing you can check is whether it is alloy steel or mild steel. Grinding sparks for mild steel are red balls whereas alloy steel gives off bright red or white star shaped sparks. The latter must be welded the heat treated or will be brittle and prone to crack.