phosgene was one of the primary fillers in gas shells during WWI thats why its often confused with mustard gas. I have never experianced that gas yet though. I have experianced chlorine gas while disolving mortar samples for analisis...nasty stuff. I have also, while doing a fire assay on sulphide ore, got a noze full of Hydrogen Sulphide gas again not pleasant. While we are on the topic of welding hazards I am surprized no one has brought up radiation hazards. Some Tig welding rods contain thorium, a radioactive element that you really do not want in your lungs. Likewise some alloys (mostly aluminum or zinc) contain thorium so care is advisable. Be carefull of some bronze and brass material also. Arsenic bronze was once commonly used for things like pipe fittings which while relatively harmless as is its a big problem if you cut grind or melt it. All this to say wearing some kind of dust mask is just smart practice.