Welding produces loads of acrid fumes from the ionized air, welding rod shields making vapor, and contamination on the steel you were welding. There is also intense UV light that can damage your retinas and the UV can burn the whites of your eyes.
That is why you wear a hood, make sure it is set correctly for the operation you are doing and that you have the right fans running to blow away the fumes.
In some shops you have to have a pressurized air line to your hood and it has to be fed by a ventilation certified compressor (usually a low pressure rig that feeds you cool air through a lightweight 1" low pressure tube).
Either way make all your setups according to AWS.