For starters, light caused by welding is extremely intense. The intensity can cause the retina to be burned/destroyed. The cells are damaged by the intense light, same reason why you don't look into a laser beam. Light caused by welding contains some ultraviolet light which is harmful. So it is best to wear long sleeved shirts and pants. And of course your welding mask!
Sputnik is correct. The spectrum of light does have a high UV component. It is also due to the fact that the light is coming form a small point source. Energy density is very high. It is also possible to do bad cornea damage as well as retinal. Uv also promotes cataracts.