Anyone who has used a welder knows that large amounts of sparks fly everywhere. They can easily ignite clothing (c'mon, who hasn't had a shirt sleeve or pants cuff start smoldering?) and flammable objects on the floor. Little molten beads of metal bounce all over the place and can easily go unnoticed while one's face is being shielded by the welding goggles or mask.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/Articles/2007/01/30/Welding-Safety-Is-No-Accident