In a small metal fabricating shop two men were welding aluminum brackets when both men felt dizzy and collapsed before they could get out of the shop. A foreman bringing his truck in to be fixed at that moment opened a bay door and drove in. Seeing the men on the floor he jumped out and called for help. By the time paramedics got there the men were fine again. The foreman thinking the men were just fooling around ordered them back to work. He was considering firing them until he saw them weld after the incident. The welds were all black and uneven looking (proper aluminum welds never look this way). Then he realized what had happened and apologized to the men.

What happened?