Worse than what was dropped in the trenches. There were only a few known deaths from mustard gas. Gangrene is death and decay of tissue that works its way into living tissue through the blood stream and causes all sorts of other bacteria to flourish. It was also the leading cause of death and ampitations before we had the understanding of germs and bacteria that we have now and had killed more people than bullets in the civil war and likely more than all enemy actions in every war pre 20th century. Mustard gas on the other hand is nothing more than a blister agent that can at least heal back up and recover.