Did you know that materials like flour and coal can cause explosions with the correct particle-to-air ratio is present?
All it takes is an intense heat source like a welding arc to explode the mixture of the fine particles in the air and the chain reaction will take place. Those workers would have been killed instantly.
This incident indicates that a flour explosion occurred in the tunnel as most probably no one bothered to check if a dangerous condition existed.
Most factory owners always rush maintenance works and short cuts are the norm. No wonder students shun away from engineering as a career.