Abstract: Employees #1 through #8, of PCS Phosphate, Inc., were using jackhammers and a water hose to remove built up deposits of tricalcium phosphate (a feed supplement) from a kiln vent duct. During this maintenance procedure a larger than normal piece of the build-up came loose and fell down the incinerator's vent chamber (plenum), creating negative air pressure. A large, explosive gust of hot air and dust escaped through both vent openings, causing heat/steam burns to employees who were using jackhammers and water hoses at the vent openings. The exhausting gases were so great that employees were knocked down from the force. Employees #5, #6 and #8 were hospitalized. The others were treated and released.