An employee was on a mezzanine to start to dismantle it. He was in the process of moving a piece of plywood that had been installed to cover an opening in the mezzanine where an exhaust stack had been previously installed. While in the process of removing the plywood, he lost his footing and fell approximately 9 feet to the concrete floor below, suffering a fractured elbow and leg, as well as a cracked pelvis and hip.