Abstract: On September 6, 2001, Employee #1 was positioned over an elevated walkway adjacent to a mixing unit. Inadequate cleaning procedure was being used to clean the mixer, which were never discussed with the manufacturer. This cleaning procedure involved the introduction of a solvent, diesel fuel, by a hand held hose through an inspection door into a hot container, a Madsen Model Number 440 Twin Shaft Pug Mill Mixer, Serial Number 533.6, which contained an ignition source of unknown origin. When the diesel fuel used as a cleaning solvent was introduced into the hot environment of the pug mill, the solvent quickly evaporated into a fog of solvent vapor and atomized fuel droplets. This mixture of liquid solvent, solvent vapor, and atomized solvent droplets was potentially flammable and explosive at any volumetric concentration between 0.3 and 10.0 when mixed with air. The mixture exploded when the volumetric concentration ran into this explosive range and an ignition occurred. Employee #1 was blown over the top rail of the walkway handrail and landed approximately 15.5 ft below. He was hospitalized for burns to his face and neck as well as sever head trauma.