Abstract: On October 2, 2007, Employees #1 through #5 and four coworkers, of RPI Coating, Inc., were staging materials in a penstock. They were using a Graco Xtreme airless sprayer, which heated a two-part epoxy paint. The sprayer was not operating correctly and the crew was directed to use approximately 5 to 10 gallons of methyl ethyl ketone (MEK) to flush the hoses. The flammable MEK vapors and liquids ignited into a flash fire. The resulting heat could have exceeded the decomposition temperature of the Duromar HPL 2510 hardener, and combustion by-products filled the occupied area of the penstock. The crew attempted to escape to a smoke-free area, but the natural ventilation of the penstock caused the smoke and vapors to enter the area into which they fled. Employees #1 through #5 were exposed to carbon monoxide, carbon dioxide and/or to chemical vapors from the decomposition of the Duromar HPL-2510 hardener and base of the epoxy paint. All five employees dies of asphyxia. There were several potential ignition sources, including: 1) the sprayer's heater, which did not appear to be de-energized prior to the use of MEK; 2) static electricity, with the sprayer recycling MEK into the hopper while the sprayer sat on a rolling scaffold; or 3) the electrical wiring, which did not appear to be approved for use in Class I-Division or Class I-Division 2 locations.