Abstract: Employees #1 through #5 were attempting to find the source of a fire that had started when cardboard, which was being used to block a vent, ignited from welding slag. The exhaust system drew the burning cardboard through the system. The high volume of air flow accelerated the burning cardboard and caused the dust collection filter to ignite. When maintenance personnel attempted to locate the source of the smoke, the filter access caps were opened, letting the smoke out into the plant area. When the supervisor realized that the fire was past the incipient stage, employees were evacuated and the fire department was called. Employees #1 through #5, who had been using a portable fire extinguisher to try to put out the blaze, were sent to the hospital as a precautionary measure. Employees #2 through #5 were checked and released. Employee #1 experienced respiratory distress from a pre-existing asthmatic condition, and he was admitted for observation. The employer does have a comprehensive safety and health policy and program; it investigates accidents and takes appropriate corrective measures when a problem is noted. Employees #1 through #5 had not been trained in the use of portable fire extinguishers and appropriate training is now being conducted.