Abstract: At 9:45 a.m. on August 20, 1987, Employees #1 and #2 were in an approximately 10 ft by 6 ft by 5 1/2 ft space, cleaning soot from the tubes in a boiler air preheater. There was an induced draft fan between the air preheater and stack gas header duct. Three other boilers, two of which were operating, shared the header duct. Due to the high air demand from cleaning the boiler burners and the startup of a paper machine, a positive pressure was created in the header, forcing stack gases from the online boilers (sulfur dioxide, carbon dioxide, and nitrous oxide) into the space where the employees were working. They had been provided only with dust respirators and the gases forced them out of the confined space. Both employees were sent to the hospital and released; one of them lost work days as a result of the accident.