Abstract: The worksite was a state penitentiary ash silo that received and stored ash from two boilers until it was transported elsewhere. The outside temperature was minus 11 degrees Fahrenheit when inmates began to empty ash out of the silo. Earlier in the week, however, several boiler tubes had ruptured, sending an unusual amount of wet ash and unburnt coal into the silo, freezing and plugging the outlet. Employee #2 decided to enter the silo and ram out the obstruction from the inside. Employee #1 saw Employee #2 collapse, and called for help before entering to rescue him. Employees #3, #4, and #7 then entered the silo to help. None of the employees realized that carbon monoxide had built up in the silo due to smoldering ash and unburnt coal. Employees #9, #10, and #25 through #29, city fire fighters, were also exposed to carbon monoxide while rescuing Employees #1 through #3. The three workers were, by then, unconscious, and they all died. Later that same day, Employees #6, #8, and #11 through #29 were exposed to carbon monoxide when the conveying system for sending ash to the silo was opened up for maintenance. Employees #6, #8, #11, and #12 were hospitalized for overexposure. The system had been in operation for 16 years and was not perceived as a hazard. Employees routinely entered the ash silo to clean it out or to unplug it.