Abstract: On June 19, Employees #1through #4 were doing electrical and mechanical repairs in the bleach plant and digester building of the Palo Mill. Employees #5 through #8 were in the carpenter shop lunchroom in the bleach plant on break. Sulfur dioxide escaped from the dump chest off gas controlling system, and all eight employees required hospitalization. The cause of the release from the sulfur dioxide controlling (recovery) system was that a valve on the #1 digester malfunctioned and remained open, letting fresh liquor flow into the dump chest. This was confirmed by the cook who saw that the valve light on his control panel was off, indicating a closed valve. The off light resulted because of the jammed dump valve, which is electrically driven and subject to being overloaded under the above condition. The #1 digester blow valve stem jammed because a journeyman pipefitter had accidentally removed its stem guide several days earlier. He and a newly hired pipefitter were supposed to have removed the blow valve stem guide from the #4, not the #1, digester. The #4 digester was down and locked out for repairs. All mill employees are required to carry half-face respirators. During this sulfur dioxide release, the respirators were inadequate.