Abstract: At approximately 11:15 a.m. on August 28, 1991, a Worthington pump compressor, model #VR-10E, serial #1-53832, in the compressor room of building #3, ruptured, releasing ammonia vapors. No work was being done in the compressor room at the time of the accident. The only possible witness in the area was the watch engineer, who was suspended and not available for an interview. The employer believes that the watch engineer should have shut down the system before it was overpressurized. The inspection was unable to determine if the accident was caused by error made by the watch engineer. Employees #1, #2, and #3, none of whom work in the compressor room, were exposed to the ammonia vapors. Employee #1, who was working in building #4 next to the leak, was informed of the leak, then exited through the east side of the building where the vapors were present. She suffered irritation of the lungs, nose, and throat with low blood oxygen. Employee #1 was hospitalized for observation (approximately 24 hours). Employee #2 was outside going to lunch; Employee #3, a machine operator in building #4, was exposed to the vapors while in the building. Employees #2 and #3 had irritation of the lungs, nose and throat, and were treated and released from the hospital the same day. No causal factors were determined except the error Employee #1 made in exiting through the east side of the building. The inspection was not able to determine why the compressor became overpressurized.