Abstract: Maintenance workers changed a seal on a votator. They pressurized the system to 60 psi to check for leaks, then opened a valve to bring a vacuum compressor on-line to lower the pressure to normal operating pressure. The compressor had been shut down and was causing internal pressure in the system to continue to build up. A safety valve vented anhydrous ammonia to the building's roof as the pressure exceeded 150 psi. Ventilation intakes on the building's roof spread the ammonia vapors throughout the entire building. All employees were evacuated as the maintenance workers brought the compressor on-line and eventually lowered the pressure to close the safety relief valve. Ammonia vapors settled into low areas around the building, overexposing nine employees. Four employees were kept overnight at the hospital. An additional employee was admitted the following day.