Abstract: Overpressurization of an ammonia refrigeration system due to the inadvertent shut-off of its compressor caused an ammonia release through an elevated safety release valve. Most of the ammonia was vented outside but some of the gas was pulled inside the plant by the ventilation system. Workers inside were evacuated, and Employees #1 through #7 had minor to moderate exposure to ammonia, causing acute illness. All seven employees were sent to area hospitals and were released the same day. None sustained long-term or permanent harm. One supervisor put on a gas mask to re-enter the compressor control room to restart the compressor system. He suffered no apparent illnesses.