Abstract: Employee #1, a security guard at an aquarium, entered an area where there had been an ozone release of unknown concentration. The ozone is used to disinfect water in the aquarium and is generated on site. A malfunctioning safety relief valve released the ozone into the immediate vicinity. Employee #2, the operator, shut down the ozone generator and opened a door to the outside to ventilate the area. This triggered a security alarm, and Employee #1 was sent to investigate. He did not know about the ozone release and did not wear respiratory protection. He walked through the area and into a control room where he learned of his exposure from the operator. Shortly, he began to feel faint and lost consciousness, due either to the ozone or to anxiety over his possible exposure. He was taken to the local hospital and released within several hours with no apparent aftereffects. Employee #2 was also exposed to the ozone release.